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: Radio occultation bending angle anomalies during tropical cyclones Atmospheric Measurement Techniques, 4, 1053-1060, 2011 Author(s): R. Biondi, T. Neubert, S. Syndergaard, and J. K. Nielsen The tropical deep convection affects the radiation balance of the atmosphere changing the water vapor mixing ratio and the temperature of the upper troposphere lower stratosphere. The aim of this work is to better understand these processes and to investigate if severe storms leave a significant signature in radio occultation profiles in the tropical tropopause layer. Using tropical cyclone best track database and data from different GPS radio occultation missions (COSMIC, GRACE, CHAMP, SACC and GPSMET), we selected 1194 profiles in a time window of 3 h and a space window of 300 km from the eye of the cyclone. We show that the bending angle anomaly of a GPS radio occultation signal is typically larger than the climatology in the upper troposphere and lower stratosphere and that a double tropopause during deep convection can easily be detected using this technique. Comparisons with co-located radiosondes, climatology of tropopause altitudes and GOES analyses are also shown to support the hypothesis that the bending angle anomaly can be used as an indicator of convective towers. The results are discussed in connection to the GPS radio occultation receiver which will be part of the Atomic Clock Ensemble in Space (ACES) payload on the International Space Station.

Description: Assimilation of GPS radio occultation data at DWD Atmospheric Measurement Techniques, 4, 1105-1113, 2011 Author(s): H. Anlauf, D. Pingel, and A. Rhodin We describe the status of the assimilation of bending angles from GPS radio occultations in the 3D-Var for DWD's operational global forecast model GME ("Global Model for Europe"). Experiments show that the assimilation of GPSRO data leads to a significant reduction of biases in the analyses of temperature, humidity and wind in the upper troposphere and the stratosphere, as well as a better r. m. s. fit in the comparison to radiosondes. The impact on forecasts is most prominent in the data sparse Southern Hemisphere, but is also quite notable in the Northern Hemisphere extra-tropics. The positive results found in the impact experiments lead to the implementation of the assimilation of GPS radio occultations from GRACE-A, FORMOSAT-3/COSMIC and GRAS/MetOp-A into the operational suite on 3 August 2010. We also show some initial results from assimilation experiments using radio occultation data from the German research satellite TerraSAR-X.

Description: CIAO: the CNR-IMAA advanced observatory for atmospheric research Atmospheric Measurement Techniques, 4, 1191-1208, 2011 Author(s): F. Madonna, A. Amodeo, A. Boselli, C. Cornacchia, V. Cuomo, G. D'Amico, A. Giunta, L. Mona, and G. Pappalardo Long-term observations of aerosol and clouds are of crucial importance to understand the weather climate system. At the Istituto di Metodologie per l'Analisi Ambientale of the Italian National Research Council (CNR-IMAA) an advanced atmospheric observatory, named CIAO, is operative. CIAO (CNR-IMAA Atmospheric Observatory) main scientific objective is the long term measurement for the climatology of aerosol and cloud properties. Its equipment addresses the state-of-the-art for the ground-based remote sensing of aerosol, water vapour and clouds including active and passive sensors, like lidars, ceilometers, radiometers, and a radar. This paper describes the CIAO infrastructure, its scientific activities as well as the observation strategy. The observation strategy is mainly organized in order to provide quality assured measurements for satellite validation and model evaluation and to fully exploit the synergy and integration of the active and passive sensors for the improvement of atmospheric profiling. Data quality is ensured both by the application of protocols and dedicated quality assurance programs mainly related to the projects and networks in which the infrastructure is involved. The paper also introduces examples of observations performed at CIAO and of the synergies and integration algorithms (using Raman lidar and microwave profiler data) developed and implemented at the observatory for the optimization and improvement of water vapour profiling. CIAO database represents an optimal basis to study the synergy between different sensors and to investigate aerosol-clouds interactions, and can give a significant contribution to the validation programs of the incoming new generation satellite missions.

Description: Airborne DOAS limb measurements of tropospheric trace gas profiles: case studies on the profile retrieval of O 4 and BrO Atmospheric Measurement Techniques, 4, 1241-1260, 2011 Author(s): C. Prados-Roman, A. Butz, T. Deutschmann, M. Dorf, L. Kritten, A. Minikin, U. Platt, H. Schlager, H. Sihler, N. Theys, M. Van Roozendael, T. Wagner, and K. Pfeilsticker A novel limb scanning mini-DOAS spectrometer for the detection of UV/vis absorbing radicals (e.g., O 3 , BrO, IO, HONO) was deployed on the DLR-Falcon (Deutsches Zentrum für Luft- und Raumfahrt) aircraft and tested during the ASTAR 2007 campaign (Arctic Study of Tropospheric Aerosol, Clouds and Radiation) that took place at Svalbard (78° N) in spring 2007. Our main objectives during this campaign were to test the instrument, and to perform spectral and profile retrievals of tropospheric trace gases, with particular interest on investigating the distribution of halogen compounds (e.g., BrO) during the so-called ozone depletion events (ODEs). In the present work, a new method for the retrieval of vertical profiles of tropospheric trace gases from tropospheric DOAS limb observations is presented. Major challenges arise from modeling the radiative transfer in an aerosol and cloud particle loaded atmosphere, and from overcoming the lack of a priori knowledge of the targeted trace gas vertical distribution (e.g., unknown tropospheric BrO vertical distribution). Here, those challenges are tackled by a mathematical inversion of tropospheric trace gas profiles using a regularization approach constrained by a retrieved vertical profile of the aerosols extinction coefficient E M . The validity and limitations of the algorithm are tested with in situ measured E M , and with an absorber of known vertical profile (O 4 ). The method is then used for retrieving vertical profiles of tropospheric BrO. Results indicate that, for aircraft ascent/descent observations, the limit for the BrO detection is roughly 1.5 pptv (pmol mol −1 ), and the BrO profiles inferred from the boundary layer up to the upper troposphere and lower stratosphere have around 10 degrees of freedom. For the ASTAR 2007 deployments during ODEs, the retrieved BrO vertical profiles consistently indicate high BrO mixing ratios (∼15 pptv) within the boundary layer, low BrO mixing ratios (≤1.5 pptv) in the free troposphere, occasionally enhanced BrO mixing ratios (∼1.5 pptv) in the upper troposphere, and increasing BrO mixing ratios with altitude in the lowermost stratosphere. These findings agree reasonably well with satellite and balloon-borne soundings of total and partial BrO atmospheric column densities.

Description: Processing of GRAS/METOP radio occultation data recorded in closed-loop and raw-sampling modes Atmospheric Measurement Techniques, 4, 1021-1026, 2011 Author(s): M. E. Gorbunov, K. B. Lauritsen, H.-H. Benzon, G. B. Larsen, S. Syndergaard, and M. B. Sørensen Instrument GRAS (Global Navigation Satellite System Receiver for Atmospheric Sounding) on-board of the Metop-A satellite was activated on 27 October 2006. Currently, Metop-A is a fully operational satellite with GRAS providing from 650–700 occultations per day. We describe our processing of GRAS data based on the modification of our OCC software, which was modified to become capable of reading and processing GRAS data. We perform a statistical comparison of bending angles and refractivities derived from GRAS data with those derived from ECMWF analyses. We conclude that GRAS data have error characteristics close to those of COSMIC data. In the height range 10–30 km, the systematic refractivity difference GRAS–ECMWF is of the order of 0.1–0.2 %, and the standard deviation is 0.3–0.6 %. In the lower troposphere GRAS refractivity and bending angle indicate a negative bias, which reaches its maximum value in the tropics. In particular the retrieved refractivity is biased by up to 2.5 %. The negative bias pattern is similar to that found in the statistical validation of COSMIC data. This makes it probable that the bias should not be attributed to the instrument design or hardware.

Description: Eight-component retrievals from ground-based MAX-DOAS observations Atmospheric Measurement Techniques, 4, 1027-1044, 2011 Author(s): H. Irie, H. Takashima, Y. Kanaya, K. F. Boersma, L. Gast, F. Wittrock, D. Brunner, Y. Zhou, and M. Van Roozendael We attempt for the first time to retrieve lower-tropospheric vertical profile information for 8 quantities from ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations. The components retrieved are the aerosol extinction coefficients at two wavelengths, 357 and 476 nm, and NO 2 , HCHO, CHOCHO, H 2 O, SO 2 , and O 3 volume mixing ratios. A Japanese MAX-DOAS profile retrieval algorithm, version 1 (JM1), is applied to observations performed at Cabauw, the Netherlands (51.97° N, 4.93° E), in June–July 2009 during the Cabauw Intercomparison campaign of Nitrogen Dioxide measuring Instruments (CINDI). Of the retrieved profiles, we focus here on the lowest-layer data (mean values at altitudes 0–1 km), where the sensitivity is usually highest owing to the longest light path. In support of the capability of the multi-component retrievals, we find reasonable overall agreement with independent data sets, including a regional chemical transport model (CHIMERE) and in situ observations performed near the surface (2–3 m) and at the 200-m height level of the tall tower in Cabauw. Plumes of enhanced HCHO and SO 2 were likely affected by biogenic and ship emissions, respectively, and an improvement in their emission strengths is suggested for better agreement between CHIMERE simulations and MAX-DOAS observations. Analysis of air mass factors indicates that the horizontal spatial representativeness of MAX-DOAS observations is about 3–15 km (depending mainly on aerosol extinction), comparable to or better than the spatial resolution of current UV-visible satellite observations and model calculations. These demonstrate that MAX-DOAS provides multi-component data useful for the evaluation of satellite observations and model calculations and can play an important role in bridging different data sets having different spatial resolutions.

Description: Preliminary validation of column-averaged volume mixing ratios of carbon dioxide and methane retrieved from GOSAT short-wavelength infrared spectra Atmospheric Measurement Techniques, 4, 1061-1076, 2011 Author(s): I. Morino, O. Uchino, M. Inoue, Y. Yoshida, T. Yokota, P. O. Wennberg, G. C. Toon, D. Wunch, C. M. Roehl, J. Notholt, T. Warneke, J. Messerschmidt, D. W. T. Griffith, N. M. Deutscher, V. Sherlock, B. Connor, J. Robinson, R. Sussmann, and M. Rettinger Column-averaged volume mixing ratios of carbon dioxide and methane retrieved from the Greenhouse gases Observing SATellite (GOSAT) Short-Wavelength InfraRed observation (GOSAT SWIR X CO 2 and X CH 4 ) were compared with the reference calibrated data obtained by ground-based high-resolution Fourier Transform Spectrometers (g-b FTSs) participating in the Total Carbon Column Observing Network (TCCON). Preliminary results are as follows: the GOSAT SWIR X CO 2 and X CH 4 (Version 01.xx) are biased low by 8.85 ± 4.75 ppm (2.3 ± 1.2 %) and 20.4 ± 18.9 ppb (1.2 ± 1.1 %), respectively. The standard deviation of the GOSAT SWIR X CO 2 and X CH 4 is about 1 % (1 σ) after correcting the negative biases of X CO 2 and X CH 4 by 8.85 ppm and 20.4 ppb, respectively. The latitudinal distributions of zonal means of the GOSAT SWIR X CO 2 and X CH 4 show similar features to those of the g-b FTS data except for the negative biases in the GOSAT data.

Description: Remote sensing of aerosols over snow using infrared AATSR observations Atmospheric Measurement Techniques, 4, 1133-1145, 2011 Author(s): L. G. Istomina, W. von Hoyningen-Huene, A. A. Kokhanovsky, E. Schultz, and J. P. Burrows Infrared (IR) retrievals of aerosol optical thickness (AOT) are challenging because of the low reflectance of aerosol layer at longer wavelengths. In this paper we present a closer analysis of this problem, performed with radiative transfer (RT) simulations for coarse and accumulation mode of four main aerosol components. It shows the strong angular dependence of aerosol IR reflectance at low solar elevations resulting from the significant asymmetry of aerosol phase function at these wavelengths. This results in detectable values of aerosol IR reflectance at certain non-nadir observation angles providing the advantage of multiangle remote sensing instruments for a retrieval of AOT at longer wavelengths. Such retrievals can be of importance e.g. in case of a very strong effect of the surface on the top of atmosphere (TOA) reflectance in the visible spectral range. In the current work, a new method to retrieve AOT of the coarse and accumulation mode particles over snow has been developed using the measurements of Advanced Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite. The algorithm uses AATSR channel at 3.7 μm and utilizes its dual-viewing observation technique, implying the forward view with an observation zenith angle of around 55 degrees and the nadir view. It includes cloud/snow discrimination, extraction of the atmospheric reflectance out of measured brightness temperature (BT) at 3.7 μm, and interpolation of look-up tables (LUTs) for a given aerosol reflectance. The algorithm uses LUTs, separately simulated with RT forward calculations. The resulting AOT at 500 nm is estimated from the value at 3.7 μm using a fixed Angström parameter. The presented method has been validated against ground-based Aerosol Robotic Network (AERONET) data for 4 high Arctic stations and shows good agreement. A case study has been performed at W-Greenland on 5 July 2008. The day before was characterized by a noticeable dust event. The retrieved AOT maps of the region show a clear increase of AOT in the Kangerlussuaq area. The area of increased AOT was detected on 5 July on the ice sheet east of Kangelussuaq, opposite to the observed north easterly wind at ground level. This position can be explained by a small scale atmospheric circulation transporting the mobilized mineral dust upslope, after its intrusion into the upper branch of the circulation. The performed study of atmospheric reflectance at 3.7 μm also shows possibilities for the detection and retrievals of cloud properties over snow surfaces.

