ALBERT

Description: Development and field testing of a rapid and ultra-stable atmospheric carbon dioxide spectrometer Atmospheric Measurement Techniques, 7, 4445-4453, 2014 Author(s): B. Xiang, D. D. Nelson, J. B. McManus, M. S. Zahniser, R. A. Wehr, and S. C. Wofsy We present field test results for a new spectroscopic instrument to measure atmospheric carbon dioxide (CO 2 ) with high precision (0.02 μmol mol −1 , or ppm at 1 Hz) and demonstrate high stability (within 0.1 ppm over more than 8 months), without the need for hourly, daily, or even monthly calibration against high-pressure gas cylinders. The technical novelty of this instrument (ABsolute Carbon dioxide, ABC) is the spectral null method using an internal quartz reference cell with known CO 2 column density. Compared to a previously described prototype, the field instrument has better stability and benefits from more precise thermal control of the optics and more accurate pressure measurements in the sample cell (at the mTorr level). The instrument has been deployed at a long-term ecological research site (the Harvard Forest, USA), where it has measured for 8 months without on-site calibration and with minimal maintenance, showing drift bounds of less than 0.1 ppm. Field measurements agree well with those of a commercially available cavity ring-down CO 2 instrument (Picarro G2301) run with a standard calibration protocol. This field test demonstrates that ABC is capable of performing high-accuracy, unattended, continuous field measurements with minimal use of reference gas cylinders.

Description: Investigating uptake of N 2 O in agricultural soils using a high-precision dynamic chamber method Atmospheric Measurement Techniques, 7, 4455-4462, 2014 Author(s): N. J. Cowan, D. Famulari, P. E. Levy, M. Anderson, D. S. Reay, and U. M. Skiba Uptake (or negative flux) of nitrous oxide (N 2 O) in agricultural soils is a controversial issue which has proved difficult to investigate in the past due to constraints such as instrumental precision and methodological uncertainties. Using a recently developed high-precision quantum cascade laser gas analyser combined with a closed dynamic chamber, a well-defined detection limit of 4 μg N 2 O-N m −2 h −1 could be achieved for individual soil flux measurements. 1220 measurements of N 2 O flux were made from a variety of UK soils using this method, of which 115 indicated uptake by the soil (i.e. a negative flux in the micrometeorological sign convention). Only four of these apparently negative fluxes were greater than the detection limit of the method, which suggests that the vast majority of reported negative fluxes from such measurements are actually due to instrument noise. As such, we suggest that the bulk of negative N 2 O fluxes reported for agricultural fields are most likely due to limits in detection of a particular flux measurement methodology and not a result of microbiological activity consuming atmospheric N 2 O.

Description: Rapid, optical measurement of the atmospheric pressure on a fast research aircraft using open-path TDLAS Atmospheric Measurement Techniques, 7, 3653-3666, 2014 Author(s): B. Buchholz, A. Afchine, and V. Ebert Because of the high travel speed, the complex flow dynamics around an aircraft, and the complex dependency of the fluid dynamics on numerous airborne parameters, it is quite difficult to obtain accurate pressure values at a specific instrument location of an aircraft's fuselage. Complex simulations using computational fluid dynamics (CFD) models can in theory computationally "transfer" pressure values from one location to another. However, for long flight patterns, this process is inconvenient and cumbersome. Furthermore, these CFD transfer models require a local experimental validation, which is rarely available. In this paper, we describe an integrated approach for a spectroscopic, calibration-free, in-flight pressure determination in an open-path White cell on an aircraft fuselage using ambient, atmospheric water vapour as the "sensor species". The presented measurements are realised with the HAI (Hygrometer for Atmospheric Investigations) instrument, built for multiphase water detection via calibration-free TDLAS (tunable diode laser absorption spectroscopy). The pressure determination is based on raw data used for H 2 O concentration measurement, but with a different post-flight evaluation method, and can therefore be conducted at deferred time intervals on any desired flight track. The spectroscopic pressure is compared in-flight with the static ambient pressure of the aircraft avionic system and a micro-mechanical pressure sensor, located next to the open-path cell, over a pressure range from 150 to 800 hPa, and a water vapour concentration range of more than 3 orders of magnitude. The correlation between the micro-mechanical pressure sensor measurements and the spectroscopic pressure measurements shows an average deviation from linearity of only 0.14% and a small offset of 9.5 hPa. For the spectroscopic pressure evaluation we derive measurement uncertainties under laboratory conditions of 3.2 and 5.1% during in-flight operation on the HALO airplane. Under certain flight conditions we quantified, for the first time, stalling-induced, dynamic pressure deviations of up to 30% (at 200 hPa) between the avionic sensor and the optical and mechanical pressure sensors integrated in HAI. Such severe local pressure deviations from the typically used avionic pressure are important to take into account for other airborne sensors employed on such fast flying platforms as the HALO aircraft.

Description: Mixing-layer height retrieval with ceilometer and Doppler lidar: from case studies to long-term assessment Atmospheric Measurement Techniques, 7, 3685-3704, 2014 Author(s): J. H. Schween, A. Hirsikko, U. Löhnert, and S. Crewell Aerosol signatures observed by ceilometers are frequently used to derive mixing-layer height (MLH) which is an essential variable for air quality modelling. However, Doppler wind lidar measurements of vertical velocity can provide a more direct estimation of MLH via simple thresholding. A case study reveals difficulties in the aerosol-based MLH retrieval during transition times when the mixing layer builds up in the morning and when turbulence decays in the afternoon. The difficulties can be explained by the fact that the aerosol distribution is related to the history of the mixing process and aerosol characteristics are modified by humidification. The results of the case study are generalized by evaluating one year of joint measurements by a Vaisala CT25K and a HALO Photonics Streamline wind lidar. On average the aerosol-based retrieval gives higher MLH than the wind lidar with an overestimation of MLH by about 300 m (600 m) in the morning (late afternoon). Also, the daily aerosol-based maximum MLH is larger and occurs later during the day and the average morning growth rates are smaller than those derived from the vertical wind. In fair weather conditions classified by less than 4 octa cloud cover the mean diurnal cycle of cloud base height corresponds well to the mixing-layer height showing potential for a simplified MLH estimation.

Description: Intercomparison of stratospheric gravity wave observations with AIRS and IASI Atmospheric Measurement Techniques, 7, 4517-4537, 2014 Author(s): L. Hoffmann, M. J. Alexander, C. Clerbaux, A. W. Grimsdell, C. I. Meyer, T. Rößler, and B. Tournier Gravity waves are an important driver for the atmospheric circulation and have substantial impact on weather and climate. Satellite instruments offer excellent opportunities to study gravity waves on a global scale. This study focuses on observations from the Atmospheric Infrared Sounder (AIRS) onboard the National Aeronautics and Space Administration Aqua satellite and the Infrared Atmospheric Sounding Interferometer (IASI) onboard the European MetOp satellites. The main aim of this study is an intercomparison of stratospheric gravity wave observations of both instruments. In particular, we analyzed AIRS and IASI 4.3 μm brightness temperature measurements, which directly relate to stratospheric temperature. Three case studies showed that AIRS and IASI provide a clear and consistent picture of the temporal development of individual gravity wave events. Statistical comparisons based on a 5-year period of measurements (2008–2012) showed similar spatial and temporal patterns of gravity wave activity. However, the statistical comparisons also revealed systematic differences of variances between AIRS and IASI that we attribute to the different spatial measurement characteristics of both instruments. We also found differences between day- and nighttime data that are partly due to the local time variations of the gravity wave sources. While AIRS has been used successfully in many previous gravity wave studies, IASI data are applied here for the first time for that purpose. Our study shows that gravity wave observations from different hyperspectral infrared sounders such as AIRS and IASI can be directly related to each other, if instrument-specific characteristics such as different noise levels and spatial resolution and sampling are carefully considered. The ability to combine observations from different satellites provides an opportunity to create a long-term record, which is an exciting prospect for future climatological studies of stratospheric gravity wave activity.

Description: Collection efficiency of the soot-particle aerosol mass spectrometer (SP-AMS) for internally mixed particulate black carbon Atmospheric Measurement Techniques, 7, 4507-4516, 2014 Author(s): M. D. Willis, A. K. Y. Lee, T. B. Onasch, E. C. Fortner, L. R. Williams, A. T. Lambe, D. R. Worsnop, and J. P. D. Abbatt The soot-particle aerosol mass spectrometer (SP-AMS) uses an intra-cavity infrared laser to vaporize refractory black carbon (rBC) containing particles, making the particle beam–laser beam overlap critical in determining the collection efficiency (CE) for rBC and associated non-refractory particulate matter (NR-PM). This work evaluates the ability of the SP-AMS to quantify rBC and NR-PM mass in internally mixed particles with different thicknesses of organic coating. Using apparent relative ionization efficiencies for uncoated and thickly coated rBC particles, we report measurements of SP-AMS sensitivity to NR-PM and rBC, for Regal Black, the recommended particulate calibration material. Beam width probe (BWP) measurements are used to illustrate an increase in sensitivity for highly coated particles due to narrowing of the particle beam, which enhances the CE of the SP-AMS by increasing the laser beam–particle beam overlap. Assuming complete overlap for thick coatings, we estimate CE for bare Regal Black particles of 0.6 ± 0.1, which suggests that previously measured SP-AMS sensitivities to Regal Black were underestimated by up to a factor of 2. The efficacy of the BWP measurements is highlighted by studies at a busy road in downtown Toronto and at a non-roadside location, which show particle beam widths similar to, but greater than that of bare Regal Black and coated Regal Black, respectively. Further BWP measurements at field locations will help to constrain the range of CE for fresh and aged rBC-containing particles. The ability of the SP-AMS to quantitatively assess the composition of internally mixed particles is validated through measurements of laboratory-generated organic coated particles, which demonstrate that the SP-AMS can quantify rBC and NR-PM over a wide range of particle compositions and rBC core sizes.

Description: Reference quality upper-air measurements: GRUAN data processing for the Vaisala RS92 radiosonde Atmospheric Measurement Techniques, 7, 4463-4490, 2014 Author(s): R. J. Dirksen, M. Sommer, F. J. Immler, D. F. Hurst, R. Kivi, and H. Vömel The GCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN) data processing for the Vaisala RS92 radiosonde was developed to meet the criteria for reference measurements. These criteria stipulate the collection of metadata, the use of well-documented correction algorithms, and estimates of the measurement uncertainty. An important and novel aspect of the GRUAN processing is that the uncertainty estimates are vertically resolved. This paper describes the algorithms that are applied in version 2 of the GRUAN processing to correct for systematic errors in radiosonde measurements of pressure, temperature, humidity, and wind, as well as how the uncertainties related to these error sources are derived. Currently, the RS92 is launched on a regular basis at 13 out of 15 GRUAN sites. An additional GRUAN requirement for performing reference measurements with the RS92 is that the manufacturer-prescribed procedure for the radiosonde's preparation, i.e. heated reconditioning of the sensors and recalibration during ground check, is followed. In the GRUAN processing however, the recalibration of the humidity sensors that is applied during ground check is removed. For the dominant error source, solar radiation, laboratory experiments were performed to investigate and model its effect on the RS92's temperature and humidity measurements. GRUAN uncertainty estimates are 0.15 K for night-time temperature measurements and approximately 0.6 K at 25 km during daytime. The other uncertainty estimates are up to 6% relative humidity for humidity, 10–50 m for geopotential height, 0.6 hPa for pressure, 0.4–1 m s −1 for wind speed, and 1° for wind direction. Daytime temperature profiles for GRUAN and Vaisala processing are comparable and consistent within the estimated uncertainty. GRUAN daytime humidity profiles are up to 15% moister than Vaisala processed profiles, of which two-thirds is due to the radiation dry bias correction and one-third is due to an additional calibration correction. Redundant measurements with frost point hygrometers (CFH and NOAA FPH) show that GRUAN-processed RS92 humidity profiles and frost point data agree within 15% in the troposphere. No systematic biases occur, apart from a 5% dry bias for GRUAN data around −40 °C at night.

Description: Deployment of a sequential two-photon laser-induced fluorescence sensor for the detection of gaseous elemental mercury at ambient levels: fast, specific, ultrasensitive detection with parts-per-quadrillion sensitivity Atmospheric Measurement Techniques, 7, 4251-4265, 2014 Author(s): D. Bauer, S. Everhart, J. Remeika, C. Tatum Ernest, and A. J. Hynes The operation of a laser-based sensor for gas-phase elemental mercury, Hg(0), is described. It utilizes sequential two-photon laser excitation with detection of blue-shifted laser-induced fluorescence (LIF) to provide a highly specific detection scheme that precludes detection of anything other than atomic mercury. It has high sensitivity, fast temporal resolution, and can be deployed for in situ measurements in the open atmosphere with essentially no perturbation of the environment. An ambient sample can also be pulled through a fluorescence cell, allowing for standard addition calibrations of the concentration. No type of preconcentration is required and there appears to be no significant interferences from other atmospheric constituents, including gas-phase oxidized mercury species. As a consequence, it is not necessary to remove oxidized mercury, commonly referred to as reactive gaseous mercury (RGM), from the air sample. The instrument has been deployed as part of an instrument intercomparison and compares well with conventional instrumentation that utilizes preconcentration on gold followed by analysis using cold-vapor atomic fluorescence spectroscopy (CVAFS). Currently, the achievable detection sensitivity is ~ 15 pg m −3 (~ 5 × 10 4 atoms cm −3 , ~ 2 ppq) at a sampling rate of 0.1 Hz, i.e., averaging 100 shots with a 10 Hz laser system. Preliminary results are described for a 50 Hz instrument that utilizes a modified excitation sequence and has monitored ambient elemental mercury with an effective sampling rate of 10 Hz. Additional work is required to produce the precision necessary to perform eddy correlation measurements. Addition of a pyrolysis channel should allow for the measurement of total gaseous mercury (TGM) and hence RGM (by difference) with good sensitivity and time resolution.

Description: Prerequisites for application of hyperbolic relaxed eddy accumulation on managed grasslands and alternative net ecosystem exchange flux partitioning Atmospheric Measurement Techniques, 7, 4237-4250, 2014 Author(s): M. Riederer, J. Hübner, J. Ruppert, W. A. Brand, and T. Foken Relaxed eddy accumulation is still applied in ecosystem sciences for measuring trace gas fluxes. On managed grasslands, the length of time between management events and the application of relaxed eddy accumulation has an essential influence on the determination of the proportionality factor b and thus on the resulting flux. In this study this effect is discussed for the first time. Also, scalar similarity between proxy scalars and scalars of interest is affected until the ecosystem has completely recovered. Against this background, CO 2 fluxes were continuously measured and 13 CO 2 isofluxes were determined with a high measurement precision on two representative days in summer 2010. Moreover, a common method for the partitioning of the net ecosystem exchange into assimilation and respiration based on temperature and light response was compared with an isotopic approach directly based on the isotope discrimination of the biosphere. This approach worked well on the grassland site and could enhance flux partitioning results by better reproducing the environmental conditions.

Description: Extending the satellite data record of tropospheric ozone profiles from Aura-TES to MetOp-IASI: characterisation of optimal estimation retrievals Atmospheric Measurement Techniques, 7, 4223-4236, 2014 Author(s): H. Oetjen, V. H. Payne, S. S. Kulawik, A. Eldering, J. Worden, D. P. Edwards, G. L. Francis, H. M. Worden, C. Clerbaux, J. Hadji-Lazaro, and D. Hurtmans We apply the Tropospheric Emission Spectrometer (TES) ozone retrieval algorithm to Infrared Atmospheric Sounding Instrument (IASI) radiances and characterise the uncertainties and information content of the retrieved ozone profiles. This study focuses on mid-latitudes for the year 2008. We validate our results by comparing the IASI ozone profiles to ozone sondes. In the sonde comparisons, we find a negative bias (1–10%) in the IASI profiles in the lower to mid-troposphere and a positive bias (up to 14%) in the upper troposphere/lower stratosphere (UTLS) region. For the described cases, the degrees of freedom for signal are on average 3.2, 0.3, 0.8, and 0.9 for the columns 0 km – top of atmosphere, (0–6), (0–11), and (8–16) km, respectively. We find that our biases with respect to sondes and our degrees of freedom for signal for ozone are comparable to previously published results from other IASI ozone algorithms. In addition to evaluating biases, we validate the retrieval errors by comparing predicted errors to the sample covariance matrix of the IASI observations themselves. For the predicted versus empirical error comparison, we find that these errors are consistent and that the measurement noise and the interference of temperature and water vapour on the retrieval together mostly explain the empirically derived random errors. In general, the precision of the IASI ozone profiles is better than 20%.

Description: Solar irradiances measured using SPN1 radiometers: uncertainties and clues for development Atmospheric Measurement Techniques, 7, 4267-4283, 2014 Author(s): J. Badosa, J. Wood, P. Blanc, C. N. Long, L. Vuilleumier, D. Demengel, and M. Haeffelin The fast development of solar radiation and energy applications, such as photovoltaic and solar thermodynamic systems, has increased the need for solar radiation measurement and monitoring, for not only the global but also the diffuse and direct components. End users look for the best compromise between getting close to state-of-the-art measurements and keeping low capital, maintenance and operating costs. Among the existing commercial options, SPN1 is a relatively low cost solar radiometer that estimates global and diffuse solar irradiances from seven thermopile sensors under a shading mask and without moving parts. This work presents a comprehensive study of SPN1 accuracy and sources of uncertainty, drawing on laboratory experiments, numerical modelling and comparison studies between measurements from this sensor and state-of-the art instruments for six diverse sites. Several clues are provided for improving the SPN1 accuracy and agreement with state-of-the art measurements.

Description: Differences in aerosol absorption Ångström exponents between correction algorithms for a particle soot absorption photometer measured on the South African Highveld Atmospheric Measurement Techniques, 7, 4285-4298, 2014 Author(s): J. Backman, A. Virkkula, V. Vakkari, J. P. Beukes, P. G. Van Zyl, M. Josipovic, S. Piketh, P. Tiitta, K. Chiloane, T. Petäjä, M. Kulmala, and L. Laakso Absorption Ångström exponents (AAEs) calculated from filter-based absorption measurements are often used to give information on the origin of the ambient aerosol, for example, to distinguish between urban pollution and biomass burning aerosol. Filter-based absorption measurements are widely used and are common at aerosol monitoring stations globally. Several correction algorithms are used to account for artefacts associated with filter-based absorption techniques. These algorithms are of profound importance when determining the absolute amount of absorption by the aerosol. However, this study shows that there are substantial differences between the AAEs calculated from these corrections. Depending on the used correction, AAEs can change by as much as 46%. The study also highlights that the difference between AAEs calculated using different corrections can lead to conflicting conclusions on the type of aerosol when using the same data set. The AAE ranged between 1.17 for non-corrected data to 1.96 for the correction that gave the greatest values. Furthermore, the study implies that the AAEs reported for a site depend on at which filter transmittance the filter is changed. In this work, the AAEs were calculated from data measured with a three-wavelength particle soot absorption photometer (PSAP) at Elandsfontein on the South African Highveld for 23 months. The sample air of the PSAP was diluted to prolong filter change intervals, by a factor of 15. The correlation coefficient between the dilution-corrected PSAP and a non-diluted Multi-Angle Absorption Photometer (MAAP) was 0.9. Thus, the study also shows that the applicability of the PSAP can be extended to remote sites that are not often visited or suffer from high levels of pollution.

