ALBERT

Description: Accounting for the effects of sastrugi in the CERES clear-sky Antarctic shortwave angular distribution models Atmospheric Measurement Techniques, 8, 3163-3175, 2015 Author(s): J. Corbett and W. Su The Cloud and the Earth's Radiant Energy System (CERES) instruments on NASA's Terra, Aqua and Soumi NPP satellites are used to provide a long-term measurement of Earth's energy budget. To accomplish this, the radiances measured by the instruments must be inverted to fluxes by the use of a scene-type-dependent angular distribution model (ADM). For permanent snow scenes over Antarctica, shortwave (SW) ADMs are created by compositing radiance measurements over the full viewing zenith and azimuth range. However, the presence of small-scale wind blown roughness features called sastrugi cause the BRDF (bidirectional reflectance distribution function) of the snow to vary significantly based upon the solar azimuth angle and location. This can result in monthly regional biases between −12 and 7.5 Wm −2 in the inverted TOA (top-of-atmosphere) SW flux. The bias is assessed by comparing the CERES shortwave fluxes derived from nadir observations with those from all viewing zenith angles, as the sastrugi affect fluxes inverted from the oblique viewing angles more than for the nadir viewing angles. In this paper we further describe the clear-sky Antarctic ADMs from Su et al. (2015). These ADMs account for the sastrugi effect by using measurements from the Multi-Angle Imaging Spectro-Radiometer (MISR) instrument to derive statistical relationships between radiance from different viewing angles. We show here that these ADMs reduce the bias and artifacts in the CERES SW flux caused by sastrugi, both locally and Antarctic-wide. The regional monthly biases from sastrugi are reduced to between −5 and 7 Wm −2 , and the monthly-mean biases over Antarctica are reduced by up to 0.64 Wm −2 , a decrease of 74 %. These improved ADMs are used as part of the Edition 4 CERES SSF (Single Scanner Footprint) data.

Description: New and improved infrared absorption cross sections for dichlorodifluoromethane (CFC-12) Atmospheric Measurement Techniques, 8, 3197-3207, 2015 Author(s): J. J. Harrison Despite its widespread commercial use throughout the twentieth century, primarily in the refrigeration industry, dichlorodifluoromethane (CFC-12) is now known to have the undesirable effect of depleting stratospheric ozone. As this long-lived molecule slowly degrades in the atmosphere, monitoring its vertical concentration profile using infrared sounders on satellite platforms crucially requires accurate laboratory spectroscopic data. This work describes new high-resolution infrared absorption cross sections of dichlorodifluoromethane over the spectral range 800–1270 cm −1 , determined from spectra recorded using a high-resolution Fourier transform spectrometer (Bruker IFS 125HR) and a 26 cm pathlength cell. Spectra of dichlorodifluoromethane/dry synthetic air mixtures were recorded at resolutions between 0.01 and 0.03 cm −1 (calculated as 0.9/MOPD; MOPD = maximum optical path difference) over a range of temperatures and pressures (7.5–761 Torr and 190–294 K) appropriate for atmospheric conditions. This new cross-section dataset improves upon the one currently available in the HITRAN and GEISA databases.

Description: Calibration of 3-D wind measurements on a single-engine research aircraft Atmospheric Measurement Techniques, 8, 3177-3196, 2015 Author(s): C. Mallaun, A. Giez, and R. Baumann An innovative calibration method for the wind speed measurement using a boom-mounted Rosemount model 858 AJ air velocity probe is introduced. The method is demonstrated for a sensor system installed on a medium-size research aircraft which is used for measurements in the atmospheric boundary layer. The method encounters a series of coordinated flight manoeuvres to directly estimate the aerodynamic influences on the probe and to calculate the measurement uncertainties. The introduction of a differential Global Positioning System (DGPS) combined with a high-accuracy inertial reference system (IRS) has brought major advances to airborne measurement techniques. The exact determination of geometrical height allows the use of the pressure signal as an independent parameter. Furthermore, the exact height information and the stepwise calibration process lead to maximum accuracy. The results show a measurement uncertainty for the aerodynamic influence of the dynamic and static pressures of 0.1 hPa. The applied parametrisation does not require any height dependencies or time shifts. After extensive flight tests a correction for the flow angles (attack and sideslip angles) was found, which is necessary for a successful wind calculation. A new method is demonstrated to correct for the aerodynamic influence on the sideslip angle. For the three-dimensional (3-D) wind vector (with 100 Hz resolution) a novel error propagation scheme is tested, which determines the measurement uncertainties to be 0.3 m s −1 for the horizontal and 0.2 m s −1 for the vertical wind components.

Description: On the consistency of 2-D video disdrometers in measuring microphysical parameters of solid precipitation Atmospheric Measurement Techniques, 8, 3251-3261, 2015 Author(s): F. Bernauer, K. Hürkamp, W. Rühm, and J. Tschiersch Detailed characterization and classification of precipitation is an important task in atmospheric research. Line scanning 2-D video disdrometer devices are well established for rain observations. The two orthogonal views taken of each hydrometeor passing the sensitive area of the instrument qualify these devices especially for detailed characterization of nonsymmetric solid hydrometeors. However, in case of solid precipitation, problems related to the matching algorithm have to be considered and the user must be aware of the limited spatial resolution when size and shape descriptors are analyzed. Clarifying the potential of 2-D video disdrometers in deriving size, velocity and shape parameters from single recorded pictures is the aim of this work. The need of implementing a matching algorithm suitable for mixed- and solid-phase precipitation is highlighted as an essential step in data evaluation. For this purpose simple reproducible experiments with solid steel spheres and irregularly shaped Styrofoam particles are conducted. Self-consistency of shape parameter measurements is tested in 38 cases of real snowfall. As a result, it was found that reliable size and shape characterization with a relative standard deviation of less than 5 % is only possible for particles larger than 1 mm. For particles between 0.5 and 1.0 mm the relative standard deviation can grow up to 22 % for the volume, 17 % for size parameters and 14 % for shape descriptors. Testing the adapted matching algorithm with a reproducible experiment with Styrofoam particles, a mismatch probability of less than 3 % was found. For shape parameter measurements in case of real solid-phase precipitation, the 2-DVD shows self-consistent behavior.

Description: Schneefernerhaus as a mountain research station for clouds and turbulence Atmospheric Measurement Techniques, 8, 3209-3218, 2015 Author(s): S. Risius, H. Xu, F. Di Lorenzo, H. Xi, H. Siebert, R. A. Shaw, and E. Bodenschatz Cloud measurements are usually carried out with airborne campaigns, which are expensive and are limited by temporal duration and weather conditions. Ground-based measurements at high-altitude research stations therefore play a complementary role in cloud study. Using the meteorological data (wind speed, direction, temperature, humidity, visibility, etc.) collected by the German Weather Service (DWD) from 2000 to 2012 and turbulence measurements recorded by multiple ultrasonic sensors (sampled at 10 Hz) in 2010, we show that the Umweltforschungsstation Schneefernerhaus (UFS) located just below the peak of Zugspitze in the German Alps, at a height of 2650 m, is a well-suited station for cloud–turbulence research. The wind at UFS is dominantly in the east–west direction and nearly horizontal. During the summertime (July and August) the UFS is immersed in warm clouds about 25 % of the time. The clouds are either from convection originating in the valley in the east, or associated with synoptic-scale weather systems typically advected from the west. Air turbulence, as measured from the second- and third-order velocity structure functions that exhibit well-developed inertial ranges, possesses Taylor microscale Reynolds numbers up to 10 4 , with the most probable value at ~ 3000. In spite of the complex topography, the turbulence appears to be nearly as isotropic as many laboratory flows when evaluated on the "Lumley triangle".

Description: Plume-based analysis of vehicle fleet air pollutant emissions and the contribution from high emitters Atmospheric Measurement Techniques, 8, 3263-3275, 2015 Author(s): J. M. Wang, C.-H. Jeong, N. Zimmerman, R. M. Healy, D. K. Wang, F. Ke, and G. J. Evans An automated identification and integration method has been developed for in-use vehicle emissions under real-world conditions. This technique was applied to high-time-resolution air pollutant measurements of in-use vehicle emissions performed under real-world conditions at a near-road monitoring station in Toronto, Canada, during four seasons, through month-long campaigns in 2013–2014. Based on carbon dioxide measurements, over 100 000 vehicle-related plumes were automatically identified and fuel-based emission factors for nitrogen oxides; carbon monoxide; particle number; black carbon; benzene, toluene, ethylbenzene, and xylenes (BTEX); and methanol were determined for each plume. Thus the automated identification enabled the measurement of an unprecedented number of plumes and pollutants over an extended duration. Emission factors for volatile organic compounds were also measured roadside for the first time using a proton transfer reaction time-of-flight mass spectrometer; this instrument provided the time resolution required for the plume capture technique. Mean emission factors were characteristic of the light-duty gasoline-dominated vehicle fleet present at the measurement site, with mean black carbon and particle number emission factors of 35 mg kg fuel −1 and 7.5 × 10 14 # kg fuel −1 , respectively. The use of the plume-by-plume analysis enabled isolation of vehicle emissions, and the elucidation of co-emitted pollutants from similar vehicle types, variability of emissions across the fleet, and the relative contribution from heavy emitters. It was found that a small proportion of the fleet ( 〈 25 %) contributed significantly to total fleet emissions: 100, 100, 81, and 77 % for black carbon, carbon monoxide, BTEX, and particle number, respectively. Emission factors of a single pollutant may help classify a vehicle as a high emitter; however, regulatory strategies to more efficiently target multi-pollutant mixtures may be better developed by considering the co-emitted pollutants as well.

Description: High-resolution measurement of cloud microphysics and turbulence at a mountaintop station Atmospheric Measurement Techniques, 8, 3219-3228, 2015 Author(s): H. Siebert, R. A. Shaw, J. Ditas, T. Schmeissner, S. P. Malinowski, E. Bodenschatz, and H. Xu Mountain research stations are advantageous not only for long-term sampling of cloud properties but also for measurements that are prohibitively difficult to perform on airborne platforms due to the large true air speed or adverse factors such as weight and complexity of the equipment necessary. Some cloud–turbulence measurements, especially Lagrangian in nature, fall into this category. We report results from simultaneous, high-resolution and collocated measurements of cloud microphysical and turbulence properties during several warm cloud events at the Umweltforschungsstation Schneefernerhaus (UFS) on Zugspitze in the German Alps. The data gathered were found to be representative of observations made with similar instrumentation in free clouds. The observed turbulence shared all features known for high-Reynolds-number flows: it exhibited approximately Gaussian fluctuations for all three velocity components, a clearly defined inertial subrange following Kolmogorov scaling (power spectrum, and second- and third-order Eulerian structure functions), and highly intermittent velocity gradients, as well as approximately lognormal kinetic energy dissipation rates. The clouds were observed to have liquid water contents on the order of 1 g m −3 and size distributions typical of continental clouds, sometimes exhibiting long positive tails indicative of large drop production through turbulent mixing or coalescence growth. Dimensionless parameters relevant to cloud–turbulence interactions, the Stokes number and settling parameter are in the range typically observed in atmospheric clouds. Observed fluctuations in droplet number concentration and diameter suggest a preference for inhomogeneous mixing. Finally, enhanced variance in liquid water content fluctuations is observed at high frequencies, and the scale break occurs at a value consistent with the independently estimated phase relaxation time from microphysical measurements.

Description: The Ice Selective Inlet: a novel technique for exclusive extraction of pristine ice crystals in mixed-phase clouds Atmospheric Measurement Techniques, 8, 3087-3106, 2015 Author(s): P. Kupiszewski, E. Weingartner, P. Vochezer, M. Schnaiter, A. Bigi, M. Gysel, B. Rosati, E. Toprak, S. Mertes, and U. Baltensperger Climate predictions are affected by high uncertainties partially due to an insufficient knowledge of aerosol–cloud interactions. One of the poorly understood processes is formation of mixed-phase clouds (MPCs) via heterogeneous ice nucleation. Field measurements of the atmospheric ice phase in MPCs are challenging due to the presence of much more numerous liquid droplets. The Ice Selective Inlet (ISI), presented in this paper, is a novel inlet designed to selectively sample pristine ice crystals in mixed-phase clouds and extract the ice residual particles contained within the crystals for physical and chemical characterization. Using a modular setup composed of a cyclone impactor, droplet evaporation unit and pumped counterflow virtual impactor (PCVI), the ISI segregates particles based on their inertia and phase, exclusively extracting small ice particles between 5 and 20 μm in diameter. The setup also includes optical particle spectrometers for analysis of the number size distribution and shape of the sampled hydrometeors. The novelty of the ISI is a droplet evaporation unit, which separates liquid droplets and ice crystals in the airborne state, thus avoiding physical impaction of the hydrometeors and limiting potential artefacts. The design and validation of the droplet evaporation unit is based on modelling studies of droplet evaporation rates and computational fluid dynamics simulations of gas and particle flows through the unit. Prior to deployment in the field, an inter-comparison of the optical particle size spectrometers and a characterization of the transmission efficiency of the PCVI was conducted in the laboratory. The ISI was subsequently deployed during the Cloud and Aerosol Characterization Experiment (CLACE) 2013 and 2014 – two extensive international field campaigns encompassing comprehensive measurements of cloud microphysics, as well as bulk aerosol, ice residual and ice nuclei properties. The campaigns provided an important opportunity for a proof of concept of the inlet design. In this work we present the setup of the ISI, including the modelling and laboratory characterization of its components, as well as field measurements demonstrating the ISI performance and validating the working principle of the inlet. Finally, measurements of biological aerosol during a Saharan dust event (SDE) are presented, showing a first indication of enrichment of bio-material in sub-2 μm ice residuals.

Description: A modification to the standard ionospheric correction method used in GPS radio occultation Atmospheric Measurement Techniques, 8, 3385-3393, 2015 Author(s): S. B. Healy and I. D. Culverwell A modification to the standard bending-angle correction used in GPS radio occultation (GPS-RO) is proposed. The modified approach should reduce systematic residual ionospheric errors in GPS radio occultation climatologies. A new second-order term is introduced in order to account for a known source of systematic error, which is generally neglected. The new term has the form κ(a) × (α L1 (a)-α L2 (a)) 2 , where a is the impact parameter and (α L1 , α L2 ) are the L1 and L2 bending angles, respectively. The variable κ is a weak function of the impact parameter, a , but it does depend on a priori ionospheric information. The theoretical basis of the new term is examined. The sensitivity of κ to the assumed ionospheric parameters is investigated in one-dimensional simulations, and it is shown that κ ≃ 10–20 rad −1 . We note that the current implicit assumption is κ=0, and this is probably adequate for numerical weather prediction applications. However, the uncertainty in κ should be included in the uncertainty estimates for the geophysical climatologies produced from GPS-RO measurements. The limitations of the new ionospheric correction when applied to CHAMP (Challenging Minisatellite Payload) measurements are noted. These arise because of the assumption that the refractive index is unity at the satellite, made when deriving bending angles from the Doppler shift values.

Description: Editorial Note "A novel Whole Air Sample Profiler (WASP) for the quantification of volatile organic compounds in the boundary layer" published in Atmos. Meas. Tech., 6, 2703–2712, 2013 Atmospheric Measurement Techniques, 8, 3405-3406, 2015 Author(s): T. Wagner, H. Harder, J. Joiner, P. Laj, and A. Richter No abstract available.

Description: Global validation of SCIAMACHY limb ozone data (versions 2.9 and 3.0, IUP Bremen) using ozonesonde measurements Atmospheric Measurement Techniques, 8, 3369-3383, 2015 Author(s): J. Jia, A. Rozanov, A. Ladstätter-Weißenmayer, and J. P. Burrows In this paper, the latest SCIAMACHY limb ozone scientific vertical profiles, namely the current V2.9 and the upcoming V3.0, are extensively compared with ozonesonde data from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) database. The comparisons are made on a global scale from 2003 to 2011, involving 61 sonde stations. The retrieval processors used to generate V2.9 and V3.0 data sets are briefly introduced. The comparisons are discussed in terms of vertical profiles and stratospheric partial columns. Our results indicate that the V2.9 ozone profile data between 20 and 30 km are in good agreement with ground-based measurements, with less than 5 % relative differences in the latitude range of 90° S–40° N (with the exception of the tropical Pacific region, where an overestimation of more than 10 % is observed), which corresponds to less than 5 DU partial-column differences. In the tropics the differences are within 3 %. However, this data set shows a significant underestimation northwards of 40° N (up to ~ 15 %). The newly developed V3.0 data set reduces this bias to below 10 % while maintaining a good agreement southwards of 40° N with slightly increased relative differences of up to 5 % in the tropics.

Description: Exploiting the sensitivity of two satellite cloud height retrievals to cloud vertical distribution Atmospheric Measurement Techniques, 8, 3419-3431, 2015 Author(s): C. K. Carbajal Henken, L. Doppler, R. Lindstrot, R. Preusker, and J. Fischer This work presents a study on the sensitivity of two satellite cloud height retrievals to cloud vertical distribution. The difference in sensitivity is exploited by relating the difference in the retrieved cloud heights to cloud vertical extent. The two cloud height retrievals, performed within the Freie Universität Berlin AATSR MERIS Cloud (FAME-C) algorithm, are based on independent measurements and different retrieval techniques. First, cloud-top temperature (CTT) is retrieved from Advanced Along Track Scanning Radiometer (AATSR) measurements in the thermal infrared. Second, cloud-top pressure (CTP) is retrieved from Medium Resolution Imaging Spectrometer (MERIS) measurements in the oxygen-A absorption band and a nearby window channel. Both CTT and CTP are converted to cloud-top height (CTH) using atmospheric profiles from a numerical weather prediction model. First, a sensitivity study using radiative transfer simulations in the near-infrared and thermal infrared was performed to demonstrate, in a quantitative manner, the larger impact of the assumed cloud vertical extinction profile, described in terms of shape and vertical extent, on MERIS than on AATSR top-of-atmosphere measurements. Consequently, cloud vertical extinction profiles will have a larger influence on the MERIS than on the AATSR cloud height retrievals for most cloud types. Second, the difference in retrieved CTH (ΔCTH) from AATSR and MERIS are related to cloud vertical extent (CVE), as observed by ground-based lidar and radar at three ARM sites. To increase the impact of the cloud vertical extinction profile on the MERIS-CTP retrievals, single-layer and geometrically thin clouds are assumed in the forward model. Similarly to previous findings, the MERIS-CTP retrievals appear to be close to pressure levels in the middle of the cloud. Assuming a linear relationship, the ΔCTH multiplied by 2.5 gives an estimate on the CVE for single-layer clouds. The relationship is stronger for single-layer clouds than for multi-layer clouds. Due to large variations of cloud vertical extinction profiles occurring in nature, a quantitative estimate of the cloud vertical extent is accompanied with large uncertainties. Yet, estimates of the CVE provide an additional parameter, next to CTH, that can be obtained from passive imager measurements and can be used to further describe cloud vertical distribution, thus contributing to the characterization of a cloudy scene. To further demonstrate the plausibility of the approach, an estimate of the CVE was applied to a case study. In light of the follow-up mission Sentinel-3 with AATSR and MERIS like instruments, Sea and Land Surface Temperature Radiometer (SLSTR) and (Ocean and Land Colour Instrument) OLCI, respectively, for which the FAME-C algorithm can be easily adapted, a more accurate estimate of the CVE can be expected. OLCI will have three channels in the oxygen-A absorption band, possibly providing enhanced information on cloud vertical distributions.

Description: Accuracy of retrieving temperature and humidity profiles by ground-based microwave radiometry in truly complex terrain Atmospheric Measurement Techniques, 8, 3355-3367, 2015 Author(s): G. Massaro, I. Stiperski, B. Pospichal, and M. W. Rotach Within the Innsbruck Box project, a ground-based microwave radiometer (RPG-HATPRO) was operated in the Inn Valley (Austria), in very complex terrain, between September 2012 and May 2013 to obtain temperature and humidity vertical profiles of the full troposphere with a specific focus on the valley boundary layer. In order to assess its performance in a deep alpine valley, the profiles obtained by the radiometer with different retrieval algorithms based on different climatologies are compared to local radiosonde data. A retrieval that is improved with respect to the one provided by the manufacturer, based on better resolved data, shows a significantly smaller root mean square error (RMSE), both for the temperature and humidity profiles. The improvement is particularly substantial at the heights close to the mountaintop level and in the upper troposphere. Lower-level inversions, common in an alpine valley, are resolved to a satisfactory degree. On the other hand, upper-level inversions (above 1200 m) still pose a significant challenge for retrieval. For this purpose, specialized retrieval algorithms were developed by classifying the radiosonde climatologies into specialized categories according to different criteria (seasons, daytime, nighttime) and using additional regressors (e.g., measurements from mountain stations). The training and testing on the radiosonde data for these specialized categories suggests that a classification of profiles that reproduces meaningful physical characteristics can yield improved targeted specialized retrievals. A novel and very promising method of improving the profile retrieval in a mountainous region is adding further information in the retrieval, such as the surface temperature at fixed levels along a topographic slope or from nearby mountaintops.

Description: Comparison of ozone retrievals from the Pandora spectrometer system and Dobson spectrophotometer in Boulder, Colorado Atmospheric Measurement Techniques, 8, 3407-3418, 2015 Author(s): J. Herman, R. Evans, A. Cede, N. Abuhassan, I. Petropavlovskikh, and G. McConville A comparison of retrieved total column ozone (TCO) amounts between the Pandora #34 spectrometer system and the Dobson #061 spectrophotometer from direct-sun observations was performed on the roof of the Boulder, Colorado, NOAA building. This paper, part of an ongoing study, covers a 1-year period starting on 17 December 2013. Both the standard Dobson and Pandora TCO retrievals required a correction, TCOcorr = TCO (1 + C(T) ), using a monthly varying effective ozone temperature, T E , derived from a temperature and ozone profile climatology. The correction is used to remove a seasonal difference caused by using a fixed temperature in each retrieval algorithm. The respective corrections C ( T E ) are C Pandora = 0.00333( T E -225) and C Dobson = -0.0013( T E -226.7) per degree K. After the applied corrections removed most of the seasonal retrieval dependence on ozone temperature, TCO agreement between the instruments was within 1 % for clear-sky conditions. For clear-sky observations, both co-located instruments tracked the day-to-day variation in total column ozone amounts with a correlation of r 2 = 0.97 and an average offset of 1.1 ± 5.8 DU. In addition, the Pandora TCO data showed 0.3 % annual average agreement with satellite overpass data from AURA/OMI (Ozone Monitoring Instrument) and 1 % annual average offset with Suomi-NPP/OMPS (Suomi National Polar-orbiting Partnership, the nadir viewing portion of the Ozone Mapper Profiler Suite).