Description: Analytical system for stable carbon isotope measurements of low molecular weight (C 2 -C 6 ) hydrocarbons Atmospheric Measurement Techniques, 4, 1161-1175, 2011 Author(s): A. Zuiderweg, R. Holzinger, and T. Röckmann We present setup, testing and initial results from a new automated system for stable carbon isotope ratio measurements on C 2 to C 6 atmospheric hydrocarbons. The inlet system allows analysis of trace gases from air samples ranging from a few liters for urban samples and samples with high mixing ratios, to many tens of liters for samples from remote unpolluted regions with very low mixing ratios. The centerpiece of the sample preparation is the separation trap, which is used to separate CO 2 and methane from the compounds of interest. The main features of the system are (i) the capability to sample up to 300 l of air, (ii) long term (since May 2009) operational δ 13 C accuracy levels in the range 0.3–0.8 ‰ (1-σ), and (iii) detection limits of order 1.5–2.5 ngC (collected amount of substance) for all reported compounds. The first application of this system was the analysis of 21 ambient air samples taken during 48 h in August 2009 in Utrecht, the Netherlands. Results obtained are generally in good agreement with those from similar urban ambient air studies. Short sample intervals allowed by the design of the instrument help to illustrate the complex diurnal behavior of hydrocarbons in an urban environment, where diverse sources, dynamical processes, and chemical reactions are present.

Description: Source brightness fluctuation correction of solar absorption fourier transform mid infrared spectra Atmospheric Measurement Techniques, 4, 1045-1051, 2011 Author(s): T. Ridder, T. Warneke, and J. Notholt The precision and accuracy of trace gas observations using solar absorption Fourier Transform infrared spectrometry depend on the stability of the light source. Fluctuations in the source brightness, however, cannot always be avoided. Current correction schemes, which calculate a corrected interferogram as the ratio of the raw DC interferogram and a smoothed DC interferogram, are applicable only to near infrared measurements. Spectra in the mid infrared spectral region below 2000 cm −1 are generally considered uncorrectable, if they are measured with a MCT detector. Such measurements introduce an unknown offset to MCT interferograms, which prevents the established source brightness fluctuation correction. This problem can be overcome by a determination of the offset using the modulation efficiency of the instrument. With known modulation efficiency the offset can be calculated, and the source brightness correction can be performed on the basis of offset-corrected interferograms. We present a source brightness fluctuation correction method which performs the smoothing of the raw DC interferogram in the interferogram domain by an application of a running mean instead of high-pass filtering the corresponding spectrum after Fourier transformation of the raw DC interferogram. This smoothing can be performed with the onboard software of commercial instruments. The improvement of MCT spectra and subsequent ozone profile and total column retrievals is demonstrated. Application to InSb interferograms in the near infrared spectral region proves the equivalence with the established correction scheme.

Description: An improved NO 2 retrieval for the GOME-2 satellite instrument Atmospheric Measurement Techniques, 4, 1147-1159, 2011 Author(s): A. Richter, M. Begoin, A. Hilboll, and J. P. Burrows Satellite observations of nitrogen dioxide (NO 2 ) provide valuable information on both stratospheric and tropospheric composition. Nadir measurements from GOME, SCIAMACHY, OMI, and GOME-2 have been used in many studies on tropospheric NO 2 burdens, the importance of different NO x emissions sources and their change over time. The observations made by the three GOME-2 instruments will extend the existing data set by more than a decade, and a high quality of the data as well as their good consistency with existing time series is of particular importance. In this paper, an improved GOME-2 NO 2 retrieval is described which reduces the scatter of the individual NO 2 columns globally but in particular in the region of the Southern Atlantic Anomaly. This is achieved by using a larger fitting window including more spectral points, and by applying a two step spike removal algorithm in the fit. The new GOME-2 data set is shown to have good consistency with SCIAMACHY NO 2 columns. Remaining small differences are shown to be linked to changes in the daily solar irradiance measurements used in both GOME-2 and SCIAMACHY retrievals. In the large retrieval window, a not previously identified spectral signature was found which is linked to deserts and other regions with bare soil. Inclusion of this empirically derived pseudo cross-section significantly improves the retrievals and potentially provides information on surface properties and desert aerosols. Using the new GOME-2 NO 2 data set, a long-term average of tropospheric columns was computed and high-pass filtered. The resulting map shows evidence for pollution from several additional shipping lanes, not previously identified in satellite observations. This illustrates the excellent signal to noise ratio achievable with the improved GOME-2 retrievals.

Description: Matching radiative transfer models and radiosonde data from the EPS/Metop Sodankylä campaign to IASI measurements Atmospheric Measurement Techniques, 4, 1177-1189, 2011 Author(s): X. Calbet, R. Kivi, S. Tjemkes, F. Montagner, and R. Stuhlmann Radiances observed from IASI are compared to calculated ones. Calculated radiances are obtained using several radiative transfer models (OSS, LBLRTM v11.3 and v11.6) on best estimates of the atmospheric state vectors. The atmospheric state vectors are derived from cryogenic frost point hygrometer and humidity dry bias corrected RS92 measurements flown on sondes launched 1 h and 5 min before IASI overpass time. The temperature and humidity best estimate profiles are obtained by interpolating or extrapolating these measurements to IASI overpass time. The IASI observed and calculated radiances match to within one sigma IASI instrument noise in the spectral region where water vapour is a strong absorber (wavenumber, ν, in the range of 1500 ≤ ν ≤ 1570 and 1615 ≤ ν ≤ 1800 cm −1 ).

Description: Eddy covariance measurements with high-resolution time-of-flight aerosol mass spectrometry: a new approach to chemically resolved aerosol fluxes Atmospheric Measurement Techniques, 4, 1275-1289, 2011 Author(s): D. K. Farmer, J. R. Kimmel, G. Phillips, K. S. Docherty, D. R. Worsnop, D. Sueper, E. Nemitz, and J. L. Jimenez Although laboratory studies show that biogenic volatile organic compounds (VOCs) yield substantial secondary organic aerosol (SOA), production of biogenic SOA as indicated by upward fluxes has not been conclusively observed over forests. Further, while aerosols are known to deposit to surfaces, few techniques exist to provide chemically-resolved particle deposition fluxes. To better constrain aerosol sources and sinks, we have developed a new technique to directly measure fluxes of chemically-resolved submicron aerosols using the high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) in a new, fast eddy covariance mode. This approach takes advantage of the instrument's ability to quantitatively identify both organic and inorganic components, including ammonium, sulphate and nitrate, at a temporal resolution of several Hz. The new approach has been successfully deployed over a temperate ponderosa pine plantation in California during the BEARPEX-2007 campaign, providing both total and chemically resolved non-refractory (NR) PM 1 fluxes. Average deposition velocities for total NR-PM 1 aerosol at noon were 2.05 ± 0.04 mm s −1 . Using a high resolution measurement of the NH 2 + and NH 3 + fragments, we demonstrate the first eddy covariance flux measurements of particulate ammonium, which show a noon-time deposition velocity of 1.9 ± 0.7 mm s −1 and are dominated by deposition of ammonium sulphate.

Description: Space and ground segment performance and lessons learned of the FORMOSAT-3/COSMIC mission: four years in orbit Atmospheric Measurement Techniques, 4, 1115-1132, 2011 Author(s): C.-J. Fong, D. Whiteley, E. Yang, K. Cook, V. Chu, B. Schreiner, D. Ector, P. Wilczynski, T.-Y. Liu, and N. Yen The FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) Mission consisting of six Low-Earth-Orbit (LEO) satellites is the world's first demonstration constellation using radio occultation signals from Global Positioning System (GPS) satellites. The atmospheric profiles derived by processing radio occultation signals are retrieved in near real-time for global weather/climate monitoring, numerical weather prediction, and space weather research. The mission has processed, on average, 1400 to 1800 high-quality atmospheric sounding profiles per day. The atmospheric radio occultation data are assimilated into operational numerical weather prediction models for global weather prediction, including typhoon/hurricane/cyclone forecasts. The radio occultation data has shown a positive impact on weather predictions at many national weather forecast centers. A follow-on mission was proposed that transitions the current experimental research mission into a significantly improved real-time operational mission, which will reliably provide 8000 radio occultation soundings per day. The follow-on mission, as planned, will consist of 12 LEO satellites (compared to 6 satellites for the current mission) with data latency requirement of 45 min (compared to 3 h for the current mission), which will provide greatly enhanced opportunities for operational forecasts and scientific research. This paper will address the FORMOSAT-3/COSMIC system and mission overview, the spacecraft and ground system performance after four years in orbit, the lessons learned from the encountered technical challenges and observations, and the expected design improvements for the spacecraft and ground system for FORMOSAT-7/COSMIC-2.

Description: Exploring Earth's atmosphere with radio occultation: contributions to weather, climate and space weather Atmospheric Measurement Techniques, 4, 1077-1103, 2011 Author(s): R. A. Anthes The launch of the proof-of-concept mission GPS/MET (Global Positioning System/Meteorology) in 1995 began a revolution in profiling Earth's atmosphere through radio occultation (RO). GPS/MET; subsequent single-satellite missions CHAMP (CHAllenging Minisatellite Payload), SAC-C (Satellite de Aplicaciones Cientificas-C), GRACE (Gravity Recovery and Climate Experiment), METOP-A, and TerraSAR-X (Beyerle et al., 2010); and the six-satellite constellation, FORMOSAT-3/COSMIC (Formosa Satellite mission {#}3/Constellation Observing System for Meteorology, Ionosphere, and Climate) have proven the theoretical capabilities of RO to provide accurate and precise profiles of electron density in the ionosphere and refractivity, containing information on temperature and water vapor, in the stratosphere and troposphere. This paper summarizes results from these RO missions and the applications of RO observations to atmospheric research and operational weather analysis and prediction.

Description: Detection of HO 2 by laser-induced fluorescence: calibration and interferences from RO 2 radicals Atmospheric Measurement Techniques, 4, 1209-1225, 2011 Author(s): H. Fuchs, B. Bohn, A. Hofzumahaus, F. Holland, K. D. Lu, S. Nehr, F. Rohrer, and A. Wahner HO 2 concentration measurements are widely accomplished by chemical conversion of HO 2 to OH including reaction with NO and subsequent detection of OH by laser-induced fluorescence. RO 2 radicals can be converted to OH via a similar radical reaction sequence including reaction with NO, so that they are potential interferences for HO 2 measurements. Here, the conversion efficiency of various RO 2 species to HO 2 is investigated. Experiments were conducted with a radical source that produces OH and HO 2 by water photolysis at 185 nm, which is frequently used for calibration of LIF instruments. The ratio of HO 2 and the sum of OH and HO 2 concentrations provided by the radical source was investigated and was found to be 0.50 ± 0.02. RO 2 radicals are produced by the reaction of various organic compounds with OH in the radical source. Interferences via chemical conversion from RO 2 radicals produced by the reaction of OH with methane and ethane (H-atom abstraction) are negligible consistent with measurements in the past. However, RO 2 radicals from OH plus alkene- and aromatic-precursors including isoprene (mainly OH-addition) are detected with a relative sensitivity larger than 80 % with respect to that for HO 2 for the configuration of the instrument with which it was operated during field campaigns. Also RO 2 from OH plus methyl vinyl ketone and methacrolein exhibit a relative detection sensitivity of 60 %. Thus, previous measurements of HO 2 radical concentrations with this instrument were biased in the presence of high RO 2 radical concentrations from isoprene, alkenes or aromatics, but were not affected by interferences in remote clean environment with no significant emissions of biogenic VOCs, when the OH reactivity was dominated by small alkanes. By reducing the NO concentration and/or the transport time between NO addition and OH detection, interference from these RO 2 species are suppressed to values below 20 % relative to the HO 2 detection sensitivity. The HO 2 conversion efficiency is also smaller by a factor of four, but this is still sufficient for atmospheric HO 2 concentration measurements for a wide range of conditions.