Description: Looking through the haze: evaluating the CALIPSO level 2 aerosol optical depth using airborne high spectral resolution lidar data Atmospheric Measurement Techniques, 7, 4317-4340, 2014 Author(s): R. R. Rogers, M. A. Vaughan, C. A. Hostetler, S. P. Burton, R. A. Ferrare, S. A. Young, J. W. Hair, M. D. Obland, D. B. Harper, A. L. Cook, and D. M. Winker The Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud–Aerosol Lidar and Pathfinder Satellite Observations (CALIPSO) spacecraft has provided over 8 yr of nearly continuous vertical profiling of Earth's atmosphere. In this paper we investigate the V3.01 and V3.02 CALIOP 532 nm aerosol layer optical depth (AOD) product (i.e the AOD of individual layers) and the column AOD product (i.e., the sum AOD of the complete column) using an extensive database of coincident measurements. The CALIOP AOD measurements and AOD uncertainty estimates are compared with collocated AOD measurements collected with the NASA High Spectral Resolution Lidar (HSRL) in the North American and Caribbean regions. In addition, the CALIOP aerosol lidar ratios are investigated using the HSRL measurements. In general, compared with the HSRL values, the CALIOP layer AOD are biased high by less than 50% for AOD 〈 0.3 with higher errors for higher AOD. Less than 60% of the HSRL AOD measurements are encompassed within the CALIOP layer 1 SD uncertainty range (around the CALIOP layer AOD), so an error estimate is created to encompass 68% of the HSRL data. Using this new metric, the CALIOP layer AOD error is estimated using the HSRL layer AOD as ±0.035 ± 0.05 · (HSRL layer AOD) at night and ±0.05 ± 0.05 · (HSRL layer AOD) during the daytime. Furthermore, the CALIOP layer AOD error is found to correlate with aerosol loading as well as aerosol subtype, with the AODs in marine and dust layers agreeing most closely with the HSRL values. The lidar ratios used by CALIOP for polluted dust, polluted continental, and biomass burning layers are larger than the values measured by the HSRL in the CALIOP layers, and therefore the AODs for these types retrieved by CALIOP were generally too large. We estimated the CALIOP column AOD error can be expressed as ±0.05 ± 0.07 · (HSRL column AOD) at night and ±0.08 ± 0.1 · (HSRL column AOD) during the daytime. Multiple sources of error contribute to both positive and negative errors in the CALIOP column AOD, including multiple layers in the column of different aerosol types, lidar ratio errors, cloud misclassification, and undetected aerosol layers. The undetected layers were further investigated and we found that the layer detection algorithm works well at night, although undetected aerosols in the free troposphere introduce a mean underestimate of 0.02 in the column AOD in the data set examined. The decreased signal-to-noise ratio (SNR) during the daytime led to poorer performance of the layer detection. This caused the daytime CALIOP column AOD to be less accurate than during the nighttime, because CALIOP frequently does not detect optically thin aerosol layers with AOD 〈 0.1. Given that the median vertical extent of aerosol detected within any column was 1.6 km during the nighttime and 1.5 km during the daytime, we can estimate the minimum extinction detection threshold to be 0.012 km −1 at night and 0.067 km −1 during the daytime in a layer median sense. This extensive validation of level 2 CALIOP AOD products extends previous validation studies to nighttime lighting conditions and provides independent measurements of the lidar ratio; thus, allowing the assessment of the effect on the CALIOP AOD of using inappropriate lidar ratio values in the extinction retrieval.

Description: The use of NO 2 absorption cross section temperature sensitivity to derive NO 2 profile temperature and stratospheric–tropospheric column partitioning from visible direct-sun DOAS measurements Atmospheric Measurement Techniques, 7, 4299-4316, 2014 Author(s): E. Spinei, A. Cede, W. H. Swartz, J. Herman, and G. H. Mount This paper presents a temperature sensitivity method (TESEM) to accurately calculate total vertical NO 2 column, atmospheric slant NO 2 profile-weighted temperature ( T ), and to separate stratospheric and tropospheric columns from direct-sun (DS), ground-based measurements using the retrieved T . TESEM is based on differential optical absorption spectroscopy (DOAS) fitting of the linear temperature-dependent NO 2 absorption cross section, σ ( T ), regression model (Vandaele et al., 2003). Separation between stratospheric and tropospheric columns is based on the primarily bimodal vertical distribution of NO 2 and an assumption that stratospheric effective temperature can be represented by temperature at 27 km ± 3 K, and tropospheric effective temperature is equal to surface temperature within 3–5 K. These assumptions were derived from the Global Modeling Initiative (GMI) chemistry-transport model (CTM) simulations over two northern midlatitude sites in 2011. TESEM was applied to the Washington State University Multi-Function DOAS instrument (MFDOAS) measurements at four midlatitude locations with low and moderate NO 2 anthropogenic emissions: (1) the Jet Propulsion Laboratory's Table Mountain Facility (JPL-TMF), CA, USA (34.38° N/117.68° W); (2) Pullman, WA, USA (46.73° N/117.17° W); (3) Greenbelt, MD, USA (38.99° N/76.84° W); and (4) Cabauw, the Netherlands (51.97° N/4.93° E) during July 2007, June–July 2009, July–August and October 2011, November 2012–May 2013, respectively. NO 2 T and total, stratospheric, and tropospheric NO 2 vertical columns were determined over each site.

Description: A Fabry–Perot interferometer-based camera for two-dimensional mapping of SO 2 distributions Atmospheric Measurement Techniques, 7, 3705-3715, 2014 Author(s): J. Kuhn, N. Bobrowski, P. Lübcke, L. Vogel, and U. Platt We examine a new imaging method for the remote sensing of volcanic gases, which relies on the regularly spaced narrow-band absorption structures in the UV–VIS of many molecules. A Fabry–Perot interferometer (FPI) is used to compare the scattered sunlight radiance at wavelengths corresponding to absorption bands with the radiance at wavelengths in between the bands, thereby identifying and quantifying the gas. In this first theoretical study, we present sample calculations for the detection of sulfur dioxide (SO 2 ). Optimum values for the FPI setup parameters are proposed. Furthermore, the performance of the FPI method is compared to SO 2 cameras. We show that camera systems using an FPI are far less influenced by changes in atmospheric radiative transfer (e.g., due to aerosol) and have a great potential as a future technique for examining emissions of SO 2 (or other gases) from volcanic sources and other point sources.

Description: Middle-atmospheric zonal and meridional wind profiles from polar, tropical and midlatitudes with the ground-based microwave Doppler wind radiometer WIRA Atmospheric Measurement Techniques, 7, 4491-4505, 2014 Author(s): R. Rüfenacht, A. Murk, N. Kämpfer, P. Eriksson, and S. A. Buehler WIRA is a ground-based microwave Doppler spectroradiometer specifically designed for the measurement of profiles of horizontal wind in the upper stratosphere and lower mesosphere region where no other continuously running measurement technique exists. A proof of principle has been delivered in a previous publication. A technical upgrade including a new high-frequency amplifier and sideband filter has improved the signal to noise ratio by a factor of 2.4. Since this upgrade the full horizontal wind field comprising zonal and meridional wind profiles is continuously measured. A completely new retrieval based on optimal estimation has been set up. Its characteristics are detailed in the present paper. Since the start of the routine operation of the first prototype in September 2010, WIRA has been measuring at four different locations at polar, mid- and tropical latitudes (67°22' N/26°38' E, 46°57' N/7°26' E, 43°56' N/5°43' E and 21°04' S/55°23' E) for time periods between 5.5 and 11 months. The data presented in this paper are daily average wind profiles with typical uncertainties and resolutions of 10 to 20 m s −1 and 10 to 16 km, respectively. A comparison between the data series from WIRA and European Centre for Medium-Range Weather Forecasts (ECMWF) model data revealed agreement within 10% in the stratospheric zonal wind. The meridional wind profiles agree within their error bars over the entire sensitive altitude range of WIRA. However, significant differences in the mesospheric zonal wind speed of up to 50% have been found.

Description: Analysis of internal gravity waves with GPS RO density profiles Atmospheric Measurement Techniques, 7, 4123-4132, 2014 Author(s): P. Šácha, U. Foelsche, and P. Pišoft GPS radio occultation (RO) data have proved to be a great tool for atmospheric monitoring and studies. In the past decade, they were frequently used for analyses of the internal gravity waves in the upper troposphere and lower stratosphere region. Atmospheric density is the first quantity of state gained in the retrieval process and is not burdened by additional assumptions. However, there are no studies elaborating in detail the utilization of GPS RO density profiles for gravity wave analyses. In this paper, we introduce a method for density background separation and a methodology for internal gravity wave analysis using the density profiles. Various background choices are discussed and the correspondence between analytical forms of the density and temperature background profiles is examined. In the stratosphere, a comparison between the power spectrum of normalized density and normalized dry temperature fluctuations confirms the suitability of the density profiles' utilization. In the height range of 8–40 km, results of the continuous wavelet transform are presented and discussed. Finally, the limits of our approach are discussed and the advantages of the density usage are listed.

Description: Characterization of model errors in the calculation of tangent heights for atmospheric infrared limb measurements Atmospheric Measurement Techniques, 7, 4117-4122, 2014 Author(s): M. Ridolfi and L. Sgheri We review the main factors driving the calculation of the tangent height of spaceborne limb measurements: the ray-tracing method, the refractive index model and the assumed atmosphere. We find that commonly used ray tracing and refraction models are very accurate, at least in the mid-infrared. The factor with largest effect in the tangent height calculation is the assumed atmosphere. Using a climatological model in place of the real atmosphere may cause tangent height errors up to ± 200 m. Depending on the adopted retrieval scheme, these errors may have a significant impact on the derived profiles.

Description: Recovering long-term aerosol optical depth series (1976–2012) from an astronomical potassium-based resonance scattering spectrometer Atmospheric Measurement Techniques, 7, 4103-4116, 2014 Author(s): A. Barreto, E. Cuevas, P. Pallé, P. M. Romero, C. Guirado, C. J. Wehrli, and F. Almansa A 37-year long-term series of monochromatic aerosol optical depth (AOD) has been recovered from solar irradiance measurements performed with the solar spectrometer Mark-I, deployed at Izaña mountain since 1976. The instrument operation is based on the method of resonant scattering, which affords wavelength absolute reference and stability (long-term stability and high precision) in comparison to other instruments based purely on interference filters. However, it has been specifically designed as a reference instrument for helioseismology, and its ability to determine AOD from transmitted and scattered monochromatic radiation at 769.9 nm inside a potassium vapour cell in the presence of a permanent magnetic field is evaluated in this paper. Particularly, the use of an exposed mirror arrangement to collect sunlight as well as the Sun–laboratory velocity dependence of the scattered component introduces some important inconveniences to overcome when we perform the instrument's calibration. We have solved this problem using a quasi-continuous Langley calibration technique and a refinement procedure to correct for calibration errors as well as for the fictitious diurnal cycle on AOD data. Our results showed similar calibration errors retrieved by means of this quasi-continuous Langley technique applied in different aerosol load events (from 0.04 to 0.3), provided aerosol concentration remains constant throughout the calibration interval. It assures the validity of this technique when it is applied in those periods with relatively high aerosol content. The comparative analysis between the recovered AOD data set from the Mark-I and collocated quasi-simultaneous data from the Cimel-AErosol RObotic NETwork (AERONET) and Precision Filter Radiometer (PFR) instruments showed an absolute mean bias ≤ 0.01 in the 10- and 12-year comparison, respectively. High correlation coefficients between AERONET and Mark-I and PFR/Mark-I pairs confirmed a very good linear relationship between instruments, proving that recovered AOD data series from Mark-I can be used together with PFR and AERONET AOD data to build a long-term AOD data series at the Izaña site (1976–now), suitable for future analysis of aerosols trends and inter-annual variability. Finally, the AOD preliminary trend analysis in the 29-year period from 1984 to 2012 with Mark-I AOD revealed no significant trends.

Description: Multistation intercomparison of column-averaged methane from NDACC and TCCON: impact of dynamical variability Atmospheric Measurement Techniques, 7, 4081-4101, 2014 Author(s): A. Ostler, R. Sussmann, M. Rettinger, N. M. Deutscher, S. Dohe, F. Hase, N. Jones, M. Palm, and B.-M. Sinnhuber Dry-air column-averaged mole fractions of methane (XCH 4 ) retrieved from ground-based solar Fourier transform infrared (FTIR) measurements provide valuable information for satellite validation, evaluation of chemical-transport models, and source-sink-inversions. In this context, Sussmann et al. (2013) have shown that midinfrared (MIR) soundings from the Network for the Detection of Atmospheric Composition Change (NDACC) can be combined with near-infrared (NIR) soundings from the Total Carbon Column Observing Network (TCCON) without the need to apply an overall intercalibration factor. However, in spite of efforts to reduce a priori impact, some residual seasonal biases were identified, and the reasons behind remained unclear. In extension to this previous work, which was based on multiannual quasi-coincident MIR and NIR measurements from the stations Garmisch (47.48° N, 11.06° E, 743 m a.s.l.) and Wollongong (34.41° S, 150.88° E, 30 m a.s.l.), we now investigate upgraded retrievals with longer temporal coverage and include three additional stations (Ny-Ålesund, 78.92° N, 11.93° E, 20 m a.s.l.; Karlsruhe, 49.08° N, 8.43° E, 110 m a.s.l.; Izaña, 28.31° N, 16.45° W, 2.370 m a.s.l.). Our intercomparison results (except for Ny-Ålesund) confirm that there is no overall bias between MIR and NIR XCH 4 retrievals, and all MIR and NIR time series reveal a quasi-periodic seasonal bias for all stations, except for Izaña. We find that dynamical variability causes MIR–NIR differences of up to ~ 30 ppb (parts per billion) for Ny-Ålesund, ~ 20 ppb for Wollongong, ~ 18 ppb for Garmisch, and ~ 12 ppb for Karlsruhe. The mechanisms behind this variability are elaborated via two case studies, one dealing with stratospheric subsidence induced by the polar vortex at Ny-Ålesund and the other with a deep stratospheric intrusion event at Garmisch. Smoothing effects caused by the dynamical variability during these events are different for MIR and NIR retrievals depending on the altitude of the perturbation area. MIR retrievals appear to be more realistic in the case of stratospheric subsidence, while NIR retrievals are more accurate in the case of stratosphere–troposphere exchange (STE) in the upper troposphere/lower stratosphere (UTLS) region. About 35% of the FTIR measurement days at Garmisch are impacted by STE, and about 23% of the measurement days at Ny-Ålesund are influenced by polar vortex subsidence. The exclusion of data affected by these dynamical situations resulted in improved agreement of MIR and NIR seasonal cycles for Ny-Ålesund and Garmisch. We found that dynamical variability is a key factor in constraining the accuracy of MIR and NIR seasonal cycles. To mitigate this impact it is necessary to use more realistic a priori profiles that take these dynamical events into account (e.g., via improved models), and/or to improve the FTIR retrievals to achieve a more uniform sensitivity at all altitudes (possibly including profile retrievals for the TCCON data).

Description: A sun-tracking method to improve the pointing accuracy of weather radar Atmospheric Measurement Techniques, 5, 547-555, 2012 Author(s): X. Muth, M. Schneebeli, and A. Berne Accurate positioning of data collected by a weather radar is of primary importance for their appropriate georeferencing, which in turn makes it possible to combine those with additional sources of information (topography, land cover maps, meteorological simulations from numerical weather models to list a few). This issue is especially acute for mobile radar systems, for which accurate and stable leveling might be difficult to ensure. The sun is a source of microwave radiation, which can be detected by weather radars and used for accurate positioning of radar data. This paper presents a technique based on the similarity between theodolites and radar systems as well as on the sun echoes to quantify and hence correct the instrumental errors which can affect the pointing accuracy of radar antenna. The proposed method is applied to data collected in the Swiss Alps using a mobile X-band radar system. The obtained instrumental bias values are evaluated by comparing the locations of the ground echoes predicted using these bias estimates with the observed ground echo locations. The very good agreement between the two confirms the accuracy of the proposed method.

Description: Smoke aerosol and its radiative effects during extreme fire event over Central Russia in summer 2010 Atmospheric Measurement Techniques, 5, 557-568, 2012 Author(s): N. Chubarova, Ye. Nezval', I. Sviridenkov, A. Smirnov, and I. Slutsker Different microphysical, optical and radiative properties of aerosol were analyzed during the severe fires in summer 2010 over Central Russia using ground measurements at two AERONET sites in Moscow (Meteorological Observatory of Moscow State University – MSU MO) and Zvenigorod (Moscow Region) and radiative measurements at the MSU MO. Volume aerosol size distribution in smoke conditions had a bimodal character with the significant prevalence of fine mode particles, for which effective radius was shifted to higher values ( r eff-fine = 0.24 μm against approximately 0.15 μm in typical conditions). For smoke aerosol, the imaginary part of refractive index (REFI) in the visible spectral region was lower than that for typical aerosol (REFI λ =675 nm = 0.006 against REFI λ =675 nm = 0.01), while single scattering albedo (SSA) was significantly higher (SSA λ =675 nm = 0.95 against SSA λ =675 nm ~ 0.9). Extremely high aerosol optical thickness at 500 nm (AOT500) was observed on 6–8 August reaching the absolute maximum on 7 August in Moscow (AOT500 = 6.4) and at Zvenigorod (AOT500 = 5.9). A dramatic attenuation of solar irradiance at ground was also recorded. Maximum irradiance loss had reached 64% for global shortwave irradiance, 91% for UV radiation 300–380 nm, and 97% for erythemally-weighted UV irradiance at relatively high solar elevation 47°. Significant spectral dependence in attenuation of solar irradiance in smoky conditions was mainly explained by higher AOT and smaller SSA in UV (0.8–0.9) compared with SSA in the visible region of spectrum. The assessments of radiative forcing effect (RFE) at the TOA indicated a significant cooling of the smoky atmosphere. Instant RFE reached −167 Wm −2 at AOT500 = 6.4, climatological RFE calculated with August 2010 monthly mean AOT was about −65 Wm −2 , compared with −20 Wm −2 for typical aerosol according to the 10 yr period of measurements in Moscow.

Description: Evaluation of Arctic broadband surface radiation measurements Atmospheric Measurement Techniques, 5, 429-438, 2012 Author(s): N. Matsui, C. N. Long, J. Augustine, D. Halliwell, T. Uttal, D. Longenecker, O. Niebergall, J. Wendell, and R. Albee The Arctic is a challenging environment for making in-situ surface radiation measurements. A standard suite of radiation sensors is typically designed to measure incoming and outgoing shortwave (SW) and thermal infrared, or longwave (LW), radiation. Enhancements may include various sensors for measuring irradiance in narrower bandwidths. Many solar radiation/thermal infrared flux sensors utilize protective glass domes and some are mounted on complex mechanical platforms (solar trackers) that keep sensors and shading devices trained on the sun along its diurnal path. High quality measurements require striking a balance between locating stations in a pristine undisturbed setting free of artificial blockage (such as from buildings and towers) and providing accessibility to allow operators to clean and maintain the instruments. Three significant sources of erroneous data in the Arctic include solar tracker malfunctions, rime/frost/snow deposition on the protective glass domes of the radiometers and operational problems due to limited operator access in extreme weather conditions. In this study, comparisons are made between the global and component sum (direct [vertical component] + diffuse) SW measurements. The difference between these two quantities (that theoretically should be zero) is used to illustrate the magnitude and seasonality of arctic radiation flux measurement problems. The problem of rime/frost/snow deposition is investigated in more detail for one case study utilizing both SW and LW measurements. Solutions to these operational problems that utilize measurement redundancy, more sophisticated heating and ventilation strategies and a more systematic program of operational support and subsequent data quality protocols are proposed.