Description: Retrieval and validation of carbon dioxide, methane and water vapor for the Canary Islands IR-laser occultation experiment Atmospheric Measurement Techniques, 8, 3315-3336, 2015 Author(s): V. Proschek, G. Kirchengast, S. Schweitzer, J. S. A. Brooke, P. F. Bernath, C. B. Thomas, J.-G. Wang, K. A. Tereszchuk, G. González Abad, R. J. Hargreaves, C. A. Beale, J. J. Harrison, P. A. Martin, V. L. Kasyutich, C. Gerbig, O. Kolle, and A. Loescher The first ground-based experiment to prove the concept of a novel space-based observation technique for microwave and infrared-laser occultation between low-Earth-orbit satellites was performed in the Canary Islands between La Palma and Tenerife. For two nights from 21 to 22 July 2011 the experiment delivered the infrared-laser differential transmission principle for the measurement of greenhouse gases (GHGs) in the free atmosphere. Such global and long-term stable measurements of GHGs, accompanied also by measurements of thermodynamic parameters and line-of-sight wind in a self-calibrating way, have become very important for climate change monitoring. The experiment delivered promising initial data for demonstrating the new observation concept by retrieving volume mixing ratios of GHGs along a ~144 km signal path at altitudes of ~2.4 km. Here, we present a detailed analysis of the measurements, following a recent publication that introduced the experiment's technical setup and first results for an example retrieval of CO 2 . We present the observational and validation data sets, the latter simultaneously measured at the transmitter and receiver sites; the measurement data handling; and the differential transmission retrieval procedure. We also determine the individual and combined uncertainties influencing the results and present the retrieval results for 12 CO 2 , 13 CO 2 , C 18 OO, H 2 O and CH 4 . The new method is found to have a reliable basis for monitoring of greenhouse gases such as CO 2 , CH 4 , and H 2 O in the free atmosphere.

Description: New calibration noise suppression techniques for the GLORIA limb imager Atmospheric Measurement Techniques, 8, 3147-3161, 2015 Author(s): T. Guggenmoser, J. Blank, A. Kleinert, T. Latzko, J. Ungermann, F. Friedl-Vallon, M. Höpfner, M. Kaufmann, E. Kretschmer, G. Maucher, T. Neubert, H. Oelhaf, P. Preusse, M. Riese, H. Rongen, M. K. Sha, O. Sumińska-Ebersoldt, and V. Tan The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) presents new opportunities for the retrieval of trace gases in the upper troposphere and lower stratosphere. The radiometric calibration of the measured signal is achieved using in-flight measurements of reference blackbody and upward-pointing "deep space" scenes. In this paper, we present techniques developed specifically to calibrate GLORIA data exploiting the instrument's imaging capability. The algorithms discussed here make use of the spatial correlation of parameters across GLORIA's detector pixels in order to mitigate the noise levels and artefacts in the calibration measurements. This is achieved by combining a priori and empirical knowledge about the instrument background radiation with noise-mitigating compression methods, specifically low-pass filtering and principal component analysis (PCA). In addition, a new software package for the processing of GLORIA data is introduced which allows us to generate calibrated spectra from raw measurements in a semi-automated data processing chain.

Description: An assessment of the performance of a 1.5 μm Doppler lidar for operational vertical wind profiling based on a 1-year trial Atmospheric Measurement Techniques, 8, 2251-2266, 2015 Author(s): E. Päschke, R. Leinweber, and V. Lehmann We present the results of a 1-year quasi-operational testing of the 1.5 μm StreamLine Doppler lidar developed by Halo Photonics from 2 October 2012 to 2 October 2013. The system was configured to continuously perform a velocity-azimuth display scan pattern using 24 azimuthal directions with a constant beam elevation angle of 75°. Radial wind estimates were selected using a rather conservative signal-to-noise ratio based threshold of −18.2 dB (0.015). A 30 min average profile of the wind vector was calculated based on the assumption of a horizontally homogeneous wind field through a Moore–Penrose pseudoinverse of the overdetermined linear system. A strategy for the quality control of the retrieved wind vector components is outlined for ensuring consistency between the Doppler lidar wind products and the inherent assumptions employed in the wind vector retrieval. Quality-controlled lidar measurements were compared with independent reference data from a collocated operational 482 MHz radar wind profiler running in a four-beam Doppler beam swinging mode and winds from operational radiosonde measurements. The intercomparison results reveal a particularly good agreement between the Doppler lidar and the radar wind profiler, with root mean square errors ranging between 0.5 and 0.7 m s −1 for wind speed and between 5 and 10° for wind direction. The median of the half-hourly averaged wind speed for the intercomparison data set is 8.2 m s −1 , with a lower quartile of 5.4 m s −1 and an upper quartile of 11.6 m s −1 .

Description: Comparison of operational satellite SO 2 products with ground-based observations in northern Finland during the Icelandic Holuhraun fissure eruption Atmospheric Measurement Techniques, 8, 2279-2289, 2015 Author(s): I. Ialongo, J. Hakkarainen, R. Kivi, P. Anttila, N. A. Krotkov, K. Yang, C. Li, S. Tukiainen, S. Hassinen, and J. Tamminen This paper shows the results of the comparison of satellite SO 2 observations from OMI (Ozone Monitoring Instrument) and OMPS (Ozone Mapping Profiler Suite) with ground-based measurements during the Icelandic Holuhraun fissure eruption in September 2014. The volcanic plume reached Finland on several days during the month of September. The SO 2 total columns from the Brewer direct sun (DS) measurements in Sodankylä (67.42° N, 26.59° E), northern Finland, are compared to the satellite data. The operational satellite SO 2 products are evaluated for high latitude conditions (e.g. large solar zenith angle, SZA). The results show that the best agreement can be found for lowest SZAs, close-to-nadir satellite pixels, cloud fraction below 0.3 and small distance between the station and the centre of the pixel. Under good retrieval conditions, the difference between satellite data and Brewer measurements remains mostly below the uncertainty on the satellite SO 2 retrievals (up to about 2 DU at high latitudes). The satellite products assuming a priori profile with SO 2 predominantly in the planetary boundary layer give total column values with the best agreement with the ground-based data. The analysis of the SO 2 surface concentrations at four air quality stations in northern Finland shows that the volcanic plume coming from Iceland was located very close to the surface. This is connected to the fact that this was a fissure eruption and most of the SO 2 was emitted into the troposphere. This is an exceptional case because the SO 2 volcanic emissions directly affect the air quality levels at surface in an otherwise pristine environment like northern Finland. The time evolution of the SO 2 concentrations peaks during the same days when large SO 2 total column values are measured by the Brewer in Sodankylä and enhanced SO 2 signal is visible over northern Finland from the satellite maps. Thus, the satellite retrievals were able to detect the spatiotemporal evolution of the volcanic plume as compared to the surface observations. Furthermore, direct-broadcast SO 2 satellite data (from both OMI and OMPS instruments) are compared for the first time against ground-based observations.

Description: Inter-comparison of laboratory smog chamber and flow reactor systems on organic aerosol yield and composition Atmospheric Measurement Techniques, 8, 2315-2332, 2015 Author(s): E. A. Bruns, I. El Haddad, A. Keller, F. Klein, N. K. Kumar, S. M. Pieber, J. C. Corbin, J. G. Slowik, W. H. Brune, U. Baltensperger, and A. S. H. Prévôt A variety of tools are used to simulate atmospheric aging, including smog chambers and flow reactors. Traditional, large-scale smog chambers age emissions over the course of hours to days, whereas flow reactors rapidly age emissions using high oxidant concentrations to reach higher degrees of oxygenation than typically attained in smog chamber experiments. The atmospheric relevance of the products generated under such rapid oxidation warrants further study. However, no previously published studies have compared the yields and chemical composition of products generated in flow reactors and smog chambers from the same starting mixture. The yields and composition of the organic aerosol formed from the photo-oxidation of α-pinene and of wood-combustion emissions in a smog chamber (SC) and two flow reactors: a potential aerosol mass reactor (PAM) and a micro-smog chamber (MSC), were determined using aerosol mass spectrometry. Reactants were sampled from the SC and aged in the MSC and the PAM using a range of hydroxyl radical (OH) concentrations and then photo-chemically aged in the SC. The chemical composition, as well as the maximum yields and emission factors, of the products in both the α-pinene and wood-combustion systems determined with the PAM and the SC agreed reasonably well. High OH exposures have been shown previously to lower yields by breaking carbon–carbon bonds and forming higher volatility species, which reside largely in the gas phase; however, fragmentation in the PAM was not observed. The yields determined using the PAM for the α-pinene system were slightly lower than in the SC, possibly from increased wall losses of gas phase species due to the higher surface area to volume ratios in the PAM, even when offset with better isolation of the sampled flow from the walls. The α-pinene SOA results for the MSC were not directly comparable, as particles were smaller than the optimal AMS transmission range. The higher supersaturation in the flow reactors resulted in more nucleation than in the SC. For the wood-combustion system, emission factors measured from the MSC were typically lower than those measured from the SC. Lower emission factors in the MSC may have been due to considerable nucleation mode particles formed in the MSC which were not detected by the AMS or due to condensational loss of gases to the walls inside or after the MSC. More comprehensive coverage of the potential particle size range is needed in future SOA measurements to improve our understanding of the differences in yields when comparing the MSC to the SC. The PAM and the SC agreed within measurement uncertainties in terms of yields and composition for the systems and conditions studied here and this agreement supports the continued use of the PAM to study atmospheric aging.

Description: Evaporation from weighing precipitation gauges: impacts on automated gauge measurements and quality assurance methods Atmospheric Measurement Techniques, 8, 2291-2300, 2015 Author(s): R. D. Leeper and J. Kochendorfer Evaporation from a precipitation gauge can cause errors in the amount of measured precipitation. For automated weighing-bucket gauges, the World Meteorological Organization (WMO) suggests the use of evaporative suppressants and frequent observations to limit these biases. However, the use of evaporation suppressants is not always feasible due to environmental hazards and the added cost of maintenance, transport, and disposal of the gauge additive. In addition, research has suggested that evaporation prior to precipitation may affect precipitation measurements from auto-recording gauges operating at sub-hourly frequencies. For further evaluation, a field campaign was conducted to monitor evaporation and its impacts on the quality of precipitation measurements from gauges used at U.S. Climate Reference Network (USCRN) stations. Two Geonor gauges were collocated, with one gauge using an evaporative suppressant (referred to as Geonor-NonEvap) and the other with no suppressant (referred to as Geonor-Evap) to evaluate evaporative losses and evaporation biases on precipitation measurements. From June to August, evaporative losses from the Geonor-Evap gauge exceeded accumulated precipitation, with an average loss of 0.12 mm h −1 . The impact of evaporation on precipitation measurements was sensitive to the choice of calculation method. In general, the pairwise method that utilized a longer time series to smooth out sensor noise was more sensitive to gauge evaporation (−4.6% bias with respect to control) than the weighted-average method that calculated depth change over a smaller window (

Description: Quantifying residual ionospheric errors in GNSS radio occultation bending angles based on ensembles of profiles from end-to-end simulations Atmospheric Measurement Techniques, 8, 2999-3019, 2015 Author(s): C. L. Liu, G. Kirchengast, K. Zhang, R. Norman, Y. Li, S. C. Zhang, J. Fritzer, M. Schwaerz, S. Q. Wu, and Z. X. Tan The radio occultation (RO) technique using signals from the Global Navigation Satellite System (GNSS), in particular from the Global Positioning System (GPS) so far, is currently widely used to observe the atmosphere for applications such as numerical weather prediction and global climate monitoring. The ionosphere is a major error source in RO measurements at stratospheric altitudes, and a linear ionospheric correction of dual-frequency RO bending angles is commonly used to remove the first-order ionospheric effect. However, the residual ionospheric error (RIE) can still be significant so that it needs to be further mitigated for high-accuracy applications, especially above about 30 km altitude where the RIE is most relevant compared to the magnitude of the neutral atmospheric bending angle. Quantification and careful analyses for better understanding of the RIE is therefore important for enabling benchmark-quality stratospheric RO retrievals. Here we present such an analysis of bending angle RIEs covering the stratosphere and mesosphere, using quasi-realistic end-to-end simulations for a full-day ensemble of RO events. Based on the ensemble simulations we assessed the variation of bending angle RIEs, both biases and standard deviations, with solar activity, latitudinal region and with or without the assumption of ionospheric spherical symmetry and co-existing observing system errors. We find that the bending angle RIE biases in the upper stratosphere and mesosphere, and in all latitudinal zones from low to high latitudes, have a clear negative tendency and a magnitude increasing with solar activity, which is in line with recent empirical studies based on real RO data although we find smaller bias magnitudes, deserving further study in the future. The maximum RIE biases are found at low latitudes during daytime, where they amount to within −0.03 to −0.05 μrad, the smallest at high latitudes (0 to −0.01 μrad; quiet space weather and winter conditions). Ionospheric spherical symmetry or asymmetries about the RO event location have only a minor influence on RIE biases. The RIE standard deviations are markedly increased both by ionospheric asymmetries and increasing solar activity and amount to about 0.3 to 0.7 μrad in the upper stratosphere and mesosphere. Taking also into account the realistic observation errors of a modern RO receiving system, amounting globally to about 0.4 μrad (unbiased; standard deviation), shows that the random RIEs are typically comparable to the total observing system error. The results help to inform future RIE mitigation schemes that will improve upon the use of the linear ionospheric correction of bending angles and also provide explicit uncertainty estimates.

Description: Kalman filter physical retrieval of surface emissivity and temperature from SEVIRI infrared channels: a validation and intercomparison study Atmospheric Measurement Techniques, 8, 2981-2997, 2015 Author(s): G. Masiello, C. Serio, S. Venafra, G. Liuzzi, F. Göttsche, I. F. Trigo, and P. Watts A Kalman filter-based approach for the physical retrieval of surface temperature and emissivity from SEVIRI (Spinning Enhanced Visible and Infrared Imager) infrared observations has been developed and validated against in situ and satellite observations. Validation for land has been provided based on in situ observations from the two permanent stations at Evora and Gobabeb operated by Karlsruhe Institute of Technology (KIT) within the framework of EUMETSAT's Satellite Application Facility on Land Surface Analysis (LSA SAF). Sea surface retrievals have been intercompared on a broad spatial scale with equivalent satellite products (MODIS, Moderate Resolution Imaging Spectroradiometer, and AVHRR, Advanced Very High Resolution Radiometer) and ECMWF (European Centre for Medium-Range Weather Forecasts) analyses. For surface temperature, the Kalman filter yields a root mean square accuracy of ≈ ±1.5 °C for the two land sites considered and ≈ ±1.0 °C for the sea. Comparisons with polar satellite instruments over the sea surface show nearly zero temperature bias. Over the land surface the retrieved emissivity follows the seasonal vegetation cycle and permits identification of desert sand regions using the SEVIRI channel at 8.7 μm due to the strong quartz reststrahlen bands around 8–9 μm. Considering the two validation stations, we have found that emissivity retrieved in SEVIRI channel 10.8 μm over the gravel plains of the Namibian desert is in excellent agreement with in situ observations. Over Evora, the seasonal variation of emissivity with vegetation is successfully retrieved and yields emissivity values for green and dry vegetation that are in good agreement with spectral library data. The algorithm has been applied to the SEVIRI full disk, and emissivity maps on that global scale have been physically retrieved for the first time.

Description: Electron density profiles probed by radio occultation of FORMOSAT-7/COSMIC-2 at 520 and 800 km altitude Atmospheric Measurement Techniques, 8, 3069-3074, 2015 Author(s): J. Y. Liu, C. Y. Lin, and H. F. Tsai The FORMOSAT-7/COSMIC-2 (F7/C2) will ultimately place 12 satellites in orbit with two launches with 24–28.5° inclination and 520–550 km altitude in 2016 and with 72° inclination and 720–750 km altitude in 2018. It would be very useful for the community to construct the global three-dimensional electron density structure by simultaneously combining the two launch observations for studying ionospheric structure and dynamics. However, to properly construct the global electron density structure, it is essential to know and evaluate differences between the ionospheric electron densities probed by the two launches. To mimic the F7/C2 observations, we examine the electron density probed at the two satellite altitudes 500 and 800 km by means of FORMOSAT-3/COSMIC (F3/C) observations at the parking orbit 500 km altitude and mission orbit 800 km altitude, as well as a corresponding observing system simulation experiment (OSSE). Observation and OSSE results show that the sounding geometries by satellite orbiting at 500 and 800 km altitudes can cause the overall differences in the electron density, the F2 peak electron density, and the F2 peak height of about 18–24, 12–28 %, and 7–19 km, respectively. Results confirm that the discrepancies mainly result from the sounding geometry and the grid (contour) bias of the electron density.

Description: Indirect estimation of absorption properties for fine aerosol particles using AATSR observations: a case study of wildfires in Russia in 2010 Atmospheric Measurement Techniques, 8, 3075-3085, 2015 Author(s): E. Rodríguez, P. Kolmonen, T. H. Virtanen, L. Sogacheva, A.-M. Sundström, and G. de Leeuw The Advanced Along-Track Scanning Radiometer (AATSR) on board the ENVISAT satellite is used to study aerosol properties. The retrieval of aerosol properties from satellite data is based on the optimized fit of simulated and measured reflectances at the top of the atmosphere (TOA). The simulations are made using a radiative transfer model with a variety of representative aerosol properties. The retrieval process utilizes a combination of four aerosol components, each of which is defined by their (lognormal) size distribution and a complex refractive index: a weakly and a strongly absorbing fine-mode component, coarse mode sea salt aerosol and coarse mode desert dust aerosol). These components are externally mixed to provide the aerosol model which in turn is used to calculate the aerosol optical depth (AOD). In the AATSR aerosol retrieval algorithm, the mixing of these components is decided by minimizing the error function given by the sum of the differences between measured and calculated path radiances at 3–4 wavelengths, where the path radiances are varied by varying the aerosol component mixing ratios. The continuous variation of the fine-mode components allows for the continuous variation of the fine-mode aerosol absorption. Assuming that the correct aerosol model (i.e. the correct mixing fractions of the four components) is selected during the retrieval process, also other aerosol properties could be computed such as the single scattering albedo (SSA). Implications of this assumption regarding the ratio of the weakly/strongly absorbing fine-mode fraction are investigated in this paper by evaluating the validity of the SSA thus obtained. The SSA is indirectly estimated for aerosol plumes with moderate-to-high AOD resulting from wildfires in Russia in the summer of 2010. Together with the AOD, the SSA provides the aerosol absorbing optical depth (AAOD). The results are compared with AERONET data, i.e. AOD level 2.0 and SSA and AAOD inversion products. The RMSE (root mean square error) is 0.03 for SSA and 0.02 for AAOD lower than 0.05. The SSA is further evaluated by comparison with the SSA retrieved from the Ozone Monitoring Instrument (OMI). The SSA retrieved from both instruments show similar features, with generally lower AATSR-estimated SSA values over areas affected by wildfires.

Description: Retrieval of vertical profiles of atmospheric refraction angles by inversion of optical dilution measurements Atmospheric Measurement Techniques, 8, 3135-3145, 2015 Author(s): D. Fussen, C. Tétard, E. Dekemper, D. Pieroux, N. Mateshvili, F. Vanhellemont, G. Franssens, and P. Demoulin In this paper, we consider occultations of celestial bodies through the atmospheric limb from low Earth orbit satellites and we show how the usual change of tangent altitude associated with atmospheric refraction is inseparably connected to a variation of the observed apparent intensity, for extended and pointlike sources. We demonstrate, in the regime of weak refraction angles, that atmospheric optical dilution and image deformation are strictly concomitant. The approach leads to the integration of a simple differential equation related to the observed transmittance in the absence of other absorbing molecules along the optical path. The algorithm does not rely on the absolute knowledge of the radiometer pointing angle that is related to the accurate knowledge of the satellite attitude. We successfully applied the proposed method to the measurements performed by two past occultation experiments: GOMOS for stellar and ORA for solar occultations. The developed algorithm (named ARID) will be applied to the imaging of solar occultations in a forthcoming pico-satellite mission.

Description: GOMOS bright limb ozone data set Atmospheric Measurement Techniques, 8, 3107-3115, 2015 Author(s): S. Tukiainen, E. Kyrölä, J. Tamminen, J. Kujanpää, and L. Blanot We have created a daytime ozone profile data set from the measurements of the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the Envisat satellite. This so-called GOMOS bright limb (GBL) data set contains ∼ 358 000 stratospheric daytime ozone profiles measured by GOMOS in 2002–2012. The GBL data set complements the widely used GOMOS nighttime data based on stellar occultation measurements. The GBL data set is based on the GOMOS daytime occultations but instead of the transmitted star light we use limb-scattered solar light. The ozone profiles retrieved from these radiance spectra cover the 18–60 km altitude range and have approximately 2–3 km vertical resolution. We show that these profiles are generally in better than 10 % agreement with the NDACC (Network for the Detection of Atmospheric Composition Change) ozonesonde profiles and with the GOMOS nighttime, MLS (Microwave Limb Sounder), and OSIRIS (Optical Spectrograph and InfraRed Imager System) satellite measurements. However, there is a 10–13 % negative bias at 40 km altitude and a 10–50 % positive bias at 50 km for solar zenith angles 〉 75°. These biases are most likely caused by stray light which is difficult to characterize and to remove entirely from the measured spectra. Nevertheless, the GBL data set approximately doubles the amount of useful GOMOS ozone profiles and improves coverage of the summer pole.