Description: Diode laser-based cavity ring-down instrument for NO 3 , N 2 O 5 , NO, NO 2 and O 3 from aircraft Atmospheric Measurement Techniques, 4, 1227-1240, 2011 Author(s): N. L. Wagner, W. P. Dubé, R. A. Washenfelder, C. J. Young, I. B. Pollack, T. B. Ryerson, and S. S. Brown This article presents a diode laser-based, cavity ring-down spectrometer for simultaneous in situ measurements of four nitrogen oxide species, NO 3 , N 2 O 5 , NO, NO 2 , as well as O 3 , designed for deployment on aircraft. The instrument measures NO 3 and NO 2 by optical extinction at 662 nm and 405 nm, respectively; N 2 O 5 is measured by thermal conversion to NO 3 , while NO and O 3 are measured by chemical conversion to NO 2 . The instrument has several advantages over previous instruments developed by our group for measurement of NO 2 , NO 3 and N 2 O 5 alone, based on a pulsed Nd:YAG and dye laser. First, the use of continuous wave diode lasers reduces the requirements for power and weight and eliminates hazardous materials. Second, detection of NO 2 at 405 nm is more sensitive than our previously reported 532 nm instrument, and does not have a measurable interference from O 3 . Third, the instrument includes chemical conversion of NO and O 3 to NO 2 to provide measurements of total NO x (= NO + NO 2 ) and O x (= NO 2 + O 3 ) on two separate channels; mixing ratios of NO and O 3 are determined by subtraction of NO 2 . Finally, all five species are calibrated against a single standard based on 254 nm O 3 absorption to provide high accuracy. Disadvantages include an increased sensitivity to water vapor on the 662 nm NO 3 and N 2 O 5 channels and a modest reduction in sensitivity for these species compared to the pulsed laser instrument. The in-flight detection limit for both NO 3 and N 2 O 5 is 3 pptv (2 σ, 1 s) and for NO, NO 2 and O 3 is 140, 90, and 120 pptv (2 σ, 1 s) respectively. Demonstrated performance of the instrument in a laboratory/ground based environment is better by approximately a factor of 2–3. The NO and NO 2 measurements are less precise than research-grade chemiluminescence instruments. However, the combination of these five species in a single instrument, calibrated to a single analytical standard, provides a complete and accurate picture of nighttime nitrogen oxide chemistry. The instrument performance is demonstrated using data acquired during a recent field campaign in California.

Description: Retrieval of water vapor vertical distributions in the upper troposphere and the lower stratosphere from SCIAMACHY limb measurements Atmospheric Measurement Techniques, 4, 933-954, 2011 Author(s): A. Rozanov, K. Weigel, H. Bovensmann, S. Dhomse, K.-U. Eichmann, R. Kivi, V. Rozanov, H. Vömel, M. Weber, and J. P. Burrows This study describes the retrieval of water vapor vertical distributions in the upper troposphere and lower stratosphere (UTLS) altitude range from space-borne observations of the scattered solar light made in limb viewing geometry. First results using measurements from SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) aboard ENVISAT (Environmental Satellite) are presented here. In previous publications, the retrieval of water vapor vertical distributions has been achieved exploiting either the emitted radiance leaving the atmosphere or the transmitted solar radiation. In this study, the scattered solar radiation is used as a new source of information on the water vapor content in the UTLS region. A recently developed retrieval algorithm utilizes the differential absorption structure of the water vapor in 1353–1410 nm spectral range and yields the water vapor content in the 11–25 km altitude range. In this study, the retrieval algorithm is successfully applied to SCIAMACHY limb measurements and the resulting water vapor profiles are compared to in situ balloon-borne observations. The results from both satellite and balloon-borne instruments are found to agree typically within 10 %.

Description: Fast NO 2 retrievals from Odin-OSIRIS limb scatter measurements Atmospheric Measurement Techniques, 4, 965-972, 2011 Author(s): A. E. Bourassa, C. A. McLinden, C. E. Sioris, S. Brohede, A. F. Bathgate, E. J. Llewellyn, and D. A. Degenstein The feasibility of retrieving vertical profiles of NO 2 from space-based measurements of limb scattered sunlight has been demonstrated using several different data sets since the 1980's. The NO 2 data product routinely retrieved from measurements made by the Optical Spectrograph and InfraRed Imaging System (OSIRIS) instrument onboard the Odin satellite uses a spectral fitting technique over the 437 to 451 nm range, over which there are 36 individual wavelength measurements. In this work we present a proof of concept technique for the retrieval of NO 2 using only 4 of the 36 OSIRIS measurements in this wavelength range, which reduces the computational cost by almost an order of magnitude. The method is an adaptation of a triplet analysis technique that is currently used for the OSIRIS retrievals of ozone at Chappuis band wavelengths. The results obtained are shown to be in very good agreement with the spectral fit method, and provide an important alternative for applications where the computational burden is very high. Additionally this provides a baseline for future instrument design in terms of cost effectiveness and reducing spectral range requirements.

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: Assessment of BSRN radiation records for the computation of monthly means Atmospheric Measurement Techniques, 4, 339-354, 2011 Author(s): A. Roesch, M. Wild, A. Ohmura, E. G. Dutton, C. N. Long, and T. Zhang The integrity of the Baseline Surface Radiation Network (BSRN) radiation monthly averages are assessed by investigating the impact on monthly means due to the frequency of data gaps caused by missing or discarded high time resolution data. The monthly statistics, especially means, are considered to be important and useful values for climate research, model performance evaluations and for assessing the quality of satellite (time- and space-averaged) data products. The study investigates the spread in different algorithms that have been applied for the computation of monthly means from 1-min values. The paper reveals that the computation of monthly means from 1-min observations distinctly depends on the method utilized to account for the missing data. The intra-method difference generally increases with an increasing fraction of missing data. We found that a substantial fraction of the radiation fluxes observed at BSRN sites is either missing or flagged as questionable. The percentage of missing data is 4.4%, 13.0%, and 6.5% for global radiation, direct shortwave radiation, and downwelling longwave radiation, respectively. Most flagged data in the shortwave are due to nighttime instrumental noise and can reasonably be set to zero after correcting for thermal offsets in the daytime data. The study demonstrates that the handling of flagged data clearly impacts on monthly mean estimates obtained with different methods. We showed that the spread of monthly shortwave fluxes is generally clearly higher than for downwelling longwave radiation. Overall, BSRN observations provide sufficient accuracy and completeness for reliable estimates of monthly mean values. However, the value of future data could be further increased by reducing the frequency of data gaps and the number of outliers. It is shown that two independent methods for accounting for the diurnal and seasonal variations in the missing data permit consistent monthly means to within less than 1 W m −2 in most cases. The authors suggest using a standardized method for the computation of monthly means which addresses diurnal variations in the missing data in order to avoid a mismatch of future published monthly mean radiation fluxes from BSRN. The application of robust statistics would probably lead to less biased results for data records with frequent gaps and/or flagged data and outliers. The currently applied empirical methods should, therefore, be completed by the development of robust methods.

Description: Characterisation of a new Fast CPC and its application for atmospheric particle measurements Atmospheric Measurement Techniques, 4, 823-833, 2011 Author(s): B. Wehner, H. Siebert, M. Hermann, F. Ditas, and A. Wiedensohler A new Fast CPC (FCPC) using butanol as working fluid has been built based on the setup described by Wang et al. (2002). In this study, we describe the new instrument. The functionality and stable operation of the FCPC in the laboratory, as well as under atmospheric conditions, is demonstrated. The counting efficiency was measured for three temperature differences between FCPC saturator and condenser, 25, 27, and 29 K, subsequently resulting in a lower detection limit between 6.1 and 8.5 nm. Above 25 nm the FCPC reached 98–100% counting efficiency compared to an electrometer used as the reference instrument. The FCPC demonstrated its ability to perform continuous measurements over a few hours in the laboratory with respect to the total particle counting. The instrument has been implemented into the airborne measurement platform ACTOS to perform measurements in the atmospheric boundary layer. Therefore, a stable operation over two hours is required. The mixing time of the new FCPC was estimated in two ways using a time series with highly fluctuating particle number concentrations. The analysis of a sharp ramp due to a concentration change results in a mixing time of 5 ms while a spectral analysis of atmospheric data demonstrates that for frequencies up to 10 Hz coherent structures can be resolved before sampling noise dominates.

Description: A network of autonomous surface ozone monitors in Antarctica: technical description and first results Atmospheric Measurement Techniques, 4, 645-658, 2011 Author(s): S. J.-B. Bauguitte, N. Brough, M. M. Frey, A. E. Jones, D. J. Maxfield, H. K. Roscoe, M. C. Rose, and E. W. Wolff A suite of 10 autonomous ozone monitoring units, each powered using renewable energy, was developed and built to study surface ozone in Antarctica during the International Polar Year (2007–2009). The monitoring systems were deployed in a network around the Weddell Sea sector of coastal Antarctica with a transect up onto the Antarctic Plateau. The aim was to measure for a full year, thus gaining a much-improved broader view of boundary layer ozone seasonality at different locations as well as of factors affecting the budget of surface ozone in Antarctica. Ozone mixing ratios were measured based on UV photometry using a modified version of the commercial 2B Technologies Inc. Model 202 instrument. All but one of the autonomous units measured successfully within its predefined duty cycle throughout the year, with some differences in performance dependent on power availability and ambient temperature. Mean data recovery after removal of outliers was on average 70% (range 44–83%) and precision varied between 1.5 and 8 ppbv, thus was sufficiently good to resolve year-round the main ozone features of scientific interest. We conclude that, with adequate power, and noting a minor communication problem, our units would be able to operate successfully at ambient temperatures down to −60 °C. Systems such as the one described in this paper, or derivatives of it, could therefore be deployed either as local or regional networks elsewhere in the Arctic or Antarctic. Here we present technical information and first results from the experiment.

Description: Analysis of the application of the optical method to the measurements of the water vapor content in the atmosphere – Part 1: Basic concepts of the measurement technique Atmospheric Measurement Techniques, 4, 843-856, 2011 Author(s): V. D. Galkin, F. Immler, G. A. Alekseeva, F.-H. Berger, U. Leiterer, T. Naebert, I. N. Nikanorova, V. V. Novikov, V. P. Pakhomov, and I. B. Sal'nikov We retrieved the total content of the atmospheric water vapor (or Integrated Water Vapor, IWV) from extensive sets of photometric data obtained since 1995 at Lindenberg Meteorological Observatory with star and sun photometers. Different methods of determination of the empirical parameters that are necessary for the retrieval are discussed. The instruments were independently calibrated using laboratory measurements made at Pulkovo Observatory with the VKM-100 multi-pass vacuum cell. The empirical parameters were also calculated by the simulation of the atmospheric absorption by water vapor, using the MODRAN-4 program package for different model atmospheres. The results are compared to those presented in the literature, obtained with different instruments and methods of the retrieval. The reliability of the empirical parameters, used for the power approximation that links the water vapor content with the observed absorption, is analyzed. Currently, the total (from measurements, calibration, and calculations) errors yield the standard uncertainty of about 10 % in the total column water vapor. We discuss the possibilities for improving the accuracy of calibration to ~1 % as indispensable condition in order to make it possible to use data obtained by optical photometry as an independent reference for other methods (GPS, MW-radiometers, lidar, etc).