Description: Profiles of CH 4 , HDO, H 2 O, and N 2 O with improved lower tropospheric vertical resolution from Aura TES radiances Atmospheric Measurement Techniques, 5, 397-411, 2012 Author(s): J. Worden, S. Kulawik, C. Frankenberg, V. Payne, K. Bowman, K. Cady-Peirara, K. Wecht, J.-E. Lee, and D. Noone Thermal infrared (IR) radiances measured near 8 microns contain information about the vertical distribution of water vapor (H 2 O), the water isotopologue HDO, and methane (CH 4 ), key gases in the water and carbon cycles. Previous versions (Version 4 or less) of the TES profile retrieval algorithm used a "spectral-window" approach to minimize uncertainty from interfering species at the expense of reduced vertical resolution and sensitivity. In this manuscript we document changes to the vertical resolution and uncertainties of the TES version 5 retrieval algorithm. In this version (Version 5), joint estimates of H 2 O, HDO, CH 4 and nitrous oxide (N 2 O) are made using radiances from almost the entire spectral region between 1100 cm −1 and 1330 cm −1 . The TES retrieval constraints are also modified in order to better use this information. The new H 2 O estimates show improved vertical resolution in the lower troposphere and boundary layer, while the new HDO/H 2 O estimates can now profile the HDO/H 2 O ratio between 925 hPa and 450 hPa in the tropics and during summertime at high latitudes. The new retrievals are now sensitive to methane in the free troposphere between 800 and 150 mb with peak sensitivity near 500 hPa; whereas in previous versions the sensitivity peaked at 200 hPa. However, the upper troposphere methane concentrations are biased high relative to the lower troposphere by approximately 4% on average. This bias is likely related to temperature, calibration, and/or methane spectroscopy errors. This bias can be mitigated by normalizing the CH 4 estimate by the ratio of the N 2 O estimate relative to the N 2 O prior, under the assumption that the same systematic error affects both the N 2 O and CH 4 estimates. We demonstrate that applying this ratio theoretically reduces the CH 4 estimate for non-retrieved parameters that jointly affect both the N 2 O and CH 4 estimates. The relative upper troposphere to lower troposphere bias is approximately 2.8% after this bias correction. Quality flags based upon the vertical variability of the methane and N 2 O estimates can be used to reduce this bias further. While these new CH 4 , HDO/H 2 O, and H 2 O estimates are consistent with previous TES retrievals in the altitude regions where the sensitivities overlap, future comparisons with independent profile measurement will be required to characterize the biases of these new retrievals and determine if the calculated uncertainties using the new constraints are consistent with actual uncertainties.

Description: Two instruments based on differential optical absorption spectroscopy (DOAS) to measure accurate ammonia concentrations in the atmosphere Atmospheric Measurement Techniques, 5, 413-427, 2012 Author(s): H. Volten, J. B. Bergwerff, M. Haaima, D. E. Lolkema, A. J. C. Berkhout, G. R. van der Hoff, C. J. M. Potma, R. J. Wichink Kruit, W. A. J. van Pul, and D. P. J. Swart We present two Differential Optical Absorption Spectroscopy (DOAS) instruments built at RIVM: the RIVM DOAS and the miniDOAS. Both instruments provide virtually interference-free measurements of NH 3 concentrations in the atmosphere, since they measure over an open path, without suffering from inlet problems or interference problems by ammonium aerosols dissociating on tubes or filters. They measure concentrations up to at least 200 μg m −3 , have a fast response, low maintenance demands, and a high up-time. The RIVM DOAS has a high accuracy of typically 0.15 μg m −3 for ammonia for 5-min averages and over a total light path of 100 m. The miniDOAS has been developed for application in measurement networks such as the Dutch National Air Quality Monitoring Network (LML). Compared to the RIVM DOAS it has a similar accuracy, but is significantly reduced in size, costs, and handling complexity. The RIVM DOAS and miniDOAS results showed excellent agreement ( R 2 = 0.996) during a field measurement campaign in Vredepeel, the Netherlands. This measurement site is located in an agricultural area and is characterized by highly variable, but on average high ammonia concentrations in the air. The RIVM-DOAS and miniDOAS results were compared to the results of the AMOR instrument, a continuous-flow wet denuder system, which is currently used in the LML. Averaged over longer time spans of typically a day, the (mini)DOAS and AMOR results agree reasonably well, although an offset of the AMOR values compared to the (mini)DOAS results exists. On short time scales, the (mini)DOAS shows a faster response and does not show the memory effects due to inlet tubing and transport of absorption fluids encountered by the AMOR. Due to its high accuracy, high uptime, low maintenance and its open path, the (mini)DOAS shows a good potential for flux measurements by using two (or more) systems in a gradient set-up and applying the aerodynamic gradient technique.

Description: Technical Note: The single particle soot photometer fails to reliably detect PALAS soot nanoparticles Atmospheric Measurement Techniques, 5, 3099-3107, 2012 Author(s): M. Gysel, M. Laborde, A. A. Mensah, J. C. Corbin, A. Keller, J. Kim, A. Petzold, and B. Sierau The single particle soot photometer (SP2) uses laser-induced incandescence (LII) for the measurement of atmospheric black carbon (BC) particles. The BC mass concentration is obtained by combining quantitative detection of BC mass in single particles with a counting efficiency of 100% above its lower detection limit. It is commonly accepted that a particle must contain at least several tenths of a femtogram BC in order to be detected by the SP2. Here we show the result that most BC particles from a PALAS spark discharge soot generator remain undetected by the SP2, even if their BC mass, as independently determined with an aerosol particle mass analyser (APM), is clearly above the typical lower detection limit of the SP2. Comparison of counting efficiency and effective density data of PALAS soot with flame generated soot (combustion aerosol standard burner, CAST), fullerene soot and carbon black particles (Cabot Regal 400R) reveals that particle morphology can affect the SP2's lower detection limit. PALAS soot particles are fractal-like agglomerates of very small primary particles with a low fractal dimension, resulting in a very low effective density. Such loosely packed particles behave like "the sum of individual primary particles" in the SP2's laser. Accordingly, most PALAS soot particles remain undetected as the SP2's laser intensity is insufficient to heat the primary particles to their vaporisation temperature because of their small size ( D pp ≈ 5–10 nm). Previous knowledge from pulsed laser-induced incandescence indicated that particle morphology might have an effect on the SP2's lower detection limit, however, an increase of the lower detection limit by a factor of ∼5–10, as reported here for PALAS soot, was not expected. In conclusion, the SP2's lower detection limit at a certain laser power depends primarily on the total BC mass per particle for compact particles with sufficiently high effective density. By contrast, the BC mass per primary particle mainly determines whether fractal-like particles with low fractal dimension and very small primary particles are detectable, while their total BC mass has only a minor influence. This effect shifts the lower detection limit to much higher BC mass, or makes them completely undetectable. Consequently, care has to be taken when using the SP2 in applications dealing with loosely packed particles that have very small primary particles as building blocks.

Description: Effect of air composition (N 2 , O 2 , Ar, and H 2 O) on CO 2 and CH 4 measurement by wavelength-scanned cavity ring-down spectroscopy: calibration and measurement strategy Atmospheric Measurement Techniques, 5, 2689-2701, 2012 Author(s): H. Nara, H. Tanimoto, Y. Tohjima, H. Mukai, Y. Nojiri, K. Katsumata, and C. W. Rella We examined potential interferences from water vapor and atmospheric background gases (N 2 , O 2 , and Ar), and biases by isotopologues of target species, on accurate measurement of atmospheric CO 2 and CH 4 by means of wavelength-scanned cavity ring-down spectroscopy (WS-CRDS). Changes of the background gas mole fractions in the sample air substantially impacted the CO 2 and CH 4 measurements: variation of CO 2 and CH 4 due to relative increase of each background gas increased as Ar 〈 O 2 〈 N 2 , suggesting similar relation for the pressure-broadening effects (PBEs) among the background gas. The pressure-broadening coefficients due to variations in O 2 and Ar for CO 2 and CH 4 are empirically determined from these experimental results. Calculated PBEs using the pressure-broadening coefficients are linearly correlated with the differences between the mole fractions of O 2 and Ar and their ambient abundances. Although the PBEs calculation showed that impact of natural variation of O 2 is negligible on the CO 2 and CH 4 measurements, significant bias was inferred for the measurement of synthetic standard gases. For gas standards balanced with purified air, the PBEs were estimated to be marginal (up to 0.05 ppm for CO 2 and 0.01 ppb for CH 4 ) although the PBEs were substantial (up to 0.87 ppm for CO 2 and 1.4 ppb for CH 4 ) for standards balanced with synthetic air. For isotopic biases on CO 2 measurements, we compared experimental results and theoretical calculations, which showed excellent agreement within their uncertainty. We derived instrument-specific water correction functions empirically for three WS-CRDS instruments (Picarro EnviroSense 3000i, G-1301, and G-2301), and evaluated the transferability of the water correction function from G-1301 among these instruments. Although the transferability was not proven, no significant difference was found in the water vapor correction function for the investigated WS-CRDS instruments as well as the instruments reported in the past studies within the typical analytical precision at sufficiently low water concentrations (

Description: Atmospheric effect on the ground-based measurements of broadband surface albedo Atmospheric Measurement Techniques, 5, 2675-2688, 2012 Author(s): T. Manninen, A. Riihelä, and G. de Leeuw Ground-based pyranometer measurements of the (clear-sky) broadband surface albedo are affected by the atmospheric conditions (mainly by aerosol particles, water vapour and ozone). A new semi-empirical method for estimating the magnitude of the effect of atmospheric conditions on surface albedo measurements in clear-sky conditions is presented. Global and reflected radiation and/or aerosol optical depth (AOD) at two wavelengths are needed to apply the method. Depending on the aerosol optical depth and the solar zenith angle values, the effect can be as large as 20%. For the cases we tested using data from the Cabauw atmospheric test site in the Netherlands, the atmosphere caused typically up to 5% overestimation of surface albedo with respect to corresponding black-sky surface albedo values.

Description: Improved Micro Rain Radar snow measurements using Doppler spectra post-processing Atmospheric Measurement Techniques, 5, 2661-2673, 2012 Author(s): M. Maahn and P. Kollias The Micro Rain Radar 2 (MRR) is a compact Frequency Modulated Continuous Wave (FMCW) system that operates at 24 GHz. The MRR is a low-cost, portable radar system that requires minimum supervision in the field. As such, the MRR is a frequently used radar system for conducting precipitation research. Current MRR drawbacks are the lack of a sophisticated post-processing algorithm to improve its sensitivity (currently at +3 dBz), spurious artefacts concerning radar receiver noise and the lack of high quality Doppler radar moments. Here we propose an improved processing method which is especially suited for snow observations and provides reliable values of effective reflectivity, Doppler velocity and spectral width. The proposed method is freely available on the web and features a noise removal based on recognition of the most significant peak. A dynamic dealiasing routine allows observations even if the Nyquist velocity range is exceeded. Collocated observations over 115 days of a MRR and a pulsed 35.2 GHz MIRA35 cloud radar show a very high agreement for the proposed method for snow, if reflectivities are larger than −5 dBz. The overall sensitivity is increased to −14 and −8 dBz, depending on range. The proposed method exploits the full potential of MRR's hardware and substantially enhances the use of Micro Rain Radar for studies of solid precipitation.

Description: Comparison of satellite microwave backscattering (ASCAT) and visible/near-infrared reflectances (PARASOL) for the estimation of aeolian aerodynamic roughness length in arid and semi-arid regions Atmospheric Measurement Techniques, 5, 2703-2712, 2012 Author(s): C. Prigent, C. Jiménez, and J. Catherinot Previous studies examined the possibility to estimate the aeolian aerodynamic roughness length from satellites, either from visible/near-infrared observations or from microwave backscattering measurements. Here we compare the potential of the two approaches and propose to merge the two sources of information to benefit from their complementary aspects, i.e. the high spatial resolution of the visible/near-infrared (6 km for PARASOL that is part of the A-Train) and the independence from atmospheric contamination of the active microwaves (ASCAT on board MetOp with a lower spatial resolution of 25 km). A global map of the aeolian aerodynamic roughness length at 6 km resolution is derived, for arid and semi-arid regions. It shows very good consistency with the existing information on the properties of these surfaces. The dataset is available to the community, for use in atmospheric dust transport models.

Description: Anisotropy of small-scale stratospheric irregularities retrieved from scintillations of a double star α-Cru observed by GOMOS/ENVISAT Atmospheric Measurement Techniques, 5, 2713-2722, 2012 Author(s): V. Kan, V. F. Sofieva, and F. Dalaudier In this paper, we discuss estimating anisotropy of air density irregularities (ratio of characteristic horizontal and vertical scales) from satellite observations of bi-chromatic scintillations of a double star whose components are not resolved by the detector. The analysis is based on fitting experimental auto- and cross-spectra of scintillations by those computed using the 3-D spectral model of atmospheric irregularities consisting of anisotropic and isotropic components. Application of the developed method to the scintillation measurements of the double star α-Cru by GOMOS (Global Ozone Monitoring by Occultation of Stars) fast photometers results in estimates of anisotropy coefficient of ~15–20 at altitudes 30–38 km, as well as other parameters of atmospheric irregularities. The obtained estimates of the anisotropy coefficient correspond to small-scale irregularities, close to the buoyancy scale.

Description: Development of a new data-processing method for SKYNET sky radiometer observations Atmospheric Measurement Techniques, 5, 2723-2737, 2012 Author(s): M. Hashimoto, T. Nakajima, O. Dubovik, M. Campanelli, H. Che, P. Khatri, T. Takamura, and G. Pandithurai In order to reduce uncertainty in the estimation of Direct Aerosol Radiative Forcing (DARF), it is important to improve the estimation of the single scattering albedo (SSA). In this study, we propose a new data processing method to improve SSA retrievals for the SKYNET sky radiometer network, which is one of the growing number of networks of sun-sky photometers, such as NASA AERONET and others. There are several reports that SSA values from SKYNET have a bias compared to those from AERONET, which is regarded to be the most accurate due to its rigorous calibration routines and data quality and cloud screening algorithms. We investigated possible causes of errors in SSA that might explain the known biases through sensitivity experiments using a numerical model, and also using real data at the SKYNET sites at Pune (18.616° N/73.800° E) in India and Beijing (39.586° N/116.229° E) in China. Sensitivity experiments showed that an uncertainty of the order of ±0.03 in the SSA value can be caused by a possible error in the ground surface albedo or solid view angle assumed for each observation site. Another candidate for possible error in the SSA was found in cirrus contamination generated by imperfect cloud screening in the SKYNET data processing. Therefore, we developed a new data quality control method to get rid of low quality or cloud contamination data, and we applied this method to the real observation data at the Pune site in SKYNET. After applying this method to the observation data, we were able to screen out a large amount of cirrus-contaminated data and to reduce the deviation in the SSA value from that of AERONET. We then estimated DARF using data screened by our new method. The result showed that the method significantly reduced the difference of 5 W m −2 that existed between the SKYNET and AERONET values of DARF before screening. The present study also suggests the necessity of preparing suitable a priori information on the distribution of coarse particles ranging in radius between 10 μm and 30 μm for the analysis of heavily dust-laden atmospheric cases.

Description: Calibration and intercomparison of acetic acid measurements using proton-transfer-reaction mass spectrometry (PTR-MS) Atmospheric Measurement Techniques, 5, 2739-2750, 2012 Author(s): K. B. Haase, W. C. Keene, A. A. P. Pszenny, H. R. Mayne, R. W. Talbot, and B. C. Sive Acetic acid is one of the most abundant organic acids in the ambient atmosphere, with maximum mixing ratios reaching into the tens of parts per billion by volume (ppbv) range. The identities and associated magnitudes of the major sources and sinks for acetic acid are poorly characterized, due in part to the limitations of available measurement techniques. This paper demonstrates that, when properly calibrated, proton-transfer-reaction mass spectrometry (PTR-MS) can be a valuable technique for fast response, accurate quantification of acetic acid in ambient air. Three different PTR-MS configurations were calibrated at low ppbv mixing ratios using permeation tubes, which yielded calibration factors between 7.0 and 10.9 normalized counts per second per ppbv (ncps ppbv −1 ) at a drift tube field strength of 132 Townsend (Td). Detection limits ranged from 0.06 to 0.32 ppbv with dwell times of 5 s. These calibration factors showed negligible humidity dependence. Acetic acid was measured with PTR-MS on Appledore B Island, ME, during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) campaign and validated based on acetic acid measured in parallel using tandem mist chambers coupled with ion chromatography (MC/IC). Mixing ratios ranged from a minimum of 0.075 ± 0.004 ppbv to 3.555 ± 0.171 ppbv, with a median mixing ratio of 0.530 ± 0.025 ppbv. An orthogonal least squares linear regression of paired data yielded a slope of 1.14 ± 0.06 (2σ), an intercept of 0.049 ± 0.020 (2σ) ppbv, and an R 2 of 0.78.

Description: Airborne observations of formic acid using a chemical ionization mass spectrometer Atmospheric Measurement Techniques, 5, 3029-3039, 2012 Author(s): M. Le Breton, M. R. McGillen, J. B. A. Muller, A. Bacak, D. E. Shallcross, P. Xiao, L. G. Huey, D. Tanner, H. Coe, and C. J. Percival The first airborne measurements of formic acid mixing ratios over the United Kingdom were measured on the FAAM BAe-146 research aircraft on 16 March 2010 with a chemical ionization mass spectrometer using I − reagent ions. The I − ionization scheme was able to measure formic acid mixing ratios at 1 Hz in the boundary layer. In-flight standard addition calibrations from a formic acid source were used to determine the instrument sensitivity of 35 ± 6 ion counts pptv −1 s −1 and a limit of detection of 25 pptv. Routine measurements were made through a scrubbed inlet to determine the instrumental background. Three plumes of formic acid were observed over the UK, originating from London, Humberside and Tyneside. The London plume had the highest formic acid mixing ratio throughout the flight, peaking at 358 pptv. No significant correlations of formic acid with NO x and ozone were found, but a positive correlation was observed between CO and HCOOH within the two plumes where coincident data were recorded. A trajectory model was employed to determine the sources of the plumes and compare modelled mixing ratios with measured values. The model underestimated formic acid concentrations by up to a factor of 2. This is explained by missing sources in the model, which were considered to be both primary emissions of formic acid of mainly anthropogenic origin and a lack of precursor emissions, such as isoprene, from biogenic sources, whose oxidation in situ would lead to formic acid formation.

Description: Applying spaceborne reflectivity measurements for calculation of the solar ultraviolet radiation at ground level Atmospheric Measurement Techniques, 5, 3041-3054, 2012 Author(s): P. N. den Outer, A. van Dijk, H. Slaper, A. V. Lindfors, H. De Backer, A. F. Bais, U. Feister, T. Koskela, and W. Josefsson Long-term analysis of cloud effects on ultraviolet (UV) radiation on the ground using spaceborne observations requires the use of instruments that have operated consecutively. The longest data record can be built from the reflectivity measurements produced by the instruments Total Ozone Mapping Spectrometers (TOMS) flown on Nimbus 7 from 1979 to 1992, TOMS on Earth Probe from 1996 to 2005, and the Ozone Monitoring Instrument (OMI) flown on EOS Aura since 2004. The reflectivity data produced by TOMS on Earth Probe is only included until 2002. A comparison is made with cloud effects inferred from ground-based pyranometer measurements at over 83 World Radiation Data Centre stations. Modelled UV irradiances utilizing the standard reflectivity are compared with measurements of UV irradiances at eight European low-elevation stations. The reflectivity data of the two TOMS instruments shows a consistent agreement, and the required corrections are of low percentage, i.e. 2–3%. In contrast, the reflectivity product of OMI requires correction of 7–10%, and a solar angle dependency therein is more pronounced. These corrections were inferred from a comparison with pyranometer data, and tested using the UV measurements. The average reduction of UV radiation due to clouds for all sites together indicates a small trend: a diminishing cloudiness, in line with ground-based UV observations. Uncorrected implementation of the reflectivity data would have indicated the opposite. An optimal area was established for reflectivity data for the calculation of daily sums of UV radiation. It measures approximately 1.25° in latitudinal direction for square-shaped areas overhead the ground-based UV stations. Such an area can be traversed within 5 to 7 h at the average wind speeds found for the West European continent.

Description: Corrigendum to "Greenhouse gas measurements over a 144 km open path in the Canary Islands" published in Atmos. Meas. Tech., 5, 2309–2319, 2012 Atmospheric Measurement Techniques, 5, 2349-2349, 2012 Author(s): J. S. A. Brooke, P. F. Bernath, G. Kirchengast, C. B. Thomas, J.-G. Wang, K. A. Tereszchuk, G. González Abad, R. J. Hargreaves, C. A. Beale, J. J. Harrison, S. Schweitzer, V. Proschek, P. A. Martin, V. L. Kasyutich, C. Gerbig, O. Kolle, and A. Loescher No abstract available.