Description: High temporal resolution estimates of columnar aerosol microphysical parameters from spectrum of aerosol optical depth by linear estimation: application to long-term AERONET and star-photometry measurements Atmospheric Measurement Techniques, 8, 3117-3133, 2015 Author(s): D. Pérez-Ramírez, I. Veselovskii, D. N. Whiteman, A. Suvorina, M. Korenskiy, A. Kolgotin, B. Holben, O. Dubovik, A. Siniuk, and L. Alados-Arboledas This work deals with the applicability of the linear estimation technique (LE) to invert spectral measurements of aerosol optical depth (AOD) provided by AERONET CIMEL sun photometers. The inversion of particle properties using only direct-sun AODs allows the evaluation of parameters such as effective radius ( r eff ) and columnar volume aerosol content ( V ) with significantly better temporal resolution than the operational AERONET algorithm which requires both direct sun and sky radiance measurements. Sensitivity studies performed demonstrate that the constraints on the range of the inversion are very important to minimize the uncertainties, and therefore estimates of r eff can be obtained with uncertainties less than 30 % and of V with uncertainties below 40 %. The LE technique is applied to data acquired at five AERONET sites influenced by different aerosol types and the retrievals are compared with the results of the operational AERONET code. Good agreement between the two techniques is obtained when the fine mode predominates, while for coarse mode cases the LE results systematically underestimate both r eff and V . The highest differences are found for cases where no mode predominates. To minimize these biases, correction functions are developed using the multi-year database of observations at selected sites, where the AERONET retrieval is used as the reference. The derived corrections are tested using data from 18 other AERONET stations offering a range of aerosol types. After correction, the LE retrievals provide better agreement with AERONET for all the sites considered. Finally, the LE approach developed here is applied to AERONET and star-photometry measurements in the city of Granada (Spain) to obtain day-to-night time evolution of columnar aerosol microphysical properties.

Description: Next-generation angular distribution models for top-of-atmosphere radiative flux calculation from CERES instruments: validation Atmospheric Measurement Techniques, 8, 3297-3313, 2015 Author(s): W. Su, J. Corbett, Z. Eitzen, and L. Liang Radiative fluxes at the top of the atmosphere (TOA) from the Clouds and the Earth's Radiant Energy System (CERES) instrument are fundamental variables for understanding the Earth's energy balance and how it changes with time. TOA radiative fluxes are derived from the CERES radiance measurements using empirical angular distribution models (ADMs). This paper evaluates the accuracy of CERES TOA fluxes using direct integration and flux consistency tests. Direct integration tests show that the overall bias in regional monthly mean TOA shortwave (SW) flux is less than 0.2 Wm −2 and the RMSE is less than 1.1 Wm −2 . The bias and RMSE are very similar between Terra and Aqua. The bias in regional monthly mean TOA LW fluxes is less than 0.5 Wm −2 and the RMSE is less than 0.8 Wm −2 for both Terra and Aqua. The accuracy of the TOA instantaneous flux is assessed by performing tests using fluxes inverted from nadir- and oblique-viewing angles using CERES along-track observations and temporally and spatially matched MODIS observations, and using fluxes inverted from multi-angle MISR observations. The averaged TOA instantaneous SW flux uncertainties from these two tests are about 2.3 % (1.9 Wm −2 ) over clear ocean, 1.6 % (4.5 Wm −2 ) over clear land, and 2.0 % (6.0 Wm −2 ) over clear snow/ice; and are about 3.3 % (9.0 Wm −2 ), 2.7 % (8.4 Wm −2 ), and 3.7 % (9.9 Wm −2 ) over ocean, land, and snow/ice under all-sky conditions. The TOA SW flux uncertainties are generally larger for thin broken clouds than for moderate and thick overcast clouds. The TOA instantaneous daytime LW flux uncertainties derived from the CERES-MODIS test are 0.5 % (1.5 Wm −2 ), 0.8 % (2.4 Wm −2 ), and 0.7 % (1.3 Wm −2 ) over clear ocean, land, and snow/ice; and are about 1.5 % (3.5 Wm −2 ), 1.0 % (2.9 Wm −2 ), and 1.1 % (2.1 Wm −2 ) over ocean, land, and snow/ice under all-sky conditions. The TOA instantaneous nighttime LW flux uncertainties are about 0.5–1 % ( 〈 2.0 Wm −2 ) for all surface types. Flux uncertainties caused by errors in scene identification are also assessed by using the collocated CALIPSO, CloudSat, CERES and MODIS data product. Errors in scene identification tend to underestimate TOA SW flux by about 0.6 Wm −2 and overestimate TOA daytime (nighttime) LW flux by 0.4 (0.2) Wm −2 when all CERES viewing angles are considered.

Description: Observations of precipitable water vapour over complex topography of Ethiopia from ground-based GPS, FTIR, radiosonde and ERA-Interim reanalysis Atmospheric Measurement Techniques, 8, 3277-3295, 2015 Author(s): G. Mengistu Tsidu, T. Blumenstock, and F. Hase Water vapour is one of the most important greenhouse gases. Long-term changes in the amount of water vapour in the atmosphere need to be monitored not only for its direct role as a greenhouse gas but also because of its role in amplifying other feedbacks such as clouds and albedo. In recent decades, monitoring of water vapour on a regular and continuous basis has become possible as a result of the steady increase in the number of deployed global positioning satellite (GPS) ground-based receivers. However, the Horn of Africa remained a data-void region in this regard until recently, when some GPS ground-receiver stations were deployed to monitor tectonic movements in the Great Rift Valley. This study seizes this opportunity and the installation of a Fourier transform infrared spectrometer (FTIR) at Addis Ababa to assess the quality and comparability of precipitable water vapour (PWV) from GPS, FTIR, radiosonde and interim ECMWF Re-Analysis (ERA-Interim) over Ethiopia. The PWV from the three instruments and the reanalysis show good correlation, with correlation coefficients in the range from 0.83 to 0.92. On average, GPS shows the highest PWV followed by FTIR and radiosonde observations. ERA-Interim is higher than all measurements with a bias of 4.6 mm compared to GPS. The intercomparison between GPS and ERA-Interim was extended to seven other GPS stations in the country. Only four out of eight GPS stations included simultaneous surface pressure observations. Uncertainty in the model surface pressure of 1 hPa can cause up to 0.35 mm error in GPS PWV. The gain obtained from using observed surface pressure in terms of reducing bias and strengthening correlation is significant but shows some variations among the GPS sites. The comparison between GPS and ERA-Interim PWV over the seven other GPS stations shows differences in the magnitude and sign of bias of ERA-Interim with respect to GPS PWV from station to station. This feature is also prevalent in diurnal and seasonal variabilities. The spatial variation in the relationship between the two data sets is partly linked to variation in the skill of the European Centre for Medium-Range Weather Forecasts (ECMWF) model over different regions and seasons. This weakness in the model is related to poor observational constraints from this part of the globe and sensitivity of its convection scheme to orography and land surface features. This is consistent with observed wet bias over some highland stations and dry bias over few lowland stations. The skill of ECMWF in reproducing realistic PWV varies with time of the day and season, showing large positive bias during warm and wet summer at most of the GPS sites.

Description: A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16° S) during the dry season Atmospheric Measurement Techniques, 8, 3229-3250, 2015 Author(s): M. Litt, J.-E. Sicart, and W. Helgason Over glaciers in the outer tropics, during the dry winter season, turbulent fluxes are an important sink of melt energy due to high sublimation rates, but measurements in stable surface layers in remote and complex terrains remain challenging. Eddy-covariance (EC) and bulk-aerodynamic (BA) methods were used to estimate surface turbulent heat fluxes of sensible ( H ) and latent heat ( LE ) in the ablation zone of the tropical Zongo Glacier, Bolivia (16° S, 5080 m a.s.l.), from 22 July to 1 September 2007. We studied the turbulent fluxes and their associated random and systematic measurement errors under the three most frequent wind regimes. For nightly, density-driven katabatic flows, and for strong downslope flows related to large-scale forcing, H generally heats the surface (i.e. is positive), while LE cools it down (i.e. is negative). On average, both fluxes exhibit similar magnitudes and cancel each other out. Most energy losses through turbulence occur for daytime upslope flows, when H is weak due to small temperature gradients and LE is strongly negative due to very dry air. Mean random errors of the BA method (6 % on net H + LE fluxes) originated mainly from large uncertainties in roughness lengths. For EC fluxes, mean random errors were due mainly to poor statistical sampling of large-scale outer-layer eddies (12 %). The BA method is highly sensitive to the method used to derive surface temperature from longwave radiation measurements and underestimates fluxes due to vertical flux divergence at low heights and nonstationarity of turbulent flow. The EC method also probably underestimates the fluxes, albeit to a lesser extent, due to underestimation of vertical wind speed and to vertical flux divergence. For both methods, when H and LE compensate each other in downslope fluxes, biases tend to cancel each other out or remain small. When the net turbulent fluxes ( H + LE ) are the largest in upslope flows, nonstationarity effects and underestimations of the vertical wind speed do not compensate, and surface temperature errors are important, so that large biases on H + LE are expected when using both the EC and the BA method.

Description: Pointing errors in solar absorption spectrometry – correction scheme and its validation Atmospheric Measurement Techniques, 8, 3715-3728, 2015 Author(s): A. Reichert, P. Hausmann, and R. Sussmann A method for quantification of sun-pointing inaccuracies in solar absorption spectrometry is presented along with a correction scheme for the resulting errors in trace gas vertical column or profile retrievals. A posteriori correction of pointing errors requires knowledge of both coordinates of the mispointing vector on the solar disk. In principle, quantitative information on the mispointing can be retrieved from Doppler shifts of solar lines derived from measured spectra. However, this yields only one component of the mispointing vector, namely the one which is perpendicular to the solar rotation axis. Missing information on the second vector component has hindered a posteriori correction of mispointing errors so far. Our idea of how to overcome this problem is to obtain estimates of both coordinates of the mispointing by combining subsequent measurements with differing orientations of the solar rotation axis relative to the zenith direction. The proposed concept is suitable in the case of systematic mispointing, i.e., if the mispointing is approximately constant within a given set of measurements. An implementation of this original concept is demonstrated using measurements from the solar absorption Fourier transform infrared (FTIR) spectrometer at the Zugspitze (47.42° N, 10.98° E, 2964 m a.s.l.). Soundings in the September 2012–September 2014 time interval were impacted by mispointing problems due to a non-optimum solar tracking optics configuration. They show a mean mispointing in the zenith direction of −0.063°. This causes biases in vertical soundings of trace gases, e.g., −2.82 ppb in monthly means of dry-air column-averaged mole fractions of methane (XCH 4 ). Measurements made with the more stable pre-September 2012 and post-September 2014 optics configurations show considerably smaller mispointing effects. Applying the mispointing correction, the April 2006–March 2014 XCH 4 trend determined from Zugspitze measurements is reduced from 6.45 [5.84, 7.04] to 6.07 [5.55, 6.59] ppb yr −1 . The correction thereby restores consistency with results from the nearby Garmisch FTIR site (47.48° N, 11.06° E, 743 m a.s.l.). The mispointing correction is applicable to solar absorption measurements in the mid-infrared and near infrared. It will be of particular benefit for refining existing records of high-accuracy-and-precision greenhouse gas soundings for the purpose of improved trend analysis or source–sink inversions.

Description: Accuracy and precision of 14 C-based source apportionment of organic and elemental carbon in aerosols using the Swiss_4S protocol Atmospheric Measurement Techniques, 8, 3729-3743, 2015 Author(s): G. O. Mouteva, S. M. Fahrni, G. M. Santos, J. T. Randerson, Y.-L. Zhang, S. Szidat, and C. I. Czimczik Aerosol source apportionment remains a critical challenge for understanding the transport and aging of aerosols, as well as for developing successful air pollution mitigation strategies. The contributions of fossil and non-fossil sources to organic carbon (OC) and elemental carbon (EC) in carbonaceous aerosols can be quantified by measuring the radiocarbon ( 14 C) content of each carbon fraction. However, the use of 14 C in studying OC and EC has been limited by technical challenges related to the physical separation of the two fractions and small sample sizes. There is no common procedure for OC/EC 14 C analysis, and uncertainty studies have largely focused on the precision of yields. Here, we quantified the uncertainty in 14 C measurement of aerosols associated with the isolation and analysis of each carbon fraction with the Swiss_4S thermal–optical analysis (TOA) protocol. We used an OC/EC analyzer (Sunset Laboratory Inc., OR, USA) coupled to a vacuum line to separate the two components. Each fraction was thermally desorbed and converted to carbon dioxide (CO 2 ) in pure oxygen (O 2 ). On average, 91 % of the evolving CO 2 was then cryogenically trapped on the vacuum line, reduced to filamentous graphite, and measured for its 14 C content via accelerator mass spectrometry (AMS). To test the accuracy of our setup, we quantified the total amount of extraneous carbon introduced during the TOA sample processing and graphitization as the sum of modern and fossil ( 14 C-depleted) carbon introduced during the analysis of fossil reference materials (adipic acid for OC and coal for EC) and contemporary standards (oxalic acid for OC and rice char for EC) as a function of sample size. We further tested our methodology by analyzing five ambient airborne particulate matter (PM 2.5 ) samples with a range of OC and EC concentrations and 14 C contents in an interlaboratory comparison. The total modern and fossil carbon blanks of our setup were 0.8 ± 0.4 and 0.67 ± 0.34 μg C, respectively, based on multiple measurements of ultra-small samples. The extraction procedure (Swiss_4S protocol and cryo-trapping only) contributed 0.37 ± 0.18 μg of modern carbon and 0.13 ± 0.07 μg of fossil carbon to the total blank of our system, with consistent estimates obtained for the two laboratories. There was no difference in the background correction between the OC and EC fractions. Our setup allowed us to efficiently isolate and trap each carbon fraction with the Swiss_4S protocol and to perform 14 C analysis of ultra-small OC and EC samples with high accuracy and low 14 C blanks.

Description: The impact of vibrational Raman scattering of air on DOAS measurements of atmospheric trace gases Atmospheric Measurement Techniques, 8, 3767-3787, 2015 Author(s): J. Lampel, U. Frieß, and U. Platt In remote sensing applications, such as differential optical absorption spectroscopy (DOAS), atmospheric scattering processes need to be considered. After inelastic scattering on N 2 and O 2 molecules, the scattered photons occur as additional intensity at a different wavelength, effectively leading to "filling-in" of both solar Fraunhofer lines and absorptions of atmospheric constituents, if the inelastic scattering happens after the absorption. Measured spectra in passive DOAS applications are typically corrected for rotational Raman scattering (RRS), also called Ring effect, which represents the main contribution to inelastic scattering. Inelastic scattering can also occur in liquid water, and its influence on DOAS measurements has been observed over clear ocean water. In contrast to that, vibrational Raman scattering (VRS) of N 2 and O 2 has often been thought to be negligible, but it also contributes. Consequences of VRS are red-shifted Fraunhofer structures in scattered light spectra and filling-in of Fraunhofer lines, additional to RRS. At 393 nm, the spectral shift is 25 and 40 nm for VRS of O 2 and N 2 , respectively. We describe how to calculate VRS correction spectra according to the Ring spectrum. We use the VRS correction spectra in the spectral range of 420–440 nm to determine the relative magnitude of the cross-sections of VRS of O 2 and N 2 and RRS of air. The effect of VRS is shown for the first time in spectral evaluations of Multi-Axis DOAS data from the SOPRAN M91 campaign and the MAD-CAT MAX-DOAS intercomparison campaign. The measurements yield in agreement with calculated scattering cross-sections that the observed VRS(N 2 ) cross-section at 393 nm amounts to 2.3 ± 0.4 % of the cross-section of RRS at 433 nm under tropospheric conditions. The contribution of VRS(O 2 ) is also found to be in agreement with calculated scattering cross-sections. It is concluded, that this phenomenon has to be included in the spectral evaluation of weak absorbers as it reduces the measurement error significantly and can cause apparent differential optical depth of up to 3 ×10 −4 . Its influence on the spectral retrieval of IO, glyoxal, water vapour and NO 2 in the blue wavelength range is evaluated for M91. For measurements with a large Ring signal a significant and systematic bias of NO 2 dSCDs (differential slant column densities) up to (−3.8 ± 0.4) × 10 14 molec cm −2 is observed if this effect is not considered. The effect is typically negligible for DOAS fits with an RMS (root mean square) larger than 4 × 10 −4 .

Description: OMI tropospheric NO 2 air mass factors over South America: effects of biomass burning aerosols Atmospheric Measurement Techniques, 8, 3831-3849, 2015 Author(s): P. Castellanos, K. F. Boersma, O. Torres, and J. F. de Haan Biomass burning is an important and uncertain source of aerosols and NO x (NO + NO 2 ) to the atmosphere. Satellite observations of tropospheric NO 2 are essential for characterizing this emissions source, but inaccuracies in the retrieval of NO 2 tropospheric columns due to the radiative effects of aerosols, especially light-absorbing carbonaceous aerosols, are not well understood. It has been shown that the O 2 –O 2 effective cloud fraction and pressure retrieval is sensitive to aerosol optical and physical properties, including aerosol optical depth (AOD). Aerosols implicitly influence the tropospheric air mass factor (AMF) calculations used in the NO 2 retrieval through the effective cloud parameters used in the independent pixel approximation. In this work, we explicitly account for the effects of biomass burning aerosols in the Ozone Monitoring Instrument (OMI) tropospheric NO 2 AMF calculation for cloud-free scenes. We do so by including collocated aerosol extinction vertical profile observations from the CALIOP instrument, and aerosol optical depth (AOD) and single scattering albedo (SSA) retrieved by the OMI near-UV aerosol algorithm (OMAERUV) in the DISAMAR radiative transfer model. Tropospheric AMFs calculated with DISAMAR were benchmarked against AMFs reported in the Dutch OMI NO 2 (DOMINO) retrieval; the mean and standard deviation of the difference was 0.6 ± 8 %. Averaged over three successive South American biomass burning seasons (2006–2008), the spatial correlation in the 500 nm AOD retrieved by OMI and the 532 nm AOD retrieved by CALIOP was 0.6, and 68 % of the daily OMAERUV AOD observations were within 30 % of the CALIOP observations. Overall, tropospheric AMFs calculated with observed aerosol parameters were on average 10 % higher than AMFs calculated with effective cloud parameters. For effective cloud radiance fractions less than 30 %, or effective cloud pressures greater than 800 hPa, the difference between tropospheric AMFs based on implicit and explicit aerosol parameters is on average 6 and 3 %, respectively, which was the case for the majority of the pixels considered in our study; 70 % had cloud radiance fraction below 30 %, and 50 % had effective cloud pressure greater than 800 hPa. Pixels with effective cloud radiance fraction greater than 30 % or effective cloud pressure less than 800 hPa corresponded with stronger shielding in the implicit aerosol correction approach because the assumption of an opaque effective cloud underestimates the altitude-resolved AMF; tropospheric AMFs were on average 30–50 % larger when aerosol parameters were included, and for individual pixels tropospheric AMFs can differ by more than a factor of 2. The observation-based approach to correcting tropospheric AMF calculations for aerosol effects presented in this paper depicts a promising strategy for a globally consistent aerosol correction scheme for clear-sky pixels.

Description: Intercomparison of two comparative reactivity method instruments inf the Mediterranean basin during summer 2013 Atmospheric Measurement Techniques, 8, 3851-3865, 2015 Author(s): N. Zannoni, S. Dusanter, V. Gros, R. Sarda Esteve, V. Michoud, V. Sinha, N. Locoge, and B. Bonsang The hydroxyl radical (OH) plays a key role in the atmosphere, as it initiates most of the oxidation processes of volatile organic compounds (VOCs), and can ultimately lead to the formation of ozone and secondary organic aerosols (SOAs). There are still uncertainties associated with the OH budget assessed using current models of atmospheric chemistry and direct measurements of OH sources and sinks have proved to be valuable tools to improve our understanding of the OH chemistry. The total first order loss rate of OH, or total OH reactivity, can be directly measured using three different methods, such as the following: total OH loss rate measurement, laser-induced pump and probe technique and comparative reactivity method. Observations of total OH reactivity are usually coupled to individual measurements of reactive compounds in the gas phase, which are used to calculate the OH reactivity. Studies using the three methods have highlighted that a significant fraction of OH reactivity is often not explained by individually measured reactive compounds and could be associated to unmeasured or unknown chemical species. Therefore accurate and reproducible measurements of OH reactivity are required. The comparative reactivity method (CRM) has demonstrated to be an advantageous technique with an extensive range of applications, and for this reason it has been adopted by several research groups since its development. However, this method also requires careful corrections to derive ambient OH reactivity. Herein we present an intercomparison exercise of two CRM instruments, CRM-LSCE (Laboratoire des Sciences du Climat et de l'Environnement) and CRM-MD (Mines Douai), conducted during July 2013 at the Mediterranean site of Ersa, Cape Corsica, France. The intercomparison exercise included tests to assess the corrections needed by the two instruments to process the raw data sets as well as OH reactivity observations. The observation was divided in three parts: 2 days of plant emissions (8–9 July), 2 days of ambient measurements (10–11 July) and 2 days (12–13 July) of plant emissions. We discuss in detail the experimental approach adopted and how the data sets were processed for both instruments. Corrections required for the two instruments lead to higher values of reactivity in ambient air; overall 20 % increase for CRM-MD and 49 % for CRM-LSCE compared to the raw data. We show that ambient OH reactivity measured by the two instruments agrees very well (correlation described by a linear least squares fit with a slope of 1 and R 2 of 0.75). This study highlights that ambient measurements of OH reactivity with differently configured CRM instruments yield consistent results in a low NO x (NO + NO 2 ), terpene rich environment, despite differential corrections relevant to each instrument. Conducting more intercomparison exercises, involving more CRM instruments operated under different ambient and instrumental settings will help in assessing the variability induced due to instrument-specific corrections further.

Description: The GOME-type Total Ozone Essential Climate Variable (GTO-ECV) data record from the ESA Climate Change Initiative Atmospheric Measurement Techniques, 8, 3923-3940, 2015 Author(s): M. Coldewey-Egbers, D. G. Loyola, M. Koukouli, D. Balis, J.-C. Lambert, T. Verhoelst, J. Granville, M. van Roozendael, C. Lerot, R. Spurr, S. M. Frith, and C. Zehner We present the new GOME-type Total Ozone Essential Climate Variable (GTO-ECV) data record which has been created within the framework of the European Space Agency's Climate Change Initiative (ESA-CCI). Total ozone column observations – based on the GOME-type Direct Fitting version 3 algorithm – from GOME (Global Ozone Monitoring Experiment), SCIAMACHY (SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY), and GOME-2 have been combined into one homogeneous time series, thereby taking advantage of the high inter-sensor consistency. The data record spans the 15-year period from March 1996 to June 2011 and it contains global monthly mean total ozone columns on a 1°× 1° grid. Geophysical ground-based validation using Brewer, Dobson, and UV–visible instruments has shown that the GTO-ECV level 3 data record is of the same high quality as the equivalent individual level 2 data products that constitute it. Both absolute agreement and long-term stability are excellent with respect to the ground-based data, for almost all latitudes apart from a few outliers which are mostly due to sampling differences between the level 2 and level 3 data. We conclude that the GTO-ECV data record is valuable for a variety of climate applications such as the long-term monitoring of the past evolution of the ozone layer, trend analysis and the evaluation of chemistry–climate model simulations.