Description: Intercomparison of atmospheric water vapor soundings from the differential absorption lidar (DIAL) and the solar FTIR system on Mt. Zugspitze Atmospheric Measurement Techniques, 4, 835-841, 2011 Author(s): H. Vogelmann, R. Sussmann, T. Trickl, and T. Borsdorff We present an intercomparison of three years of measurements of integrated water vapor (IWV) performed by the mid-infrared solar FTIR (Fourier Transform Infra-Red) instrument on the summit of Mt. Zugspitze (2964 m a.s.l.) and by the nearby near-infrared differential absorption lidar (DIAL) at the Schneefernerhaus research station (2675 m a.s.l.). The solar FTIR was shown to be one of the most accurate and precise IWV sounders in recent work (Sussmann et al., 2009) and is taken as the reference here. By calculating the FTIR-DIAL correlation (22 min coincidence interval, 15 min integration time) we derive an almost ideal slope of 0.996 (10), a correlation coefficient of R = 0.99, an IWV intercept of −0.039 (42) mm (−1.2 % of the mean), and a bias of −0.052 (26) mm (−1.6 % of the mean) from the scatter plot. By selecting a subset of coincidences with an optimum temporal and spatial matching between DIAL and FTIR, we obtain a conservative estimate of the precision of the DIAL in measuring IWV which is better than 0.1 mm (3.2 % of the mean). We found that for a temporal coincidence interval of 22 min the difference in IWV measured by these two systems is dominated by the volume mismatch (horizontal distance: 680 m). The outcome from this paper is twofold: (1) the IWV soundings by FTIR and DIAL agree very well in spite of the differing wavelength regions with different spectroscopic line parameters and retrieval algorithms used. (2) In order to derive an estimate of the precision of state-of-the-art IWV sounders from intercomparison experiments, it is necessary to use a temporal matching on time scales shorter than 10 min and a spatial matching on the 100-m scale.

Description: An effective method for the detection of trace species demonstrated using the MetOp Infrared Atmospheric Sounding Interferometer Atmospheric Measurement Techniques, 4, 1567-1580, 2011 Author(s): J. C. Walker, A. Dudhia, and E. Carboni Fast and reliable methods for the detection of atmospheric trace species are needed for near-real-time applications including volcanic hazard avoidance. One common approach using hyperspectral instruments is to measure the difference in brightness temperature between a small number of target sensitive and background channels to determine the presence of the target species. Although fast and robust, current brightness temperature difference methods do not fully exploit the spectral range and resolution of hyperspectral instruments, and the noise associated with the measurements remains high. In this paper, we describe a way to make full use of the spectral information from hyperspectral sounders allowing the presence of the target species to be determined with much better sensitivity in near-real-time if required. The technique is demonstrated using the MetOp Infrared Atmospheric Sounding Interferometer considering two case studies: (a) the detection of sulphur dioxide from the eruption of the Kasatochi volcano in Alaska in August 2008, and (b) the detection of ammonia emissions related to agriculture over Southern Asia in May 2008. The performance of this method is compared against that of existing brightness temperature difference methods. It is found that the sensitivity of the detection of these trace species is improved by up to an order of magnitude.

Description: Statistically optimized inversion algorithm for enhanced retrieval of aerosol properties from spectral multi-angle polarimetric satellite observations Atmospheric Measurement Techniques, 4, 975-1018, 2011 Author(s): O. Dubovik, M. Herman, A. Holdak, T. Lapyonok, D. Tanré, J. L. Deuzé, F. Ducos, A. Sinyuk, and A. Lopatin The proposed development is an attempt to enhance aerosol retrieval by emphasizing statistical optimization in inversion of advanced satellite observations. This optimization concept improves retrieval accuracy relying on the knowledge of measurement error distribution. Efficient application of such optimization requires pronounced data redundancy (excess of the measurements number over number of unknowns) that is not common in satellite observations. The POLDER imager on board the PARASOL micro-satellite registers spectral polarimetric characteristics of the reflected atmospheric radiation at up to 16 viewing directions over each observed pixel. The completeness of such observations is notably higher than for most currently operating passive satellite aerosol sensors. This provides an opportunity for profound utilization of statistical optimization principles in satellite data inversion. The proposed retrieval scheme is designed as statistically optimized multi-variable fitting of all available angular observations obtained by the POLDER sensor in the window spectral channels where absorption by gas is minimal. The total number of such observations by PARASOL always exceeds a hundred over each pixel and the statistical optimization concept promises to be efficient even if the algorithm retrieves several tens of aerosol parameters. Based on this idea, the proposed algorithm uses a large number of unknowns and is aimed at retrieval of extended set of parameters affecting measured radiation. The algorithm is designed to retrieve complete aerosol properties globally. Over land, the algorithm retrieves the parameters of underlying surface simultaneously with aerosol. In all situations, the approach is anticipated to achieve a robust retrieval of complete aerosol properties including information about aerosol particle sizes, shape, absorption and composition (refractive index). In order to achieve reliable retrieval from PARASOL observations even over very reflective desert surfaces, the algorithm was designed as simultaneous inversion of a large group of pixels within one or several images. Such multi-pixel retrieval regime takes advantage of known limitations on spatial and temporal variability in both aerosol and surface properties. Specifically the variations of the retrieved parameters horizontally from pixel-to-pixel and/or temporary from day-to-day are enforced to be smooth by additional a priori constraints. This concept is expected to provide satellite retrieval of higher consistency, because the retrieval over each single pixel will be benefiting from coincident aerosol information from neighboring pixels, as well, from the information about surface reflectance (over land) obtained in preceding and consequent observations over the same pixel. The paper provides in depth description of the proposed inversion concept, illustrates the algorithm performance by a series of numerical tests and presents the examples of preliminary retrieval results obtained from actual PARASOL observations. It should be noted that many aspects of the described algorithm design considerably benefited from experience accumulated in the preceding effort on developments of currently operating AERONET and PARASOL retrievals, as well as several core software components were inherited from those earlier algorithms.

Description: An investigation of atmospheric temperature profiles in the Australian region using collocated GPS radio occultation and radiosonde data Atmospheric Measurement Techniques, 4, 2087-2092, 2011 Author(s): K. Zhang, E. Fu, D. Silcock, Y. Wang, and Y. Kuleshov GPS radio occultation (RO) has been recognised as an alternative atmospheric upper air observation technique due to its distinct features and technological merits. The CHAllenging Minisatellite Payload (CHAMP) RO satellite and FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) RO constellation together have provided about ten years of high quality global coverage RO atmospheric profiles. This technique is best used for meteorological studies in the difficult-to-access areas such as deserts and oceans. To better understand and use RO data, effective quality assessment using independent radiosonde data and its associated collocation criteria used in tempo-spatial domain are important. This study compares GPS RO retrieved temperature profiles from both CHAMP (between May 2001 and October 2008) and FORMOSAT-3/COSMIC (between July 2006 and December 2009) with radiosonde data from 38 Australian radiosonde stations. The overall results show a good agreement between the two data sets. Different collocation criteria within 3 h and 300 km between the profile pairs have been applied and the impact of these different collocation criteria on the evaluation results is found statistically insignificantly. The CHAMP and FORMOSAT-3/COSMIC temperature profiles have been evaluated at 16 different pressure levels and the differences between GPS RO and radiosonde at different levels of the atmosphere have been studied. The result shows that the mean temperature difference between radiosonde and CHAMP is 0.39 °C (with a standard deviation of 1.20 °C) and the one between radiosonde and FORMOSAT-3/COSMIC is 0.37 °C (with a standard deviation of 1.24 °C). Different collocation criteria have been applied and insignificant differences were identified amongst the results.

Description: A high volume sampling system for isotope determination of volatile halocarbons and hydrocarbons Atmospheric Measurement Techniques, 4, 2073-2086, 2011 Author(s): E. Bahlmann, I. Weinberg, R. Seifert, C. Tubbesing, and W. Michaelis The isotopic composition of volatile organic compounds (VOCs) can provide valuable information on their sources and fate not deducible from mixing ratios alone. In particular the reported carbon stable isotope ratios of chloromethane and bromomethane from different sources cover a δ 13 C-range of almost 100‰ making isotope ratios a very promising tool for studying the biogeochemistry of these compounds. So far, the determination of the isotopic composition of C 1 and C 2 halocarbons others than chloromethane is hampered by their low mixing ratios. In order to determine the carbon isotopic composition of C 1 and C 2 halocarbons with mixing ratios as low as 1 pptv (i) a field suitable cryogenic high volume sampling system and (ii) a chromatographic set up for processing these samples have been developed and validated. The sampling system was tested at two different sampling sites, an urban and a coastal location in Northern Germany. The average δ 13 C-values for bromomethane at the urban site were −42.9 ± 1.1‰ and agreed well with previously published results. But at the coastal site bromomethane was substantially enriched in 13 C by almost 10‰. Less pronounced differences were observed for chlorodifluoromethane, 1,1,1-trichloroethane and chloromethane. We suggest that these differences are related to the turnover of these compounds in ocean surface waters. Furthermore we report first carbon isotope ratios for iodomethane (−40.4‰ to −79.8‰), bromoform (−13.8‰ to 22.9‰), and other halocarbons.

Description: Processing and validation of refractivity from GRAS radio occultation data Atmospheric Measurement Techniques, 4, 2065-2071, 2011 Author(s): K. B. Lauritsen, S. Syndergaard, H. Gleisner, M. E. Gorbunov, F. Rubek, M. B. Sørensen, and H. Wilhelmsen We discuss the processing of GRAS radio occultation (RO) data done at the GRAS Satellite Application Facility. The input data consists of operational near-real time bending angles from December 2010 from the Metop-A satellite operated by EUMETSAT. The data are processed by an Abel inversion algorithm in combination with statistical optimization based on a two-parameter fit to an MSIS climatology. We compare retrieved refractivity to analyses from ECMWF. It is found that for global averages, the mean differences to ECMWF analyses are smaller than 0.2% below 30 km (except near the surface), with standard deviations around 0.5% for altitudes between 8 and 25 km. The current processing is limited by several factors, which are discussed. In particular, the penetration depth for rising occultations is generally poor, which is related to the tracking of the L2 signal. Extrapolation of the difference between the L1 and L2 signals below the altitude where L2 is lost is possible and would generally allow deeper penetration of retrieved refractivity profiles into the lower troposphere.

Description: Greenhouse gas profiling by infrared-laser and microwave occultation: retrieval algorithm and demonstration results from end-to-end simulations Atmospheric Measurement Techniques, 4, 2035-2058, 2011 Author(s): V. Proschek, G. Kirchengast, and S. Schweitzer Measuring greenhouse gas (GHG) profiles with global coverage and high accuracy and vertical resolution in the upper troposphere and lower stratosphere (UTLS) is key for improved monitoring of GHG concentrations in the free atmosphere. In this respect a new satellite mission concept adding an infrared-laser part to the already well studied microwave occultation technique exploits the joint propagation of infrared-laser and microwave signals between Low Earth Orbit (LEO) satellites. This synergetic combination, referred to as LEO-LEO microwave and infrared-laser occultation (LMIO) method, enables to retrieve thermodynamic profiles (pressure, temperature, humidity) and accurate altitude levels from the microwave signals and GHG profiles from the simultaneously measured infrared-laser signals. However, due to the novelty of the LMIO method, a retrieval algorithm for GHG profiling is not yet available. Here we introduce such an algorithm for retrieving GHGs from LEO-LEO infrared-laser occultation (LIO) data, applied as a second step after retrieving thermodynamic profiles from LEO-LEO microwave occultation (LMO) data. We thoroughly describe the LIO retrieval algorithm and unveil the synergy with the LMO-retrieved pressure, temperature, and altitude information. We furthermore demonstrate the effective independence of the GHG retrieval results from background (a priori) information in discussing demonstration results from LMIO end-to-end simulations for a representative set of GHG profiles, including carbon dioxide (CO 2 ), water vapor (H 2 O), methane (CH 4 ), and ozone (O 3 ). The GHGs except for ozone are well retrieved throughout the UTLS, while ozone is well retrieved from about 10 km to 15 km upwards, since the ozone layer resides in the lower stratosphere. The GHG retrieval errors are generally smaller than 1% to 3% r.m.s., at a vertical resolution of about 1 km. The retrieved profiles also appear unbiased, which points to the climate benchmarking capability of the LMIO method. This performance, found here for clear-air atmospheric conditions, is unprecedented for vertical profiling of GHGs in the free atmosphere and encouraging for future LMIO implementation. Subsequent work will examine GHG retrievals in cloudy air, addressing retrieval performance when scanning through intermittent upper tropospheric cloudiness.

Description: Catalytic oxidation of H 2 on platinum: a robust method for generating low mixing ratio H 2 O standards Atmospheric Measurement Techniques, 4, 2059-2064, 2011 Author(s): A. W. Rollins, T. D. Thornberry, R.-S. Gao, B. D. Hall, and D. W. Fahey Standard reference samples of water vapor suitable for in situ calibration of atmospheric hygrometers are not currently widespread, leading to difficulties in unifying the calibrations of these hygrometers and potentially contributing to observed measurement discrepancies. We describe and evaluate a system for reliably and quantitatively converting mixtures of H 2 in air to H 2 O on a heated platinum surface, providing a compact, portable, adjustable source of water vapor. The technique is shown to be accurate and can be used to easily and reliably produce a wide range of water vapor concentrations (≈1 ppm −2%) on demand. The result is a H 2 O standard that is expected to be suitable for in situ calibration of aircraft hygrometers, with an accuracy nearly that of the available H 2 standards (≈±2%).