Description: NO 2 observations over the western Pacific and Indian Ocean by MAX-DOAS on Kaiyo , a Japanese research vessel Atmospheric Measurement Techniques, 5, 2351-2360, 2012 Author(s): H. Takashima, H. Irie, Y. Kanaya, and F. Syamsudin Nitrogen dioxide (NO 2 ) profile retrievals were performed by ship-borne Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) using a compact/low-power spectrometer on the Japanese research vessel Kaiyo during two ocean cruises around Japan and Japan–Bali (Indonesia)–Indian Ocean. DOAS analysis using a 425–450 nm fitting window revealed a clear land–ocean contrast in NO 2 differential slant column densities (DSCDs) but poor fitting results and negative values, especially at low elevation angles at low latitudes (

Description: Greenhouse gas measurements over a 144 km open path in the Canary Islands Atmospheric Measurement Techniques, 5, 2309-2319, 2012 Author(s): J. S. A. Brooke, P. F. Bernath, G. Kirchengast, C. B. Thomas, J.-G. Wang, K. A. Tereszchuk, G. González Abad, R. J. Hargreaves, C. A. Beale, J. J. Harrison, S. Schweitzer, V. Proschek, P. A. Martin, V. L. Kasyutich, C. Gerbig, O. Kolle, and A. Loescher A new technique for the satellite remote sensing of greenhouse gases in the atmosphere via the absorption of short-wave infrared laser signals transmitted between counter-rotating satellites in low Earth orbit has recently been proposed; this would enable the acquisition of a long-term, stable, global set of altitude-resolved concentration measurements. We present the first ground-based experimental demonstration of this new infrared-laser occultation method, in which the atmospheric absorption of CO 2 near 2.1 μm was measured over a ~144 km path length between two peaks in the Canary Islands (at an altitude of ~2.4 km), using relatively low power diode lasers (~4 to 10 mW). The retrieved CO 2 volume mixing ratio of 400 ppm (±15 ppm) is consistent within experimental uncertainty with simultaneously recorded in situ validation measurements. We conclude that the new method has a sound basis for monitoring CO 2 in the free atmosphere; other greenhouse gases such as methane, nitrous oxide and water vapour can be monitored in the same way.

Description: An empirical model of optical and radiative characteristics of the tropospheric aerosol over West Siberia in summer Atmospheric Measurement Techniques, 5, 1513-1527, 2012 Author(s): M. V. Panchenko, T. B. Zhuravleva, S. A. Terpugova, V. V. Polkin, and V. S. Kozlov An empirical model of the vertical profiles of aerosol optical characteristics is described. This model was developed based on data acquired from multi-year airborne sensing of optical and microphysical characteristics of the tropospheric aerosol over West Siberia. The main initial characteristics for the creation of the model were measurement data of the vertical profiles of the aerosol angular scattering coefficients in the visible wavelength range, particle size distribution functions and mass concentrations of black carbon (BC). The proposed model allows us to retrieve the aerosol optical and radiative characteristics in the visible and near-IR wavelength range, using the season, air mass type and time of day as input parameters. The columnar single scattering albedo and asymmetry factor of the aerosol scattering phase function, calculated using the average vertical profiles, are in good agreement with data from the AERONET station located in Tomsk. For solar radiative flux calculations, this empirical model has been tested for typical summer conditions. The available experimental database obtained for the regional features of West Siberia and the model developed on this basis are shown to be sufficient for performing these calculations.

Description: Calibration of an all-sky camera for obtaining sky radiance at three wavelengths Atmospheric Measurement Techniques, 5, 2013-2024, 2012 Author(s): R. Román, M. Antón, A. Cazorla, A. de Miguel, F. J. Olmo, J. Bilbao, and L. Alados-Arboledas This paper proposes a method to obtain spectral sky radiances, at three wavelengths (464, 534 and 626 nm), from hemispherical sky images. Images are registered with the All-Sky Imager installed at the Andalusian Center for Environmental Research (CEAMA) in Granada (Spain). The methodology followed in this work for the absolute calibration in radiance of this instrument is based on the comparison of its output measurements with modelled sky radiances derived from the LibRadtran/UVSPEC radiative transfer code under cloud-free conditions. Previously, in order to check the goodness of the simulated radiances, these are compared with experimental values recorded by a CIMEL sunphotometer. In general, modelled radiances are in agreement with experimental data, showing mean differences lower than 20% except for the pixels located next to the Sun position that show larger errors. The relationship between the output signal of the All-Sky Imager and the modelled sky radiances provides a calibration matrix for each image. The variability of the matrix coefficients is analyzed, showing no significant changes along a period of 5 months. Therefore, a unique calibration matrix per channel is obtained for all selected images (a total of 705 images per channel). Camera radiances are compared with CIMEL radiances, finding mean absolute differences between 2% and 15% except for pixels near to the Sun and high scattering angles. We apply these calibration matrices to three images in order to study the sky radiance distributions for three different sky conditions: cloudless, overcast and partially cloudy. Horizon brightening under cloudless conditions has been observed together with the enhancement effect of individual clouds on sky radiance.

Description: Aircraft measurements of carbon dioxide and methane for the calibration of ground-based high-resolution Fourier Transform Spectrometers and a comparison to GOSAT data measured over Tsukuba and Moshiri Atmospheric Measurement Techniques, 5, 2003-2012, 2012 Author(s): T. Tanaka, Y. Miyamoto, I. Morino, T. Machida, T. Nagahama, Y. Sawa, H. Matsueda, D. Wunch, S. Kawakami, and O. Uchino Aircraft measurements of carbon dioxide and methane over Tsukuba (36.05° N, 140.12° E) (February 2010) and Moshiri (44.36° N, 142.26° E) (August 2009) were made to calibrate ground-based high-resolution Fourier Transform Spectrometers (g-b FTSs) and to compare with the Greenhouse gases Observing SATellite (GOSAT). The aircraft measurements over Tsukuba in February 2010 were successful in synchronizing with both the g-b FTS and GOSAT for the first time. Airborne in situ and flask-sampling instruments were mounted on the aircraft, and measurements were carried out between altitudes of 0.5 and 7 km to obtain vertical profiles of carbon dioxide (CO 2 ), methane (CH 4 ), and other gaseous species. By comparing the g-b FTS measurements with the airborne measurements, the column-averaged dry air mole fractions of CO 2 (X CO 2 ) and CH 4 (X CH 4 ) retrieved from the g-b FTS measurements at Tsukuba were biased low by 0.33 ± 0.11% for X CO 2 and 0.69 ± 0.29% for X CH 4 . The g-b FTS values at Moshiri were biased low by 1.24% for X CO 2 and 2.11% for X CH 4 . The GOSAT data show biases that are 3.1% ± 1.7% lower for X CO 2 and 2.5% ± 0.8% lower for X CH 4 than the aircraft measurements obtained over Tsukuba.

Description: Quantitative bias estimates for tropospheric NO 2 columns retrieved from SCIAMACHY, OMI, and GOME-2 using a common standard for East Asia Atmospheric Measurement Techniques, 5, 2403-2411, 2012 Author(s): H. Irie, K. F. Boersma, Y. Kanaya, H. Takashima, X. Pan, and Z. F. Wang For the intercomparison of tropospheric nitrogen dioxide (NO 2 ) vertical column density (VCD) data from three different satellite sensors (SCIAMACHY, OMI, and GOME-2), we use a common standard to quantitatively evaluate the biases for the respective data sets. As the standard, a regression analysis using a single set of collocated ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations at several sites in Japan and China from 2006–2011 is adopted. Examinations of various spatial coincidence criteria indicates that the slope of the regression line can be influenced by the spatial distribution of NO 2 over the area considered. While the slope varies systematically with the distance between the MAX-DOAS and satellite observation points around Tokyo in Japan, such a systematic dependence is not clearly seen and correlation coefficients are generally higher in comparisons at sites in China. On the basis of these results, we focus mainly on comparisons over China and estimate the biases in SCIAMACHY, OMI, and GOME-2 data (TM4NO2A and DOMINO version 2 products) against the MAX-DOAS observations to be −5 ± 14%, −10 ± 14%, and +1 ± 14%, respectively, which are all small and insignificant. We suggest that these small biases now allow for analyses combining these satellite data for air quality studies, which are more systematic and quantitative than previously possible.

Description: Combined wind measurements by two different lidar instruments in the Arctic middle atmosphere Atmospheric Measurement Techniques, 5, 2433-2445, 2012 Author(s): J. Hildebrand, G. Baumgarten, J. Fiedler, U.-P. Hoppe, B. Kaifler, F.-J. Lübken, and B. P. Williams During a joint campaign in January 2009, the Rayleigh/Mie/Raman (RMR) lidar and the sodium lidar at the ALOMAR Observatory (69° N, 16° E) in Northern Norway were operated simultaneously for more than 40 h, collecting data for wind measurements in the middle atmosphere from 30 up to 110 km altitude. As both lidars share the same receiving telescopes, the upper altitude range of the RMR lidar and the lower altitude range of the sodium lidar overlap in the altitude region of ≈80–85 km. For this overlap region we are thus able to present the first simultaneous wind measurements derived from two different lidar instruments. The comparison of winds derived by RMR and sodium lidar is excellent for long integration times of 10 h as well as shorter ones of 1 h. Combination of data from both lidars allows identifying wavy structures between 30 and 110 km altitude, whose amplitudes increase with height. We have also performed vertical wind measurements and measurements of the same horizontal wind component using two independent lasers and telescopes of the RMR lidar and show how to use this data to calibrate and validate the wind retrieval. For the latter configuration we found a good agreement of the results but also identified inhomogeneities in the horizontal wind at about 55 km altitude of up to 20 ms −1 for an integration time of nearly 4 h. Such small-scale inhomogeneities in the horizontal wind field are an essential challenge when comparing data from different instruments.

Description: Global distributions of C 2 H 6 , C 2 H 2 , HCN, and PAN retrieved from MIPAS reduced spectral resolution measurements Atmospheric Measurement Techniques, 5, 723-734, 2012 Author(s): A. Wiegele, N. Glatthor, M. Höpfner, U. Grabowski, S. Kellmann, A. Linden, G. Stiller, and T. von Clarmann Vertical profiles of mixing ratios of C 2 H 6 , C 2 H 2 , HCN, and PAN were retrieved from MIPAS reduced spectral resolution nominal mode limb emission measurements. The retrieval strategy follows that of the analysis of MIPAS high resolution measurements, with occasional adjustments to cope with the reduced spectral resolution under which MIPAS is operated since 2005. MIPAS measurements from January 2005 to January 2010 have been analyzed with special emphasis on October 2007. Largest mixing ratios are found in the troposphere, and reach 1.2 ppbv for C 2 H 6 , 1 ppbv for HCN, 600 pptv for PAN, and 450 pptv for C 2 H 2 . The estimated precisions in case of significantly enhanced mixing ratios (including measurement noise and propagation of uncertain parameters randomly varying in the time domain) and altitude resolution are typically 10%, 3–4.5 km for C 2 H 6 , 15%, 4–6 km for HCN, 6%, 2.5–3.5 km for PAN, and 7%, 2.5–4 km for C 2 H 2 .

Description: DOAS measurements of NO 2 from an ultralight aircraft during the Earth Challenge expedition Atmospheric Measurement Techniques, 5, 2057-2068, 2012 Author(s): A. Merlaud, M. Van Roozendael, J. van Gent, C. Fayt, J. Maes, X. Toledo-Fuentes, O. Ronveaux, and M. De Mazière We report on airborne Differential Optical Absorption Spectroscopy (DOAS) measurements of NO 2 tropospheric columns above South Asia, the Arabic peninsula, North Africa, and Italy in November and December 2009. The DOAS instrument was installed on an ultralight aircraft involved in the Earth Challenge project, an expedition of seven pilots flying on four ultralight aircraft between Australia and Belgium. The instrument recorded spectra in limb geometry with a large field of view, a set-up which provides a high sensitivity to the boundary layer NO 2 while minimizing the uncertainties related to the attitude variations. We compare our measurements with OMI (Ozone Monitoring Instrument) and GOME-2 (Global Ozone Monitoring Experiment 2) tropospheric NO 2 products when the latter are available. Above Rajasthan and the Po Valley, two areas where the NO 2 field is homogeneous, data sets agree very well. Our measurements in these areas are 0.1 ± 0.1 to 3 ± 1 × 10 15 molec cm −2 and 2.6 ± 0.8 × 10 16 molec cm −2 , respectively. Flying downwind of Riyadh, our NO 2 measurements show the structure of the megacity's exhaust plume with a higher spatial resolution than OMI. Moreover, our measurements are larger (up to 40%) than those seen by satellites. We also derived tropospheric columns when no satellite data were available if it was possible to get information on the visibility from satellite measurements of aerosol optical thickness. This experiment also provides a confirmation for the recent finding of a soil signature above desert.

Description: Evaluating nighttime CALIOP 0.532 μm aerosol optical depth and extinction coefficient retrievals Atmospheric Measurement Techniques, 5, 2143-2160, 2012 Author(s): J. R. Campbell, J. L. Tackett, J. S. Reid, J. Zhang, C. A. Curtis, E. J. Hyer, W. R. Sessions, D. L. Westphal, J. M. Prospero, E. J. Welton, A. H. Omar, M. A. Vaughan, and D. M. Winker NASA Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) Version 3.01 5-km nighttime 0.532 μm aerosol optical depth (AOD) datasets from 2007 are screened, averaged and evaluated at 1° × 1° resolution versus corresponding/co-incident 0.550 μm AOD derived using the US Navy Aerosol Analysis and Prediction System (NAAPS), featuring two-dimensional variational assimilation of quality-assured NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR) AOD. In the absence of sunlight, since passive radiometric AOD retrievals rely overwhelmingly on scattered radiances, the model represents one of the few practical global estimates available from which to attempt such a validation. Daytime comparisons, though, provide useful context. Regional-mean CALIOP vertical profiles of night/day 0.532 μm extinction coefficient are compared with 0.523/0.532 μm ground-based lidar measurements to investigate representativeness and diurnal variability. In this analysis, mean nighttime CALIOP AOD are mostly lower than daytime (0.121 vs. 0.126 for all aggregated data points, and 0.099 vs. 0.102 when averaged globally per normalised 1° × 1° bin), though the relationship is reversed over land and coastal regions when the data are averaged per normalised bin (0.134/0.108 vs. 0140/0.112, respectively). Offsets assessed within single bins alone approach ±20%. CALIOP AOD, both day and night, are higher than NAAPS over land (0.137 vs. 0.124) and equal over water (0.082 vs. 0.083) when averaged globally per normalised bin. However, for all data points inclusive, NAAPS exceeds CALIOP over land, coast and ocean, both day and night. Again, differences assessed within single bins approach 50% in extreme cases. Correlation between CALIOP and NAAPS AOD is comparable during both day and night. Higher correlation is found nearest the equator, both as a function of sample size and relative signal magnitudes inherent at these latitudes. Root mean square deviation between CALIOP and NAAPS varies between 0.1 and 0.3 globally during both day/night. Averaging of CALIOP along-track AOD data points within a single NAAPS grid bin improves correlation and RMSD, though day/night and land/ocean biases persist and are believed systematic. Vertical profiles of extinction coefficient derived in the Caribbean compare well with ground-based lidar observations, though potentially anomalous selection of a priori lidar ratios for CALIOP retrievals is likely inducing some discrepancies. Mean effective aerosol layer top heights are stable between day and night, indicating consistent layer-identification diurnally, which is noteworthy considering the potential limiting effects of ambient solar noise during day.

Description: The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds Atmospheric Measurement Techniques, 5, 2039-2055, 2012 Author(s): D. Rosenfeld, E. Williams, M. O. Andreae, E. Freud, U. Pöschl, and N. O. Rennó The cloud-mediated aerosol radiative forcing is widely recognized as the main source of uncertainty in our knowledge of the anthropogenic forcing on climate. The current challenges for improving our understanding are (1) global measurements of cloud condensation nuclei (CCN) in the cloudy boundary layer from space, and (2) disentangling the effects of aerosols from the thermodynamic and meteorological effects on the clouds. Here, we present a new conceptual framework to help us overcome these two challenges, using relatively simple passive satellite measurements in the visible and infared (IR). The idea is to use the clouds themselves as natural CCN chambers by retrieving simultaneously the number of activated aerosols at cloud base, N a , and the cloud base updraft speed. The N a is obtained by analyzing the distribution of cloud drop effective radius in convective elements as a function of distance above cloud base. The cloud base updraft velocities are estimated by double stereoscopic viewing and tracking of the evolution of cloud surface features just above cloud base. In order to resolve the vertical dimension of the clouds, the field of view will be 100 m for the microphysical retrievals, and 50 m for the stereoscopic measurements. The viewing geometry will be eastward and 30 degrees off nadir, with the Sun in the back at 30 degrees off zenith westward, requiring a Sun-synchronous orbit at 14 LST. Measuring simultaneously the thermodynamic environment, the vertical motions of the clouds, their microstructure and the CCN concentration will allow separating the dynamics from the CCN effects. This concept is being applied in the proposed satellite mission named Clouds, Hazards and Aerosols Survey for Earth Researchers (CHASER).

Description: Evaluation of continuous water vapor δD and δ 18 O measurements by off-axis integrated cavity output spectroscopy Atmospheric Measurement Techniques, 5, 2069-2080, 2012 Author(s): N. Kurita, B. D. Newman, L. J. Araguas-Araguas, and P. Aggarwal Recent commercially available laser spectroscopy systems enabled us to continuously and reliably measure the δD and δ 18 O of atmospheric water vapor. The use of this new technology is becoming popular because of its advantages over the conventional approach based on cold trap collection. These advantages include much higher temporal resolution/continuous monitoring and the ability to make direct measurements of both isotopes in the field. Here, we evaluate the accuracy and precision of the laser based water vapor isotope instrument through a comparison of measurements with those found using the conventional cold trap method. A commercially available water vapor isotope analyzer (WVIA) with the vaporization system of a liquid water standard (Water Vapor Isotope Standard Source, WVISS) from Los Gatos Research (LGR) Inc. was used for this study. We found that the WVIA instrument can provide accurate results if (1) correction is applied for time-dependent isotope drift, (2) normalization to the VSMOW/SLAP scale is implemented, and (3) the water vapor concentration dependence of the isotopic ratio is also corrected. In addition, since the isotopic value of water vapor generated by the WVISS is also dependent on the concentration of water vapor, this effect must be considered to determine the true water vapor concentration effect on the resulting isotope measurement. To test our calibration procedure, continuous water vapor isotope measurements using both a laser instrument and a cold trap system were carried out at the IAEA Isotope Hydrology Laboratory in Vienna from August to December 2011. The calibrated isotopic values measured using the WVIA agree well with those obtained via the cold trap method. The standard deviation of the isotopic difference between both methods is about 1.4‰ for δD and 0.28‰ for δ 18 O. This precision allowed us to obtain reliable values for d -excess. The day-to-day variation of d -excess measured by WVIA also agrees well with that found using the cold trap method. These results demonstrate that a coupled system, using commercially available WVIA and WVISS instruments can provide continuous and accurate isotope data, with results achieved similar to those obtained using the conventional method, but with drastically improved temporal resolution.