Description: PTRwid: A new widget tool for processing PTR-TOF-MS data Atmospheric Measurement Techniques, 8, 3903-3922, 2015 Author(s): R. Holzinger PTRwid is a fast and user friendly tool that has been developed to process data from proton-transfer-reaction time-of-flight mass spectrometers (PTR-TOF-MS) that use HTOF (high-resolution time-of-flight) mass spectrometers from Tofwerk AG (Switzerland). PTRwid is designed for a comprehensive evaluation of whole laboratory or field-based studies. All processing runs autonomously, and entire laboratory or field campaigns can, in principle, be processed with a few mouse clicks. Unique features of PTRwid include (i) an autonomous and accurate mass scale calibration, (ii) the computation of a "unified mass list" that – in addition to a uniform data structure – provides a robust method to determine the precision of attributed peak masses, and (iii) fast data analysis due to well considered choices in data processing.

Description: Correction of water vapor absorption for aerosol remote sensing with ceilometers Atmospheric Measurement Techniques, 8, 3971-3984, 2015 Author(s): M. Wiegner and J. Gasteiger In recent years attention was increasingly paid to backscatter profiles of ceilometers as a new source of aerosol information. Several case studies have shown that – although originally intended for cloud detection only – ceilometers can provide the planetary boundary layer height and even quantitative information such as the aerosol backscatter coefficient β p , provided that the signals have been calibrated. It is expected that the retrieval of aerosol parameters will become widespread as the number of ceilometers is steadily increasing, and continuous and unattended operation is provided. In this context however one should be aware of the fact that the majority of ceilometers provides signals that are influenced by atmospheric water vapor. As a consequence, profiles of aerosol parameters can only be retrieved if water vapor absorption is taken into account. In this paper we describe the influence of water vapor absorption on ceilometer signals at wavelengths around λ = 910 nm. Spectrally high-resolved absorption coefficients are calculated from HITRAN on the basis of realistic emission spectra of ceilometers. These results are used as a reference to develop a methodology ("WAPL") for routine and near-real time corrections of the water vapor influence. Comparison of WAPL with the reference demonstrates its very high accuracy. Extensive studies with simulations based on measurements reveal that the error when water vapor absorption is ignored in the β p -retrieval can be in the order of 20 % for mid-latitudes and more than 50 % for the tropics. It is concluded that the emission spectrum of the laser source should be provided by the manufacturer to increase the accuracy of WAPL, and that 910 nm is better suited than 905 nm. With WAPL systematic errors can be avoided, that would exceed the inherent errors of the Klett solutions by far.

Description: Characterizing black carbon in rain and ice cores using coupled tangential flow filtration and transmission electron microscopy Atmospheric Measurement Techniques, 8, 3959-3969, 2015 Author(s): A. Ellis, R. Edwards, M. Saunders, R. K. Chakrabarty, R. Subramanian, A. van Riessen, A. M. Smith, D. Lambrinidis, L. J. Nunes, P. Vallelonga, I. D. Goodwin, A. D. Moy, M. A. J. Curran, and T. D. van Ommen Antarctic ice cores have been used to study the history of black carbon (BC), but little is known with regards to the physical and chemical characteristics of these particles in the remote atmosphere. Characterization remains limited by ultra-trace concentrations in ice core samples and the lack of adequate methods to isolate the particles unaltered from the melt water. To investigate the physical and chemical characteristics of these particles, we have developed a tangential flow filtration (TFF) method combined with transmission electron microscopy (TEM). Tests using ultrapure water and polystyrene latex particle standards resulted in excellent blanks and significant particle recovery. This approach has been applied to melt water from Antarctic ice cores as well as tropical rain from Darwin, Australia with successful results: TEM analysis revealed a variety of BC particle morphologies, insoluble coatings, and the attachment of BC to mineral dust particles. The TFF-based concentration of these particles has proven to give excellent results for TEM studies of BC particles in Antarctic ice cores and can be used for future studies of insoluble aerosols in rainwater and ice core samples.

Description: OMI total column ozone: extending the long-term data record Atmospheric Measurement Techniques, 8, 4845-4850, 2015 Author(s): R. D. McPeters, S. Frith, and G. J. Labow The ozone data record from the Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura satellite has proven to be very stable over the 10-plus years of operation. The OMI total column ozone processed through the Total Ozone Mapping Spectrometer (TOMS) ozone retrieval algorithm (version 8.5) has been compared with ground-based measurements and with ozone from a series of SBUV/2 (Solar Backscatter Ultraviolet) instruments. Comparison with an ensemble of Brewer–Dobson sites shows an absolute offset of about 1.5 % and almost no relative trend. Comparison with a merged ozone data set (MOD) created by combining data from a series of SBUV/2 instruments again shows an offset, of about 1 %, and a relative trend of less than 0.5 % over 10 years. The offset is mostly due to the use of the old Bass–Paur ozone cross sections in the OMI retrievals rather than the Brion–Daumont–Malicet cross sections that are now recommended. The bias in the Southern Hemisphere is smaller than that in the Northern Hemisphere, 0.9 % vs. 1.5 %, for reasons that are not completely understood. When OMI was compared with the European realization of a multi-instrument ozone time series, the GTO (GOME type Total Ozone) data set, there was a small trend of about −0.85 % decade −1 . Since all the comparisons of OMI relative to other ozone measuring systems show relative trends that are less than 1 % decade −1 , we conclude that the OMI total column ozone data are sufficiently stable that they can be used in studies of ozone trends.

Description: Sensitivity of remotely sensed trace gas concentrations to polarisation Atmospheric Measurement Techniques, 8, 4917-4930, 2015 Author(s): D. M. O'Brien, I. N. Polonsky, and J. B. Kumer Current and proposed space missions estimate column-averaged concentrations of trace gases (CO 2 , CH 4 and CO) from high resolution spectra of reflected sunlight in absorption bands of the gases. The radiance leaving the top of the atmosphere is partially polarised by both reflection at the surface and scattering within the atmosphere. Generally, the polarisation state is unknown and could degrade the accuracy of the concentration measurements. The sensitivity to polarisation is modelled for the proposed geoCARB instrument, which will include neither polarisers nor polarisation scramblers to select particular polarisation states from the incident radiation. The radiometric and polarimetric calibrations proposed for geoCARB are outlined, and a model is developed for the polarisation properties of the geoCARB spectrographs. This model depends principally upon the efficiencies of the gratings to polarisations parallel and perpendicular to the rulings of the gratings. Next, an ensemble of polarised spectra is simulated for geoCARB observing targets in India, China and Australia from geostationary orbit at longitude 110° E. The spectra are analysed to recover the trace gas concentrations in two modes, the first denied access to the polarimetric calibration and the second with access. The retrieved concentrations using the calibration data are almost identical to those that would be obtained with polarisation scramblers, while the retrievals without calibration data contain outliers that do not meet the accuracies demanded by the mission.

Description: Evaluation of the operational Aerosol Layer Height retrieval algorithm for Sentinel-5 Precursor: application to O 2 A band observations from GOME-2A Atmospheric Measurement Techniques, 8, 4947-4977, 2015 Author(s): A. F. J. Sanders, J. F. de Haan, M. Sneep, A. Apituley, P. Stammes, M. O. Vieitez, L. G. Tilstra, O. N. E. Tuinder, C. E. Koning, and J. P. Veefkind An algorithm setup for the operational Aerosol Layer Height product for TROPOMI on the Sentinel-5 Precursor mission is described and discussed, applied to GOME-2A data, and evaluated with lidar measurements. The algorithm makes a spectral fit of reflectance at the O 2 A band in the near-infrared and the fit window runs from 758 to 770 nm. The aerosol profile is parameterised by a scattering layer with constant aerosol volume extinction coefficient and aerosol single scattering albedo and with a fixed pressure thickness. The algorithm's target parameter is the height of this layer. In this paper, we apply the algorithm to observations from GOME-2A in a number of systematic and extensive case studies, and we compare retrieved aerosol layer heights with lidar measurements. Aerosol scenes cover various aerosol types, both elevated and boundary layer aerosols, and land and sea surfaces. The aerosol optical thicknesses for these scenes are relatively moderate. Retrieval experiments with GOME-2A spectra are used to investigate various sensitivities, in which particular attention is given to the role of the surface albedo. From retrieval simulations with the single-layer model, we learn that the surface albedo should be a fit parameter when retrieving aerosol layer height from the O 2 A band. Current uncertainties in surface albedo climatologies cause biases and non-convergences when the surface albedo is fixed in the retrieval. Biases disappear and convergence improves when the surface albedo is fitted, while precision of retrieved aerosol layer pressure is still largely within requirement levels. Moreover, we show that fitting the surface albedo helps to ameliorate biases in retrieved aerosol layer height when the assumed aerosol model is inaccurate. Subsequent retrievals with GOME-2A spectra confirm that convergence is better when the surface albedo is retrieved simultaneously with aerosol parameters. However, retrieved aerosol layer pressures are systematically low (i.e., layer high in the atmosphere) to the extent that retrieved values no longer realistically represent actual extinction profiles. When the surface albedo is fixed in retrievals with GOME-2A spectra, convergence deteriorates as expected, but retrieved aerosol layer pressures become much higher (i.e., layer lower in atmosphere). The comparison with lidar measurements indicates that retrieved aerosol layer heights are indeed representative of the underlying profile in that case. Finally, subsequent retrieval simulations with two-layer aerosol profiles show that a model error in the assumed profile (two layers in the simulation but only one in the retrieval) is partly absorbed by the surface albedo when this parameter is fitted. This is expected in view of the correlations between errors in fit parameters and the effect is relatively small for elevated layers (less than 100 hPa). If one of the scattering layers is near the surface (boundary layer aerosols), the effect becomes surprisingly large, in such a way that the retrieved height of the single layer is above the two-layer profile. Furthermore, we find that the retrieval solution, once retrieval converges, hardly depends on the starting values for the fit. Sensitivity experiments with GOME-2A spectra also show that aerosol layer height is indeed relatively robust against inaccuracies in the assumed aerosol model, even when the surface albedo is not fitted. We show spectral fit residuals, which can be used for further investigations. Fit residuals may be partly explained by spectroscopic uncertainties, which is suggested by an experiment showing the improvement of convergence when the absorption cross section is scaled in agreement with Butz et al. (2013) and Crisp et al. (2012), and a temperature offset to the a priori ECMWF temperature profile is fitted. Retrieved temperature offsets are always negative and quite large (ranging between −4 and −8 K), which is not expected if temperature offsets absorb remaining inaccuracies in meteorological data. Other sensitivity experiments investigate fitting of stray light and fluorescence emissions. We find negative radiance offsets and negative fluorescence emissions, also for non-vegetated areas, but from the results it is not clear whether fitting these parameters improves the retrieval. Based on the present results, the operational baseline for the Aerosol Layer Height product currently will not fit the surface albedo. The product will be particularly suited for elevated, optically thick aerosol layers. In addition to its scientific value in climate research, anticipated applications of the product for TROPOMI are providing aerosol height information for aviation safety and improving interpretation of the Absorbing Aerosol Index.

Description: Radiometric consistency assessment of hyperspectral infrared sounders Atmospheric Measurement Techniques, 8, 4831-4844, 2015 Author(s): L. Wang, Y. Han, X. Jin, Y. Chen, and D. A. Tremblay The radiometric and spectral consistency among the Atmospheric Infrared Sounder (AIRS), the Infrared Atmospheric Sounding Interferometer (IASI), and the Cross-track Infrared Sounder (CrIS) is fundamental for the creation of long-term infrared (IR) hyperspectral radiance benchmark data sets for both intercalibration and climate-related studies. In this study, the CrIS radiance measurements on Suomi National Polar-orbiting Partnership (SNPP) satellite are directly compared with IASI on MetOp-A and MetOp-B at the finest spectral scale and with AIRS on Aqua in 25 selected spectral regions through simultaneous nadir overpass (SNO) observations in 2013, to evaluate radiometric consistency of these four hyperspectral IR sounders. The spectra from different sounders are paired together through strict spatial and temporal collocation. The uniform scenes are selected by examining the collocated Visible Infrared Imaging Radiometer Suite (VIIRS) pixels. Their brightness temperature (BT) differences are then calculated by converting the spectra onto common spectral grids. The results indicate that CrIS agrees well with IASI on MetOp-A and IASI on MetOp-B at the long-wave IR (LWIR) and middle-wave IR (MWIR) bands with 0.1–0.2 K differences. There are no apparent scene-dependent patterns for BT differences between CrIS and IASI for individual spectral channels. CrIS and AIRS are compared at the 25 spectral regions for both polar and tropical SNOs. The combined global SNO data sets indicate that the CrIS–AIRS BT differences are less than or around 0.1 K among 21 of 25 spectral regions and they range from 0.15 to 0.21 K in the remaining four spectral regions. CrIS–AIRS BT differences in some comparison spectral regions show weak scene-dependent features.

Description: HO x radical chemistry in oxidation flow reactors with low-pressure mercury lamps systematically examined by modeling Atmospheric Measurement Techniques, 8, 4863-4890, 2015 Author(s): Z. Peng, D. A. Day, H. Stark, R. Li, J. Lee-Taylor, B. B. Palm, W. H. Brune, and J. L. Jimenez Oxidation flow reactors (OFRs) using OH produced from low-pressure Hg lamps at 254 nm (OFR254) or both 185 and 254 nm (OFR185) are commonly used in atmospheric chemistry and other fields. OFR254 requires the addition of externally formed O 3 since OH is formed from O 3 photolysis, while OFR185 does not since O 2 can be photolyzed to produce O 3 , and OH can also be formed from H 2 O photolysis. In this study, we use a plug-flow kinetic model to investigate OFR properties under a very wide range of conditions applicable to both field and laboratory studies. We show that the radical chemistry in OFRs can be characterized as a function of UV light intensity, H 2 O concentration, and total external OH reactivity (OHR ext , e.g., from volatile organic compounds (VOCs), NO x , and SO 2 ). OH exposure is decreased by added external OH reactivity. OFR185 is especially sensitive to this effect at low UV intensity due to low primary OH production. OFR254 can be more resilient against OH suppression at high injected O 3 (e.g., 70 ppm), as a larger primary OH source from O 3 , as well as enhanced recycling of HO 2 to OH, make external perturbations to the radical chemistry less significant. However if the external OH reactivity in OFR254 is much larger than OH reactivity from injected O 3 , OH suppression can reach 2 orders of magnitude. For a typical input of 7 ppm O 3 (OHR O 3 = 10 s −1 ), 10-fold OH suppression is observed at OHR ext ~ 100 s −1 , which is similar or lower than used in many laboratory studies. The range of modeled OH suppression for literature experiments is consistent with the measured values except for those with isoprene. The finding on OH suppression may have important implications for the interpretation of past laboratory studies, as applying OH exp measurements acquired under different conditions could lead to over a 1-order-of-magnitude error in the estimated OH exp . The uncertainties of key model outputs due to uncertainty in all rate constants and absorption cross-sections in the model are within ±25 % for OH exposure and within ±60 % for other parameters. These uncertainties are small relative to the dynamic range of outputs. Uncertainty analysis shows that most of the uncertainty is contributed by photolysis rates of O 3 , O 2 , and H 2 O and reactions of OH and HO 2 with themselves or with some abundant species, i.e., O 3 and H 2 O 2 . OH exp calculated from direct integration and estimated from SO 2 decay in the model with laminar and measured residence time distributions (RTDs) are generally within a factor of 2 from the plug-flow OH exp . However, in the models with RTDs, OH exp estimated from SO 2 is systematically lower than directly integrated OH exp in the case of significant SO 2 consumption. We thus recommended using OH exp estimated from the decay of the species under study when possible, to obtain the most appropriate information on photochemical aging in the OFR. Using HO x -recycling vs. destructive external OH reactivity only leads to small changes in OH exp under most conditions. Changing the identity (rate constant) of external OH reactants can result in substantial changes in OH exp due to different reductions in OH suppression as the reactant is consumed. We also report two equations for estimating OH exposure in OFR254. We find that the equation estimating OH exp from measured O 3 consumption performs better than an alternative equation that does not use it, and thus recommend measuring both input and output O 3 concentrations in OFR254 experiments. This study contributes to establishing a firm and systematic understanding of the gas-phase HO x and O x chemistry in these reactors, and enables better experiment planning and interpretation as well as improved design of future reactors.

Description: EARLINET Single Calculus Chain – overview on methodology and strategy Atmospheric Measurement Techniques, 8, 4891-4916, 2015 Author(s): G. D'Amico, A. Amodeo, H. Baars, I. Binietoglou, V. Freudenthaler, I. Mattis, U. Wandinger, and G. Pappalardo In this paper we describe the EARLINET Single Calculus Chain (SCC), a tool for the automatic analysis of lidar measurements. The development of this tool started in the framework of EARLINET-ASOS (European Aerosol Research Lidar Network – Advanced Sustainable Observation System); it was extended within ACTRIS (Aerosol, Clouds and Trace gases Research InfraStructure Network), and it is continuing within ACTRIS-2. The main idea was to develop a data processing chain that allows all EARLINET stations to retrieve, in a fully automatic way, the aerosol backscatter and extinction profiles starting from the raw lidar data of the lidar systems they operate. The calculus subsystem of the SCC is composed of two modules: a pre-processor module which handles the raw lidar data and corrects them for instrumental effects and an optical processing module for the retrieval of aerosol optical products from the pre-processed data. All input parameters needed to perform the lidar analysis are stored in a database to keep track of all changes which may occur for any EARLINET lidar system over the time. The two calculus modules are coordinated and synchronized by an additional module (daemon) which makes the whole analysis process fully automatic. The end user can interact with the SCC via a user-friendly web interface. All SCC modules are developed using open-source and freely available software packages. The final products retrieved by the SCC fulfill all requirements of the EARLINET quality assurance programs on both instrumental and algorithm levels. Moreover, the manpower needed to provide aerosol optical products is greatly reduced and thus the near-real-time availability of lidar data is improved. The high-quality of the SCC products is proven by the good agreement between the SCC analysis, and the corresponding independent manual retrievals. Finally, the ability of the SCC to provide high-quality aerosol optical products is demonstrated for an EARLINET intense observation period.

Description: Performance assessment of a triple-frequency spaceborne cloud–precipitation radar concept using a global cloud-resolving model Atmospheric Measurement Techniques, 8, 3493-3517, 2015 Author(s): J. Leinonen, M. D. Lebsock, S. Tanelli, K. Suzuki, H. Yashiro, and Y. Miyamoto Multi-frequency radars offer enhanced detection of clouds and precipitation compared to single-frequency systems, and are able to make more accurate retrievals when several frequencies are available simultaneously. An evaluation of a spaceborne three-frequency Ku-/Ka-/W-band radar system is presented in this study, based on modeling radar reflectivities from the results of a global cloud-resolving model with a 875 m grid spacing. To produce the reflectivities, a scattering model has been developed for each of the hydrometeor types produced by the model, as well as for melting snow. The effects of attenuation and multiple scattering on the radar signal are modeled using a radiative transfer model, while nonuniform beam filling is reproduced with spatial averaging. The combined effects of these are then quantified both globally and in six localized case studies. Two different orbital scenarios using the same radar are compared. Overall, based on the results, it is expected that the proposed radar would detect a high-quality signal in most clouds and precipitation. The main exceptions are the thinnest clouds that are below the detection threshold of the W-band channel, and at the opposite end of the scale, heavy convective rainfall where a combination of attenuation, multiple scattering and nonuniform beam filling commonly cause significant deterioration of the signal; thus, while the latter can be generally detected, the quality of the retrievals is likely to be degraded.

Description: Design and application of a mobile ground-based observatory for continuous measurements of atmospheric trace gas and criteria pollutant species Atmospheric Measurement Techniques, 8, 3481-3492, 2015 Author(s): S. E. Bush, F. M. Hopkins, J. T. Randerson, C.-T. Lai, and J. R. Ehleringer Ground-based measurements of atmospheric trace gas species and criteria pollutants are essential for understanding emissions dynamics across space and time. Gas composition in the lower 50 m of the atmosphere has the greatest direct impacts on human health as well as ecosystem processes; hence data at this level are necessary for addressing carbon-cycle- and public-health-related questions. However, such surface data are generally associated with stationary measurement towers, where spatial representation is limited due to the high cost of establishing and maintaining an extensive network of measurement stations. We describe here a compact mobile laboratory equipped to provide high-precision, high-frequency, continuous, on-road synchronous measurements of CO 2 , CO, CH 4 , H 2 O, NO x , O 3 , aerosol, meteorological, and geospatial position data. The mobile laboratory has been deployed across the western USA. In addition to describing the vehicle and its capacity, we present data that illustrate the use of the laboratory as a powerful tool for investigating the spatial structure of urban trace gas emissions and criteria pollutants at spatial scales ranging from single streets to whole ecosystem and regional scales. We assess the magnitude of known point sources of CH 4 and also identify fugitive urban CH 4 emissions. We illustrate how such a mobile laboratory can be used to better understand emissions dynamics and quantify emissions ratios associated with trace gas emissions from wildfire incidents. Lastly, we discuss additional mobile laboratory applications in health and urban metabolism.

Description: Development of a photochemical source for the production and calibration of acyl peroxynitrate compounds Atmospheric Measurement Techniques, 8, 2225-2231, 2015 Author(s): P. R. Veres and J. M. Roberts A dynamic system for the calibration of acyl peroxynitrate compounds (APNs) has been developed in the laboratory to reduce the difficulty, required time, and instability of laboratory-produced standards for difficult-to-synthesize APN species. In this work we present a photochemical source for the generation of APN standards: acetyl peroxynitrate (PAN), propionyl peroxynitrate (PPN), acryloyl peroxynitrate (APAN), methacryloyl peroxynitrate (MPAN), and crotonyl peroxynitrate (CPAN). APNs are generated via photolysis of a mixture of acyl chloride (RC(O)Cl) and ketone (RC(=O)R) precursor compounds in the presence of O 2 and NO 2 . Subsequent separation by a prep-scale gas chromatograph and detection with a total NO y instrument serve to quantify the output of the APN source. Validation of the APN products was performed using iodide ion chemical ionization mass spectroscopy (I - CIMS). This method of standard production is an efficient and accurate technique for the calibration of instrumentation used to measure PAN, PPN, APAN, MPAN, and CPAN.