Description: The Level 2 research product algorithms for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) Atmospheric Measurement Techniques, 4, 2105-2124, 2011 Author(s): P. Baron, J. Urban, H. Sagawa, J. Möller, D. P. Murtagh, J. Mendrok, E. Dupuy, T. O. Sato, S. Ochiai, K. Suzuki, T. Manabe, T. Nishibori, K. Kikuchi, R. Sato, M. Takayanagi, Y. Murayama, M. Shiotani, and Y. Kasai This paper describes the algorithms of the level-2 research (L2r) processing chain developed for the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES). The chain has been developed in parallel to the operational chain for conducting researches on calibration and retrieval algorithms. L2r chain products are available to the scientific community. The objective of version 2 is the retrieval of the vertical distribution of trace gases in the altitude range of 18–90 km. A theoretical error analysis is conducted to estimate the retrieval feasibility of key parameters of the processing: line-of-sight elevation tangent altitudes (or angles), temperature and ozone profiles. While pointing information is often retrieved from molecular oxygen lines, there is no oxygen line in the SMILES spectra, so the strong ozone line at 625.371 GHz has been chosen. The pointing parameters and the ozone profiles are retrieved from the line wings which are measured with high signal to noise ratio, whereas the temperature profile is retrieved from the optically thick line center. The main systematic component of the retrieval error was found to be the neglect of the non-linearity of the radiometric gain in the calibration procedure. This causes a temperature retrieval error of 5–10 K. Because of these large temperature errors, it is not possible to construct a reliable hydrostatic pressure profile. However, as a consequence of the retrieval of pointing parameters, pressure induced errors are significantly reduced if the retrieved trace gas profiles are represented on pressure levels instead of geometric altitude levels. Further, various setups of trace gas retrievals have been tested. The error analysis for the retrieved HOCl profile demonstrates that best results for inverting weak lines can be obtained by using narrow spectral windows.

Description: Observation of strato-mesospheric CO above Kiruna with ground-based microwave radiometry – retrieval and satellite comparison Atmospheric Measurement Techniques, 4, 2389-2408, 2011 Author(s): C. G. Hoffmann, U. Raffalski, M. Palm, B. Funke, S. H. W. Golchert, G. Hochschild, and J. Notholt CO serves as a tracer for dynamics in the polar winter middle atmosphere. This work presents the retrieval and the characterization of ground-based CO measurements from the winters 2008/2009 and 2009/2010 by the Kiruna microwave radiometer KIMRA, located in northern Sweden (67.8° N, 20.4° E). Furthermore, the dataset is used for an extensive comparison to the recent satellite instruments MLS on Aura, ACE-FTS, and MIPAS on Envisat. The vmr profiles are retrieved using the optimal estimation approach. A detailed analysis of the averaging kernel functions is carried out, showing sensitivity of the measurements between 40 and 80 km altitude, a vertical resolution of 16 to 22 km, as well as a residual influence of the region up to 130 km altitude. An error assessment reveals a total error of the retrieved profile that increases with altitude and is approx. ±0.1 ppmv at 50 km altitude and ±3 ppmv at 80 km altitude. The main contributions to this total error arise from the measurement noise and the uncertainty of the used temperature profiles. The expected dynamical features of the polar winter middle atmosphere are qualitatively identified in the retrieved time series, but are not quantitatively analyzed here. The dense MLS dataset is used to investigate the influence of the collocation criteria on the satellite comparison, showing that relaxing the distance criterion causes a high bias for MLS. The comparison including the other instruments is difficult because of the small number of coincidences. However, it suggests that there is a general agreement between KIMRA and the satellite instruments below 65 km altitude, but a high bias for KIMRA above this altitude. Furthermore, the shape of the KIMRA profile appears to be systematically different from the satellite profiles.

Description: On the quantification of atmospheric carbonate carbon by thermal/optical analysis protocols Atmospheric Measurement Techniques, 4, 2409-2419, 2011 Author(s): A. Karanasiou, E. Diapouli, F. Cavalli, K. Eleftheriadis, M. Viana, A. Alastuey, X. Querol, and C. Reche The objective of this work was to study the possibility of identifying and quantifying atmospheric carbonate carbon (CC) by thermal-optical analysis. Three different temperature protocols, two modified NIOSH-like protocols (RT-QUARTZ-840 and RT-QUARTZ-700), and the EUSAAR_2 protocol were tested on filter samples containing known amounts of CC with the semi-continuous Sunset analyzer. Carbonate was quantified by the manual integration of the sharp peak appeared at the maximum temperature step of the inert mode. High recoveries of CC were achieved by all the thermal protocols. Using the EUSAAR_2 thermal protocol, more than 95% of CC evolved as OC during the maximum temperature step in inert atmosphere for CC amounts up to 56 μg. Using the RT-QUARTZ-840 protocol specifically developed for on-line analyses, CC completely evolves as OC during the maximum temperature step in the inert node, regardless of the CC concentration. However, the quantification of CC by the RT-QUARTZ-840 protocol suitable for the semi-continuous analyzer implies a high level of uncertainty (manual integration, residual contribution of organic carbon). Therefore, it is advisable to determine CC with an independent method (e.g. by acidic decomposition of CO 3 2- and subsequent detection of CO 2 ) when other sample aliquots are available. The comparison of the peak integration method with the direct determination of the CC sample content by acidic CO 2 release showed that the peak integration method provides always higher CC concentrations of about 33%. Nevertheless, the determination of CC with the RT-QUARTZ-840 protocol may be considered in cases where on line monitoring instruments are used and for areas where CC concentrations are expected to be significant e.g. Southern European countries. This case study suggests that users of the semi continuous Sunset analyzer can manually integrate the sharp peak (if present) at the maximum temperature step of the He mode (between 128–130 and 160–165 s when using the RT-QUARTZ-840 protocol) and calculate the CC concentration though with a rather high error.

Description: The CU ground MAX-DOAS instrument: characterization of RMS noise limitations and first measurements near Pensacola, FL of BrO, IO, and CHOCHO Atmospheric Measurement Techniques, 4, 2421-2439, 2011 Author(s): S. Coburn, B. Dix, R. Sinreich, and R. Volkamer We designed and assembled the University of Colorado Ground Multi AXis Differential Optical Absorption Spectroscopy (CU GMAX-DOAS) instrument to retrieve bromine oxide (BrO), iodine oxide (IO), formaldehyde (HCHO), glyoxal (CHOCHO), nitrogen dioxide (NO 2 ) and the oxygen dimer (O 4 ) in the coastal atmosphere of the Gulf of Mexico. The detection sensitivity of DOAS measurements is proportional to the root mean square (RMS) of the residual spectrum that remains after all absorbers have been subtracted. Here we describe the CU GMAX-DOAS instrument and demonstrate that the hardware is capable of attaining RMS of ∼6 × 10 −6 from solar stray light noise tests using high photon count spectra (compatible within a factor of two with photon shot noise). Laboratory tests revealed two critical instrument properties that, in practice, can limit the RMS: (1) detector non-linearity noise, RMS NLin , and (2) temperature fluctuations that cause variations in optical resolution (full width at half the maximum, FWHM, of atomic emission lines) and give rise to optical resolution noise, RMS FWHM . The non-linearity of our detector is low (∼10 −2 ) yet – unless actively controlled – is sufficiently large to create RMS NLin of up to 2 × 10 −4 . The optical resolution is sensitive to temperature changes (0.03 detector pixels °C −1 at 334 nm), and temperature variations of 0.1°C can cause RMS FWHM of ~1 × 10 −4 . Both factors were actively addressed in the design of the CU GMAX-DOAS instrument. With an integration time of 60 s the instrument can reach RMS noise of 3 × 10 −5 , and typical RMS in field measurements ranged from 6 × 10 −5 to 1.4 × 10 −4 . The CU GMAX-DOAS was set up at a coastal site near Pensacola, Florida, where we detected BrO, IO and CHOCHO in the marine boundary layer (MBL), with daytime average tropospheric vertical column densities (average of data above the detection limit), VCDs, of ∼2 × 10 13 molec cm −2 , 8 × 10 12 molec cm −2 and 4 × 10 14 molec cm −2 , respectively. HCHO and NO 2 were also detected with typical MBL VCDs of 1 × 10 16 and 3 × 10 15 molec cm −2 . These are the first measurements of BrO, IO and CHOCHO over the Gulf of Mexico. The atmospheric implications of these observations for elevated mercury wet deposition rates in this area are briefly discussed. The CU GMAX-DOAS has great potential to investigate RMS-limited problems, like the abundance and variability of trace gases in the MBL and possibly the free troposphere (FT).

Description: Nitrous oxide emissions from managed grassland: a comparison of eddy covariance and static chamber measurements Atmospheric Measurement Techniques, 4, 2179-2194, 2011 Author(s): S. K. Jones, D. Famulari, C. F. Di Marco, E. Nemitz, U. M. Skiba, R. M. Rees, and M. A. Sutton Managed grasslands are known to be an important source of N 2 O with estimated global losses of 2.5 Tg N 2 O-N yr −1 . Chambers are to date the most widely used method to measure N 2 O fluxes, but also micrometeorological methods are successfully applied. In this paper we present a comparison of N 2 O fluxes measured by non-steady state chambers and eddy covariance (EC) (using an ultra-sonic anemometer coupled with a tunable diode laser) from an intensively grazed and fertilised grassland site in South East Scotland. The measurements were taken after fertilisation events in 2003, 2007 and 2008. In four out of six comparison periods, a short-lived increase of N 2 O emissions was observed after mineral N application, returning to background level within 2–6 days. Highest fluxes were measured by both methods in July 2007 with maximum values of 1438 ng N 2 O-N m −2 s −1 (EC) and 651 ng N 2 O-N m −2 s −1 (chamber method). Negative fluxes above the detection limit were observed in all comparison periods by EC, while with chambers, the recorded negative fluxes were always below detection limit. Median and average fluxes over each period were always positive. Over all 6 comparison periods, 69% of N 2 O fluxes measured by EC at the time of chamber closure were within the range of the chamber measurements. N 2 O fluxes measured by EC during the time of chamber closure were not consistently smaller, neither larger, compared to those measured by chambers: this reflects the fact that the different techniques integrate fluxes over different spatial and temporal scales. Large fluxes measured by chambers may be representing local hotspots providing a small contribution to the flux measured by the EC method which integrates over a larger area. The spatial variability from chamber measurements was high, as shown by a coefficient of variation of up to 139%. No diurnal pattern of N 2 O fluxes was observed, possibly due to the small diurnal variations of soil temperature. The calculation of cumulative fluxes using different integration methods showed EC data provide generally lower estimates of N 2 O emissions than chambers.

Description: Simultaneous stable isotope analysis of methane and nitrous oxide on ice core samples Atmospheric Measurement Techniques, 4, 2607-2618, 2011 Author(s): C. J. Sapart, C. van der Veen, I. Vigano, M. Brass,, R. S. W. van de Wal, M. Bock, H. Fischer, T. Sowers, C. Buizert, P. Sperlich, T. Blunier, M. Behrens, J. Schmitt, B. Seth, and T. Röckmann Methane and nitrous oxide are important greenhouse gases which show a strong increase in atmospheric mixing ratios since pre-industrial time as well as large variations during past climate changes. The understanding of their biogeochemical cycles can be improved using stable isotope analysis. However, high-precision isotope measurements on air trapped in ice cores are challenging because of the high susceptibility to contamination and fractionation. Here, we present a dry extraction system for combined CH 4 and N 2 O stable isotope analysis from ice core air, using an ice grating device. The system allows simultaneous analysis of δD(CH 4 ) or δ 13 C(CH 4 ), together with δ 15 N(N 2 O), δ 18 O(N 2 O) and δ 15 N(NO + fragment) on a single ice core sample, using two isotope mass spectrometry systems. The optimum quantity of ice for analysis is about 600 g with typical "Holocene" mixing ratios for CH 4 and N 2 O. In this case, the reproducibility (1σ ) is 2.1‰ for δD(CH 4 ), 0.18‰ for δ 13 C(CH 4 ), 0.51‰ for δ 15 N(N 2 O), 0.69‰ for δ 18 O(N 2 O) and 1.12‰ for δ 15 N(NO + fragment). For smaller amounts of ice the standard deviation increases, particularly for N 2 O isotopologues. For both gases, small-scale intercalibrations using air and/or ice samples have been carried out in collaboration with other institutes that are currently involved in isotope measurements of ice core air. Significant differences are shown between the calibration scales, but those offsets are consistent and can therefore be corrected for.