Description: A disjunct eddy accumulation system for the measurement of BVOC fluxes: instrument characterizations and field deployment Atmospheric Measurement Techniques, 5, 2115-2132, 2012 Author(s): G. D. Edwards, D. K. Martins, T. Starn, K. Pratt, and P. B. Shepson Biological volatile organic compounds (BVOCs), such as isoprene and monoterpenes, are emitted in large amounts from forests. Quantification of the flux of BVOCs is critical in the evaluation of the impact of these compounds on the concentrations of atmospheric oxidants and on the production of secondary organic aerosol. A disjunct eddy accumulation (DEA) sampler system was constructed for the measurement of speciated BVOC fluxes. Unlike traditional eddy covariance (EC), the relatively new technique of disjunct sampling differs by taking short, discrete samples that allow for slower sampling frequencies. Disjunct sample airflow is directed into cartridges containing sorbent materials at sampling rates proportional to the magnitude of the vertical wind. Compounds accumulated on the cartridges are then quantified by thermal desorption and gas chromatography. Herein, we describe our initial tests to evaluate the disjunct sampler including the application of vertical wind measurements to create optimized sampling thresholds. Measurements of BVOC fluxes obtained from DEA during its deployment above a mixed hardwood forest at the University of Michigan Biological Station (Pellston, MI) during the 2009 CABINEX field campaign are reported. Daytime (09:00 a.m. to 05:00 p.m. LT) isoprene fluxes, when averaged over the footprint of the tower, were 1.31 mg m −2 h −1 which are comparable to previous flux measurements at this location. Speciated monoterpene fluxes are some of the first to be reported from this site. Daytime averages were 26.7 μg m −2 h −1 for α-pinene and 10.6 μg m −2 h −1 for β-pinene. These measured concentrations and fluxes were compared to the output of an atmospheric chemistry model, and were found to be consistent with our knowledge of the variables that control BVOCs fluxes at this site.

Description: Validation of six years of SCIAMACHY carbon monoxide observations using MOZAIC CO profile measurements Atmospheric Measurement Techniques, 5, 2133-2142, 2012 Author(s): A. T. J. de Laat, R. Dijkstra, H. Schrijver, P. Nédélec, and I. Aben This paper presents a validation study of SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY (SCIAMACHY) carbon monoxide (CO) total column measurements from the Iterative Maximum Likelihood Method (IMLM) algorithm using vertically integrated profile aircraft measurements obtained within the MOZAIC project for the six year time period of 2003–2008. Overall we find a good agreement between SCIAMACHY and airborne measurements for both mean values – also on a year-to-year basis – as well as seasonal variations. Several locations show large biases that are attributed to local effects like orography and proximity of large emission sources. Differences were detected for individual years: 2003, 2004 and 2006 have larger biases than 2005, 2007 and 2008, which appear to be related to SCIAMACHY instrumental issues but require more research. Results from this study are consistent with, and complementary to, findings from a previous validation study using ground-based measurements (de Laat et al., 2010b). According to this study, the SCIAMACHY data, if individual measurements are of sufficient quality – good signal-to-noise, can be used to determine the spatial distribution and seasonal cycles of CO total columns over clean areas. Biases found over areas with strong emissions (Africa, China) could be explained by low sensitivity of the instrument in the boundary layer and users are recommended to avoid using the SCIAMACHY data while trying to quantify CO burden and/or retrieve CO emissions in such areas.

Description: Corrigendum to "Trend analysis of aerosol optical thickness and Ångström exponent derived from the global AERONET spectral observations" published in Atmos. Meas. Tech., 5, 1271–1299, 2012 Atmospheric Measurement Techniques, 5, 2113-2113, 2012 Author(s): J. Yoon, W. von Hoyningen-Huene, A. A. Kokhanovsky, M. Vountas, and J. P. Burrows No abstract available.

Description: Using sonic anemometer temperature to measure sensible heat flux in strong winds Atmospheric Measurement Techniques, 5, 2095-2111, 2012 Author(s): S. P. Burns, T. W. Horst, L. Jacobsen, P. D. Blanken, and R. K. Monson Sonic anemometers simultaneously measure the turbulent fluctuations of vertical wind ( w ') and sonic temperature ( T s '), and are commonly used to measure sensible heat flux ( H ). Our study examines 30-min heat fluxes measured with a Campbell Scientific CSAT3 sonic anemometer above a subalpine forest. We compared H calculated with T s to H calculated with a co-located thermocouple and found that, for horizontal wind speed ( U ) less than 8 m s −1 , the agreement was around ±30 W m −2 . However, for U ≈ 8 m s −1 , the CSAT H had a generally positive deviation from H calculated with the thermocouple, reaching a maximum difference of ≈250 W m −2 at U ≈ 18 m s −1 . With version 4 of the CSAT firmware, we found significant underestimation of the speed of sound and thus T s in high winds (due to a delayed detection of the sonic pulse), which resulted in the large CSAT heat flux errors. Although this T s error is qualitatively similar to the well-known fundamental correction for the crosswind component, it is quantitatively different and directly related to the firmware estimation of the pulse arrival time. For a CSAT running version 3 of the firmware, there does not appear to be a significant underestimation of T s ; however, a T s error similar to that of version 4 may occur if the CSAT is sufficiently out of calibration. An empirical correction to the CSAT heat flux that is consistent with our conceptual understanding of the T s error is presented. Within a broader context, the surface energy balance is used to evaluate the heat flux measurements, and the usefulness of side-by-side instrument comparisons is discussed.

Description: Remote sensing of near-infrared chlorophyll fluorescence from space in scattering atmospheres: implications for its retrieval and interferences with atmospheric CO 2 retrievals Atmospheric Measurement Techniques, 5, 2081-2094, 2012 Author(s): C. Frankenberg, C. O'Dell, L. Guanter, and J. McDuffie With the advent of dedicated greenhouse gas space-borne spectrometers sporting high resolution spectra in the O 2 A-band spectral region (755–774 nm), the retrieval of chlorophyll fluorescence has become feasible on a global scale. If unaccounted for, however, fluorescence can indirectly perturb the greenhouse gas retrievals as it perturbs the oxygen absorption features. As atmospheric CO 2 measurements are used to invert net fluxes at the land–atmosphere interface, a bias caused by fluorescence can be crucial as it will spatially correlate with the fluxes to be inverted. Avoiding a bias and retrieving fluorescence accurately will provide additional constraints on both the net and gross fluxes in the global carbon cycle. We show that chlorophyll fluorescence, if neglected, systematically interferes with full-physics multi-band X CO 2 retrievals using the O 2 A-band. Systematic biases in X CO 2 can amount to +1 ppm if fluorescence constitutes 1% to the continuum level radiance. We show that this bias can be largely eliminated by simultaneously fitting fluorescence in a full-physics based retrieval. If fluorescence is the primary target, a dedicated but very simple retrieval based purely on Fraunhofer lines is shown to be more accurate and very robust even in the presence of large scattering optical depths. We find that about 80% of the surface fluorescence is retained at the top-of-atmosphere, even for cloud optical thicknesses around 2–5. We further show that small instrument modifications to future O 2 A-band spectrometer spectral ranges can result in largely reduced random errors in chlorophyll fluorescence, paving the way towards a more dedicated instrument exploiting solar absorption features only.

Description: Airborne intercomparison of HO x measurements using laser-induced fluorescence and chemical ionization mass spectrometry during ARCTAS Atmospheric Measurement Techniques, 5, 2025-2037, 2012 Author(s): X. Ren, J. Mao, W. H. Brune, C. A. Cantrell, R. L. Mauldin III, R. S. Hornbrook, E. Kosciuch, J. R. Olson, J. H. Crawford, G. Chen, and H. B. Singh The hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals, collectively called HO x , play central roles in tropospheric chemistry. Accurate measurements of OH and HO 2 are critical to examine our understanding of atmospheric chemistry. Intercomparisons of different techniques for detecting OH and HO 2 are vital to evaluate their measurement capabilities. Three instruments that measured OH and/or HO 2 radicals were deployed on the NASA DC-8 aircraft throughout Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) in the spring and summer of 2008. One instrument was the Penn State Airborne Tropospheric Hydrogen Oxides Sensor (ATHOS) for OH and HO 2 measurements based on Laser-Induced Fluorescence (LIF) spectroscopy. A second instrument was the NCAR Selected-Ion Chemical Ionization Mass Spectrometer (SI-CIMS) for OH measurement. A third instrument was the NCAR Peroxy Radical Chemical Ionization Mass Spectrometer (PeRCIMS) for HO 2 measurement. Formal intercomparison of LIF and CIMS was conducted for the first time on a same aircraft platform. The three instruments were calibrated by quantitative photolysis of water vapor by ultraviolet (UV) light at 184.9 nm with three different calibration systems. The absolute accuracies were ±32% (2σ) for the LIF instrument, ±65% (2σ) for the SI-CIMS instrument, and ±50% (2σ) for the PeRCIMS instrument. In general, good agreement was obtained between the CIMS and LIF measurements of both OH and HO 2 measurements. Linear regression of the entire data set yields [OH] CIMS = 0.89 × [OH] LIF + 2.8 × 10 4 cm −3 with a correlation coefficient r 2 = 0.72 for OH, and [HO 2 ] CIMS = 0.86 × [HO 2 ] LIF + 3.9 parts per trillion by volume (pptv, equivalent to pmol mol −1 ) with a correlation coefficient r 2 = 0.72 for HO 2 . In general, the difference between CIMS and LIF instruments for OH and HO 2 measurements can be explained by their combined measurement uncertainties. Comparison with box model results shows some similarities for both the CIMS and LIF measurements. First, the observed-to-modeled HO 2 ratio increases greatly for higher NO mixing ratios, indicating that the model may not properly account for HO x sources that correlate with NO. Second, the observed-to-modeled OH ratio increases with increasing isoprene mixing ratios, suggesting either incomplete understanding of isoprene chemistry in the model or interferences in the measurements in environments where biogenic emissions dominate ambient volatile organic compounds.

Description: Consistency between Fourier transform and small-volume few-wave decomposition for spectral and spatial variability of gravity waves above a typhoon Atmospheric Measurement Techniques, 5, 1637-1651, 2012 Author(s): C. I. Lehmann, Y.-H. Kim, P. Preusse, H.-Y. Chun, M. Ern, and S.-Y. Kim Convective gravity wave (GW) sources are spatially localized and emit at the same time waves with a wide spectrum of phase speeds. Any wave analysis therefore compromises between spectral and spatial resolution. Future satellite borne limb imagers will for a first time provide real 3-D volumes of observations. These volumes will be however limited which will impose further constraints on the analysis technique. In this study a three dimensional few-wave approach fitting sinusoidal waves to limited 3-D volumes is introduced. The method is applied to simulated GWs above typhoon Ewiniar and GW momentum flux is estimated from temperature fluctuations. Phase speed spectra as well as average profiles of positive, negative and net momentum fluxes are compared to momentum flux estimated by Fourier transform as well as spatial averaging of wind fluctuations. The results agree within 10–20%. The few-wave method can also reveal the spatial orientation of the GWs with respect to the source. The relevance of the results for different types of measurements as well as its applicability to model data is discussed.

Description: Satellite retrieval of the liquid water fraction in tropical clouds between −20 and −38 °C Atmospheric Measurement Techniques, 5, 1683-1698, 2012 Author(s): D. L. Mitchell and R. P. d'Entremont This study describes a satellite remote sensing method for directly retrieving the liquid water fraction in mixed phase clouds, and appears unique in this respect. The method uses MODIS split-window channels for retrieving the liquid fraction from cold clouds where the liquid water fraction is less than 50% of the total condensate. This makes use of the observation that clouds only containing ice exhibit effective 12-to-11 μm absorption optical thickness ratios (β eff ) that are quasi-constant with retrieved cloud temperature T . This observation was made possible by using two CO 2 channels to retrieve T and then using the 12 and 11 μm channels to retrieve emissivities and β eff . Thus for T 〈 −40 °C, β eff is constant, but for T 〉 −40 °C, β eff slowly increases due to the presence of liquid water, revealing mean liquid fractions of ~ 10% around −22 °C from tropical clouds identified as cirrus by the cloud mask. However, the uncertainties for these retrievals are large, and extensive in situ measurements are needed to refine and validate these retrievals. Such liquid levels are shown to reduce the cloud effective diameter D e such that cloud optical thickness will increase by more than 50% for a given water path, relative to D e corresponding to pure ice clouds. Such retrieval information is needed for validation of the cloud microphysics in climate models. Since low levels of liquid water can dominate cloud optical properties, tropical clouds between −25 and −20 °C may be susceptible to the first aerosol indirect effect.

Description: Carbon monoxide measurements onboard the CARIBIC passenger aircraft using UV resonance fluorescence Atmospheric Measurement Techniques, 5, 1753-1760, 2012 Author(s): D. Scharffe, F. Slemr, C. A. M. Brenninkmeijer, and A. Zahn Goal of the project CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrumented Container) is to carry out regular and detailed observations of atmospheric composition (particles and gases) at cruising altitudes of passenger aircraft, i.e. at 9–12 km. Continuous, fast measurement of CO is indispensable for the chemical characterization of encountered air masses, for the detection of plumes of polluted air and for studying troposphere-stratosphere transport. CO is measured by a commercial resonance fluorescence UV instrument modified for the use onboard passenger aircraft. Modifications were necessary to optimize the instrument reliability allowing unattended operation for several days. The instrument has a precision of 1–2 ppbv at an integration time of 1 s. The response time to reach 63.2% signal strength is 2 s. We describe the modifications of the instrument, the experiences made during its operation since December 2004, the quality control of CO measurements onboard CARIBIC, and suggest a regular service routine that guarantees long-term high-quality data.

Description: Evaluation of turbulent dissipation rate retrievals from Doppler Cloud Radar Atmospheric Measurement Techniques, 5, 1375-1385, 2012 Author(s): M. D. Shupe, I. M. Brooks, and G. Canut Turbulent dissipation rate retrievals from cloud radar Doppler velocity measurements are evaluated using independent, in situ observations in Arctic stratocumulus clouds. In situ validation data sets of dissipation rate are derived using sonic anemometer measurements from a tethered balloon and high frequency pressure variation observations from a research aircraft, both flown in proximity to stationary, ground-based radars. Modest biases are found among the data sets in particularly low- or high-turbulence regimes, but in general the radar-retrieved values correspond well with the in situ measurements. Root mean square differences are typically a factor of 4–6 relative to any given magnitude of dissipation rate. These differences are no larger than those found when comparing dissipation rates computed from tethered-balloon and meteorological tower-mounted sonic anemometer measurements made at spatial distances of a few hundred meters. Temporal lag analyses suggest that approximately half of the observed differences are due to spatial sampling considerations, such that the anticipated radar-based retrieval uncertainty is on the order of a factor of 2–3. Moreover, radar retrievals are clearly able to capture the vertical dissipation rate structure observed by the in situ sensors, while offering substantially more information on the time variability of turbulence profiles. Together these evaluations indicate that radar-based retrievals can, at a minimum, be used to determine the vertical structure of turbulence in Arctic stratocumulus clouds.

Description: CH 4 , CO, and H 2 O spectroscopy for the Sentinel-5 Precursor mission: an assessment with the Total Carbon Column Observing Network measurements Atmospheric Measurement Techniques, 5, 1387-1398, 2012 Author(s): A. Galli, A. Butz, R. A. Scheepmaker, O. Hasekamp, J. Landgraf, P. Tol, D. Wunch, N. M. Deutscher, G. C. Toon, P. O. Wennberg, D. W. T. Griffith, and I. Aben The TROPOspheric Monitoring Instrument (TROPOMI) will be part of ESA's Sentinel-5 Precursor (S5P) satellite platform scheduled for launch in 2015. TROPOMI will monitor methane and carbon monoxide concentrations in the Earth's atmosphere by measuring spectra of back-scattered sunlight in the short-wave infrared (SWIR). S5P will be the first satellite mission to rely uniquely on the spectral window at 4190–4340 cm −1 (2.3 μm) to retrieve CH 4 and CO. In this study, we investigated if the absorption features of the three relevant molecules CH 4 , CO, and H 2 O are adequately known. To this end, we retrieved total columns of CH 4 , CO, and H 2 O from absorption spectra measured by two ground-based Fourier transform spectrometers that are part of the Total Carbon Column Observing Network (TCCON). The retrieval results from the 4190–4340 cm −1 range at the TROPOMI resolution (0.45 cm −1 ) were then compared to the CH 4 results obtained from the 6000 cm −1 region, and the CO results obtained from the 4190–4340 cm −1 region at the higher TCCON resolution (0.02 cm −1 ). For TROPOMI-like settings, we were able to reproduce the CH 4 columns to an accuracy of 0.3% apart from a constant bias of 1%. The CO retrieval accuracy was, through interference, systematically influenced by the shortcomings of the CH 4 and H 2 O spectroscopy. In contrast to CH 4 , the CO column error also varied significantly with atmospheric H 2 O content. Unaddressed, this would introduce seasonal and latitudinal biases to the CO columns retrieved from TROPOMI measurements. We therefore recommend further effort from the spectroscopic community to be directed at the H 2 O and CH 4 spectroscopy in the 4190–4340 cm −1 region.

Description: New Aura Microwave Limb Sounder observations of BrO and implications for Br y Atmospheric Measurement Techniques, 5, 1741-1751, 2012 Author(s): L. Millán, N. Livesey, W. Read, L. Froidevaux, D. Kinnison, R. Harwood, I. A. MacKenzie, and M. P. Chipperfield This paper introduces a new inversion algorithm for retrievals of stratospheric BrO from the Aura Microwave Limb Sounder. This version is based on the algorithm described by Livesey et al. (2006a) but uses a more realistic atmospheric state to constrain the retrieval. A description of the methodology and an error analysis are presented. Single daily profile precision uncertainty, when taking the ascending-descending (day-night) difference, was found to be up to 40 pptv while systematic error biases were estimated to be less than about 3 pptv. Monthly mean comparisons show broad agreement with other measurements as well as with state-of-the-art numerical models. We infer a 2005 yearly total inorganic Br y using the measured MLS BrO to be 20.3 ± 4.5 pptv, which implies a contribution from very short lived substances to the stratospheric bromine budget of 5 ± 4.5 pptv.

Description: Multi-wavelength Raman lidar, sun photometric and aircraft measurements in combination with inversion models for the estimation of the aerosol optical and physico-chemical properties over Athens, Greece Atmospheric Measurement Techniques, 5, 1793-1808, 2012 Author(s): R. E. Mamouri, A. Papayannis, V. Amiridis, D. Müller, P. Kokkalis, S. Rapsomanikis, E. T. Karageorgos, G. Tsaknakis, A. Nenes, S. Kazadzis, and E. Remoundaki A novel procedure has been developed to retrieve, simultaneously, the optical, microphysical and chemical properties of tropospheric aerosols with a multi-wavelength Raman lidar system in the troposphere over an urban site (Athens, Greece: 37.9° N, 23.6° E, 200 m a.s.l.) using data obtained during the European Space Agency (ESA) THERMOPOLIS project, which took place between 15–31 July 2009 over the Greater Athens Area (GAA). We selected to apply our procedure for a case study of intense aerosol layers that occurred on 20–21 July 2009. The National Technical University of Athens (NTUA) EOLE 6-wavelength Raman lidar system has been used to provide the vertical profiles of the optical properties of aerosols (extinction and backscatter coefficients, lidar ratio) and the water vapor mixing ratio. An inversion algorithm was used to derive the mean aerosol microphysical properties (mean effective radius ( r eff ), single-scattering albedo ω ) and mean complex refractive index ( m )) at selected heights in the 2–3 km height region. We found that r eff was 0.14–0.4 (±0.14) μm, ω was 0.63–0.88 (±0.08) (at 532 nm) and m ranged from 1.44 (±0.10) + 0.01 (±0.01) i to 1.55 (±0.12) + 0.06 (±0.02) i , in good agreement (only for the r eff values) with in situ aircraft measurements. The water vapor and temperature profiles were incorporated into the ISORROPIA II model to propose a possible in situ aerosol composition consistent with the retrieved m and ω values. The retrieved aerosol chemical composition in the 2–3 km height region gave a variable range of sulfate (0–60%) and organic carbon (OC) content (0–50%), although the OC content increased (up to 50%) and the sulfate content dropped (up to 30%) around 3 km height; the retrieved low ω value (0.63), indicates the presence of absorbing biomass burning smoke mixed with urban haze. Finally, the retrieved aerosol microphysical properties were compared with column-integrated sun photometer CIMEL data.