Description: Ceilometer aerosol profiling versus Raman lidar in the frame of the INTERACT campaign of ACTRIS Atmospheric Measurement Techniques, 8, 2207-2223, 2015 Author(s): F. Madonna, F. Amato, J. Vande Hey, and G. Pappalardo Despite their differences from more advanced and more powerful lidars, the low construction and operation cost of ceilometers (originally designed for cloud base height monitoring) has fostered their use for the quantitative study of aerosol properties. The large number of ceilometers available worldwide represents a strong motivation to investigate both the extent to which they can be used to fill in the geographical gaps between advanced lidar stations and also how their continuous data flow can be linked to existing networks of the more advanced lidars, like EARLINET (European Aerosol Research Lidar Network). In this paper, multi-wavelength Raman lidar measurements are used to investigate the capability of ceilometers to provide reliable information about atmospheric aerosol properties through the INTERACT (INTERcomparison of Aerosol and Cloud Tracking) campaign carried out at the CNR-IMAA Atmospheric Observatory (760 m a.s.l., 40.60° N, 15.72° E), in the framework of the ACTRIS (Aerosol Clouds Trace gases Research InfraStructure) FP7 project. This work is the first time that three different commercial ceilometers with an advanced Raman lidar are compared over a period of 6 months. The comparison of the attenuated backscatter coefficient profiles from a multi-wavelength Raman lidar and three ceilometers (CHM15k, CS135s, CT25K) reveals differences due to the expected discrepancy in the signal to noise ratio (SNR) but also due to changes in the ambient temperature on the short and mid-term stability of ceilometer calibration. Therefore, technological improvements are needed to move ceilometers towards operational use in the monitoring of atmospheric aerosols in the low and free troposphere.

Description: Continuous measurements of greenhouse gases and atmospheric oxygen at the Namib Desert Atmospheric Observatory Atmospheric Measurement Techniques, 8, 2233-2250, 2015 Author(s): E. J. Morgan, J. V. Lavrič, T. Seifert, T. Chicoine, A. Day, J. Gomez, R. Logan, J. Sack, T. Shuuya, E. G. Uushona, K. Vincent, U. Schultz, E.-G. Brunke, C. Labuschagne, R. L. Thompson, S. Schmidt, A. C. Manning, and M. Heimann A new coastal background site has been established for observations of greenhouse gases (GHGs) in the central Namib Desert at Gobabeb, Namibia. The location of the site was chosen to provide observations for a data-poor region in the global sampling network for GHGs. Semi-automated continuous measurements of carbon dioxide, methane, nitrous oxide, carbon monoxide, atmospheric oxygen, and basic meteorology are made at a height of 21 m a.g.l., 50 km from the coast at the northern border of the Namib Sand Sea. Atmospheric oxygen is measured with a differential fuel cell analyzer (DFCA). Carbon dioxide and methane are measured with an early-model cavity ring-down spectrometer (CRDS); nitrous oxide and carbon monoxide are measured with an off-axis integrated cavity output spectrometer (OA-ICOS). Instrument-specific water corrections are employed for both the CRDS and OA-ICOS instruments in lieu of drying. The performance and measurement uncertainties are discussed in detail. As the station is located in a remote desert environment, there are some particular challenges, namely fine dust, high diurnal temperature variability, and minimal infrastructure. The gas handling system and calibration scheme were tailored to best fit the conditions of the site. The CRDS and DFCA provide data of acceptable quality when base requirements for operation are met, specifically adequate temperature control in the laboratory and regular supply of electricity. In the case of the OA-ICOS instrument, performance is significantly improved through the implementation of a drift correction through frequent measurements of a reference cylinder.

Description: Estimating reflectivity values from wind turbines for analyzing the potential impact on weather radar services Atmospheric Measurement Techniques, 8, 2183-2193, 2015 Author(s): I. Angulo, O. Grande, D. Jenn, D. Guerra, and D. de la Vega The World Meteorological Organization (WMO) has repeatedly expressed concern over the increasing number of impact cases of wind turbine farms on weather radars. Current signal processing techniques to mitigate wind turbine clutter (WTC) are scarce, so the most practical approach to this issue is the assessment of the potential interference from a wind farm before it is installed. To do so, and in order to obtain a WTC reflectivity model, it is crucial to estimate the radar cross section (RCS) of the wind turbines to be built, which represents the power percentage of the radar signal that is backscattered to the radar receiver. For the proposed model, a representative scenario has been chosen in which both the weather radar and the wind farm are placed on clear areas; i.e., wind turbines are supposed to be illuminated only by the lowest elevation angles of the radar beam. This paper first characterizes the RCS of wind turbines in the weather radar frequency bands by means of computer simulations based on the physical optics theory and then proposes a simplified model to estimate wind turbine RCS values. This model is of great help in the evaluation of the potential impact of a certain wind farm on the weather radar operation.

Description: On the microwave optical properties of randomly oriented ice hydrometeors Atmospheric Measurement Techniques, 8, 1913-1933, 2015 Author(s): P. Eriksson, M. Jamali, J. Mendrok, and S. A. Buehler Microwave remote sensing is important for observing the mass of ice hydrometeors. One of the main error sources of microwave ice mass retrievals is that approximations around the shape of the particles are unavoidable. One common approach to represent particles of irregular shape is the soft particle approximation (SPA). We show that it is possible to define a SPA that mimics mean optical particles of available reference data over narrow frequency ranges, considering a single observation technique at the time, but that SPA does not work in a broader context. Most critically, the required air fraction varies with frequency and application, as well as with particle size. In addition, the air fraction matching established density parameterisations results in far too soft particles, at least for frequencies above 90 GHz. That is, alternatives to SPA must be found. One alternative was recently presented by Geer and Baordo (2014). They used a subset of the same reference data and simply selected as "shape model" the particle type giving the best overall agreement with observations. We present a way to perform the same selection of a representative particle shape but without involving assumptions on particle size distribution and actual ice mass contents. Only an assumption on the occurrence frequency of different particle shapes is still required. Our analysis leads to the same selection of representative shape as found by Geer and Baordo (2014). In addition, we show that the selected particle shape has the desired properties at higher frequencies as well as for radar applications. Finally, we demonstrate that in this context the assumption on particle shape is likely less critical when using mass equivalent diameter to characterise particle size compared to using maximum dimension, but a better understanding of the variability of size distributions is required to fully characterise the advantage. Further advancements on these subjects are presently difficult to achieve due to a lack of reference data. One main problem is that most available databases of precalculated optical properties assume completely random particle orientation, while for certain conditions a horizontal alignment is expected. In addition, the only database covering frequencies above 340 GHz has a poor representation of absorption as it is based on outdated refractive index data as well as only covering particles having a maximum dimension below 2 mm and a single temperature.

Description: Using self-organising maps to explore ozone profile validation results – SCIAMACHY limb compared to ground-based lidar observations Atmospheric Measurement Techniques, 8, 1951-1963, 2015 Author(s): J. A. E. van Gijsel, R. Zurita-Milla, P. Stammes, S. Godin-Beekmann, T. Leblanc, M. Marchand, I. S. McDermid, K. Stebel, W. Steinbrecht, and D. P. J. Swart Traditional validation of atmospheric profiles is based on the intercomparison of two or more data sets in predefined ranges or classes of a given observational characteristic such as latitude or solar zenith angle. In this study we trained a self-organising map (SOM) with a full time series of relative difference profiles of SCIAMACHY limb v5.02 and lidar ozone profiles from seven observation sites. Each individual observation characteristic was then mapped to the obtained SOM to investigate to which degree variation in this characteristic is explanatory for the variation seen in the SOM map. For the studied data sets, altitude-dependent relations for the global data set were found between the difference profiles and studied variables. From the lowest altitude studied (18 km) ascending, the most influencing factors were found to be longitude, followed by solar zenith angle and latitude, sensor age and again solar zenith angle together with the day of the year at the highest altitudes studied here (up to 45 km). After accounting for both latitude and longitude, residual partial correlations with a reduced magnitude are seen for various factors. However, (partial) correlations cannot point out which (combination) of the factors drives the observed differences between the ground-based and satellite ozone profiles as most of the factors are inter-related. Clustering into three classes showed that there are also some local dependencies, with for instance one cluster having a much stronger correlation with the sensor age (days since launch) between 36 and 42 km. The proposed SOM-based approach provides a powerful tool for the exploration of differences between data sets without being limited to a priori defined data subsets.

Description: Comparing satellite- to ground-based automated and manual cloud coverage observations – a case study Atmospheric Measurement Techniques, 8, 2001-2015, 2015 Author(s): A. Werkmeister, M. Lockhoff, M. Schrempf, K. Tohsing, B. Liley, and G. Seckmeyer In this case study we compare cloud fractional cover measured by radiometers on polar satellites (AVHRR) and on one geostationary satellite (SEVIRI) to ground-based manual (SYNOP) and automated observations by a cloud camera (Hemispherical Sky Imager, HSI). These observations took place in Hannover, Germany, and in Lauder, New Zealand, over time frames of 3 and 2 months, respectively. Daily mean comparisons between satellite derivations and the ground-based HSI found the deviation to be 6 ± 14% for AVHRR and 8 ± 16% for SEVIRI, which can be considered satisfactory. AVHRR's instantaneous differences are smaller (2 ± 22%) than instantaneous SEVIRI cloud fraction estimates (8 ± 29%) when compared to HSI due to resolution and scenery effect issues. All spaceborne observations show a very good skill in detecting completely overcast skies (cloud cover ≥ 6 oktas) with probabilities between 92 and 94% and false alarm rates between 21 and 29% for AVHRR and SEVIRI in Hannover, Germany. In the case of a clear sky (cloud cover lower than 3 oktas) we find good skill with detection probabilities between 72 and 76%. We find poor skill, however, whenever broken clouds occur (probability of detection is 32% for AVHRR and 12% for SEVIRI in Hannover, Germany). In order to better understand these discrepancies we analyze the influence of algorithm features on the satellite-based data. We find that the differences between SEVIRI and HSI cloud fractional cover (CFC) decrease (from a bias of 8 to almost 0%) with decreasing number of spatially averaged pixels and decreasing index which determines the cloud coverage in each "cloud-contaminated" pixel of the binary map. We conclude that window size and index need to be adjusted in order to improve instantaneous SEVIRI and AVHRR estimates. Due to its automated operation and its spatial, temporal and spectral resolution, we recommend as well that more automated ground-based instruments in the form of cloud cameras should be installed as they cover larger areas of the sky than other automated ground-based instruments. These cameras could be an essential supplement to SYNOP observation as they cover the same spectral wavelengths as the human eye.

Description: The "dual-spot" Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation Atmospheric Measurement Techniques, 8, 1965-1979, 2015 Author(s): L. Drinovec, G. Močnik, P. Zotter, A. S. H. Prévôt, C. Ruckstuhl, E. Coz, M. Rupakheti, J. Sciare, T. Müller, A. Wiedensohler, and A. D. A. Hansen Aerosol black carbon is a unique primary tracer for combustion emissions. It affects the optical properties of the atmosphere and is recognized as the second most important anthropogenic forcing agent for climate change. It is the primary tracer for adverse health effects caused by air pollution. For the accurate determination of mass equivalent black carbon concentrations in the air and for source apportionment of the concentrations, optical measurements by filter-based absorption photometers must take into account the "filter loading effect". We present a new real-time loading effect compensation algorithm based on a two parallel spot measurement of optical absorption. This algorithm has been incorporated into the new Aethalometer model AE33. Intercomparison studies show excellent reproducibility of the AE33 measurements and very good agreement with post-processed data obtained using earlier Aethalometer models and other filter-based absorption photometers. The real-time loading effect compensation algorithm provides the high-quality data necessary for real-time source apportionment and for determination of the temporal variation of the compensation parameter k .

Description: A model sensitivity study of the impact of clouds on satellite detection and retrieval of volcanic ash Atmospheric Measurement Techniques, 8, 1935-1949, 2015 Author(s): A. Kylling, N. Kristiansen, A. Stohl, R. Buras-Schnell, C. Emde, and J. Gasteiger Volcanic ash is commonly observed by infrared detectors on board Earth-orbiting satellites. In the presence of ice and/or liquid-water clouds, the detected volcanic ash signature may be altered. In this paper the sensitivity of detection and retrieval of volcanic ash to the presence of ice and liquid-water clouds was quantified by simulating synthetic equivalents to satellite infrared images with a 3-D radiative transfer model. The sensitivity study was made for the two recent eruptions of Eyjafjallajökull (2010) and Grímsvötn (2011) using realistic water and ice clouds and volcanic ash clouds. The water and ice clouds were taken from European Centre for Medium-Range Weather Forecast (ECMWF) analysis data and the volcanic ash cloud fields from simulations by the Lagrangian particle dispersion model FLEXPART. The radiative transfer simulations were made both with and without ice and liquid-water clouds for the geometry and channels of the Spinning Enhanced Visible and Infrared Imager (SEVIRI). The synthetic SEVIRI images were used as input to standard reverse absorption ash detection and retrieval methods. Ice and liquid-water clouds were on average found to reduce the number of detected ash-affected pixels by 6–12%. However, the effect was highly variable and for individual scenes up to 40% of pixels with mass loading 〉0.2 g m −2 could not be detected due to the presence of water and ice clouds. For coincident pixels, i.e. pixels where ash was both present in the FLEXPART (hereafter referred to as "Flexpart") simulation and detected by the algorithm, the presence of clouds overall increased the retrieved mean mass loading for the Eyjafjallajökull (2010) eruption by about 13%, while for the Grímsvötn (2011) eruption ash-mass loadings the effect was a 4% decrease of the retrieved ash-mass loading. However, larger differences were seen between scenes (standard deviations of ±30 and ±20% for Eyjafjallajökull and Grímsvötn, respectively) and even larger ones within scenes. The impact of ice and liquid-water clouds on the detection and retrieval of volcanic ash, implies that to fully appreciate the location and amount of ash, hyperspectral and spectral band measurements by satellite instruments should be combined with ash dispersion modelling.

Description: Measurements of methane emissions from natural gas gathering facilities and processing plants: measurement methods Atmospheric Measurement Techniques, 8, 2017-2035, 2015 Author(s): J. R. Roscioli, T. I. Yacovitch, C. Floerchinger, A. L. Mitchell, D. S. Tkacik, R. Subramanian, D. M. Martinez, T. L. Vaughn, L. Williams, D. Zimmerle, A. L. Robinson, S. C. Herndon, and A. J. Marchese Increased natural gas production in recent years has spurred intense interest in methane (CH 4 ) emissions associated with its production, gathering, processing, transmission, and distribution. Gathering and processing facilities (G&P facilities) are unique in that the wide range of gas sources (shale, coal-bed, tight gas, conventional, etc.) results in a wide range of gas compositions, which in turn requires an array of technologies to prepare the gas for pipeline transmission and distribution. We present an overview and detailed description of the measurement method and analysis approach used during a 20-week field campaign studying CH 4 emissions from the natural gas G&P facilities between October 2013 and April 2014. Dual-tracer flux measurements and on-site observations were used to address the magnitude and origins of CH 4 emissions from these facilities. The use of a second tracer as an internal standard revealed plume-specific uncertainties in the measured emission rates of 20–47%, depending upon plume classification. Combining downwind methane, ethane (C 2 H 6 ), carbon monoxide (CO), carbon dioxide (CO 2 ), and tracer gas measurements with on-site tracer gas release allows for quantification of facility emissions and in some cases a more detailed picture of source locations.

Description: Tomographic retrieval of water vapour and temperature around polar mesospheric clouds using Odin-SMR Atmospheric Measurement Techniques, 8, 1981-1999, 2015 Author(s): O. M. Christensen, P. Eriksson, J. Urban, D. Murtagh, K. Hultgren, and J. Gumbel A special observation mode of the Odin satellite provides the first simultaneous measurements of water vapour, temperature and polar mesospheric cloud (PMC) brightness over a large geographical area while still resolving both horizontal and vertical structures in the clouds and background atmosphere. The observation mode was activated during June, July and August of 2010 and 2011, and for latitudes between 50 and 82° N. This paper focuses on the water vapour and temperature measurements carried out with Odin's sub-millimetre radiometer (SMR). The tomographic retrieval approach used provides water vapour and temperature between 75 and 90 km with a vertical resolution of about 2.5 km and a horizontal resolution of about 200 km. The precision of the measurements is estimated to 0.2 ppmv for water vapour and 2 K for temperature. Due to limited information about the pressure at the measured altitudes, the results have large uncertainties (〉 3 ppmv) in the retrieved water vapour. These errors, however, influence mainly the mean atmosphere retrieved for each orbit, and variations around this mean are still reliably captured by the measurements. SMR measurements are performed using two different mixer chains, denoted as frequency mode 19 and 13. Systematic differences between the two frontends have been noted. A first comparison with the Solar Occultation For Ice Experiment instrument (SOFIE) on-board the Aeronomy of Ice in the Mesosphere (AIM) satellite and the Fourier Transform Spectrometer of the Atmospheric Chemistry Experiment (ACE-FTS) on-board SCISAT indicates that the measurements using the frequency mode 19 have a significant low bias in both temperature (〉 15 K) and water vapour (〉 0.5 ppmv), while the measurements using frequency mode 13 agree with the other instruments considering estimated errors. PMC brightness data is provided by OSIRIS, Odin's other sensor. Combined SMR and OSIRIS data for some example orbits is considered. For these orbits, effects of PMCs on the water vapour distribution are clearly seen. Areas depleted of water vapour are found above layers with PMC, while regions of enhanced water vapour due to ice particle sedimentation are primarily placed between and under the clouds.

Description: Airborne in situ vertical profiling of HDO / H 2 16 O in the subtropical troposphere during the MUSICA remote sensing validation campaign Atmospheric Measurement Techniques, 8, 2037-2049, 2015 Author(s): C. Dyroff, S. Sanati, E. Christner, A. Zahn, M. Balzer, H. Bouquet, J. B. McManus, Y. González-Ramos, and M. Schneider Vertical profiles of water vapor (H 2 O) and its isotope ratio D / H expressed as δD(H 2 O) were measured in situ by the ISOWAT II diode-laser spectrometer during the MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) airborne campaign. We present recent modifications of the instrument design. The instrument calibration on the ground as well as in flight is described. Based on the calibration measurements, the humidity-dependent uncertainty of our airborne data is determined. For the majority of the airborne data we achieved an accuracy (uncertainty of the mean) of Δ(δD) ≈10‰. Vertical profiles between 150 and ~7000 m were obtained during 7 days in July and August 2013 over the subtropical North Atlantic Ocean near Tenerife. The flights were coordinated with ground-based (Network for the Detection of Atmospheric Composition Change, NDACC) and space-based (Infrared Atmospheric Sounding Interferometer, IASI) FTIR remote sensing measurements of δD(H 2 O) as a means to validate the remote sensing humidity and δD(H 2 O) data products. The results of the validation are presented in detail in a separate paper (Schneider et al., 2014). The profiles were obtained with a high vertical resolution of around 3 m. By analyzing humidity and δD(H 2 O) correlations we were able to identify different layers of air masses with specific isotopic signatures. The results are discussed.

Description: Methodology for determining multilayered temperature inversions Atmospheric Measurement Techniques, 8, 2051-2060, 2015 Author(s): G. J. Fochesatto Temperature sounding of the atmospheric boundary layer (ABL) and lower troposphere exhibits multilayered temperature inversions specially in high latitudes during extreme winters. These temperature inversion layers are originated based on the combined forcing of local- and large-scale synoptic meteorology. At the local scale, the thermal inversion layer forms near the surface and plays a central role in controlling the surface radiative cooling and air pollution dispersion; however, depending upon the large-scale synoptic meteorological forcing, an upper level thermal inversion can also exist topping the local ABL. In this article a numerical methodology is reported to determine thermal inversion layers present in a given temperature profile and deduce some of their thermodynamic properties. The algorithm extracts from the temperature profile the most important temperature variations defining thermal inversion layers. This is accomplished by a linear interpolation function of variable length that minimizes an error function. The algorithm functionality is demonstrated on actual radiosonde profiles to deduce the multilayered temperature inversion structure with an error fraction set independently.

Description: Performance of WVSS-II hygrometers on the FAAM research aircraft Atmospheric Measurement Techniques, 8, 1617-1625, 2015 Author(s): A. K. Vance, S. J. Abel, R. J. Cotton, and A. M. Woolley We compare the performance of five hygrometers fitted to the Facility for Airborne Atmospheric Measurement's (FAAM) BAe 146-301 research aircraft using data from approximately 100 flights executed over the course of 2 years under a wide range of conditions. Bulk comparison of cloud free data show good agreement between chilled mirror hygrometers and a WVSS-II fed from a modified Rosemount inlet, but that a WVSS-II fed from the standard flush inlet appears to over-read compared to the other instruments, except at higher humidities. Statistical assessment of hygrometer performance in cloudy conditions is problematic due to the variable nature of clouds, so a number of case studies are used instead to investigate the performance of the hygrometers in sub-optimal conditions. It is found that the flush inlet is not susceptible to either liquid or solid water but that the Rosemount inlet has a significant susceptibility to liquid water and may also be susceptible to ice. In all conditions the WVSS-II responds much more rapidly than the chilled mirror devices, with the flush inlet-fed WVSS-II being more rapid than that connected to the Rosemount.

Description: Towards validation of ammonia (NH 3 ) measurements from the IASI satellite Atmospheric Measurement Techniques, 8, 1575-1591, 2015 Author(s): M. Van Damme, L. Clarisse, E. Dammers, X. Liu, J. B. Nowak, C. Clerbaux, C. R. Flechard, C. Galy-Lacaux, W. Xu, J. A. Neuman, Y. S. Tang, M. A. Sutton, J. W. Erisman, and P. F. Coheur Limited availability of ammonia (NH 3 ) observations is currently a barrier for effective monitoring of the nitrogen cycle. It prevents a full understanding of the atmospheric processes in which this trace gas is involved and therefore impedes determining its related budgets. Since the end of 2007, the Infrared Atmospheric Sounding Interferometer (IASI) satellite has been observing NH 3 from space at a high spatio-temporal resolution. This valuable data set, already used by models, still needs validation. We present here a first attempt to validate IASI-NH 3 measurements using existing independent ground-based and airborne data sets. The yearly distributions reveal similar patterns between ground-based and space-borne observations and highlight the scarcity of local NH 3 measurements as well as their spatial heterogeneity and lack of representativity. By comparison with monthly resolved data sets in Europe, China and Africa, we show that IASI-NH 3 observations are in fair agreement, but they are characterized by a smaller variation in concentrations. The use of hourly and airborne data sets to compare with IASI individual observations allows investigations of the impact of averaging as well as the representativity of independent observations for the satellite footprint. The importance of considering the latter and the added value of densely located airborne measurements at various altitudes to validate IASI-NH 3 columns are discussed. Perspectives and guidelines for future validation work on NH 3 satellite observations are presented.