Description: Volcanic ash detection and retrievals using MODIS data by means of neural networks Atmospheric Measurement Techniques, 4, 2619-2631, 2011 Author(s): M. Picchiani, M. Chini, S. Corradini, L. Merucci, P. Sellitto, F. Del Frate, and S. Stramondo Volcanic ash clouds detection and retrieval represent a key issue for aviation safety due to the harming effects on aircraft. A lesson learned from the recent Eyjafjallajokull eruption is the need to obtain accurate and reliable retrievals on a real time basis. In this work we have developed a fast and accurate Neural Network (NN) approach to detect and retrieve volcanic ash cloud properties from the Moderate Resolution Imaging Spectroradiometer (MODIS) data in the Thermal InfraRed (TIR) spectral range. Some measurements collected during the 2001, 2002 and 2006 Mt. Etna volcano eruptions have been considered as test cases. The ash detection and retrievals obtained from the Brightness Temperature Difference (BTD) algorithm are used as training for the NN procedure that consists in two separate steps: ash detection and ash mass retrieval. The ash detection is reduced to a classification problem by identifying two classes: "ashy" and "non-ashy" pixels in the MODIS images. Then the ash mass is estimated by means of the NN, replicating the BTD-based model performances. A segmentation procedure has also been tested to remove the false ash pixels detection induced by the presence of high meteorological clouds. The segmentation procedure shows a clear advantage in terms of classification accuracy: the main drawback is the loss of information on ash clouds distal part. The results obtained are very encouraging; indeed the ash detection accuracy is greater than 90%, while a mean RMSE equal to 0.365 t km −2 has been obtained for the ash mass retrieval. Moreover, the NN quickness in results delivering makes the procedure extremely attractive in all the cases when the rapid response time of the system is a mandatory requirement.

Description: Method for determination of stable carbon isotope ratio of methylnitrophenols in atmospheric particulate matter Atmospheric Measurement Techniques, 4, 2453-2464, 2011 Author(s): S. Moukhtar, M. Saccon, A. Kornilova, S. Irei, L. Huang, and J. Rudolph A technique for the measurement of the stable isotope ratio of methylnitrophenols in atmospheric particulate matter is presented. Atmospheric samples from rural and suburban areas were collected for evaluation of the procedure. Particulate matter was collected on quartz fibre filters using dichotomous high volume air samplers. Methylnitrophenols were extracted from the filters using acetonitrile. The sample was then purified using a combination of high-performance liquid chromatography and solid phase extraction. The final solution was then divided into two aliquots. To one aliquot, a derivatising agent, Bis(trimethylsilyl)trifluoroacetamide, was added for Gas Chromatography-Mass Spectrometry analysis. The second half of the sample was stored in a refrigerator. For samples with concentrations exceeding 1 ng μl −1 , the second half of the sample was used for measurement of stable carbon isotope ratios by Gas Chromatography-Isotope Ratio Mass Spectrometry. The procedure described in this paper provides a method for the analysis of methylnitrophenols in atmospheric particulate matter at concentrations as low as 0.3 pg m −3 and for stable isotope ratios with an accuracy of better than ±0.5‰ for concentrations exceeding 100 pg m −3 . In all atmospheric particulate matter samples analysed, 2-methyl-4-nitrophenol was found to be the most abundant methylnitrophenol, with concentrations ranging from the low pg m −3 range in rural areas to more than 200 pg m −3 in some samples from a suburban location.

Description: Stratospheric methane profiles from SCIAMACHY solar occultation measurements derived with onion peeling DOAS Atmospheric Measurement Techniques, 4, 2567-2577, 2011 Author(s): S. Noël, K. Bramstedt, A. Rozanov, H. Bovensmann, and J. P. Burrows Stratospheric methane (CH 4 ) profiles have been derived from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on ENVISAT with an updated version of the Onion Peeling DOAS (ONPD) method. The SCIAMACHY solar occultation measurements cover the latitudinal range between about 50° N and 70° N. Currently, reasonable results are obtained between 20 and 40 km altitude. Comparisons with correlative ACE-FTS measurements show an average agreement within the expected accuracy of the ACE-FTS data of about 10%. To demonstrate the capability of SCIAMACHY solar occultation measurements in the context of greenhouse gas monitoring, time series of stratospheric CH 4 profiles covering the period from 2003 to 2010 have been generated. The SCIAMACHY CH 4 profile solar occultation temporal series shows a strong seasonal cycle. This is attributed to the variations in both time and space of the retrieved data set. At lower altitudes, the observed temporal variations are explained by variations of the tropopause height. The temporal data set is also impacted by variations of the size and duration of the polar vortex in the northern hemisphere. The data set provides unique information about CH 4 changes in the stratosphere at mid to high latitudes.

Description: Water isotopic ratios from a continuously melted ice core sample Atmospheric Measurement Techniques, 4, 2531-2542, 2011 Author(s): V. Gkinis, T. J. Popp, T. Blunier, M. Bigler, S. Schüpbach, E. Kettner, and S. J. Johnsen A new technique for on-line high resolution isotopic analysis of liquid water, tailored for ice core studies is presented. We built an interface between a Wavelength Scanned Cavity Ring Down Spectrometer (WS-CRDS) purchased from Picarro Inc. and a Continuous Flow Analysis (CFA) system. The system offers the possibility to perform simultaneuous water isotopic analysis of δ 18 O and δD on a continuous stream of liquid water as generated from a continuously melted ice rod. Injection of sub μl amounts of liquid water is achieved by pumping sample through a fused silica capillary and instantaneously vaporizing it with 100% efficiency in a~home made oven at a temperature of 170 °C. A calibration procedure allows for proper reporting of the data on the VSMOW–SLAP scale. We apply the necessary corrections based on the assessed performance of the system regarding instrumental drifts and dependance on the water concentration in the optical cavity. The melt rates are monitored in order to assign a depth scale to the measured isotopic profiles. Application of spectral methods yields the combined uncertainty of the system at below 0.1‰ and 0.5‰ for δ 18 O and δD, respectively. This performance is comparable to that achieved with mass spectrometry. Dispersion of the sample in the transfer lines limits the temporal resolution of the technique. In this work we investigate and assess these dispersion effects. By using an optimal filtering method we show how the measured profiles can be corrected for the smoothing effects resulting from the sample dispersion. Considering the significant advantages the technique offers, i.e. simultaneuous measurement of δ 18 O and δD, potentially in combination with chemical components that are traditionally measured on CFA systems, notable reduction on analysis time and power consumption, we consider it as an alternative to traditional isotope ratio mass spectrometry with the possibility to be deployed for field ice core studies. We present data acquired in the field during the 2010 season as part of the NEEM deep ice core drilling project in North Greenland.

Description: A 3-D tomographic retrieval approach with advection compensation for the air-borne limb-imager GLORIA Atmospheric Measurement Techniques, 4, 2509-2529, 2011 Author(s): J. Ungermann, J. Blank, J. Lotz, K. Leppkes, L. Hoffmann, T. Guggenmoser, M. Kaufmann, P. Preusse, U. Naumann, and M. Riese Infrared limb sounding from aircraft can provide 2-D curtains of multiple trace gas species. However, conventional limb sounders view perpendicular to the aircraft axis and are unable to resolve the observed airmass along their line-of-sight. GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) is a new remote sensing instrument that is able to adjust its horizontal view angle with respect to the aircraft flight direction from 45° to 135°. This will allow for tomographic measurements of mesoscale structures for a wide variety of atmospheric constituents. Many flights of the GLORIA instrument will not follow closed curves that allow measuring an airmass from all directions. Consequently, it is examined by means of simulations, what spatial resolution can be expected under ideal conditions from tomographic evaluation of measurements made during a straight flight. It is demonstrated that the achievable horizontal resolution in the line-of-sight direction could be reduced from over 200 km to around 70 km compared to conventional retrievals and that the tomographic retrieval is also more robust against horizontal gradients in retrieved quantities in this direction. In a second step, it is shown that the incorporation of channels exhibiting different optical depth can further enhance the spatial resolution of 3-D retrievals enabling the exploitation of spectral samples usually not used for limb sounding due to their opacity. A second problem for tomographic retrievals is that advection, which can be neglected for conventional retrievals, plays an important role for the time-scales involved in a tomographic measurement flight. This paper presents a method to diagnose the effect of a time-varying atmosphere on a 3-D retrieval and demonstrates an effective way to compensate for effects of advection by incorporating wind-fields from meteorological datasets as a priori information.

Description: Design of a new multi-phase experimental simulation chamber for atmospheric photosmog, aerosol and cloud chemistry research Atmospheric Measurement Techniques, 4, 2465-2494, 2011 Author(s): J. Wang, J. F. Doussin, S. Perrier, E. Perraudin, Y. Katrib, E. Pangui, and B. Picquet-Varrault A new simulation chamber has been built at the Interuniversitary Laboratory of Atmospheric Systems (LISA). The CESAM chamber (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) is designed to allow research in multiphase atmospheric (photo-) chemistry which involves both gas phase and condensed phase processes including aerosol and cloud chemistry. CESAM has the potential to carry out variable temperature and pressure experiments under a very realistic artificial solar irradiation. It consists of a 4.2 m 3 stainless steel vessel equipped with three high pressure xenon arc lamps which provides a controlled and steady environment. Initial characterization results, all carried out at 290–297 K under dry conditions, concerning lighting homogeneity, mixing efficiency, ozone lifetime, radical sources, NO y wall reactivity, particle loss rates, background PM, aerosol formation and cloud generation are given. Photolysis frequencies of NO 2 and O 3 related to chamber radiation system were found equal to (4.2 × 10 −3 s −1 ) for J NO 2 and (1.4 × 10 −5 s −1 ) for J O 1 D which is comparable to the solar radiation in the boundary layer. An auxiliary mechanism describing NO y wall reactions has been developed. Its inclusion in the Master Chemical Mechanism allowed us to adequately model the results of experiments on the photo-oxidation of propene-NO x -Air mixtures. Aerosol yields for the α-pinene + O 3 system chosen as a reference were determined and found in good agreement with previous studies. Particle lifetime in the chamber ranges from 10 h to 4 days depending on particle size distribution which indicates that the chamber can provide high quality data on aerosol aging processes and their effects. Being evacuable, it is possible to generate in this new chamber clouds by fast expansion or saturation with or without the presence of pre-existing particles, which will provide a multiphase environment for aerosol-droplet interaction.

Description: Onboard measurement system of atmospheric carbon monoxide in the Pacific by voluntary observing ships Atmospheric Measurement Techniques, 4, 2495-2507, 2011 Author(s): H. Nara, H. Tanimoto, Y. Nojiri, H. Mukai, T. Machida, and Y. Tohjima Long-term monitoring of carbon monoxide (CO) mixing ratios in the atmosphere over the Pacific Ocean is being carried out on commercial cargo vessels participating in the National Institute for Environmental Studies Voluntary Observing Ships program. The program provides a regular platform for measurement of atmospheric CO along four cruise routes: from Japan to Oceania, the United States, Canada, and Southeast Asia. Flask samples are collected during every cruise for subsequent analysis in the laboratory, and in 2005, continuous shipboard CO measurements were initiated on three of the routes. Here, we describe the system we developed for onboard measurement of CO mixing ratios with a commercially available gas filter correlation CO analyzer. The fully automated system measures CO in ambient air, and the detector sensitivity and background signals are calibrated by referencing the measurements to a CO-in-air standard gas (~1 ppmv) and to CO-free air scrubbed with a catalyst, respectively. We examined the artificial production of CO in the high-pressure working gas standards during storage by referencing the measurements to CO standard gases maintained as our primary scale before and after use on the ships. The onboard performance of the continuous CO measurement system was evaluated by comparing its data with data from laboratory analyses of flask samples using gas chromatography with a reduction gas detector. The reasonably good consistency between the two independent measurement methods demonstrated the good performance of both methods over the course of 3–5 years. The continuous measurement system was more useful than the flask sampling method for regionally polluted air masses, which were often encountered on Southeast Asian cruises.