Description: The RAMNI airborne lidar for cloud and aerosol research Atmospheric Measurement Techniques, 5, 1779-1792, 2012 Author(s): F. Cairo, G. Di Donfrancesco, L. Di Liberto, and M. Viterbini We describe an airborne lidar for the characterization of atmospheric aerosol. The system has been set up in response to the need to monitor extended regions where the air traffic may be posed at risk by the presence of potentially harmful volcanic ash, and to study the characteristics of volcanic emissions both near the source region and when transported over large distances. The lidar provides backscatter and linear depolarization profiles at 532 nm, from which aerosol and cloud properties can be derived. The paper presents the characteristics and capabilities of the lidar system and gives examples of its airborne deployment. Observations from three flights, aimed at assessing the system capabilities in unperturbed atmospheric conditions, and at characterizing the emissions near a volcanic ash source (Mt. Etna) and transported far away from the source, are presented and discussed.

Description: Characterization of atmospheric aerosol in the US Southeast from ground- and space-based measurements over the past decade Atmospheric Measurement Techniques, 5, 1667-1682, 2012 Author(s): E. J. Alston, I. N. Sokolik, and O. V. Kalashnikova This study examines how aerosols measured from the ground and space over the US Southeast change temporally over a regional scale during the past decade. PM 2.5 (particulate matter with aerodynamic diameter 〉2.5 micrometers) data consist of two datasets that represent the measurements that are used for regulatory purposes by the US EPA (Environmental Protection Agency) and continuous measurements used for quickly disseminating air quality information. AOD (aerosol optical depth) data come from three NASA sensors: the MODIS sensors onboard Terra and Aqua satellites and the MISR sensor onboard the Terra satellite. We analyze all available data over the state of Georgia from 2000–2009 of both types of aerosol data. The analysis reveals that during the summer the large metropolitan area of Atlanta has average PM 2.5 concentrations that are 50% more than the remainder of the state. Strong seasonality is detected in both the AOD and PM 2.5 datasets, as evidenced by a threefold increase of AOD from mean winter values to mean summer values, and the increase in PM 2.5 concentrations is almost twofold over the same period. Additionally, there is agreement between MODIS and MISR onboard the Terra satellite during the spring and summer, having correlation coefficients of 0.64 and 0.71, respectively. Monthly anomalies were used to determine the presence of a trend in all considered aerosol datasets. We found negative linear trends for both the monthly AOD anomalies from MODIS onboard Terra and the PM 2.5 datasets, which are statistically significant. Decreasing trends were also found for MISR onboard Terra and MODIS onboard Aqua, but those trends were not statistically significant. The observed decrease in AOD and PM 2.5 concentrations may be indicative of the brightening over the study region during the past decade.

Description: Eddy-covariance flux measurements with a weight-shift microlight aircraft Atmospheric Measurement Techniques, 5, 1699-1717, 2012 Author(s): S. Metzger, W. Junkermann, M. Mauder, F. Beyrich, K. Butterbach-Bahl, H. P. Schmid, and T. Foken The objective of this study is to assess the feasibility and quality of eddy-covariance flux measurements from a weight-shift microlight aircraft (WSMA). Firstly, we investigate the precision of the wind measurement (σ u,v ≤ 0.09 m s −1 , σ w = 0.04 m s −1 ), the lynchpin of flux calculations from aircraft. From here, the smallest resolvable changes in friction velocity (0.02 m s −1 ), and sensible- (5 W m −2 ) and latent (3 W m −2 ) heat flux are estimated. Secondly, a seven-day flight campaign was performed near Lindenberg (Germany). Here we compare measurements of wind, temperature, humidity and respective fluxes between a tall tower and the WSMA. The maximum likelihood functional relationship (MLFR) between tower and WSMA measurements considers the random error in the data, and shows very good agreement of the scalar averages. The MLFRs for standard deviations (SDs, 2–34%) and fluxes (17–21%) indicate higher estimates of the airborne measurements compared to the tower. Considering the 99.5% confidence intervals, the observed differences are not significant, with exception of the temperature SD. The comparison with a large-aperture scintillometer reveals lower sensible heat flux estimates at both tower (−40 to −25%) and WSMA (−25–0%). We relate the observed differences to (i) inconsistencies in the temperature and wind measurement at the tower and (ii) the measurement platforms' differing abilities to capture contributions from non-propagating eddies. These findings encourage the use of WSMA as a low cost and highly versatile flux measurement platform.

Description: Global and regional evaluation of over-land spectral aerosol optical depth retrievals from SeaWiFS Atmospheric Measurement Techniques, 5, 1761-1778, 2012 Author(s): A. M. Sayer, N. C. Hsu, C. Bettenhausen, M.-J. Jeong, B. N. Holben, and J. Zhang This study evaluates a new spectral aerosol optical depth (AOD) dataset derived from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) measurements over land. First, the data are validated against Aerosol Robotic Network (AERONET) direct-sun AOD measurements and found to compare well on a global basis. If only data with the highest quality flag are used, the correlation is 0.86 and 72% of matchups fall within an expected absolute uncertainty of 0.05 + 20% (for the wavelength of 550 nm). The quality is similar at other wavelengths and stable over the 13-yr (1997–2010) mission length. Performance tends to be better over vegetated, low-lying terrain with typical AOD of 0.3 or less, such as found over much of North America and Eurasia. Performance tends to be poorer for low-AOD conditions near backscattering geometries, where SeaWiFS overestimates AOD, or optically-thick cases of absorbing aerosol, where SeaWiFS tends to underestimate AOD. Second, the SeaWiFS data are compared with midvisible AOD derived from the Moderate Resolution Imaging Spectrometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR). All instruments show similar spatial and seasonal distributions of AOD, although there are regional and seasonal offsets between them. At locations where AERONET data are available, these offsets are largely consistent with the known validation characteristics of each dataset. With the results of this study in mind, the SeaWiFS over-land AOD record is suitable for quantitative scientific use.

Description: Critical surface albedo and its implications to aerosol remote sensing Atmospheric Measurement Techniques, 5, 1653-1665, 2012 Author(s): F. C. Seidel and C. Popp We analyse the critical surface albedo (CSA) and its implications to aerosol remote sensing. CSA is defined as the surface albedo where the reflectance at top-of-atmosphere (TOA) does not depend on aerosol optical depth (AOD). AOD retrievals are therefore inaccurate at the CSA. The CSA is obtained by derivatives of the TOA reflectance with respect to AOD using a radiative transfer code. We present the CSA and the effect of surface albedo uncertainties on AOD retrieval and atmospheric correction as a function of aerosol single-scattering albedo, illumination and observation geometry, wavelength and AOD. In general, increasing aerosol absorption and increasing scattering angles lead to lower CSA. In contrast to the strict definition of the CSA, we show that the CSA can also slightly depend on AOD and therefore rather represent a small range of surface albedo values. This was often neglected in previous studies. The following implications to aerosol remote sensing applications were found: (i) surface albedo uncertainties result in large AOD retrieval errors, particularly close to the CSA; (ii) AOD retrievals of weakly or non-absorbing aerosols require dark surfaces, while strongly absorbing aerosols can be retrieved more accurately over bright surfaces; (iii) the CSA may help to estimate aerosol absorption; and (iv) the presented sensitivity of the reflectance at TOA to AOD provides error estimations to optimise AOD retrieval algorithms.

Description: Assessment of two aerosol optical thickness retrieval algorithms applied to MODIS Aqua and Terra measurements in Europe Atmospheric Measurement Techniques, 5, 1727-1740, 2012 Author(s): P. Glantz and M. Tesche The aim of the present study is to validate AOT (aerosol optical thickness) and Ångström exponent (α), obtained from MODIS (MODerate resolution Imaging Spectroradiometer) Aqua and Terra calibrated level 1 data (1 km horizontal resolution at ground) with the SAER (Satellite AErosol Retrieval) algorithm and with MODIS Collection 5 (c005) standard product retrievals (10 km horizontal resolution), against AERONET (AErosol RObotic NETwork) sun photometer observations over land surfaces in Europe. An inter-comparison of AOT at 0.469 nm obtained with the two algorithms has also been performed. The time periods investigated were chosen to enable a validation of the findings of the two algorithms for a maximal possible variation in sun elevation. The satellite retrievals were also performed with a significant variation in the satellite-viewing geometry, since Aqua and Terra passed the investigation area twice a day for several of the cases analyzed. The validation with AERONET shows that the AOT at 0.469 and 0.555 nm obtained with MODIS c005 is within the expected uncertainty of one standard deviation of the MODIS c005 retrievals (ΔAOT = ± 0.05 ± 0.15 · AOT). The AOT at 0.443 nm retrieved with SAER, but with a much finer spatial resolution, also agreed reasonably well with AERONET measurements. The majority of the SAER AOT values are within the MODIS c005 expected uncertainty range, although somewhat larger average absolute deviation occurs compared to the results obtained with the MODIS c005 algorithm. The discrepancy between AOT from SAER and AERONET is, however, substantially larger for the wavelength 488 nm. This means that the values are, to a larger extent, outside of the expected MODIS uncertainty range. In addition, both satellite retrieval algorithms are unable to estimate α accurately, although the MODIS c005 algorithm performs better. Based on the inter-comparison of the SAER and MODIS c005 algorithms, it was found that SAER on the whole is able to obtain results within the expected uncertainty range of MODIS Aqua and Terra observations.

Description: On-line determination of ammonia at low pptv mixing ratios in the CLOUD chamber Atmospheric Measurement Techniques, 5, 1719-1725, 2012 Author(s): F. Bianchi, J. Dommen, S. Mathot, and U. Baltensperger A new instrument for the on-line determination of ammonia was developed. Since ammonia is a rather sticky compound, sampling losses were minimised with a new sampling device where the ammonia was transferred to the liquid phase only 5 mm after the inlet tip. The liquid phase was then analyzed by long pathlength absorption spectrophotometry using the Berthelot reaction with phenol and hypochlorite as reagents. The measurements were made during the CLOUD3 campaign at CERN where the influence of ammonia on the nucleation rate was studied. At stable conditions the detection limit reached with this instrument was 35 pptv (air flow rate of 2 l min −1 , liquid flow rate of 0.3 ml min −1 ), although occasionally the instrument was affected by background problems. The range of mixing ratios during this campaign was varied from the background contamination (

Description: Dimethylamine and ammonia measurements with ion chromatography during the CLOUD4 campaign Atmospheric Measurement Techniques, 5, 2161-2167, 2012 Author(s): A. P. Praplan, F. Bianchi, J. Dommen, and U. Baltensperger The CLOUD project investigates the influence of galactic cosmic rays on the nucleation of new particles in an environmental chamber at CERN. Dimethylamine (DMA) was injected intentionally into the CLOUD chamber to reach atmospherically relevant levels away from sources (up to 100 pptv) in order to study its effect on nucleation with sulphuric acid and water at 278 K. Quantification of DMA and also background ammonia (NH 3 ) was performed with ion chromatography (IC). The IC method used together with the sampling line developed for CLOUD in order to measure NH 3 and DMA at low pptv levels is described; the overall sampling efficiency of the method is discussed; and, finally, mixing ratios of NH 3 and DMA measured during CLOUD4 are reported.

Description: Geophysical validation and long-term consistency between GOME-2/MetOp-A total ozone column and measurements from the sensors GOME/ERS-2, SCIAMACHY/ENVISAT and OMI/Aura Atmospheric Measurement Techniques, 5, 2169-2181, 2012 Author(s): M. E. Koukouli, D. S. Balis, D. Loyola, P. Valks, W. Zimmer, N. Hao, J.-C. Lambert, M. Van Roozendael, C. Lerot, and R. J. D. Spurr The main aim of the paper is to assess the consistency of five years of Global Ozone Monitoring Experiment-2/Metop-A [GOME-2] total ozone columns and the long-term total ozone satellite monitoring database already in existence through an extensive inter-comparison and validation exercise using as reference Brewer and Dobson ground-based measurements. The behaviour of the GOME-2 measurements is being weighed against that of GOME (1995–2011), Ozone Monitoring Experiment [OMI] (since 2004) and the Scanning Imaging Absorption spectroMeter for Atmospheric CartograpHY [SCIAMACHY] (since 2002) total ozone column products. Over the background truth of the ground-based measurements, the total ozone columns are inter-evaluated using a suite of established validation techniques; the GOME-2 time series follow the same patterns as those observed by the other satellite sensors. In particular, on average, GOME-2 data underestimate GOME data by about 0.80%, and underestimate SCIAMACHY data by 0.37% with no seasonal dependence of the differences between GOME-2, GOME and SCIAMACHY. The latter is expected since the three datasets are based on similar DOAS algorithms. This underestimation of GOME-2 is within the uncertainty of the reference data used in the comparisons. Compared to the OMI sensor, on average GOME-2 data underestimate OMI_DOAS (collection 3) data by 1.28%, without any significant seasonal dependence of the differences between them. The lack of seasonality might be expected since both the GOME data processor [GDP] 4.4 and OMI_DOAS are DOAS-type algorithms and both consider the variability of the stratospheric temperatures in their retrievals. Compared to the OMI_TOMS (collection 3) data, no bias was found. We hence conclude that the GOME-2 total ozone columns are well suitable to continue the long-term global total ozone record with the accuracy needed for climate monitoring studies.

Description: Polarimetric X-band weather radar measurements in the tropics: radome and rain attenuation correction Atmospheric Measurement Techniques, 5, 2183-2199, 2012 Author(s): M. Schneebeli, J. Sakuragi, T. Biscaro, C. F. Angelis, I. Carvalho da Costa, C. Morales, L. Baldini, and L. A. T. Machado A polarimetric X-band radar has been deployed during one month (April 2011) for a field campaign in Fortaleza, Brazil, together with three additional laser disdrometers. The disdrometers are capable of measuring the raindrop size distributions (DSDs), hence making it possible to forward-model theoretical polarimetric X-band radar observables at the point where the instruments are located. This set-up allows to thoroughly test the accuracy of the X-band radar measurements as well as the algorithms that are used to correct the radar data for radome and rain attenuation. For the campaign in Fortaleza it was found that radome attenuation dominantly affects the measurements. With an algorithm that is based on the self-consistency of the polarimetric observables, the radome induced reflectivity offset was estimated. Offset corrected measurements were then further corrected for rain attenuation with two different schemes. The performance of the post-processing steps was analyzed by comparing the data with disdrometer-inferred polarimetric variables that were measured at a distance of 20 km from the radar. Radome attenuation reached values up to 14 dB which was found to be consistent with an empirical radome attenuation vs. rain intensity relation that was previously developed for the same radar type. In contrast to previous work, our results suggest that radome attenuation should be estimated individually for every view direction of the radar in order to obtain homogenous reflectivity fields.

Description: A compact, fast UV photometer for measurement of ozone from research aircraft Atmospheric Measurement Techniques, 5, 2201-2210, 2012 Author(s): R. S. Gao, J. Ballard, L. A. Watts, T. D. Thornberry, S. J. Ciciora, R. J. McLaughlin, and D. W. Fahey In situ measurements of atmospheric ozone (O 3 ) are performed routinely from many research aircraft platforms. The most common technique depends on the strong absorption of ultraviolet (UV) light by ozone. As atmospheric science advances to the widespread use of unmanned aircraft systems (UASs), there is an increasing requirement for minimizing instrument space, weight, and power while maintaining instrument accuracy, precision and time response. The design and use of a new, dual-beam, UV photometer instrument for in situ O 3 measurements is described. A polarization optical-isolator configuration is utilized to fold the UV beam inside the absorption cells, yielding a 60-cm absorption length with a 30-cm cell. The instrument has a fast sampling rate (2 Hz at

Description: High-resolution NO 2 remote sensing from the Airborne Prism EXperiment (APEX) imaging spectrometer Atmospheric Measurement Techniques, 5, 2211-2225, 2012 Author(s): C. Popp, D. Brunner, A. Damm, M. Van Roozendael, C. Fayt, and B. Buchmann We present and evaluate the retrieval of high spatial resolution maps of NO 2 vertical column densities (VCD) from the Airborne Prism EXperiment (APEX) imaging spectrometer. APEX is a novel instrument providing airborne measurements of unique spectral and spatial resolution and coverage as well as high signal stability. In this study, we use spectrometer data acquired over Zurich, Switzerland, in the morning and late afternoon during a flight campaign on a cloud-free summer day in June 2010. NO 2 VCD are derived with a two-step approach usually applied to satellite NO 2 retrievals, i.e. a DOAS analysis followed by air mass factor calculations based on radiative transfer computations. Our analysis demonstrates that APEX is clearly sensitive to NO 2 VCD above typical European tropospheric background abundances (〉1 × 10 15 molec cm −2 ). The two-dimensional maps of NO 2 VCD reveal a very convincing spatial distribution with strong gradients around major NO x sources (e.g. Zurich airport, waste incinerator, motorways) and low NO 2 in remote areas. The morning overflights resulted in generally higher NO 2 VCD and a more distinct pattern than the afternoon overflights which can be attributed to the meteorological conditions prevailing during that day with stronger winds and hence larger dilution in the afternoon. The remotely sensed NO 2 VCD are also in reasonably good agreement with ground-based in-situ measurements from air quality networks considering the limitations of comparing column integrals with point measurements. Airborne NO 2 remote sensing using APEX will be valuable to detect NO 2 emission sources, to provide input for NO 2 emission modelling, and to establish links between in-situ measurements, air quality models, and satellite NO 2 products.

Description: Intercomparison of desert dust optical depth from satellite measurements Atmospheric Measurement Techniques, 5, 1973-2002, 2012 Author(s): E. Carboni, G. E. Thomas, A. M. Sayer, R. Siddans, C. A. Poulsen, R. G. Grainger, C. Ahn, D. Antoine, S. Bevan, R. Braak, H. Brindley, S. DeSouza-Machado, J. L. Deuzé, D. Diner, F. Ducos, W. Grey, C. Hsu, O. V. Kalashnikova, R. Kahn, P. R. J. North, C. Salustro, A. Smith, D. Tanré, O. Torres, and B. Veihelmann This work provides a comparison of satellite retrievals of Saharan desert dust aerosol optical depth (AOD) during a strong dust event through March 2006. In this event, a large dust plume was transported over desert, vegetated, and ocean surfaces. The aim is to identify the differences between current datasets. The satellite instruments considered are AATSR, AIRS, MERIS, MISR, MODIS, OMI, POLDER, and SEVIRI. An interesting aspect is that the different algorithms make use of different instrument characteristics to obtain retrievals over bright surfaces. These include multi-angle approaches (MISR, AATSR), polarisation measurements (POLDER), single-view approaches using solar wavelengths (OMI, MODIS), and the thermal infrared spectral region (SEVIRI, AIRS). Differences between instruments, together with the comparison of different retrieval algorithms applied to measurements from the same instrument, provide a unique insight into the performance and characteristics of the various techniques employed. As well as the intercomparison between different satellite products, the AODs have also been compared to co-located AERONET data. Despite the fact that the agreement between satellite and AERONET AODs is reasonably good for all of the datasets, there are significant differences between them when compared to each other, especially over land. These differences are partially due to differences in the algorithms, such as assumptions about aerosol model and surface properties. However, in this comparison of spatially and temporally averaged data, it is important to note that differences in sampling, related to the actual footprint of each instrument on the heterogeneous aerosol field, cloud identification and the quality control flags of each dataset can be an important issue.

Description: A Raman lidar at La Reunion (20.8° S, 55.5° E) for monitoring water vapour and cirrus distributions in the subtropical upper troposphere: preliminary analyses and description of a future system Atmospheric Measurement Techniques, 5, 1333-1348, 2012 Author(s): C. Hoareau, P. Keckhut, J.-L. Baray, L. Robert, Y. Courcoux, J. Porteneuve, H. Vömel, and B. Morel A ground-based Rayleigh lidar has provided continuous observations of tropospheric water vapour profiles and cirrus cloud using a preliminary Raman channels setup on an existing Rayleigh lidar above La Reunion over the period 2002–2005. With this instrument, we performed a first measurement campaign of 350 independent water vapour profiles. A statistical study of the distribution of water vapour profiles is presented and some investigations concerning the calibration are discussed. Analysis regarding the cirrus clouds is presented and a classification has been performed showing 3 distinct classes. Based on these results, the characteristics and the design of a future lidar system, to be implemented at the new Reunion Island altitude observatory (2200 m) for long-term monitoring, is presented and numerical simulations of system performance have been realised to compare both instruments.