Description: The impact of the microphysical properties of aerosol on the atmospheric correction of hyperspectral data in coastal waters Atmospheric Measurement Techniques, 8, 1593-1604, 2015 Author(s): C. Bassani, C. Manzo, F. Braga, M. Bresciani, C. Giardino, and L. Alberotanza Hyperspectral imaging provides quantitative remote sensing of ocean colour by the high spectral resolution of the water features. The HICO™ (Hyperspectral Imager for the Coastal Ocean) is suitable for coastal studies and monitoring. The accurate retrieval of hyperspectral water-leaving reflectance from HICO™ data is still a challenge. The aim of this work is to retrieve the water-leaving reflectance from HICO™ data with a physically based algorithm, using the local microphysical properties of the aerosol in order to overcome the limitations of the standard aerosol types commonly used in atmospheric correction processing. The water-leaving reflectance was obtained using the HICO@CRI (HICO ATmospherically Corrected Reflectance Imagery) atmospheric correction algorithm by adapting the vector version of the Second Simulation of a Satellite Signal in the Solar Spectrum (6SV) radiative transfer code. The HICO@CRI algorithm was applied on to six HICO™ images acquired in the northern Mediterranean basin, using the microphysical properties measured by the Acqua Alta Oceanographic Tower (AAOT) AERONET site. The HICO@CRI results obtained with AERONET products were validated with in situ measurements showing an accuracy expressed by r 2 = 0.98. Additional runs of HICO@CRI on the six images were performed using maritime, continental and urban standard aerosol types to perform the accuracy assessment when standard aerosol types implemented in 6SV are used. The results highlight that the microphysical properties of the aerosol improve the accuracy of the atmospheric correction compared to standard aerosol types. The normalized root mean square (NRMSE) and the similar spectral value (SSV) of the water-leaving reflectance show reduced accuracy in atmospheric correction results when there is an increase in aerosol loading. This is mainly when the standard aerosol type used is characterized with different optical properties compared to the local aerosol. The results suggest that if a water quality analysis is needed the microphysical properties of the aerosol need to be taken into consideration in the atmospheric correction of hyperspectral data over coastal environments, because aerosols influence the accuracy of the retrieved water-leaving reflectance.

Description: Meso-scale modelling and radiative transfer simulations of a snowfall event over France at microwaves for passive and active modes and evaluation with satellite observations Atmospheric Measurement Techniques, 8, 1605-1616, 2015 Author(s): V. S. Galligani, C. Prigent, E. Defer, C. Jimenez, P. Eriksson, J.-P. Pinty, and J.-P. Chaboureau Microwave passive and active radiative transfer simulations are performed with the Atmospheric Radiative Transfer Simulator (ARTS) for a mid-latitude snowfall event, using outputs from the Meso-NH mesoscale cloud model. The results are compared to the corresponding microwave observations available from MHS and CloudSat. The spatial structures of the simulated and observed brightness temperatures show an overall agreement since the large-scale dynamical structure of the cloud system is reasonably well captured by Meso-NH. However, with the initial assumptions on the single-scattering properties of snow, there is an obvious underestimation of the strong scattering observed in regions with large frozen hydrometeor quantities. A sensitivity analysis of both active and passive simulations to the microphysical parametrizations is conducted. Simultaneous analysis of passive and active calculations provides strong constraints on the assumptions made to simulate the observations. Good agreements are obtained with both MHS and CloudSat observations when the single-scattering properties are calculated using the "soft sphere" parametrization from Liu (2004), along with the Meso-NH outputs. This is an important step toward building a robust data set of simulated measurements to train a statistically based retrieval scheme.

Description: Techniques for analyses of trends in GRUAN data Atmospheric Measurement Techniques, 8, 1673-1684, 2015 Author(s): G. E. Bodeker and S. Kremser The Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) provides reference quality RS92 radiosonde measurements of temperature, pressure and humidity. A key attribute of reference quality measurements, and hence GRUAN data, is that each datum has a well characterized and traceable estimate of the measurement uncertainty. The long-term homogeneity of the measurement records, and their well characterized uncertainties, make these data suitable for reliably detecting changes in global and regional climate on decadal time scales. Considerable effort is invested in GRUAN operations to (i) describe and analyse all sources of measurement uncertainty to the extent possible, (ii) quantify and synthesize the contribution of each source of uncertainty to the total measurement uncertainty, and (iii) verify that the evaluated net uncertainty is within the required target uncertainty. However, if the climate science community is not sufficiently well informed on how to capitalize on this added value, the significant investment in estimating meaningful measurement uncertainties is largely wasted. This paper presents and discusses the techniques that will need to be employed to reliably quantify long-term trends in GRUAN data records. A pedagogical approach is taken whereby numerical recipes for key parts of the trend analysis process are explored. The paper discusses the construction of linear least squares regression models for trend analysis, boot-strapping approaches to determine uncertainties in trends, dealing with the combined effects of autocorrelation in the data and measurement uncertainties in calculating the uncertainty on trends, best practice for determining seasonality in trends, how to deal with co-linear basis functions, and interpreting derived trends. Synthetic data sets are used to demonstrate these concepts which are then applied to a first analysis of temperature trends in RS92 radiosonde upper air soundings at the GRUAN site at Lindenberg, Germany (52.21° N, 14.12° E).

Description: Retrievals of formaldehyde from ground-based FTIR and MAX-DOAS observations at the Jungfraujoch station and comparisons with GEOS-Chem and IMAGES model simulations Atmospheric Measurement Techniques, 8, 1733-1756, 2015 Author(s): B. Franco, F. Hendrick, M. Van Roozendael, J.-F. Müller, T. Stavrakou, E. A. Marais, B. Bovy, W. Bader, C. Fayt, C. Hermans, B. Lejeune, G. Pinardi, C. Servais, and E. Mahieu As an ubiquitous product of the oxidation of many volatile organic compounds (VOCs), formaldehyde (HCHO) plays a key role as a short-lived and reactive intermediate in the atmospheric photo-oxidation pathways leading to the formation of tropospheric ozone and secondary organic aerosols. In this study, HCHO profiles have been successfully retrieved from ground-based Fourier transform infrared (FTIR) solar spectra and UV-visible Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) scans recorded during the July 2010–December 2012 time period at the Jungfraujoch station (Swiss Alps, 46.5° N, 8.0° E, 3580 m a.s.l.). Analysis of the retrieved products has revealed different vertical sensitivity between both remote sensing techniques. Furthermore, HCHO amounts simulated by two state-of-the-art chemical transport models (CTMs), GEOS-Chem and IMAGES v2, have been compared to FTIR total columns and MAX-DOAS 3.6–8 km partial columns, accounting for the respective vertical resolution of each ground-based instrument. Using the CTM outputs as the intermediate, FTIR and MAX-DOAS retrievals have shown consistent seasonal modulations of HCHO throughout the investigated period, characterized by summertime maximum and wintertime minimum. Such comparisons have also highlighted that FTIR and MAX-DOAS provide complementary products for the HCHO retrieval above the Jungfraujoch station. Finally, tests have revealed that the updated IR parameters from the HITRAN 2012 database have a cumulative effect and significantly decrease the retrieved HCHO columns with respect to the use of the HITRAN 2008 compilation.

Description: The micro-orifice uniform deposit impactor–droplet freezing technique (MOUDI-DFT) for measuring concentrations of ice nucleating particles as a function of size: improvements and initial validation Atmospheric Measurement Techniques, 8, 2449-2462, 2015 Author(s): R. H. Mason, C. Chou, C. S. McCluskey, E. J. T. Levin, C. L. Schiller, T. C. J. Hill, J. A. Huffman, P. J. DeMott, and A. K. Bertram The micro-orifice uniform deposit impactor–droplet freezing technique (MOUDI-DFT) combines particle collection by inertial impaction (via the MOUDI) and a microscope-based immersion freezing apparatus (the DFT) to measure atmospheric concentrations of ice nucleating particles (INPs) as a function of size and temperature. In the first part of this study we improved upon this recently introduced technique. Using optical microscopy, we investigated the non-uniformity of MOUDI aerosol deposits at spatial resolutions of 1, 0.25 mm, and for some stages when necessary 0.10 mm. The results from these measurements show that at a spatial resolution of 1 mm and less, the concentration of particles along the MOUDI aerosol deposits can vary by an order of magnitude or more. Since the total area of a MOUDI aerosol deposit ranges from 425 to 605 mm 2 and the area analyzed by the DFT is approximately 1.2 mm 2 , this non-uniformity needs to be taken into account when using the MOUDI-DFT to determine atmospheric concentrations of INPs. Measurements of the non-uniformity of the MOUDI aerosol deposits were used to select positions on the deposits that had relatively small variations in particle concentration and to build substrate holders for the different MOUDI stages. These substrate holders improve reproducibility by holding the substrate in the same location for each measurement and ensure that DFT analysis is only performed on substrate regions with relatively small variations in particle concentration. In addition, the deposit non-uniformity was used to determine correction factors that take the non-uniformity into account when determining atmospheric concentrations of INPs. In the second part of this study, the MOUDI-DFT utilizing the new substrate holders was compared to the continuous flow diffusion chamber (CFDC) technique of Colorado State University. The intercomparison was done using INP concentrations found by the two instruments during ambient measurements of continental aerosols. Results from two sampling periods were compared, and the INP concentrations determined by the two techniques agreed within experimental uncertainty. The agreement observed here is commensurate with the level of agreement found in other studies where CFDC results were compared to INP concentrations measured with other methods.

Description: Validation of first chemistry mode retrieval results from the new limb-imaging FTS GLORIA with correlative MIPAS-STR observations Atmospheric Measurement Techniques, 8, 2509-2520, 2015 Author(s): W. Woiwode, O. Sumińska-Ebersoldt, H. Oelhaf, M. Höpfner, G. V. Belyaev, A. Ebersoldt, F. Friedl-Vallon, J.-U. Grooß, T. Gulde, M. Kaufmann, A. Kleinert, M. Krämer, E. Kretschmer, T. Kulessa, G. Maucher, T. Neubert, C. Piesch, P. Preusse, M. Riese, H. Rongen, C. Sartorius, G. Schardt, A. Schönfeld, D. Schuettemeyer, M. K. Sha, F. Stroh, J. Ungermann, C. M. Volk, and J. Orphal We report first chemistry mode retrieval results from the new airborne limb- imaging infrared FTS (Fourier transform spectrometer) GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) and comparisons with observations by the conventional airborne limb- scanning infrared FTS MIPAS-STR (Michelson Interferometer for Passive Atmospheric Sounding – STRatospheric aircraft). For GLORIA, the flights aboard the high-altitude research aircraft M55 Geophysica during the ESSenCe campaign (ESa Sounder Campaign 2011) were the very first in field deployment after several years of development. The simultaneous observations of GLORIA and MIPAS-STR during the flight on 16 December 2011 inside the polar vortex and under conditions of optically partially transparent polar stratospheric clouds (PSCs) provided us the first opportunity to compare the observations by two different infrared FTS generations directly. We validate the GLORIA results with MIPAS-STR based on the lower vertical resolution of MIPAS-STR and compare the vertical resolutions of the instruments derived from their averaging kernels. The retrieval results of temperature, HNO 3 , O 3 , H 2 O, CFC-11 and CFC-12 show reasonable agreement of GLORIA with MIPAS-STR and collocated in situ observations. For the horizontally binned hyperspectral limb images, the GLORIA sampling outnumbered the horizontal cross-track sampling of MIPAS-STR by up to 1 order of magnitude. Depending on the target parameter, typical vertical resolutions of 0.5 to 2.0 km were obtained for GLORIA and are typically a factor of 2 to 4 better compared to MIPAS-STR. While the improvement of the performance, characterization and data processing of GLORIA are the subject of ongoing work, the presented first results already demonstrate the considerable gain in sampling and vertical resolution achieved with GLORIA.

Description: Probing ice-nucleation processes on the molecular level using second harmonic generation spectroscopy Atmospheric Measurement Techniques, 8, 3519-3526, 2015 Author(s): A. Abdelmonem, J. Lützenkirchen, and T. Leisner We present and characterize a novel setup to apply second harmonic generation (SHG) spectroscopy in total internal reflection geometry (TIR) to heterogeneous freezing research. It allows to monitor the evolution of water structuring at solid surfaces at low temperatures prior to heterogeneous ice nucleation. Apart from the possibility of investigating temperature dependence, a major novelty in our setup is the ability of measuring sheet-like samples in TIR geometry in a direct way. As a main experimental result, we find that our method can discriminate between good and poor ice nucleating surfaces. While at the sapphire basal plane, which is known to be a poor ice nucleator, no structural rearrangement of the water molecules is found prior to freezing, the basal plane surface of mica, an analogue to ice active mineral dust surfaces, exhibits a strong change in the nonlinear optical properties at temperatures well above the freezing transition. This is interpreted as a pre-activation, i.e. an increase in the local ordering of the interfacial water which is expected to facilitate the crystallization of ice at the surface. The results are in line with recent predictions by molecular dynamics simulations on a similar system.

Description: Detailed characterizations of the new Mines Douai comparative reactivity method instrument via laboratory experiments and modeling Atmospheric Measurement Techniques, 8, 3537-3553, 2015 Author(s): V. Michoud, R. F. Hansen, N. Locoge, P. S. Stevens, and S. Dusanter The hydroxyl (OH) radical is an important oxidant in the troposphere, which controls the lifetime of most air quality- and climate-related trace gases. However, there are still uncertainties concerning its atmospheric budget, and integrated measurements of OH sinks have been valuable to improve this aspect. Among the analytical tools used for measuring total OH reactivity in ambient air, the comparative reactivity method (CRM) is spreading rapidly in the atmospheric community. However, measurement artifacts have been highlighted for this technique, and additional work is needed to fully characterize them. In this study, we present the new Mines Douai CRM instrument, with an emphasis on the corrections that need to be applied to ambient measurements of total OH reactivity. Measurement artifacts identified in the literature have been investigated, including (1) a correction for a change in relative humidity between the measurement steps leading to different OH levels, (2) the formation of spurious OH in the sampling reactor when hydroperoxy radicals (HO 2 ) react with nitrogen monoxide (NO), (3) not operating the CRM under pseudo-first-order kinetics, and (4) the dilution of ambient air inside the reactor. The dependences of these artifacts on various measurable parameters, such as the pyrrole-to-OH ratio and the bimolecular reaction rate constants of ambient trace gases with OH, have also been studied. Based on these observations, parameterizations are proposed to correct ambient OH reactivity measurements. On average, corrections of 5.2 ± 3.2, 9.2 ± 15.7, and 8.5 ± 5.8 s −1 were respectively observed for (1), (2) and (3) during a field campaign performed in Dunkirk, France (summer 2014). Numerical simulations have been performed using a box model to check whether experimental observations mentioned above are consistent with our understanding of the chemistry occurring in the CRM reactor. Two different chemical mechanisms have been shown to reproduce the magnitude of corrections (2) and (3). In addition, these simulations reproduce their dependences on the pyrrole-to-OH ratio and on bimolecular reaction rate constants of gases reacting with OH. The good agreement found between laboratory experiments and model simulations gives us confidence in the proposed parameterizations. However, it is worth noting that the numerical values given in this study are suitable for the Mines Douai instrument and may not be appropriate for other CRM instruments. It is recommended that each group characterize its own instrument following the recommendations given in this study. An assessment of performances for the Mines Douai instrument, including a propagation of errors from the different corrections, indicates a limit of detection of 3.0 s −1 and total uncertainties of 17–25 % for OH reactivity values higher than 15 s −1 and NO x mixing ratios lower than 30 ppbv.

Description: An assessment of the stray light in 25 years of Dobson total ozone data at Athens, Greece Atmospheric Measurement Techniques, 8, 3037-3046, 2015 Author(s): J. Christodoulakis, C. Varotsos, A. P. Cracknell, C. Tzanis, and A. Neofytos In this study, we investigated the susceptibility of the Dobson spectrophotometer No. 118 to stray light interference. In this regard, a series of total ozone content measurements were carried out in Athens, Greece for air-mass values ( μ ) extending up to μ = 5. The monochromatic-heterochromatic stray light derived by Basher's model was used in order to evaluate the specific instrumental parameters which determine if this instrument suffers from this problem or not. The results obtained indicate that the measurements made by the Dobson instrument of the Athens station for air mass values up to 2.5, underestimates the total ozone content by 3.5 DU in average, or about 1 % of the station's mean total ozone content (TOC). The comparison of the values of the same parameters measured 15 years ago with the present ones indicates the good maintenance of the Dobson spectrophotometer No. 118. This fact is of crucial importance because the variability of the daily total ozone observations collected by the Athens Dobson Station since 1989 has proved to be representative to the variability of the mean total ozone observed over the whole mid-latitude zone of the Northern Hemisphere. This stresses the point that the Athens total ozone station, being the unique Dobson station in south-eastern Europe, may be assumed as a ground truth station for the reliable conversion of the satellite radiance observations to total ozone measurements.

Description: Extended and refined multi sensor reanalysis of total ozone for the period 1970–2012 Atmospheric Measurement Techniques, 8, 3021-3035, 2015 Author(s): R. J. van der A, M. A. F. Allaart, and H. J. Eskes The ozone multi-sensor reanalysis (MSR) is a multi-decadal ozone column data record constructed using all available ozone column satellite data sets, surface Brewer and Dobson observations and a data assimilation technique with detailed error modelling. The result is a high-resolution time series of 6-hourly global ozone column fields and forecast error fields that may be used for ozone trend analyses as well as detailed case studies. The ozone MSR is produced in two steps. First, the latest reprocessed versions of all available ozone column satellite data sets are collected and then are corrected for biases as a function of solar zenith angle (SZA), viewing zenith angle (VZA), time (trend), and stratospheric temperature using surface observations of the ozone column from Brewer and Dobson spectrophotometers from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). Subsequently the de-biased satellite observations are assimilated within the ozone chemistry and data assimilation model TMDAM. The MSR2 (MSR version 2) reanalysis upgrade described in this paper consists of an ozone record for the 43-year period 1970–2012. The chemistry transport model and data assimilation system have been adapted to improve the resolution, error modelling and processing speed. Backscatter ultraviolet (BUV) satellite observations have been included for the period 1970–1977. The total record is extended by 13 years compared to the first version of the ozone multi sensor reanalysis, the MSR1. The latest total ozone retrievals of 15 satellite instruments are used: BUV-Nimbus4, TOMS-Nimbus7, TOMS-EP, SBUV-7, -9, -11, -14, -16, -17, -18, -19, GOME, SCIAMACHY, OMI and GOME-2. The resolution of the model runs, assimilation and output is increased from 2° × 3° to 1° × 1°. The analysis is driven by 3-hourly meteorology from the ERA-Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) starting from 1979, and ERA-40 before that date. The chemistry parameterization has been updated. The performance of the MSR2 analysis is studied with the help of observation-minus-forecast (OmF) departures from the data assimilation, by comparisons with the individual station observations and with ozone sondes. The OmF statistics show that the mean bias of the MSR2 analyses is less than 1 % with respect to de-biased satellite observations after 1979.

Description: Eddy-covariance data with low signal-to-noise ratio: time-lag determination, uncertainties and limit of detection Atmospheric Measurement Techniques, 8, 4197-4213, 2015 Author(s): B. Langford, W. Acton, C. Ammann, A. Valach, and E. Nemitz All eddy-covariance flux measurements are associated with random uncertainties which are a combination of sampling error due to natural variability in turbulence and sensor noise. The former is the principal error for systems where the signal-to-noise ratio of the analyser is high, as is usually the case when measuring fluxes of heat, CO 2 or H 2 O. Where signal is limited, which is often the case for measurements of other trace gases and aerosols, instrument uncertainties dominate. Here, we are applying a consistent approach based on auto- and cross-covariance functions to quantify the total random flux error and the random error due to instrument noise separately. As with previous approaches, the random error quantification assumes that the time lag between wind and concentration measurement is known. However, if combined with commonly used automated methods that identify the individual time lag by looking for the maximum in the cross-covariance function of the two entities, analyser noise additionally leads to a systematic bias in the fluxes. Combining data sets from several analysers and using simulations, we show that the method of time-lag determination becomes increasingly important as the magnitude of the instrument error approaches that of the sampling error. The flux bias can be particularly significant for disjunct data, whereas using a prescribed time lag eliminates these effects (provided the time lag does not fluctuate unduly over time). We also demonstrate that when sampling at higher elevations, where low frequency turbulence dominates and covariance peaks are broader, both the probability and magnitude of bias are magnified. We show that the statistical significance of noisy flux data can be increased (limit of detection can be decreased) by appropriate averaging of individual fluxes, but only if systematic biases are avoided by using a prescribed time lag. Finally, we make recommendations for the analysis and reporting of data with low signal-to-noise and their associated errors.

Description: Error estimation for localized signal properties: application to atmospheric mixing height retrievals Atmospheric Measurement Techniques, 8, 4215-4230, 2015 Author(s): G. Biavati, D. G. Feist, C. Gerbig, and R. Kretschmer The mixing height is a key parameter for many applications that relate surface–atmosphere exchange fluxes to atmospheric mixing ratios, e.g., in atmospheric transport modeling of pollutants. The mixing height can be estimated with various methods: profile measurements from radiosondes as well as remote sensing (e.g., optical backscatter measurements). For quantitative applications, it is important to estimate not only the mixing height itself but also the uncertainty associated with this estimate. However, classical error propagation typically fails on mixing height estimates that use thresholds in vertical profiles of some measured or measurement-derived quantity. Therefore, we propose a method to estimate the uncertainty of an estimation of the mixing height. The uncertainty we calculate is related not to the physics of the boundary layer (e.g., entrainment zone thickness) but to the quality of the analyzed signals. The method relies on the concept of statistical confidence and on the knowledge of the measurement errors. It can also be applied to problems outside atmospheric mixing height retrievals where properties have to be assigned to a specific position, e.g., the location of a local extreme.