Description: Measurements of Humidity in the Atmosphere and Validation Experiments (MOHAVE)-2009: overview of campaign operations and results Atmospheric Measurement Techniques, 4, 2579-2605, 2011 Author(s): T. Leblanc, T. D. Walsh, I. S. McDermid, G. C. Toon, J.-F. Blavier, B. Haines, W. G. Read, B. Herman, E. Fetzer, S. Sander, T. Pongetti, D. N. Whiteman, T. G. McGee, L. Twigg, G. Sumnicht, D. Venable, M. Calhoun, A. Dirisu, D. Hurst, A. Jordan, E. Hall, L. Miloshevich, H. Vömel, C. Straub, N. Kampfer, G. E. Nedoluha, R. M. Gomez, K. Holub, S. Gutman, J. Braun, T. Vanhove, G. Stiller, and A. Hauchecorne The Measurements of Humidity in the Atmosphere and Validation Experiment (MOHAVE) 2009 campaign took place on 11–27 October 2009 at the JPL Table Mountain Facility in California (TMF). The main objectives of the campaign were to (1) validate the water vapor measurements of several instruments, including, three Raman lidars, two microwave radiometers, two Fourier-Transform spectrometers, and two GPS receivers (column water), (2) cover water vapor measurements from the ground to the mesopause without gaps, and (3) study upper tropospheric humidity variability at timescales varying from a few minutes to several days. A total of 58 radiosondes and 20 Frost-Point hygrometer sondes were launched. Two types of radiosondes were used during the campaign. Non negligible differences in the readings between the two radiosonde types used (Vaisala RS92 and InterMet iMet-1) made a small, but measurable impact on the derivation of water vapor mixing ratio by the Frost-Point hygrometers. As observed in previous campaigns, the RS92 humidity measurements remained within 5% of the Frost-point in the lower and mid-troposphere, but were too dry in the upper troposphere. Over 270 h of water vapor measurements from three Raman lidars (JPL and GSFC) were compared to RS92, CFH, and NOAA-FPH. The JPL lidar profiles reached 20 km when integrated all night, and 15 km when integrated for 1 h. Excellent agreement between this lidar and the frost-point hygrometers was found throughout the measurement range, with only a 3% (0.3 ppmv) mean wet bias for the lidar in the upper troposphere and lower stratosphere (UTLS). The other two lidars provided satisfactory results in the lower and mid-troposphere (2–5% wet bias over the range 3–10 km), but suffered from contamination by fluorescence (wet bias ranging from 5 to 50% between 10 km and 15 km), preventing their use as an independent measurement in the UTLS. The comparison between all available stratospheric sounders allowed to identify only the largest biases, in particular a 10% dry bias of the Water Vapor Millimeter-wave Spectrometer compared to the Aura-Microwave Limb Sounder. No other large, or at least statistically significant, biases could be observed. Total Precipitable Water (TPW) measurements from six different co-located instruments were available. Several retrieval groups provided their own TPW retrievals, resulting in the comparison of 10 different datasets. Agreement within 7% (0.7 mm) was found between all datasets. Such good agreement illustrates the maturity of these measurements and raises confidence levels for their use as an alternate or complementary source of calibration for the Raman lidars. Tropospheric and stratospheric ozone and temperature measurements were also available during the campaign. The water vapor and ozone lidar measurements, together with the advected potential vorticity results from the high-resolution transport model MIMOSA, allowed the identification and study of a deep stratospheric intrusion over TMF. These observations demonstrated the lidar strong potential for future long-term monitoring of water vapor in the UTLS.

Description: Improving measurements of SF 6 for the study of atmospheric transport and emissions Atmospheric Measurement Techniques, 4, 2441-2451, 2011 Author(s): B. D. Hall, G. S. Dutton, D. J. Mondeel, J. D. Nance, M. Rigby, J. H. Butler, F. L. Moore, D. F. Hurst, and J. W. Elkins Sulfur hexafluoride (SF 6 ) is a potent greenhouse gas and useful atmospheric tracer. Measurements of SF 6 on global and regional scales are necessary to estimate emissions and to verify or examine the performance of atmospheric transport models. Typical precision for common gas chromatographic methods with electron capture detection (GC-ECD) is 1–2%. We have modified a common GC-ECD method to achieve measurement precision of 0.5% or better. Global mean SF 6 measurements were used to examine changes in the growth rate of SF 6 and corresponding SF 6 emissions. Global emissions and mixing ratios from 2000–2008 are consistent with recently published work. More recent observations show a 10% decline in SF 6 emissions in 2008–2009, which seems to coincide with a decrease in world economic output. This decline was short-lived, as the global SF 6 growth rate has recently increased to near its 2007–2008 maximum value of 0.30±0.03 pmol mol −1 (ppt) yr −1 (95% C.L.).

Description: Description and validation of an AOT product over land at the 0.6 μm channel of the SEVIRI sensor onboard MSG Atmospheric Measurement Techniques, 4, 2543-2565, 2011 Author(s): E. Bernard, C. Moulin, D. Ramon, D. Jolivet, J. Riedi, and J.-M. Nicolas The Spinning Enhanced Visible and InfraRed Imager (SEVIRI) aboard Meteosat Second Generation (MSG) launched in 2003 by EUMETSAT is dedicated to the Nowcasting applications and Numerical Weather Prediction and to the provision of observations for climate monitoring and research. We use the data in visible and near infrared (NIR) channels to derive the aerosol optical thickness (AOT) over land. The algorithm is based on the assumption that the top of the atmosphere (TOA) reflectance increases with the aerosol load. This is a reasonable assumption except in case of absorbing aerosols above bright surfaces. We assume that the minimum in a 14-days time series of the TOA reflectance is, once corrected from gaseous scattering and absorption, representative of the surface reflectance. The AOT and the aerosol model (a set of 5 models is used), are retrieved by matching the simulated TOA reflectance with the TOA reflectances measured by SEVIRI in its visible and NIR spectral bands. The high temporal resolution of the data acquisition by SEVIRI allows to retrieve the AOT every 15 min with a spatial resolution of 3 km at sub-satellite point, over the entire SEVIRI disk covering Europe, Africa and part of South America. The resulting AOT, a level 2 product at the native temporal and spatial SEVIRI resolutions, is presented and evaluated in this paper. The AOT has been validated using ground based measurements from AErosol RObotic NETwork (AERONET), a sun-photometer network, focusing over Europe for 3 months in 2006. The SEVIRI estimates correlate well with the AERONET measurements, r = 0.64, with a slight overestimate, bias = −0.017. The sources of errors are mainly the cloud contamination and the bad estimation of the surface reflectance. The temporal evolutions exhibited by both datasets show very good agreement which allows to conclude that the AOT Level 2 product from SEVIRI can be used to quantify the aerosol content and to monitor its daily evolution with a high temporal frequency. The comparison with daily maps of Moderate Resolution Imaging Spectroradiometer (MODIS) AOT level 3 product shows qualitative good agreement in the retrieved geographic patterns of AOT. Given the high spatial and temporal resolutions obtained with this approach, our results have clear potential for applications ranging from air quality monitoring to climate studies. This paper presents a first evaluation and validation of the derived AOT over Europe in order to document the overall quality of a product that will be made publicly available to the users of the aforementioned research communities.

Description: Atmospheric influences on infrared-laser signals used for occultation measurements between Low Earth Orbit satellites Atmospheric Measurement Techniques, 4, 2273-2292, 2011 Author(s): S. Schweitzer, G. Kirchengast, and V. Proschek LEO-LEO infrared-laser occultation (LIO) is a new occultation technique between Low Earth Orbit (LEO) satellites, which applies signals in the short wave infrared spectral range (SWIR) within 2 μm to 2.5 μm. It is part of the LEO-LEO microwave and infrared-laser occultation (LMIO) method that enables to retrieve thermodynamic profiles (pressure, temperature, humidity) and altitude levels from microwave signals and profiles of greenhouse gases and further variables such as line-of-sight wind speed from simultaneously measured LIO signals. Due to the novelty of the LMIO method, detailed knowledge of atmospheric influences on LIO signals and of their suitability for accurate trace species retrieval did not yet exist. Here we discuss these influences, assessing effects from refraction, trace species absorption, aerosol extinction and Rayleigh scattering in detail, and addressing clouds, turbulence, wind, scattered solar radiation and terrestrial thermal radiation as well. We show that the influence of refractive defocusing, foreign species absorption, aerosols and turbulence is observable, but can be rendered small to negligible by use of the differential transmission principle with a close frequency spacing of LIO absorption and reference signals within 0.5%. The influences of Rayleigh scattering and terrestrial thermal radiation are found negligible. Cloud-scattered solar radiation can be observable under bright-day conditions, but this influence can be made negligible by a close time spacing (within 5 ms) of interleaved laser-pulse and background signals. Cloud extinction loss generally blocks SWIR signals, except very thin or sub-visible cirrus clouds, which can be addressed by retrieving a cloud layering profile and exploiting it in the trace species retrieval. Wind can have a small influence on the trace species absorption, which can be made negligible by using a simultaneously retrieved or a moderately accurate background wind speed profile. We conclude that the set of SWIR channels proposed for implementing the LMIO method (Kirchengast and Schweitzer, 2011) provides adequate sensitivity to accurately retrieve eight trace species of key importance to climate and atmospheric chemistry (H 2 O, CO 2 , 13 CO 2 , C 18 OO, CH 4 , N 2 O, O 3 , CO) in the upper troposphere/lower stratosphere region outside clouds under all atmospheric conditions. Two further species (HDO, H 2 18 O) can be retrieved in the upper troposphere.

Description: Tropospheric and total ozone columns over Paris (France) measured using medium-resolution ground-based solar-absorption Fourier-transform infrared spectroscopy Atmospheric Measurement Techniques, 4, 2323-2331, 2011 Author(s): C. Viatte, B. Gaubert, M. Eremenko, F. Hase, M. Schneider, T. Blumenstock, M. Ray, P. Chelin, J.-M. Flaud, and J. Orphal Ground-based Fourier-transform infrared (FTIR) solar absorption spectroscopy is a powerful remote sensing technique providing information on the vertical distribution of various atmospheric constituents. This work presents the first evaluation of a mid-resolution ground-based FTIR to measure tropospheric ozone, independently of stratospheric ozone. This is demonstrated using a new atmospheric observatory (named OASIS for "Observations of the Atmosphere by Solar absorption Infrared Spectroscopy"), installed in Créteil (France). The capacity of the technique to separate stratospheric and tropospheric ozone is demonstrated. Daily mean tropospheric ozone columns derived from the Infrared Atmospheric Sounding Interferometer (IASI) and from OASIS measurements are compared for summer 2009 and a good agreement of −5.6 (±16.1) % is observed. Also, a qualitative comparison between in-situ surface ozone measurements and OASIS data reveals OASIS's capacity to monitor seasonal tropospheric ozone variations, as well as ozone pollution episodes in summer 2009 around Paris. Two extreme pollution events are identified (on the 1 July and 6 August 2009) for which ozone partial columns from OASIS and predictions from a regional air-quality model (CHIMERE) are compared following strict criteria of temporal and spatial coincidence. An average bias of 0.2%, a mean square error deviation of 7.6%, and a correlation coefficient of 0.91 is found between CHIMERE and OASIS, demonstrating the potential of a mid-resolution FTIR instrument in ground-based solar absorption geometry for tropospheric ozone monitoring.

Description: Evaluation of the flux gradient technique for measurement of ozone surface fluxes over snowpack at Summit, Greenland Atmospheric Measurement Techniques, 4, 2305-2321, 2011 Author(s): F. Bocquet, D. Helmig, B. A. Van Dam, and C. W. Fairall A multi-step procedure for investigating ozone surface fluxes over polar snow by the tower gradient method was developed and evaluated. These measurements were then used to obtain five months (April–August 2004) of turbulent ozone flux data at the Summit research camp located in the center of the Greenland ice shield. Turbulent fluxes were determined by the gradient method incorporating tower measurements of (a) ozone gradients measured by commercial ultraviolet absorption analyzers, (b) ambient temperature gradients using aspirated thermocouple sensors, and (c) wind speed gradients determined by cup anemometers. All gradient instruments were regularly inter-compared by bringing sensors or inlets to the same measurement height. The developed protocol resulted in an uncertainty on the order of 0.1 ppbv for 30-min averaged ozone gradients that were used for the ozone flux calculations. This protocol facilitated a lower sensitivity threshold for the ozone flux determination of ∼8 × 10 −3 μg m −2 s −1 , respectively ∼0.01 cm s −1 for the ozone deposition velocity for typical environmental conditions encountered at Summit. Uncertainty in the 30-min ozone exchange measurements (evaluated by the Monte Carlo statistical approach) was on the order of 10 −2 cm s −1 . This uncertainty typically accounted to ~20–100% of the ozone exchange velocities that were determined. These measurements are among the most sensitive ozone deposition determinations reported to date. This flux experiment allowed for measurements of the relatively low ozone uptake rates encountered for polar snow, and thereby the study of their environmental and spring-versus-summer dependencies.