Description: Observing ice clouds in the submillimeter spectral range: the CloudIce mission proposal for ESA's Earth Explorer 8 Atmospheric Measurement Techniques, 5, 1529-1549, 2012 Author(s): S. A. Buehler, E. Defer, F. Evans, S. Eliasson, J. Mendrok, P. Eriksson, C. Lee, C. Jiménez, C. Prigent, S. Crewell, Y. Kasai, R. Bennartz, and A. J. Gasiewski Passive submillimeter-wave sensors are a way to obtain urgently needed global data on ice clouds, particularly on the so far poorly characterized "essential climate variable" ice water path (IWP) and on ice particle size. CloudIce was a mission proposal to the European Space Agency ESA in response to the call for Earth Explorer 8 (EE8), which ran in 2009/2010. It proposed a passive submillimeter-wave sensor with channels ranging from 183 GHz to 664 GHz. The article describes the CloudIce mission proposal, with particular emphasis on describing the algorithms for the data-analysis of submillimeter-wave cloud ice data (retrieval algorithms) and demonstrating their maturity. It is shown that we have a robust understanding of the radiative properties of cloud ice in the millimeter/submillimeter spectral range, and that we have a proven toolbox of retrieval algorithms to work with these data. Although the mission was not selected for EE8, the concept will be useful as a reference for other future mission proposals.

Description: Site selective real-time measurements of atmospheric N 2 O isotopomers by laser spectroscopy Atmospheric Measurement Techniques, 5, 1601-1609, 2012 Author(s): J. Mohn, B. Tuzson, A. Manninen, N. Yoshida, S. Toyoda, W. A. Brand, and L. Emmenegger We describe the first high precision real-time analysis of the N 2 O site-specific isotopic composition at ambient mixing ratios. Our technique is based on mid-infrared quantum cascade laser absorption spectroscopy (QCLAS) combined with an automated preconcentration unit. The QCLAS allows for simultaneous and specific analysis of the three main stable N 2 O isotopic species, 14 N 15 N 16 O, 15 N 14 N 16 O, 14 N 14 N 16 O, and the respective site-specific relative isotope ratio differences δ 15 N α and δ 15 N β . Continuous, stand-alone operation is achieved by using liquid nitrogen free N 2 O preconcentration, a quasi-room-temperature quantum cascade laser (QCL), quantitative sample transfer to the QCLAS and an optimized calibration algorithm. The N 2 O site-specific isotopic composition (δ 15 N α and δ 15 N β ) can be analysed with a long-term precision of 0.2‰. The potential of this analytical tool is illustrated by continuous N 2 O isotopomer measurements above a grassland plot over a three week period, which allowed identification of microbial source and sink processes.

Description: Cloud screening and quality control algorithm for star photometer data: assessment with lidar measurements and with all-sky images Atmospheric Measurement Techniques, 5, 1585-1599, 2012 Author(s): D. Pérez-Ramírez, H. Lyamani, F. J. Olmo, D. N. Whiteman, F. Navas-Guzmán, and L. Alados-Arboledas This paper presents the development and set up of a cloud screening and data quality control algorithm for a star photometer based on CCD camera as detector. These algorithms are necessary for passive remote sensing techniques to retrieve the columnar aerosol optical depth, δ Ae (λ), and precipitable water vapor content, W , at nighttime. This cloud screening procedure consists of calculating moving averages of δ Ae (λ) and W under different time-windows combined with a procedure for detecting outliers. Additionally, to avoid undesirable δ Ae (λ) and W fluctuations caused by the atmospheric turbulence, the data are averaged on 30 min. The algorithm is applied to the star photometer deployed in the city of Granada (37.16° N, 3.60° W, 680 m a.s.l.; South-East of Spain) for the measurements acquired between March 2007 and September 2009. The algorithm is evaluated with correlative measurements registered by a lidar system and also with all-sky images obtained at the sunset and sunrise of the previous and following days. Promising results are obtained detecting cloud-affected data. Additionally, the cloud screening algorithm has been evaluated under different aerosol conditions including Saharan dust intrusion, biomass burning and pollution events.

Description: Comparison of OH concentration measurements by DOAS and LIF during SAPHIR chamber experiments at high OH reactivity and low NO concentration Atmospheric Measurement Techniques, 5, 1611-1626, 2012 Author(s): H. Fuchs, H.-P. Dorn, M. Bachner, B. Bohn, T. Brauers, S. Gomm, A. Hofzumahaus, F. Holland, S. Nehr, F. Rohrer, R. Tillmann, and A. Wahner During recent field campaigns, hydroxyl radical (OH) concentrations that were measured by laser-induced fluorescence (LIF) were up to a factor of ten larger than predicted by current chemical models for conditions of high OH reactivity and low NO concentration. These discrepancies, which were observed in forests and urban-influenced rural environments, are so far not entirely understood. In summer 2011, a series of experiments was carried out in the atmosphere simulation chamber SAPHIR in Jülich, Germany, in order to investigate the photochemical degradation of isoprene, methyl-vinyl ketone (MVK), methacrolein (MACR) and aromatic compounds by OH. Conditions were similar to those experienced during the PRIDE-PRD2006 campaign in the Pearl River Delta (PRD), China, in 2006, where a large difference between OH measurements and model predictions was found. During experiments in SAPHIR, OH was simultaneously detected by two independent instruments: LIF and differential optical absorption spectroscopy (DOAS). Because DOAS is an inherently calibration-free technique, DOAS measurements are regarded as a reference standard. The comparison of the two techniques was used to investigate potential artifacts in the LIF measurements for PRD-like conditions of OH reactivities of 10 to 30 s −1 and NO mixing ratios of 0.1 to 0.3 ppbv. The analysis of twenty experiment days shows good agreement. The linear regression of the combined data set (averaged to the DOAS time resolution, 2495 data points) yields a slope of 1.02 ± 0.01 with an intercept of (0.10 ± 0.03) × 10 6 cm −3 and a linear correlation coefficient of R 2 = 0.86. This indicates that the sensitivity of the LIF instrument is well-defined by its calibration procedure. No hints for artifacts are observed for isoprene, MACR, and different aromatic compounds. LIF measurements were approximately 30–40% (median) larger than those by DOAS after MVK (20 ppbv) and toluene (90 ppbv) had been added. However, this discrepancy has a large uncertainty and requires further laboratory investigation. Observed differences between LIF and DOAS measurements are far too small to explain the unexpected high OH concentrations during the PRIDE-PRD2006 campaign.

Description: Remote sensing of CO 2 and CH 4 using solar absorption spectrometry with a low resolution spectrometer Atmospheric Measurement Techniques, 5, 1627-1635, 2012 Author(s): C. Petri, T. Warneke, N. Jones, T. Ridder, J. Messerschmidt, T. Weinzierl, M. Geibel, and J. Notholt Throughout the last few years solar absorption Fourier Transform Spectrometry (FTS) has been further developed to measure the total columns of CO 2 and CH 4 . The observations are performed at high spectral resolution, typically at 0.02 cm −1 . The precision currently achieved is generally better than 0.25%. However, these high resolution instruments are quite large and need a dedicated room or container for installation. We performed these observations using a smaller commercial interferometer at its maximum possible resolution of 0.11 cm −1 . The measurements have been performed at Bremen and have been compared to observations using our high resolution instrument also situated at the same location. The high resolution instrument has been successfully operated as part of the Total Carbon Column Observing Network (TCCON). The precision of the low resolution instrument is 0.32% for XCO 2 and 0.46% for XCH 4 . A comparison of the measurements of both instruments yields an average deviation in the retrieved daily means of ≤0.2% for CO 2 . For CH 4 an average bias between the instruments of 0.47% was observed. For test cases, spectra recorded by the high resolution instrument have been truncated to the resolution of 0.11 cm −1 . This study gives an offset of 0.03% for CO 2 and 0.26% for CH 4 . These results indicate that for CH 4 more than 50% of the difference between the instruments results from the resolution dependent retrieval. We tentatively assign the offset to an incorrect a-priori concentration profile or the effect of interfering gases, which may not be treated correctly.

Description: Areal-averaged trace gas emission rates from long-range open-path measurements in stable boundary layer conditions Atmospheric Measurement Techniques, 5, 1571-1583, 2012 Author(s): K. Schäfer, R. H. Grant, S. Emeis, A. Raabe, C. von der Heide, and H. P. Schmid Measurements of land-surface emission rates of greenhouse and other gases at large spatial scales (10 000 m 2 ) are needed to assess the spatial distribution of emissions. This can be readily done using spatial-integrating micro-meteorological methods like flux-gradient methods which were evaluated for determining land-surface emission rates of trace gases under stable boundary layers. Non-intrusive path-integrating measurements are utilized. Successful application of a flux-gradient method requires confidence in the gradients of trace gas concentration and wind, and in the applicability of boundary-layer turbulence theory; consequently the procedures to qualify measurements that can be used to determine the flux is critical. While there is relatively high confidence in flux measurements made under unstable atmospheres with mean winds greater than 1 m s −1 , there is greater uncertainty in flux measurements made under free convective or stable conditions. The study of N 2 O emissions of flat grassland and NH 3 emissions from a cattle lagoon involves quality-assured determinations of fluxes under low wind, stable or night-time atmospheric conditions when the continuous "steady-state" turbulence of the surface boundary layer breaks down and the layer has intermittent turbulence. Results indicate that following the Monin-Obukhov similarity theory (MOST) flux-gradient methods that assume a log-linear profile of the wind speed and concentration gradient incorrectly determine vertical profiles and thus flux in the stable boundary layer. An alternative approach is considered on the basis of turbulent diffusivity, i.e. the measured friction velocity as well as height gradients of horizontal wind speeds and concentrations without MOST correction for stability. It is shown that this is the most accurate of the flux-gradient methods under stable conditions.

Description: Seven years of global retrieval of cloud properties using space-borne data of GOME Atmospheric Measurement Techniques, 5, 1551-1570, 2012 Author(s): L. Lelli, A. A. Kokhanovsky, V. V. Rozanov, M. Vountas, A. M. Sayer, and J. P. Burrows We present a global and regional multi-annual (June 1996–May 2003) analysis of cloud properties (spherical cloud albedo – CA, cloud optical thickness – COT and cloud top height – CTH) of optically thick (COT 〉 5) clouds, derived using measurements from the GOME instrument on board the ESA ERS-2 space platform. We focus on cloud top height, which is obtained from top-of-atmosphere backscattered solar light measurements in the O 2 A-band using the Semi-Analytical CloUd Retrieval Algorithm SACURA. The physical framework relies on the asymptotic equations of radiative transfer. The dataset has been validated against independent ground- and satellite-based retrievals and is aimed to support trace-gases retrievals as well as to create a robust long-term climatology together with SCIAMACHY and GOME-2 ensuing retrievals. We observed the El Niño-Southern Oscillation anomaly in the 1997–1998 record through CTH values over the Pacific Ocean. The global average CTH as derived from GOME is 5.6 ± 3.2 km, for a corresponding average COT of 19.1 ± 13.9.

Description: A simple empirical model estimating atmospheric CO 2 background concentrations Atmospheric Measurement Techniques, 5, 1349-1357, 2012 Author(s): M. Reuter, M. Buchwitz, O. Schneising, F. Hase, J. Heymann, S. Guerlet, A. J. Cogan, H. Bovensmann, and J. P. Burrows A simple empirical CO 2 model (SECM) is presented to estimate column-average dry-air mole fractions of atmospheric CO 2 (XCO 2 ) as well as mixing ratio profiles. SECM is based on a simple equation depending on 17 empirical parameters, latitude, and date. The empirical parameters have been determined by least squares fitting to NOAA's (National Oceanic and Atmospheric Administration) assimilation system CarbonTracker version 2010 (CT2010). Comparisons with TCCON (total carbon column observing network) FTS (Fourier transform spectrometer) measurements show that SECM XCO 2 agrees quite well with reality. The synthetic XCO 2 values have a standard error of 1.39 ppm and systematic station-to-station biases of 0.46 ppm. Typical column averaging kernels of the TCCON FTS, a SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY), and two GOSAT (Greenhouse gases Observing SATellite) XCO 2 retrieval algorithms have been used to assess the smoothing error introduced by using SECM profiles instead of CT2010 profiles as a priori. The additional smoothing error amounts to 0.17 ppm for a typical SCIAMACHY averaging kernel and is most times much smaller for the other instruments (e.g. 0.05 ppm for a typical TCCON FTS averaging kernel). Therefore, SECM is well suited to provide a priori information for state-of-the-art ground-based (FTS) and satellite-based (GOSAT, SCIAMACHY) XCO 2 retrievals. Other potential applications are: (i) near real-time processing systems (that cannot make use of models like CT2010 operated in delayed mode), (ii) "CO 2 proxy" methods for XCH 4 retrievals (as correction for the XCO 2 background), and (iii) observing system simulation experiments especially for future satellite missions.

Description: PHOCUS radiometer Atmospheric Measurement Techniques, 5, 1359-1373, 2012 Author(s): O. Nyström, D. Murtagh, and V. Belitsky PHOCUS – Particles, Hydrogen and Oxygen Chemistry in the Upper Summer Mesosphere is a Swedish sounding rocket experiment, launched in July 2011, with the main goal of investigating the upper atmosphere in the altitude range 50–110 km. This paper describes the SondRad instrument in the PHOCUS payload, a radiometer comprising two frequency channels (183 GHz and 557 GHz) aimed at exploring the water vapour concentration distribution in connection with the appearance of noctilucent (night shining) clouds. The design of the radiometer system has been done in a collaboration between Omnisys Instruments AB and the Group for Advanced Receiver Development (GARD) at Chalmers University of Technology where Omnisys was responsible for the overall design, implementation, and verification of the radiometers and backend, whereas GARD was responsible for the radiometer optics and calibration systems. The SondRad instrument covers the water absorption lines at 183 GHz and 557 GHz. The 183 GHz channel is a side-looking radiometer, while the 557 GHz radiometer is placed along the rocket axis looking in the forward direction. Both channels employ sub-harmonically pumped Schottky mixers and Fast Fourier Transform Spectrometers (FFTS) backends with 67 kHz resolution. The radiometers include novel calibration systems specifically adjusted for use with each frequency channel. The 183 GHz channel employs a continuous wave CW pilot signal calibrating the entire receiving chain, while the intermediate frequency chain (the IF-chain) of the 557 GHz channel is calibrated by injecting a signal from a reference noise source through a directional coupler. The instrument collected complete spectra for both the 183 GHz and the 557 GHz with 300 Hz data rate for the 183 GHz channel and 10 Hz data rate for the 557 GHz channel for about 60 s reaching the apogee of the flight trajectory and 100 s after that. With lossless data compression using variable resolution over the spectrum, the data set was reduced to 2 × 12 MByte. The first results indicate that the instrument successfully performed measurements of the mesospheric water profile as planned. However, the temperature environment for the instruments showed more extreme behaviour than expected and accounted for. Consequently, the results of the calibration and the final data reduction will need careful treatment. Further, simulations through finite element method (FEM), modelling and direct measurements of the simulated thermal environment and its impact on the instrument performance are described, as well as suggestions for improvements in the design for future flights.

Description: Temperature profiles with bi-static Doppler-RASS and their correction Atmospheric Measurement Techniques, 5, 1399-1408, 2012 Author(s): B. Hennemuth, G. Peters, and H.-J. Kirtzel The technique of atmospheric temperature profiling by Doppler-RASS is discussed. The set up with bi-static (separated transmit and receiving) antennas implies a range dependent scattering angle. The retrieval scheme developed by Kon for such antenna geometry is reviewed and its limits of validity are discussed. Empirical tuning of the effective antenna aperture is proposed to fit the retrieved temperature profiles to reality. The method is based on the assumption that potential temperature profiles under presumedly neutral conditions are constant with height. Examples of application of the measuring technique for atmospheric boundary layer characterization are presented.

Description: An intercomparison of radar-based liquid cloud microphysics retrievals and implications for model evaluation studies Atmospheric Measurement Techniques, 5, 1409-1424, 2012 Author(s): D. Huang, C. Zhao, M. Dunn, X. Dong, G. G. Mace, M. P. Jensen, S. Xie, and Y. Liu This paper presents a statistical comparison of three cloud retrieval products of the Atmospheric Radiation Measurement (ARM) program at the Southern Great Plains (SGP) site from 1998 to 2006: MICROBASE, University of Utah (UU), and University of North Dakota (UND) products. The probability density functions of the various cloud liquid water content (LWC) retrievals appear to be consistent with each other. While the mean MICROBASE and UU cloud LWC retrievals agree well in the middle of cloud, the discrepancy increases to about 0.03 gm −3 at cloud top and cloud base. Alarmingly large differences are found in the droplet effective radius ( r e ) retrievals. The mean MICROBASE r e is more than 6 μm lower than the UU r e , whereas the discrepancy is reduced to within 1 μm if columns containing raining and/or mixed-phase layers are excluded from the comparison. A suite of stratified comparisons and retrieval experiments reveal that the LWC difference stems primarily from rain contamination, partitioning of total liquid later path (LWP) into warm and supercooled liquid, and the input cloud mask and LWP. The large discrepancy among the r e retrievals is mainly due to rain contamination and the presence of mixed-phase layers. Since rain or ice particles are likely to dominate radar backscattering over cloud droplets, the large discrepancy found in this paper can be thought of as a physical limitation of single-frequency radar approaches. It is therefore suggested that data users should use the retrievals with caution when rain and/or mixed-phase layers are present in the column.

Description: Long-term validation of tropospheric column-averaged CH 4 mole fractions obtained by mid-infrared ground-based FTIR spectrometry Atmospheric Measurement Techniques, 5, 1425-1441, 2012 Author(s): E. Sepúlveda, M. Schneider, F. Hase, O. E. García, A. Gomez-Pelaez, S. Dohe, T. Blumenstock, and J. C. Guerra At the Izaña Atmospheric Research Center, high-resolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH 4 ) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH 4 ) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH 4 contribution and a posteriori correct the totXCH 4 data of a profile scaling retrieval accordingly (troXCH 4 post ); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH 4 retr ) from retrieved CH 4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH 4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%). We compare the different FTIR CH 4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH 4 data (CH 4 GAW ), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH 4 amounts. Concerning totXCH 4 , the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH 4 is not a valid free tropospheric CH 4 proxy, as it is significantly affected by the varying stratospheric CH 4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH 4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH 4 amounts. Results of the profile retrieval analysis correlate well with the CH 4 GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%), and both data sets show very similar annual cycles and trend behaviour for the 2001–2010 time period. Furthermore, we find a very good absolute agreement between the troXCH 4 retr and CH 4 GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data. Our study strongly supports the value of mid-infrared ground-based FTIR CH 4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH 4 data of high quality.

Description: CRISTA-NF measurements with unprecedented vertical resolution during the RECONCILE aircraft campaign Atmospheric Measurement Techniques, 5, 1173-1191, 2012 Author(s): J. Ungermann, C. Kalicinsky, F. Olschewski, P. Knieling, L. Hoffmann, J. Blank, W. Woiwode, H. Oelhaf, E. Hösen, C. M. Volk, A. Ulanovsky, F. Ravegnani, K. Weigel, F. Stroh, and M. Riese The Cryogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers (CRISTA-NF), an airborne infrared limb-sounder, was operated aboard the high-flying Russian research aircraft M55-Geophysica during the Arctic RECONCILE campaign from January to March 2010. This paper describes the calibration process of the instrument and the retrieval algorithm employed and then proceeds to present retrieved trace gas volume mixing ratio cross-sections for one specific flight in this campaign. We are able to resolve the uppermost troposphere/lower stratosphere for several trace gas species for several kilometres below the flight altitude (16 to 19 km) with an unprecedented vertical resolution of 400 to 500 m for the limb-sounding technique. The instrument points sideways with respect to the flight direction. Therefore, the observations are also characterised by a rather high horizontal sampling along the flight track, which provides a full vertical profile every ≈15 km. Assembling the vertical trace gas profiles derived from CRISTA-NF measurements to cross-sections shows filaments of vortex and extra-vortex air masses in the vicinity of the polar vortex. During this campaign, the M55-Geophysica carried further instruments enabling trace gas volume mixing ratios derived from CRISTA-NF to be validated by comparing them with measurements by the in situ instruments HAGAR and FOZAN and observations by MIPAS-STR. This validation suggests that the retrieved trace gas volume mixing ratios are both qualitatively and quantitatively reliable.