Description: Intercomparison of the comparative reactivity method (CRM) and pump–probe technique for measuring total OH reactivity in an urban environment Atmospheric Measurement Techniques, 8, 4243-4264, 2015 Author(s): R. F. Hansen, M. Blocquet, C. Schoemaecker, T. Léonardis, N. Locoge, C. Fittschen, B. Hanoune, P. S. Stevens, V. Sinha, and S. Dusanter The investigation of hydroxyl radical (OH) chemistry during intensive field campaigns has led to the development of several techniques dedicated to ambient measurements of total OH reactivity, which is the inverse of the OH lifetime. Three techniques are currently used during field campaigns, including the total OH loss rate method, the pump–probe method, and the comparative reactivity method. However, no formal intercomparison of these techniques has been published so far, and there is a need to ensure that measurements of total OH reactivity are consistent among the different techniques. An intercomparison of two OH reactivity instruments, one based on the comparative reactivity method (CRM) and the other based on the pump–probe method, was performed in October 2012 in a NO x -rich environment, which is known to be challenging for the CRM technique. This study presents an extensive description of the two instruments, the CRM instrument from Mines Douai (MD-CRM) and the pump–probe instrument from the University of Lille (UL-FAGE), and highlights instrumental issues associated with the two techniques. It was found that the CRM instrument used in this study underestimates ambient OH reactivity by approximately 20 % due to the photolysis of volatile organic compounds (VOCs) inside the sampling reactor; this value is dependent on the position of the lamp within the reactor. However, this issue can easily be fixed, and the photolysis of VOCs was successfully reduced to a negligible level after this intercomparison campaign. The UL-FAGE instrument may also underestimate ambient OH reactivity due to the difficulty to accurately measure the instrumental zero. It was found that the measurements are likely biased by approximately 2 s -1 , due to impurities in humid zero air. Two weeks of ambient sampling indicate that the measurements performed by the two OH reactivity instruments are in agreement, within the measurement uncertainties for each instrument, for NO x mixing ratios up to 100 ppbv. The CRM technique has hitherto mainly been used in low-NO x environments, i.e. environments with ambient NO x mixing ratios lower than a few ppbv, due to a measurement artifact generated by ambient NO inside the sampling reactor. However, this study shows that this technique can also be used under NO x -rich conditions if a NO x -dependent correction is carefully applied on the OH reactivity measurements. A full suite of 52 VOCs, NO x , and other inorganic species were monitored during this intercomparison. An investigation of the OH reactivity budget for this urban site suggests that this suite of trace gases can account for the measured total OH reactivity.

Description: Relative drifts and biases between six ozone limb satellite measurements from the last decade Atmospheric Measurement Techniques, 8, 4369-4381, 2015 Author(s): N. Rahpoe, M. Weber, A. V. Rozanov, K. Weigel, H. Bovensmann, J. P. Burrows, A. Laeng, G. Stiller, T. von Clarmann, E. Kyrölä, V. F. Sofieva, J. Tamminen, K. Walker, D. Degenstein, A. E. Bourassa, R. Hargreaves, P. Bernath, J. Urban, and D. P. Murtagh As part of European Space Agency's (ESA) climate change initiative, high vertical resolution ozone profiles from three instruments all aboard ESA's Envisat (GOMOS, MIPAS, SCIAMACHY) and ESA's third party missions (OSIRIS, SMR, ACE-FTS) are to be combined in order to create an essential climate variable data record for the last decade. A prerequisite before combining data is the examination of differences and drifts between the data sets. In this paper, we present a detailed analysis of ozone profile differences based on pairwise collocated measurements, including the evolution of the differences with time. Such a diagnosis is helpful to identify strengths and weaknesses of each data set that may vary in time and introduce uncertainties in long-term trend estimates. The analysis reveals that the relative drift between the sensors is not statistically significant for most pairs of instruments. The relative drift values can be used to estimate the added uncertainty in physical trends. The added drift uncertainty is estimated at about 3 % decade −1 (1σ). Larger differences and variability in the differences are found in the lowermost stratosphere (below 20 km) and in the mesosphere.

Description: Real-time remote detection and measurement for airborne imaging spectroscopy: a case study with methane Atmospheric Measurement Techniques, 8, 4383-4397, 2015 Author(s): D. R. Thompson, I. Leifer, H. Bovensmann, M. Eastwood, M. Fladeland, C. Frankenberg, K. Gerilowski, R. O. Green, S. Kratwurst, T. Krings, B. Luna, and A. K. Thorpe Localized anthropogenic sources of atmospheric CH 4 are highly uncertain and temporally variable. Airborne remote measurement is an effective method to detect and quantify these emissions. In a campaign context, the science yield can be dramatically increased by real-time retrievals that allow operators to coordinate multiple measurements of the most active areas. This can improve science outcomes for both single- and multiple-platform missions. We describe a case study of the NASA/ESA CO 2 and MEthane eXperiment (COMEX) campaign in California during June and August/September 2014. COMEX was a multi-platform campaign to measure CH 4 plumes released from anthropogenic sources including oil and gas infrastructure. We discuss principles for real-time spectral signature detection and measurement, and report performance on the NASA Next Generation Airborne Visible Infrared Spectrometer (AVIRIS-NG). AVIRIS-NG successfully detected CH 4 plumes in real-time at Gb s −1 data rates, characterizing fugitive releases in concert with other in situ and remote instruments. The teams used these real-time CH 4 detections to coordinate measurements across multiple platforms, including airborne in situ, airborne non-imaging remote sensing, and ground-based in situ instruments. To our knowledge this is the first reported use of real-time trace-gas signature detection in an airborne science campaign, and presages many future applications. Post-analysis demonstrates matched filter methods providing noise-equivalent (1σ) detection sensitivity for 1.0 % CH4 column enhancements equal to 141 ppm m.

Description: Ambient measurements of aromatic and oxidized VOCs by PTR-MS and GC-MS: intercomparison between four instruments in a boreal forest in Finland Atmospheric Measurement Techniques, 8, 4453-4473, 2015 Author(s): M. K. Kajos, P. Rantala, M. Hill, H. Hellén, J. Aalto, J. Patokoski, R. Taipale, C. C. Hoerger, S. Reimann, T. M. Ruuskanen, J. Rinne, and T. Petäjä Proton transfer reaction mass spectrometry (PTR-MS) and gas chromatography mass spectrometry GC-MS) are commonly used methods for automated in situ measurements of various volatile organic compounds (VOCs) in the atmosphere. In order to investigate the reliability of such measurements, we operated four automated analyzers using their normal field measurement protocol side by side at a boreal forest site. We measured methanol, acetaldehyde, acetone, benzene and toluene by two PTR-MS and two GC-MS instruments. The measurements were conducted in southern Finland between 13 April and 14 May 2012. This paper presents correlations and biases between the concentrations measured using the four instruments. A very good correlation was found for benzene and acetone measurements between all instruments (the mean R value was 0.88 for both compounds), while for acetaldehyde and toluene the correlation was weaker (with a mean R value of 0.50 and 0.62, respectively). For some compounds, notably for methanol, there were considerable systematic differences in the mixing ratios measured by the different instruments, despite the very good correlation between the instruments (mean R = 0.90). The systematic difference manifests as a difference in the linear regression slope between measurements conducted between instruments, rather than as an offset. This mismatch indicates that the systematic uncertainty in the sensitivity of a given instrument can lead to an uncertainty of 50–100 % in the methanol emissions measured by commonly used methods.

Description: Spatial mapping of ground-based observations of total ozone Atmospheric Measurement Techniques, 8, 4487-4505, 2015 Author(s): K.-L. Chang, S. Guillas, and V. E. Fioletov Total column ozone variations estimated using ground-based stations provide important independent source of information in addition to satellite-based estimates. This estimation has been vigorously challenged by data inhomogeneity in time and by the irregularity of the spatial distribution of stations, as well as by interruptions in observation records. Furthermore, some stations have calibration issues and thus observations may drift. In this paper we compare the spatial interpolation of ozone levels using the novel stochastic partial differential equation (SPDE) approach with the covariance-based kriging. We show how these new spatial predictions are more accurate, less uncertain and more robust. We construct long-term zonal means to investigate the robustness against the absence of measurements at some stations as well as instruments drifts. We conclude that time series analyzes can benefit from the SPDE approach compared to the covariance-based kriging when stations are missing, but the positive impact of the technique is less pronounced in the case of drifts.

Description: Joint retrievals of cloud and drizzle in marine boundary layer clouds using ground-based radar, lidar and zenith radiances Atmospheric Measurement Techniques, 8, 2663-2683, 2015 Author(s): M. D. Fielding, J. C. Chiu, R. J. Hogan, G. Feingold, E. Eloranta, E. J. O'Connor, and M. P. Cadeddu Active remote sensing of marine boundary-layer clouds is challenging as drizzle drops often dominate the observed radar reflectivity. We present a new method to simultaneously retrieve cloud and drizzle vertical profiles in drizzling boundary-layer clouds using surface-based observations of radar reflectivity, lidar attenuated backscatter, and zenith radiances under conditions when precipitation does not reach the surface. Specifically, the vertical structure of droplet size and water content of both cloud and drizzle is characterised throughout the cloud. An ensemble optimal estimation approach provides full error statistics given the uncertainty in the observations. To evaluate the new method, we first perform retrievals using synthetic measurements from large-eddy simulation snapshots of cumulus under stratocumulus, where cloud water path is retrieved with an error of 31 g m −2 . The method also performs well in non-drizzling clouds where no assumption of the cloud profile is required. We then apply the method to observations of marine stratocumulus obtained during the Atmospheric Radiation Measurement MAGIC deployment in the Northeast Pacific. Here, retrieved cloud water path agrees well with independent three-channel microwave radiometer retrievals, with a root mean square difference of 10–20 g m −2 .

Description: The effect of radiometer placement and view on inferred directional and hemispheric radiometric temperatures of an urban canopy Atmospheric Measurement Techniques, 8, 2699-2714, 2015 Author(s): C. Adderley, A. Christen, and J. A. Voogt Any radiometer at a fixed location has a biased view when observing a convoluted, three-dimensional surface such as an urban canopy. The goal of this contribution is to determine the bias of various sensors views observing a simple urban residential neighbourhood (nadir, oblique, hemispherical) over a 24 hour cycle under clear weather conditions. The error in measuring a longwave radiation flux density ( L ) and/or inferring surface temperatures ( T 0 ) is quantified for different times over a diurnal cycle. Panoramic time-sequential thermography (PTST) data were recorded by a thermal camera on a hydraulic mast above a residential canyon in Vancouver, BC. The data set resolved sub-facet temperature variability of all representative urban facets in a 360° swath repetitively over a 24-hour cycle. This data set is used along with computer graphics and vision techniques to project measured fields of L for a given time and pixel onto texture sheets of a three-dimensional urban surface model at a resolution of centimetres. The resulting data set attributes L of each pixel on the texture sheets to different urban facets and associates facet location, azimuth, slope, material, and sky view factor. The texture sheets of L are used to calculate the complete surface temperature ( T 0,C ) and to simulate the radiation in the field of view (FOV) of narrow and hemispheric radiometers observing the same urban surface (in absence of emissivity and atmospheric effects). The simulated directional ( T 0,d ) and hemispheric ( T 0,h ) radiometric temperatures inferred from various biased views are compared to T 0,C . For a range of simulated off-nadir (φ) and azimuth (Ω) angles, T 0,d (φ,Ω) and T 0,C differ between −2.6 and +2.9 K over the course of the day. The effects of effective anisotropy are highest in the daytime, particularly around sunrise and sunset when different views can lead to differences in T 0,d (φ,Ω) that are as high as 3.5 K. For a sensor with a narrow FOV in the nadir of the urban surface, T 0,d (φ=0) differs from T 0,C by +1.9 K (day) and by −1.6 K (night). Simulations of the FOV of hemispherical, downward-facing pyrgeometers at 270 positions show considerable variations in the measured L and inferred hemispherical radiometeric temperature T 0,h as a function of both horizontal placement and height. The root mean squared error (RMSE) between different horizontal positions in retrieving outgoing longwave emittance L ↑ decreased exponentially with height, and was 11.2, 6.3 and 2.0 W m −2 at 2, 3, and 5 times the mean building height z b . Generally, above 3.5 z b the horizontal positional error is less than the typical accuracy of common pyrgeometers. The average T 0,h over 24 h determined from the hemispherical radiometer sufficiently above an urban surface is in close agreement with the average T 0,C . However, over the course of the day, the difference between T 0,h and T 0,C shows an RMSE of 1.7 K (9.4 W m −2 ) because the relative contributions of facets within the projected FOV of a pyrgeometer do not correspond to their fractions of the complete urban surface.

Description: Impacts of atmospheric state uncertainty on O 2 measurement requirements for the ASCENDS mission Atmospheric Measurement Techniques, 8, 2685-2697, 2015 Author(s): S. Crowell, P. Rayner, S. Zaccheo, and B. Moore Remotely sensed observations of atmospheric composition require an estimate of surface pressure. This estimate can either come from an instrument with sensitivity in an O 2 absorption feature in the spectrum, or it can be provided by a numerical weather prediction (NWP) model. In this work, the authors outline an information-based methodology for setting measurement requirements for an active lidar measurement of O 2 in the context of the Active Sensing of Carbon Emissions over Nights, Days and Seasons (ASCENDS) mission. The results indicate that the impacts of correlations in the environmentally induced vertical weighting function errors between CO 2 and O 2 measurements are nontrivial and that the choice of CO 2 and O 2 wavelengths can lead to a stricter or looser requirement than that of surface pressure considerations alone, which would indicate about a 0.1 % precision for 1mb accuracy. Furthermore, the less sensitive the CO 2 measurement is to surface pressure errors, the more difficult it will be for an O 2 observation to provide a useful measurement.

Description: Efficient photochemical generation of peroxycarboxylic nitric anhydrides with ultraviolet light-emitting diodes Atmospheric Measurement Techniques, 8, 2737-2748, 2015 Author(s): N. D. Rider, Y. M. Taha, C. A. Odame-Ankrah, J. A. Huo, T. W. Tokarek, E. Cairns, S. G. Moussa, J. Liggio, and H. D. Osthoff Photochemical sources of peroxycarboxylic nitric anhydrides (PANs) are utilized in many atmospheric measurement techniques for calibration or to deliver an internal standard. Conventionally, such sources rely on phosphor-coated low-pressure mercury (Hg) lamps to generate the UV light necessary to photo-dissociate a dialkyl ketone (usually acetone) in the presence of a calibrated amount of nitric oxide (NO) and oxygen (O 2 ). In this manuscript, a photochemical PAN source in which the Hg lamp has been replaced by arrays of ultraviolet light-emitting diodes (UV-LEDs) is described. The output of the UV-LED source was analyzed by gas chromatography (PAN-GC) and thermal dissociation cavity ring-down spectroscopy (TD-CRDS). Using acetone, diethyl ketone (DIEK), diisopropyl ketone (DIPK), or di-n-propyl ketone (DNPK), respectively, the source produces peroxyacetic (PAN), peroxypropionic (PPN), peroxyisobutanoic (PiBN), or peroxy-n-butanoic nitric anhydride (PnBN) from NO in high yield (〉 90 %). Box model simulations with a subset of the Master Chemical Mechanism (MCM) were carried out to rationalize product yields and to identify side products. The present work demonstrates that UV-LED arrays are a viable alternative to current Hg lamp setups.

Description: Use of radio occultation to probe the high-latitude ionosphere Atmospheric Measurement Techniques, 8, 2789-2800, 2015 Author(s): A. J. Mannucci, B. T. Tsurutani, O. Verkhoglyadova, A. Komjathy, and X. Pi We have explored the use of COSMIC data to provide valuable scientific information on the ionospheric impacts of energetic particle precipitation during geomagnetic storms. Ionospheric electron density in the E region, and hence ionospheric conductivity, is significantly altered by precipitating particles from the magnetosphere. This has global impacts on the thermosphere–ionosphere because of the important role of conductivity on high-latitude Joule heating. Two high-speed stream (HSS) and two coronal mass ejection (CME) storms are examined with the COSMIC data. We find clear correlation between geomagnetic activity and electron density retrievals from COSMIC. At nighttime local times, the number of profiles with maximum electron densities in the E layer (below 200 km altitude) is well correlated with geomagnetic activity. We interpret this to mean that electron density increases due to precipitation are captured by the COSMIC profiles. These "E-layer-dominant ionosphere" (ELDI) profiles have geomagnetic latitudes that are consistent with climatological models of the auroral location. For the two HSS storms that occurred in May of 2011 and 2012, a strong hemispheric asymmetry is observed, with nearly all the ELDI profiles found in the Southern, less sunlit, Hemisphere. Stronger aurora and precipitation have been observed before in winter hemispheres, but the degree of asymmetry deserves further study. For the two CME storms, occurring in July and November of 2012, large increases in the number of ELDI profiles are found starting in the storm's main phase but continuing for several days into the recovery phase. Analysis of the COSMIC profiles was extended to all local times for the July 2012 CME storm by relaxing the ELDI criterion and instead visually inspecting all profiles above 50° magnetic latitude for signatures of precipitation in the E region. For 9 days during the July 2012 period, we find a signature of precipitation occurs nearly uniformly in local time, although the magnitude of electron density increase may vary with local time. The latitudinal extent of the precipitation layers is generally consistent with auroral climatology. However, after the storm main phase on 14 July 2012 the precipitation tended to be somewhat more equatorward than the climatology (by about 5–10° latitude) and equatorward of the auroral boundary data acquired from the SSUSI sensor onboard the F18 DMSP satellite. We conclude that, if analyzed appropriately, high-latitude COSMIC profiles have the potential to contribute to our understanding of MI coupling processes and extend and improve existing models of the auroral region.

Description: Evaluation of MAX-DOAS aerosol retrievals by coincident observations using CRDS, lidar, and sky radiometer inTsukuba, Japan Atmospheric Measurement Techniques, 8, 2775-2788, 2015 Author(s): H. Irie, T. Nakayama, A. Shimizu, A. Yamazaki, T. Nagai, A. Uchiyama, Y. Zaizen, S. Kagamitani, and Y. Matsumi Coincident aerosol observations of multi-axis differential optical absorption spectroscopy (MAX-DOAS), cavity ring-down spectroscopy (CRDS), lidar, and sky radiometer were conducted in Tsukuba, Japan, on 5–18 October 2010. MAX-DOAS aerosol retrieval (for aerosol extinction coefficient and aerosol optical depth at 476 nm) was evaluated from the viewpoint of the need for a correction factor for oxygen collision complexes (O 4 or O 2 –O 2 ) absorption. The present study strongly supports this need, as systematic residuals at relatively high elevation angles (20 and 30°) were evident in MAX-DOAS profile retrievals conducted without the correction. However, adopting a single number for the correction factor ( f O 4 = 1.25) for all of the elevation angles led to systematic overestimation of near-surface aerosol extinction coefficients, as reported in the literature. To achieve agreement with all three observations, we limited the set of elevation angles to ≤10° and adopted an elevation-angle-dependent correction factor for practical profile retrievals with scattered light observations by a ground-based MAX-DOAS. With these modifications, we expect to minimize the possible effects of temperature-dependent O 4 absorption cross section and uncertainty in DOAS fit on an aerosol profile retrieval, although more efforts are encouraged to quantitatively identify a physical explanation for the need of a correction factor.

Description: An automatic collector to monitor insoluble atmospheric deposition: application for mineral dust deposition Atmospheric Measurement Techniques, 8, 2801-2811, 2015 Author(s): B. Laurent, R. Losno, S. Chevaillier, J. Vincent, P. Roullet, E. Bon Nguyen, N. Ouboulmane, S. Triquet, M. Fornier, P. Raimbault, and G. Bergametti Deposition is one of the key terms of the mineral dust cycle. However, dust deposition remains poorly constrained in transport models simulating the atmospheric dust cycle. This is mainly due to the limited number of relevant deposition measurements. This paper aims to present an automatic collector (CARAGA), specially developed to sample the total (dry and wet) atmospheric deposition of insoluble dust in remote areas. The autonomy of the CARAGA can range from 25 days to almost 1 year depending on the programmed sampling frequency (from 1 day to 2 weeks respectively). This collector is used to sample atmospheric deposition of Saharan dust on the Frioul islands in the Gulf of Lions in the Western Mediterranean. To quantify the mineral dust mass in deposition samples, a weighing and ignition protocol is applied. Almost 2 years of continuous deposition measurements performed on a weekly sampling basis on Frioul Island are presented and discussed with air mass trajectories and satellite observations of dust. Insoluble mineral deposition measured on Frioul Island was 2.45 g m −2 for February to December 2011 and 3.16 g m −2 for January to October 2012. Nine major mineral deposition events, measured during periods with significant MODIS aerosol optical depths, were associated with air masses coming from the southern Mediterranean Basin and North Africa.

Description: Methane emission estimates using chamber and tracer release experiments for a municipal waste water treatment plant Atmospheric Measurement Techniques, 8, 2853-2867, 2015 Author(s): C. E. Yver Kwok, D. Müller, C. Caldow, B. Lebègue, J. G. Mønster, C. W. Rella, C. Scheutz, M. Schmidt, M. Ramonet, T. Warneke, G. Broquet, and P. Ciais This study presents two methods for estimating methane emissions from a waste water treatment plant (WWTP) along with results from a measurement campaign at a WWTP in Valence, France. These methods, chamber measurements and tracer release, rely on Fourier transform infrared spectroscopy and cavity ring-down spectroscopy instruments. We show that the tracer release method is suitable for quantifying facility- and some process-scale emissions, while the chamber measurements provide insight into individual process emissions. Uncertainties for the two methods are described and discussed. Applying the methods to CH 4 emissions of the WWTP, we confirm that the open basins are not a major source of CH 4 on the WWTP (about 10 % of the total emissions), but that the pretreatment and sludge treatment are the main emitters. Overall, the waste water treatment plant is representative of an average French WWTP.

Description: Automated rain rate estimates using the Ka-band ARM zenith radar (KAZR) Atmospheric Measurement Techniques, 8, 3685-3699, 2015 Author(s): A. Chandra, C. Zhang, P. Kollias, S. Matrosov, and W. Szyrmer The use of millimeter wavelength radars for probing precipitation has recently gained interest. However, estimation of precipitation variables is not straightforward due to strong signal attenuation, radar receiver saturation, antenna wet radome effects and natural microphysical variability. Here, an automated algorithm is developed for routinely retrieving rain rates from the profiling Ka-band (35-GHz) ARM (Atmospheric Radiation Measurement) zenith radars (KAZR). A 1-dimensional, simple, steady state microphysical model is used to estimate impacts of microphysical processes and attenuation on the profiles of radar observables at 35-GHz and thus provide criteria for identifying situations when attenuation or microphysical processes dominate KAZR observations. KAZR observations are also screened for signal saturation and wet radome effects. The algorithm is implemented in two steps: high rain rates are retrieved by using the amount of attenuation in rain layers, while low rain rates are retrieved from the reflectivity–rain rate ( Z e – R ) relation. Observations collected by the KAZR, rain gauge, disdrometer and scanning precipitating radars during the DYNAMO/AMIE field campaign at the Gan Island of the tropical Indian Ocean are used to validate the proposed approach. The differences in the rain accumulation from the proposed algorithm are quantified. The results indicate that the proposed algorithm has a potential for deriving continuous rain rate statistics in the tropics.

Description: A perspective on the fundamental quality of GPS radio occultation data Atmospheric Measurement Techniques, 8, 4281-4294, 2015 Author(s): T.-K. Wee and Y.-H. Kuo Radio occultation (RO) is a promising source of observation for weather and climate applications. However, the uncertainties arising from imperfect retrieval algorithms may weaken the overall confidence in the data and discourage their use. As an alternative approach of assessing the quality of RO data while avoiding the nuisance of retrieval errors, this study proposes to use minimally processed data (measurement) instead of derived RO data. This study compares measured phase paths with their model counterparts, simulated with an effective ray tracer for which the refractive indices along the complete ray path linking the transmitter and the receiver are realistically specified. The comparison of phase measurements with the European Centre for Medium-Range Weather Forecasts (ECMWF) data made in the observation space shows that the RO measurements are of sufficient accuracy to uncover regional-scale systematic errors in ECMWF's operational analysis and the 45-year reanalysis (ERA40), and to clearly depict the error growth of short-term ERA40 forecasts. In the southern hemispheric stratosphere, in particular, the RO measurements served as a robust reference against which both of the two analyses were significantly biased in opposite directions even though they were produced by the same center using virtually the same set of data. The measurement and ECMWF analyses showed a close agreement in the standard deviation except for the regions and heights that the quality of the ECMWF data is controversial. This confirms the high precision of RO measurements and also indicates that the main problem of the ECMWF analyses lies in their systematic error.

Description: A new method for the absolute radiance calibration for UV–vis measurements of scattered sunlight Atmospheric Measurement Techniques, 8, 4265-4280, 2015 Author(s): T. Wagner, S. Beirle, S. Dörner, M. Penning de Vries, J. Remmers, A. Rozanov, and R. Shaiganfar Absolute radiometric calibrations are important for measurements of the atmospheric spectral radiance. Such measurements can be used to determine actinic fluxes, the properties of aerosols and clouds, and the shortwave energy budget. Conventional calibration methods in the laboratory are based on calibrated light sources and reflectors and are expensive, time consuming and subject to relatively large uncertainties. Also, the calibrated instruments might change during transport from the laboratory to the measurement sites. Here we present a new calibration method for UV–vis instruments that measure the spectrally resolved sky radiance, for example zenith sky differential optical absorption spectroscopy (DOAS) instruments or multi-axis (MAX)-DOAS instruments. Our method is based on the comparison of the solar zenith angle dependence of the measured zenith sky radiance with radiative transfer simulations. For the application of our method, clear-sky measurements during periods with almost constant aerosol optical depth are needed. The radiative transfer simulations have to take polarisation into account. We show that the calibration results are almost independent from the knowledge of the aerosol optical properties and surface albedo, which causes a rather small uncertainty of about 〈 7 %. For wavelengths below about 330 nm it is essential that the ozone column density during the measurements be constant and known.

Description: Impacts of AMSU-A, MHS and IASI data assimilation on temperature and humidity forecasts with GSI–WRF over the western United States Atmospheric Measurement Techniques, 8, 4231-4242, 2015 Author(s): Y. Bao, J. Xu, A. M. Powell Jr., M. Shao, J. Min, and Y. Pan Using NOAA's Gridpoint Statistical Interpolation (GSI) data assimilation system and NCAR's Advanced Research WRF (Weather Research and Forecasting) (ARW-WRF) regional model, six experiments are designed by (1) a control experiment (CTRL) and five data assimilation (DA) experiments with different data sets, including (2) conventional data only (CON); (3) microwave data (AMSU-A + MHS) only (MW); (4) infrared data (IASI) only (IR); (5) a combination of microwave and infrared data (MWIR); and (6) a combination of conventional, microwave and infrared observation data (ALL). One-month experiments in July 2012 and the impacts of the DA on temperature and moisture forecasts at the surface and four vertical layers over the western United States have been investigated. The four layers include lower troposphere (LT) from 800 to 1000 hPa, middle troposphere (MT) from 400 to 800 hPa, upper troposphere (UT) from 200 to 400 hPa, and lower stratosphere (LS) from 50 to 200 hPa. The results show that the regional GSI–WRF system is underestimating the observed temperature in the LT and overestimating in the UT and LS. The MW DA reduced the forecast bias from the MT to the LS within 30 h forecasts, and the CON DA kept a smaller forecast bias in the LT for 2-day forecasts. The largest root mean square error (RMSE) is observed in the LT and at the surface (SFC). Compared to the CTRL, the MW DA produced the most positive contribution in the UT and LS, and the CON DA mainly improved the temperature forecasts at the SFC. However, the IR DA gave a negative contribution in the LT. Most of the observed humidity in the different vertical layers is overestimated in the humidity forecasts except in the UT. The smallest bias in the humidity forecast occurred at the SFC and in the UT. The DA experiments apparently reduced the bias from the LT to UT, especially for the IR DA experiment, but the RMSEs are not reduced in the humidity forecasts. Compared to the CTRL, the IR DA experiment has a larger RMSE in the moisture forecast, although the smallest bias is found in the LT and MT.

Description: A new method of measuring aerosol optical properties from digital twilight photographs Atmospheric Measurement Techniques, 8, 4295-4311, 2015 Author(s): M. Saito and H. Iwabuchi An optimal-estimation algorithm for inferring aerosol optical properties from digital twilight photographs is proposed. The sensitivity of atmospheric components and surface characteristics to brightness and color of twilight sky is investigated, and the results suggest that tropospheric and stratospheric aerosol optical thickness (AOT) are sensitive to condition of the twilight sky. The coarse–fine particle volume ratio is moderately sensitive to the sky condition near the horizon under a clean-atmosphere condition. A radiative transfer model that takes into account a spherical-shell atmosphere, refraction, and multiple scattering is used as a forward model. Error analysis shows that the tropospheric and stratospheric AOT can be retrieved without significant bias. Comparisons with results from other ground-based instruments exhibit reasonable agreement on AOT. A case study suggests that the AOT retrieval method can be applied to atmospheric conditions with varying aerosol vertical profiles and vertically inhomogeneous species in the troposphere.

Description: An examination of the long-term CO records from MOPITT and IASI: comparison of retrieval methodology Atmospheric Measurement Techniques, 8, 4313-4328, 2015 Author(s): M. George, C. Clerbaux, I. Bouarar, P.-F. Coheur, M. N. Deeter, D. P. Edwards, G. Francis, J. C. Gille, J. Hadji-Lazaro, D. Hurtmans, A. Inness, D. Mao, and H. M. Worden Carbon monoxide (CO) is a key atmospheric compound that can be remotely sensed by satellite on the global scale. Fifteen years of continuous observations are now available from the MOPITT/Terra mission (2000 to present). Another 15 and more years of observations will be provided by the IASI/MetOp instrument series (2007–2023 〉). In order to study long-term variability and trends, a homogeneous record is required, which is not straightforward as the retrieved quantities are instrument and processing dependent. The present study aims at evaluating the consistency between the CO products derived from the MOPITT and IASI missions, both for total columns and vertical profiles, during a 6-year overlap period (2008–2013). The analysis is performed by first comparing the available 2013 versions of the retrieval algorithms (v5T for MOPITT and v20100815 for IASI), and second using a dedicated reprocessing of MOPITT CO profiles and columns using the same a priori information as the IASI product. MOPITT total columns are generally slightly higher over land (bias ranging from 0 to 13 %) than IASI data. When IASI and MOPITT data are retrieved with the same a priori constraints, correlation coefficients are slightly improved. Large discrepancies (total column bias over 15 %) observed in the Northern Hemisphere during the winter months are reduced by a factor of 2 to 2.5. The detailed analysis of retrieved vertical profiles compared with collocated aircraft data from the MOZAIC-IAGOS network, illustrates the advantages and disadvantages of a constant vs. a variable a priori. On one hand, MOPITT agrees better with the aircraft profiles for observations with persisting high levels of CO throughout the year due to pollution or seasonal fire activity (because the climatology-based a priori is supposed to be closer to the real atmospheric state). On the other hand, IASI performs better when unexpected events leading to high levels of CO occur, due to a larger variability associated with the a priori.

Description: Quantitative evaluation of seven optical sensors for cloud microphysical measurements at the Puy-de-Dôme Observatory, France Atmospheric Measurement Techniques, 8, 4347-4367, 2015 Author(s): G. Guyot, C. Gourbeyre, G. Febvre, V. Shcherbakov, F. Burnet, J.-C. Dupont, K. Sellegri, and O. Jourdan Clouds have an important role in Earth's radiative budget. Since the late 1970s, considerable instrumental developments have been made in order to quantify cloud microphysical and optical properties, for both airborne and ground-based applications. Intercomparison studies have been carried out in the past to assess the reliability of cloud microphysical properties inferred from various measurement techniques. However, observational uncertainties still exist, especially for droplet size distribution measurements and need to be reduced. In this work, we discuss results from an intercomparison campaign, performed at the Puy de Dôme in May 2013. During this campaign, a unique set of cloud instruments was operating simultaneously in ambient air conditions and in a wind tunnel. A Particle Volume Monitor (PVM-100), a Forward Scattering Spectrometer Probe (FSSP), a Fog Monitor (FM-100), and a Present Weather Detector (PWD) were sampling on the roof of the station. Within a wind tunnel located underneath the roof, two Cloud Droplet Probes (CDPs) and a modified FSSP (SPP-100) were operating. The main objectives of this paper are (1) to study the effects of wind direction and speed on ground-based cloud observations, (2) to quantify the cloud parameters discrepancies observed by the different instruments, and (3) to develop methods to improve the quantification of the measurements. The results revealed that all instruments showed a good agreement in their sizing abilities, both in terms of amplitude and variability. However, some of them, especially the FM-100, the FSSP and the SPP, displayed large discrepancies in their capability to assess the magnitude of the total number concentration of the cloud droplets. As a result, the total liquid water content can differ by up to a factor of 5 between the probes. The use of a standardization procedure, based on data of integrating probes (PVM-100 or visibilimeter) and extinction coefficient comparison substantially enhanced the instrumental agreement. During this experiment, the total concentration agreed in variations with the visibilimeter, except for the FSSP, so a corrective factor can be applied and it ranges from 0.44 to 2.2. This intercomparison study highlights the necessity to have an instrument which provides a bulk measurement of cloud microphysical or optical properties during cloud ground-based campaigns. Moreover, the FM and FSSP orientation was modified with an angle ranging from 30 to 90° angle with wind speeds from 3 to 7 m s −1 . The results show that the induced number concentration loss is between 29 and 98 % for the FSSP and between 15 and 68 % for the FM-100. In particular, FSSP experiments showed strong discrepancies when the wind speed was lower than 3 m s −1 and/or when the angle between the wind direction and the orientation of the instruments is greater than 30°. An inadequate orientation of the FSSP towards the wind direction leads to an underestimation of the measured effective diameter.

Description: The direct fitting approach for total ozone column retrievals: a sensitivity study on GOME-2/MetOp-A measurements Atmospheric Measurement Techniques, 8, 4429-4451, 2015 Author(s): A. Wassmann, T. Borsdorff, J. M. J. aan de Brugh, O. P. Hasekamp, I. Aben, and J. Landgraf We present a sensitivity study of the direct fitting approach to retrieve total ozone columns from the clear sky Global Ozone Monitoring Experiment 2/MetOp-A (GOME-2/MetOp-A) measurements between 325 and 335 nm in the period 2007–2010. The direct fitting of the measurement is based on adjusting the scaling of a reference ozone profile and requires accurate simulation of GOME-2 radiances. In this context, we study the effect of three aspects that introduce forward model errors if not addressed appropriately: (1) the use of a clear sky model atmosphere in the radiative transfer demanding cloud filtering, (2) different approximations of Earth's sphericity to address the influence of the solar zenith angle, and (3) the need of polarization in radiative transfer modeling. We conclude that cloud filtering using the operational GOME-2 FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A band) cloud product, which is part of level 1B data, and the use of pseudo-spherical scalar radiative transfer is fully sufficient for the purpose of this retrieval. A validation with ground-based measurements at 36 stations confirms this showing a global mean bias of −0.1 % with a standard deviation (SD) of 2.7 %. The regularization effect inherent to the profile scaling approach is thoroughly characterized by the total column averaging kernel for each individual retrieval. It characterizes the effect of the particular choice of the ozone profile to be scaled by the inversion and is part of the retrieval product. Two different interpretations of the data product are possible: first, regarding the retrieval product as an estimate of the true column, a direct comparison of the retrieved column with total ozone columns from ground-based measurements can be done. This requires accurate a priori knowledge of the reference ozone profile and the column averaging kernel is not needed. Alternatively, the retrieval product can be interpreted as an effective column defined by the total column averaging kernel. This interpretation relies much less on the a priori knowledge of the reference ozone profile; however, for its validation, measurements of the vertical ozone distribution are needed. The different manners of data interpretation are demonstrated for simulated and real measurements using on-ground ozone column and ozonesonde measurements for validation.

Description: An analytical system for the measurement of stable hydrogen isotopes in ambient volatile organic compounds Atmospheric Measurement Techniques, 8, 4475-4486, 2015 Author(s): T. Meisehen, F. Bühler, R. Koppmann, and M. Krebsbach Stable isotope measurements in atmospheric volatile organic compounds (VOCs) are an excellent tool to analyse chemical and dynamical processes in the atmosphere. While up to now isotope studies of VOCs in ambient air have mainly focussed on carbon isotopes, we herein present a new measurement system to investigate hydrogen isotope ratios in atmospheric VOCs. This system, consisting of a gas chromatography pyrolysis isotope ratio mass spectrometer (GC-P-IRMS) and a pre-concentration system, was thoroughly characterised using a VOC test mixture. A precision of better than 9 ‰ (in δ 2 H) is achieved for n -pentane, 2-methyl-1,3-butadiene (isoprene), n -heptane, 4-methyl-pentane-2-one (4-methyl-2-pentanone), methylbenzene (toluene), n -octane, ethylbenzene, m / p -xylene and 1,2,4-trimethylbenzene. A comparison with independent measurements via elemental analysis shows an accuracy of better than 9 ‰ for n -pentane, n -heptane, 4-methyl-2-pentanone, toluene and n -octane. Above a minimum required pre-concentrated compound mass the obtained δ 2 H values are constant within the standard deviations. In addition, a remarkable influence of the pyrolysis process on the isotope ratios is found and discussed. Reliable measurements are only possible if the ceramic tube used for the pyrolysis is sufficiently conditioned, i.e. the inner surface is covered with a carbon layer. It is essential to verify this conditioning regularly and to renew it if required. Furthermore, influences of a necessary H 3 + correction and the pyrolysis temperature on the isotope ratios are discussed. Finally, the applicability to measure hydrogen isotope ratios in VOCs at ambient levels is demonstrated with measurements of outside air on 5 different days in February and March 2015. The measured hydrogen isotope ratios range from −136 to −105 ‰ for n -pentane, from −86 to −63 ‰ for toluene, from −39 to −15 ‰ for ethylbenzene, from −99 to −68 ‰ for m / p -xylene and from −45 to −34 ‰ for o -xylene.

Description: Complex experiment on studying the microphysical, chemical, and optical properties of aerosol particles and estimating the contribution of atmospheric aerosol-to-earth radiation budget Atmospheric Measurement Techniques, 8, 4507-4520, 2015 Author(s): G. G. Matvienko, B. D. Belan, M. V. Panchenko, O. A. Romanovskii, S. M. Sakerin, D. M. Kabanov, S. A. Turchinovich, Y. S. Turchinovich, T. A. Eremina, V. S. Kozlov, S. A. Terpugova, V. V. Pol'kin, E. P. Yausheva, D. G. Chernov, T. B. Zhuravleva, T. V. Bedareva, S. L. Odintsov, V. D. Burlakov, A. V. Nevzorov, M. Y. Arshinov, G. A. Ivlev, D. E. Savkin, A. V. Fofonov, V. A. Gladkikh, A. P. Kamardin, Y. S. Balin, G. P. Kokhanenko, I. E. Penner, S. V. Samoilova, P. N. Antokhin, V. G. Arshinova, D. K. Davydov, A. V. Kozlov, D. A. Pestunov, T. M. Rasskazchikova, D. V. Simonenkov, T. K. Sklyadneva, G. N. Tolmachev, S. B. Belan, V. P. Shmargunov, A. S. Kozlov, and S. B. Malyshkin The primary objective of this complex aerosol experiment was the measurement of microphysical, chemical, and optical properties of aerosol particles in the surface air layer and free atmosphere. The measurement data were used to retrieve the whole set of aerosol optical parameters, necessary for radiation calculations. Three measurement cycles were performed within the experiment during 2013: in spring, when the aerosol generation is maximal; in summer (July), when atmospheric boundary layer altitude and, hence, mixing layer altitude are maximal; and in late summer/early autumn, during the period of nucleation of secondary particles. Thus, independently obtained data on the optical, meteorological, and microphysical parameters of the atmosphere allow intercalibration and inter-complement of the data and thereby provide for qualitatively new information which explains the physical nature of the processes that form the vertical structure of the aerosol field.

Description: APOLLO_NG – a probabilistic interpretation of the APOLLO legacy for AVHRR heritage channels Atmospheric Measurement Techniques, 8, 4155-4170, 2015 Author(s): L. Klüser, N. Killius, and G. Gesell The cloud processing scheme APOLLO (AVHRR Processing scheme Over cLouds, Land and Ocean) has been in use for cloud detection and cloud property retrieval since the late 1980s. The physics of the APOLLO scheme still build the backbone of a range of cloud detection algorithms for AVHRR (Advanced Very High Resolution Radiometer) heritage instruments. The APOLLO_NG (APOLLO_NextGeneration) cloud processing scheme is a probabilistic interpretation of the original APOLLO method. It builds upon the physical principles that have served well in the original APOLLO scheme. Nevertheless, a couple of additional variables have been introduced in APOLLO_NG. Cloud detection is no longer performed as a binary yes/no decision based on these physical principles. It is rather expressed as cloud probability for each satellite pixel. Consequently, the outcome of the algorithm can be tuned from being sure to reliably identify clear pixels to conditions of reliably identifying definitely cloudy pixels, depending on the purpose. The probabilistic approach allows retrieving not only the cloud properties (optical depth, effective radius, cloud top temperature and cloud water path) but also their uncertainties. APOLLO_NG is designed as a standalone cloud retrieval method robust enough for operational near-realtime use and for application to large amounts of historical satellite data. The radiative transfer solution is approximated by the same two-stream approach which also had been used for the original APOLLO. This allows the algorithm to be applied to a wide range of sensors without the necessity of sensor-specific tuning. Moreover it allows for online calculation of the radiative transfer (i.e., within the retrieval algorithm) giving rise to a detailed probabilistic treatment of cloud variables. This study presents the algorithm for cloud detection and cloud property retrieval together with the physical principles from the APOLLO legacy it is based on. Furthermore a couple of example results from NOAA-18 are presented.

Description: On the relative absorption strengths of water vapour in the blue wavelength range Atmospheric Measurement Techniques, 8, 4329-4346, 2015 Author(s): J. Lampel, D. Pöhler, J. Tschritter, U. Frieß, and U. Platt In recent updates of the HITRAN water vapour H 2 O spectroscopic compilation covering the blue spectral region (here: 394–480 nm) significant changes for the absorption bands at 416 and 426 nm were reported. In order to investigate the consistency of the different cross-sections calculated from these compilations, H 2 O vapour column density ratios for different spectral intervals were retrieved from long-path and multi-axis differential optical absorption spectroscopy (DOAS) measurements. We observed a significant improvement of the DOAS evaluation when using the updated HITRAN water vapour absorption cross-sections for the calculation of the reference spectra. In particular the magnitudes of the residual spectra as well as the fit errors were reduced. However, we also found that the best match between measurement and model is reached when the absorption cross-section of groups of lines are scaled by factors ranging from 0.5 to 1.9, suggesting that the HITRAN water vapour absorption compilation still needs significant corrections. For this spectral region we present correction factors for HITRAN 2009, HITRAN 2012, HITEMP and BT2 derived from field measurements. Additionally, upper limits for water vapour absorption in the UV-A range from 330 to 390 nm are given.

Description: On the interpretation of the loading correction of the aethalometer Atmospheric Measurement Techniques, 8, 4415-4427, 2015 Author(s): A. Virkkula, X. Chi, A. Ding, Y. Shen, W. Nie, X. Qi, L. Zheng, X. Huang, Y. Xie, J. Wang, T. Petäjä, and M. Kulmala Aerosol optical properties were measured with a seven-wavelength aethalometer and a three-wavelength nephelometer at the suburban site SORPES in Nanjing, China, in September 2013–January 2015. The aethalometer compensation parameter k , calculated with the Virkkula et al. (2007) method depended on the backscatter fraction, measured with an independent method, the integrating nephelometer. At λ = 660 nm the daily averaged compensation parameter k ≈ 0.0017 ± 0.0002 and 0.0042 ± 0.0013 when backscatter fraction at λ = 635 nm was in the ranges of 0.100 ± 0.005 and 0.160 ± 0.005, respectively. Also, the wavelength dependency of the compensation parameter depended on the backscatter fraction: when b (λ = 525 nm) was less than approximately 0.13 the compensation parameter decreased with wavelength and at larger b it increased with wavelength. This dependency has not been considered in any of the algorithms that are currently used for processing aethalometer data. The compensation parameter also depended on the single-scattering albedo ω 0 so that k decreased with increasing ω 0 . For the green light (λ = 520 nm) in the ω 0 range 0.870 ± 0.005, the average (± standard deviation) k ≈ 0.0047 ± 0.006 and in the ω 0 range 0.960 ± 0.005, k ≈ 0.0028 ± 0.0007. This difference was larger for the near-infrared light (λ = 880 nm): in the ω 0 range 0.860 ± 0.005, k ≈ 0.0055 ± 0.0023 and in the ω 0 range 0.960 ± 0.005, k ≈ 0.0019 ± 0.0011. The negative dependence of k on ω 0 was also shown with a simple theoretical analysis.