Description: Characterisation and airborne deployment of a new counterflow virtual impactor inlet Atmospheric Measurement Techniques, 5, 1259-1269, 2012 Author(s): T. Shingler, S. Dey, A. Sorooshian, F. J. Brechtel, Z. Wang, A. Metcalf, M. Coggon, J. Mülmenstädt, L. M. Russell, H. H. Jonsson, and J. H. Seinfeld A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (∼15 l min −1 ) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.

Description: A gas chromatographic instrument for measurement of hydrogen cyanide in the lower atmosphere Atmospheric Measurement Techniques, 5, 1229-1240, 2012 Author(s): J. L. Ambrose, Y. Zhou, K. Haase, H. R. Mayne, R. Talbot, and B. C. Sive A gas-chromatographic (GC) instrument was developed for measuring hydrogen cyanide (HCN) in the lower atmosphere. The main features of the instrument are (1) a cryogen-free cooler for sample dehumidification and enrichment, (2) a porous polymer PLOT column for analyte separation, (3) a flame thermionic detector (FTD) for sensitive and selective detection, and (4) a dynamic dilution system for calibration. We deployed the instrument for a ∼4 month period from January–June, 2010 at the AIRMAP atmospheric monitoring station Thompson Farm 2 (THF2) in rural Durham, NH. A subset of measurements made during 3–31 March is presented here with a detailed description of the instrument features and performance characteristics. The temporal resolution of the measurements was ~20 min, with a 75 s sample capture time. The 1σ measurement precision was

Description: MIPAS-STR measurements in the Arctic UTLS in winter/spring 2010: instrument characterization, retrieval and validation Atmospheric Measurement Techniques, 5, 1205-1228, 2012 Author(s): W. Woiwode, H. Oelhaf, T. Gulde, C. Piesch, G. Maucher, A. Ebersoldt, C. Keim, M. Höpfner, S. Khaykin, F. Ravegnani, A. E. Ulanovsky, C. M. Volk, E. Hösen, A. Dörnbrack, J. Ungermann, C. Kalicinsky, and J. Orphal The mid-infrared FTIR-limb-sounder Michelson Interferometer for Passive Atmospheric Sounding–STRatospheric aircraft (MIPAS-STR) was deployed onboard the research aircraft M55 Geophysica during the RECONCILE campaign (Reconciliation of Essential Process Parameters for an Enhanced Predictability of Arctic Stratospheric Ozone Loss and its Climate Interactions) in the Arctic winter/spring 2010. From the MIPAS-STR measurements, vertical profiles and 2-dimensional vertical cross-sections of temperature and trace gases are retrieved. Detailed mesoscale structures of polar vortex air, extra vortex air and vortex filaments are identified in the results at typical vertical resolutions of 1 to 2 km and typical horizontal sampling densities of 45 or 25 km, depending on the sampling programme. Results are shown for the RECONCILE flight 11 on 2 March 2010 and are validated with collocated in-situ measurements of temperature, O 3 , CFC-11, CFC-12 and H 2 O. Exceptional agreement is found for the in-situ comparisons of temperature and O 3 , with mean differences (vertical profile/along flight track) of 0.2/−0.2 K for temperature and −0.01/0.05 ppmv for O 3 and corresponding sample standard deviations of the mean differences of 0.7/0.6 K and 0.1/0.3 ppmv. The comparison of the retrieved vertical cross-sections of HNO 3 from MIPAS-STR and the infrared limb-sounder Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere–New Frontiers (CRISTA–NF) indicates a high degree of agreement. We discuss MIPAS-STR in its current configuration, the spectral and radiometric calibration of the measurements and the retrieval of atmospheric parameters from the spectra. The MIPAS-STR measurements are significantly affected by continuum-like contributions, which are attributed to background aerosol and broad spectral signatures from interfering trace gases, and are important for mid-infrared limb-sounding in the Upper Troposphere/Lower Stratosphere (UTLS) region. Taking into consideration continuum-like effects, we present a scheme suitable for accurate retrievals of temperature and an extended set of trace gases, including the correction of a systematic line-of-sight offset.

Description: Influence of aerosol and surface reflectance variability on hyperspectral observed radiance Atmospheric Measurement Techniques, 5, 1193-1203, 2012 Author(s): C. Bassani, R. M. Cavalli, and P. Antonelli Current aerosol retrievals based on visible and near infrared remote-sensing, are prone to loss of accuracy, where the assumptions of the applied algorithm are violated. This happens mostly over land and it is related to misrepresentation of specific aerosol conditions or surface properties. New satellite missions, based on high spectral resolution instruments, such as PRISMA (Hyperspectral Precursor of the Application Mission), represent a valuable opportunity to improve the accuracy of τ a 550 retrievable from a remote-sensing system developing new atmospheric measurement techniques. This paper aims to address the potential of these new observing systems in more accurate retrieving τ a 550 , specifically over land in heterogeneous and/or homogeneous areas composed by dark and bright targets. The study shows how the variation of the hyperspectral observed radiance can be addressed to recognise a variation of Δτ a 550 = 0.02. The goal has been achieved by using simulated radiances by combining two aerosol models (urban and continental) and two reflecting surfaces: dark (represented by water) and bright (represented by sand) for the PRISMA instrument, considering the environmental contribution of the observed radiance, i.e., the adjacency effect. Results showed that, in the continental regime, the expected instrument sensitivity would allow for retrieval accuracy of the aerosol optical thickness at 550 nm of 0.02 or better, with a dark surface surrounded by dark areas. The study also showed that for the urban regime, the surface plays a more significant role, with a bright surface surrounded by dark areas providing favourable conditions for the aerosol load retrievals, and dark surfaces representing less suitable situations for inversion independently of the surroundings. However, over all, the results obtained provide evidence that high resolution observations of Earth spectrum between 400 and 1000 nm would allow for a significant improvement of the accuracy of the τ a 550 for anthropogenic/natural aerosols over land.

Description: Comparison of methods for the determination of NO-O 3 -NO 2 fluxes and chemical interactions over a bare soil Atmospheric Measurement Techniques, 5, 1241-1257, 2012 Author(s): P. Stella, B. Loubet, P. Laville, E. Lamaud, M. Cazaunau, S. Laufs, F. Bernard, B. Grosselin, N. Mascher, R. Kurtenbach, A. Mellouki, J. Kleffmann, and P. Cellier Tropospheric ozone (O 3 ) is a known greenhouse gas responsible for impacts on human and animal health and ecosystem functioning. In addition, O 3 plays an important role in tropospheric chemistry, together with nitrogen oxides. The determination of surface-atmosphere exchange fluxes of these trace gases is a prerequisite to establish their atmospheric budget and evaluate their impact onto the biosphere. In this study, O 3 , nitric oxide (NO) and nitrogen dioxide (NO 2 ) fluxes were measured using the aerodynamic gradient method over a bare soil in an agricultural field. Ozone and NO fluxes were also measured using eddy-covariance and automatic chambers, respectively. The aerodynamic gradient measurement system, composed of fast response sensors, was capable to measure significant differences in NO and O 3 mixing ratios between heights. However, due to local advection, NO 2 mixing ratios were highly non-stationary and NO 2 fluxes were, therefore, not significantly different from zero. The chemical reactions between O 3 , NO and NO 2 led to little ozone flux divergence between the surface and the measurement height (less than 1% of the flux on average), whereas the NO flux divergence was about 10% on average. The use of fast response sensors allowed reducing the flux uncertainty. The aerodynamic gradient and the eddy-covariance methods gave comparable O 3 fluxes. The chamber NO fluxes were down to 70% lower than the aerodynamic gradient fluxes, probably because of either the spatial heterogeneity of the soil NO emissions or the perturbation due to the chamber itself.

Description: Global and long-term comparison of SCIAMACHY limb ozone profiles with correlative satellite data (2002–2008) Atmospheric Measurement Techniques, 5, 771-788, 2012 Author(s): S. Mieruch, M. Weber, C. von Savigny, A. Rozanov, H. Bovensmann, J. P. Burrows, P. F. Bernath, C. D. Boone, L. Froidevaux, L. L. Gordley, M. G. Mlynczak, J. M. Russell III, L. W. Thomason, K. A. Walker, and J. M. Zawodny SCIAMACHY limb scatter ozone profiles from 2002 to 2008 have been compared with MLS (2005–2008), SABER (2002–2008), SAGE II (2002–2005), HALOE (2002–2005) and ACE-FTS (2004–2008) measurements. The comparison is performed for global zonal averages and heights from 10 to 50 km in one km steps. The validation was performed by comparing monthly mean zonal means and by comparing averages over collocated profiles within a zonal band and month. Both approaches yield similar results. For most of the stratosphere SCIAMACHY agrees to within 10% or better with other correlative data. A systematic bias of SCIAMACHY ozone of up to 100% between 10 and 20 km in the tropics points to some remaining issues with regard to convective cloud interference. Statistical hypothesis testing reveals at which altitudes and in which region differences between SCIAMACHY and other satellite data are statistically significant. We also estimated linear trends from monthly mean data for different periods where SCIAMACHY has common observations with other satellite data using a classical trend model with QBO and seasonal terms in order to draw conclusions on potential instrumental drifts as a function of latitude and altitude. Since the time periods considered here are rather short these trend estimates are only used to identify potential instrumental issues with the SCIAMACHY data. As a result SCIAMACHY exhibits a statistically significant negative trend in the range of of about 1–3% per year depending on latitude during the period 2002–2005 (overlapping with HALOE and SAGE II) and somewhat less during 2002–2008 (overlapping with SABER) in the altitude range of 30–40 km, while in the period 2004–2008 (overlapping with MLS and ACE-FTS) no significant trends are observed. Since all correlative satellite instruments do not show to a very large extent statistically significant trends in any of the time periods considered here, the negative trends observed with SCIAMACHY data point at some remaining instrumental artifact which is most likely related to residual errors in the tangent height registration of SCIAMACHY.

Description: Operational considerations to improve total ozone measurements with a Microtops II ozone monitor Atmospheric Measurement Techniques, 5, 759-769, 2012 Author(s): J. L. Gómez-Amo, V. Estellés, A. di Sarra, R. Pedrós, M. P. Utrillas, J. A. Martínez- Lozano, C. González-Frias, E. Kyrö, and J. M. Vilaplana A Microtops II "ozone monitor" with UV channels centered at 305.5, 312.5, and 320 nm has been used routinely in six experimental campaigns carried out in several geographic locations and seasons, covering latitudes from 35 to 68° N during the last ten years (2001–2011). The total ozone content is retrieved by Microtops II by using different combinations (Channel I, 305.5/312.5 nm; Channel II, 312.5/320 nm; and Channel III, 305.5/312.5/320 nm) of the signals at the three ultraviolet wavelengths. The long-term performance of the total ozone content determination has been studied taking into account the sensitivities to the calibration, airmass, temperature and aerosols. When a calibration was used and the airmass limit was fixed to 3, the root mean square deviations of the relative differences produced by Microtops II with respect to several Brewers are 0.9, 2, and 2% respectively for the Channel I, Channel II, and Channel III retrieval. The performance of the Microtops retrieval has been stable during the last ten years. Channel I represents the best option to determine the instantaneous total ozone content. Channels II and III values appear weakly sensitive to temperature, ozone content, and aerosols. Channel II is more stable than Channel I for airmasses larger than 2.6. The conclusions do not show any dependence on latitude and season.

Description: Trend analysis of aerosol optical thickness and Ångström exponent derived from the global AERONET spectral observations Atmospheric Measurement Techniques, 5, 1271-1299, 2012 Author(s): J. Yoon, W. von Hoyningen-Huene, A. A. Kokhanovsky, M. Vountas, and J. P. Burrows Regular aerosol observations based on well-calibrated instruments have led to a better understanding of the aerosol radiative budget on Earth. In recent years, these instruments have played an important role in the determination of the increase of anthropogenic aerosols by means of long-term studies. Only few investigations regarding long-term trends of aerosol optical characteristics (e.g. aerosol optical thickness (AOT) and Ångström exponent (ÅE)) have been derived from ground-based observations. This paper aims to derive and discuss linear trends of AOT (440, 675, 870, and 1020 nm) and ÅE (440–870 nm) using AErosol RObotic NETwork (AERONET) level 2.0 spectral observations. Additionally, temporal trends of coarse- and fine-mode dominant AOTs (CdAOT and FdAOT) have been estimated by applying an aerosol classification based on accurate ÅE and Ångström exponent difference (ÅED). In order to take into account the fact that cloud disturbance is having a significant influence on the trend analysis of aerosols, we introduce a weighted least squares regression depending on two weights: (1) monthly standard deviation (σ t ) and (2) number of observations per month ( n t ). Temporal increase of FdAOTs (440 nm) prevails over newly industrializing countries in East Asia (weighted trends; +6.23% yr −1 at Beijing) and active agricultural burning regions in South Africa (+1.89% yr −1 at Mongu). On the other hand, insignificant or negative trends for FdAOTs are detected over Western Europe (+0.25% yr −1 at Avignon and −2.29% yr −1 at Ispra) and North America (−0.52% yr −1 for GSFC and −0.01% yr −1 at MD_Science_Center). Over desert regions, both increase and decrease of CdAOTs (+3.37% yr −1 at Solar_Village and −1.18% yr −1 at Ouagadougou) are observed depending on meteorological conditions.

Description: Filling-in of near-infrared solar lines by terrestrial fluorescence and other geophysical effects: simulations and space-based observations from SCIAMACHY and GOSAT Atmospheric Measurement Techniques, 5, 809-829, 2012 Author(s): J. Joiner, Y. Yoshida, A. P. Vasilkov, E. M. Middleton, P. K. E. Campbell, Y. Yoshida, A. Kuze, and L. A. Corp Global mapping of terrestrial vegetation fluorescence from space has recently been accomplished with high spectral resolution (ν/Δν 〉 35 000) measurements from the Japanese Greenhouse gases Observing SATellite (GOSAT). These data are of interest because they can potentially provide global information on the functional status of vegetation including light-use efficiency and global primary productivity that can be used for global carbon cycle modeling. Quantifying the impact of fluorescence on the O 2 -A band is important as this band is used for photon pathlength characterization in cloud- and aerosol-contaminated pixels for trace-gas retrievals including CO 2 . Here, we examine whether fluorescence information can be derived from space using potentially lower-cost hyperspectral instrumentation, i.e., more than an order of magnitude less spectral resolution (ν/Δν ~ 1600) than GOSAT, with a relatively simple algorithm. We discuss laboratory measurements of fluorescence near one of the few wide and deep solar Fraunhofer lines in the long-wave tail of the fluorescence emission region, the calcium (Ca) II line at 866 nm that is observable with a spectral resolution of ~0.5 nm. The filling-in of the Ca II line due to additive signals from various atmospheric and terrestrial effects, including fluorescence, is simulated. We then examine filling-in of this line using the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) satellite instrument. In order to interpret the satellite measurements, we developed a general approach to correct for various instrumental artifacts that produce false filling-in of solar lines in satellite measurements. The approach is applied to SCIAMACHY at the 866 nm Ca II line and to GOSAT at 758 and 770 nm on the shoulders of the O 2 -A feature where there are several strong solar Fraunhofer lines that are filled in primarily by vegetation fluorescence. Finally, we compare temporal and spatial variations of SCIAMACHY additive signals with those of GOSAT and the Enhanced Vegetation Index (EVI) from the MODerate-resolution Imaging Spectroradiometer (MODIS). Although the derived additive signals from SCIAMACHY are extremely weak at 866 nm, their spatial and temporal variations are consistent with chlorophyll a fluorescence or another vegetation-related source. We also show that filling-in occurs at 866 nm over some barren areas, possibly originating from luminescent minerals in rock and soil.

Description: Tracking of urban aerosols using combined LIDAR-based remote sensing and ground-based measurements Atmospheric Measurement Techniques, 5, 891-900, 2012 Author(s): T.-Y. He, S. Stanič, F. Gao, K. Bergant, D. Veberič, X.-Q. Song, and A. Dolžan A measuring campaign was performed over the neighboring towns of Nova Gorica in Slovenia and Gorizia in Italy on 24 and 25 May 2010, to investigate the concentration and distribution of urban aerosols. Tracking of two-dimensional spatial and temporal aerosol distributions was performed using scanning elastic LIDAR, operating at 1064 nm. In addition, PM 10 concentrations of particles, NO x concentrations and meteorological data were continuously monitored within the LIDAR scanning region. Based on the data we collected, we investigated the flow dynamics and the aerosol concentrations within the lower troposphere and found an evidence for daily aerosol cycles. We observed a number of cases with spatially localized increased LIDAR returns, which are associated with the presence of point sources of particulate matter. Daily aerosol concentration cycles were also clearly visible with a peak in aerosol concentration during the morning rush hours and daily plateau at around 17:00 Central European Time. We also found that horizontal atmospheric extinction at the height of 200 m, averaged in limited region with a radius of 300 m directly above the ground-based measuring site, was linearly correlated to the PM 10 concentration with a correlation coefficient of 0.84. When considering the average of the horizontal atmospheric extinction over the entire scanning region, a strong dependence on traffic conditions (concentration of NO x ) in the vicinity of the ground-based measuring site was observed.

Description: Progress in turbulence detection via GNSS occultation data Atmospheric Measurement Techniques, 5, 789-808, 2012 Author(s): L. B. Cornman, R. K. Goodrich, P. Axelrad, and E. Barlow The increased availability of radio occultation (RO) data offers the ability to detect and study turbulence in the Earth's atmosphere. An analysis of how RO data can be used to determine the strength and location of turbulent regions is presented. This includes the derivation of a model for the power spectrum of the log-amplitude and phase fluctuations of the permittivity (or index of refraction) field. The bulk of the paper is then concerned with the estimation of the model parameters. Parameter estimators are introduced and some of their statistical properties are studied. These estimators are then applied to simulated log-amplitude RO signals. This includes the analysis of global statistics derived from a large number of realizations, as well as case studies that illustrate various specific aspects of the problem. Improvements to the basic estimation methods are discussed, and their beneficial properties are illustrated. The estimation techniques are then applied to real occultation data. Only two cases are presented, but they illustrate some of the salient features inherent in real data.

Description: How to average logarithmic retrievals? Atmospheric Measurement Techniques, 5, 831-841, 2012 Author(s): B. Funke and T. von Clarmann Calculation of mean trace gas contributions from profiles obtained by retrievals of the logarithm of the abundance rather than retrievals of the abundance itself are prone to biases. By means of a system simulator, biases of linear versus logarithmic averaging were evaluated for both maximum likelihood and maximum a priori retrievals, for various signal to noise ratios and atmospheric variabilities. These biases can easily reach ten percent or more. As a rule of thumb we found for maximum likelihood retrievals that linear averaging better represents the true mean value in cases of large local natural variability and high signal to noise ratios, while for small local natural variability logarithmic averaging often is superior. In the case of maximum a posteriori retrievals, the mean is dominated by the a priori information used in the retrievals and the method of averaging is of minor concern. For larger natural variabilities, the appropriateness of the one or the other method of averaging depends on the particular case because the various biasing mechanisms partly compensate in an unpredictable manner. This complication arises mainly because of the fact that in logarithmic retrievals the weight of the prior information depends on abundance of the gas itself. No simple rule was found on which kind of averaging is superior, and instead of suggesting simple recipes we cannot do much more than to create awareness of the traps related with averaging of mixing ratios obtained from logarithmic retrievals.