Description: Airborne formaldehyde measurements using PTR-MS: calibration, humidity dependence, inter-comparison and initial results Atmospheric Measurement Techniques, 4, 2345-2358, 2011 Author(s): C. Warneke, P. Veres, J. S. Holloway, J. Stutz, C. Tsai, S. Alvarez, B. Rappenglueck, F. C. Fehsenfeld, M. Graus, J. B. Gilman, and J. A. de Gouw We present quantitative, fast time response measurements of formaldehyde (HCHO) onboard an aircraft using a Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS) instrument. The HCHO measurement by PTR-MS is strongly humidity dependent and therefore airborne measurements are difficult and have not been reported. The PTR-MS instrument was run in the standard PTR-MS operating mode (de Gouw and Warneke, 2007), where about 15 volatile organic compounds (VOCs) are measured together with HCHO onboard the NOAA WP-3 aircraft during the CalNex 2010 campaign in California. We compare the humidity dependence determined in the laboratory with in-flight calibrations of HCHO and calculate the HCHO mixing ratio during all flights using the results from both. The detection limit ( S/N = 1) for HCHO was between 100 pptv in the dry free troposphere and 300 pptv in the humid marine boundary layer for a one second acquisition time every 17 s. The PTR-MS measurements are compared with HCHO measurements using a DOAS instrument and a Hantzsch monitor at a ground site in Pasadena. The PTR-MS agreed with the DOAS within the stated uncertainties and was just outside the uncertainties with the Hantzsch. We also compare HCHO enhancement ratios in the Los Angeles basin and in the free troposphere with literature values and find good agreement. The usefulness of the PTR-MS HCHO measurements in atmospheric observations is demonstrated by following an isolated anthropogenic plume. The photochemical production of HCHO can be observed simultaneously with production of acetaldehyde and the photochemical degradation of aromatic compounds using the PTR-MS. The results show that PTR-MS seems a useful instrument to measure HCHO, but more inter-comparisons are needed.

Description: Sensitivity of Dobson and Brewer Umkehr ozone profile retrievals to ozone cross-sections and stray light effects Atmospheric Measurement Techniques, 4, 1841-1853, 2011 Author(s): I. Petropavlovskikh, R. Evans, G. McConville, S. Oltmans, D. Quincy, K. Lantz, P. Disterhoft, M. Stanek, and L. Flynn Remote sounding methods are used to derive ozone profile and column information from various ground-based and satellite measurements. Vertical ozone profiles measured in Dobson units (DU) are currently retrieved based on laboratory measurements of the ozone absorption cross-section spectrum between 270 and 400 nm published in 1985 by Bass and Paur (BP). Recently, the US National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) proposed using the set of ozone cross-section measurements made at the Daumont laboratory in 1992 (BDM) for revising the Aura Ozone Monitoring Instrument (OMI) and Global Ozone Monitoring Experiment (GOME) satellite ozone profiles and total ozone column retrievals. Dobson Umkehr zenith sky data have been collected by NOAA ground-based stations at Boulder, CO (BDR) and Mauna Loa Observatory, HI (MLO) since the 1980s. The UMK04 algorithm is based on the BP ozone cross-section data. It is currently used for all Dobson Umkehr data processing submitted to the World Ozone and Ultraviolet radiation Data Centre (WOUDC) under the Global Atmosphere Watch (GAW) program of the World Meteorological Organization (WMO). Ozone profiles are also retrieved from measurements by the Mark IV Brewers operated by the NOAA-EPA Brewer Spectrophotometer UV and Ozone Network (NEUBrew) using a modified UMK04 algorithm (O3BUmkehr v.2.6, Martin Stanek). This paper describes the sensitivity of the Umkehr retrievals with respect to the proposed ozone cross-section changes. It is found that the ozone cross-section choice only minimally (within the retrieval accuracy) affects the Dobson and the Brewer Umkehr retrievals. On the other hand, significantly larger errors were found in the MLO and Boulder Umkehr ozone data (−8 and +5% bias in stratosphere and troposphere respectively) when the out-of-band (OOB) stray light contribution to the Umkehr measurement is not taken into account (correction is currently not included in the UMK04). The vertical distribution of OOB effect in the retrieved profile can be related to the local ozone climatology, instrument degradation, and optical characteristics of the instrument. Nonetheless, recurring OOB errors do not contribute to the long-term ozone trends.

Description: Comparison of HDO measurements from Envisat/MIPAS with observations by Odin/SMR and SCISAT/ACE-FTS Atmospheric Measurement Techniques, 4, 1855-1874, 2011 Author(s): S. Lossow, J. Steinwagner, J. Urban, E. Dupuy, C. D. Boone, S. Kellmann, A. Linden, M. Kiefer, U. Grabowski, N. Glatthor, M. Höpfner, T. Röckmann, D. P. Murtagh, K. A. Walker, P. F. Bernath, T. von Clarmann, and G. P. Stiller Measurements of thermal emission in the mid-infrared by Envisat/MIPAS allow the retrieval of HDO information roughly in the altitude range between 10 km and 50 km. From June 2002 to March 2004 MIPAS performed measurements in the full spectral resolution mode. To assess the quality of the HDO data set obtained during that period comparisons with measurements by Odin/SMR and SCISAT/ACE-FTS were performed. Comparisons were made on profile-to-profile basis as well as using seasonal and monthly averages. All in all the comparisons yield favourable results. The largest deviations between MIPAS and ACE-FTS are observed below 15 km, where relative deviations can occasionally exceed 100%. Despite these deviations in the absolute amount of HDO the latitudinal structures observed by both instruments are consistent in this altitude range. Between 15 km and 20 km there is less good agreement, in particular in the Antarctic during winter and spring. Also in the tropics some deviations are found. Above 20 km there is a high consistency in the structures observed by all three instruments. MIPAS and ACE-FTS typically agree within 10%, with MIPAS mostly showing higher abundances than ACE-FTS. Both data sets show considerably more HDO than SMR. This bias can be explained basically by uncertainties in spectroscopic parameters. Above 40 km, where the MIPAS HDO retrieval reaches its limits, still good agreement with the structures observed by SMR is found for most seasons. This puts some confidence in the MIPAS data at these altitudes.

Description: An improved tropospheric NO 2 column retrieval algorithm for the Ozone Monitoring Instrument Atmospheric Measurement Techniques, 4, 1905-1928, 2011 Author(s): K. F. Boersma, H. J. Eskes, R. J. Dirksen, R. J. van der A, J. P. Veefkind, P. Stammes, V. Huijnen, Q. L. Kleipool, M. Sneep, J. Claas, J. Leitão, A. Richter, Y. Zhou, and D. Brunner We present an improved tropospheric nitrogen dioxide column retrieval algorithm (DOMINO v2.0) for OMI based on better air mass factors (AMFs) and a correction for across-track stripes resulting from calibration errors in the OMI backscattered reflectances. Since October 2004, NO 2 retrievals from the Ozone Monitoring Instrument (OMI), a UV/Vis nadir spectrometer onboard NASA's EOS-Aura satellite, have been used with success in several scientific studies focusing on air quality monitoring, detection of trends, and NO x emission estimates. Dedicated evaluations of previous DOMINO tropospheric NO 2 retrievals indicated their good quality, but also suggested that the tropospheric columns were susceptible to high biases (by 0–40%), probably because of errors in the air mass factor calculations. Here we update the DOMINO air mass factor approach. We calculate a new look-up table (LUT) for altitude-dependent AMFs based on more realistic atmospheric profile parameters, and include more surface albedo and surface pressure reference points than before. We improve the sampling of the TM4 model, resulting in a priori NO 2 profiles that are better mixed throughout the boundary layer. We evaluate the NO 2 profiles simulated with the improved TM4 sampling as used in the AMF calculations and show that they are highly consistent with in situ NO 2 measurements from aircraft during the INTEX-A and INTEX-B campaigns in 2004 and 2006. Our air mass factor calculations are further updated by the implementation of a high-resolution terrain height and a high-resolution surface albedo climatology based on OMI measurements. Together with a correction for across-track stripes, the overall impact of the improved terrain height and albedo descriptions is modest (

Description: A versatile and reproducible automatic injection system for liquid standard introduction: application to in-situ calibration Atmospheric Measurement Techniques, 4, 1937-1942, 2011 Author(s): G. Isaacman, N. M. Kreisberg, D. R. Worton, S. V. Hering, and A. H. Goldstein The quantitation of trace organic compounds in ambient organic aerosol is difficult due to the chemical complexity of these mixtures, but is needed to provide insight into their sources and formation processes. Compound-level characterization of organic aerosols is typically performed through sample collection followed by gas or liquid chromatography. With these methods, introduction of liquid standards has long been used as an effective means of quantifying trace compounds, but automating this technique for use with in-situ instrumentation has not previously been achieved. Here we develop an automatic injection system (AutoInject) for the introduction of liquids into a custom collection and analysis cell for improved quantitation in chromatographic measurements. The system consists of chilled reservoirs containing liquid standards from which a sample loop is loaded and then injected into the cell. The AutoInject is shown to be reproducible over 106 injections with a relative standard deviation of 1.5%, and have negligible injection-to-injection carryover. A 6-port selector allows injection of different liquid standards separately or simultaneously. Additionally, automatic injection of multiple sample loops is shown to generate a linear multi-point calibration curve. Tests conducted in this work focus on use with the T hermal desorption A erosol G as chromatograph (TAG), but the flexibility of the system allows it to be used for a variety of applications.

Description: Observation of slant column NO 2 using the super-zoom mode of AURA-OMI Atmospheric Measurement Techniques, 4, 1929-1935, 2011 Author(s): L. C. Valin, A. R. Russell, E. J. Bucsela, J. P. Veefkind, and R. C. Cohen We retrieve slant column NO 2 from the super-zoom mode of the Ozone Monitoring Instrument (OMI) to explore its utility for understanding NO x emissions and variability. Slant column NO 2 is operationally retrieved from OMI (Boersma et al., 2007; Bucsela et al., 2006) with a nadir footprint of 13 × 24 km 2 , the result of averaging eight detector elements on board the instrument. For 85 orbits in late 2004, OMI reported observations from individual "super-zoom" detector elements (spaced at 13 × 3 km 2 at nadir). We assess the spatial response of these individual detector elements in-flight and determine an upper-bound on spatial resolution of 9 km, in good agreement with on-ground calibration (7 km FWHM). We determine the precision of the super-zoom mode to be 2.1 × 10 15 molecules cm −2 , approximately a factor of √8 lower than an identical retrieval at operational scale as expected if random noise dominates the uncertainty. We retrieve slant column NO 2 over the Satpura power plant in India; Seoul, South Korea; Dubai, United Arab Emirates; and a set of large point sources on the Rihand Reservoir in India using differential optical absorption spectroscopy (DOAS). Over these sources, the super-zoom mode of OMI observes variation in slant column NO 2 of up to 30 × the instrumental precision within one operational footprint.

Description: Quantifying uncertainty in climatological fields from GPS radio occultation: an empirical-analytical error model Atmospheric Measurement Techniques, 4, 2019-2034, 2011 Author(s): B. Scherllin-Pirscher, G. Kirchengast, A. K. Steiner, Y.-H. Kuo, and U. Foelsche Due to the measurement principle of the radio occultation (RO) technique, RO data are highly suitable for climate studies. RO profiles can be used to build climatological fields of different atmospheric parameters like bending angle, refractivity, density, pressure, geopotential height, and temperature. RO climatologies are affected by random (statistical) errors, sampling errors, and systematic errors, yielding a total climatological error. Based on empirical error estimates, we provide a simple analytical error model for these error components, which accounts for vertical, latitudinal, and seasonal variations. The vertical structure of each error component is modeled constant around the tropopause region. Above this region the error increases exponentially, below the increase follows an inverse height power-law. The statistical error strongly depends on the number of measurements. It is found to be the smallest error component for monthly mean 10° zonal mean climatologies with more than 600 measurements per bin. Due to smallest atmospheric variability, the sampling error is found to be smallest at low latitudes equatorwards of 40°. Beyond 40°, this error increases roughly linearly, with a stronger increase in hemispheric winter than in hemispheric summer. The sampling error model accounts for this hemispheric asymmetry. However, we recommend to subtract the sampling error when using RO climatologies for climate research since the residual sampling error remaining after such subtraction is estimated to be only about 30% of the original one or less. The systematic error accounts for potential residual biases in the measurements as well as in the retrieval process and generally dominates the total climatological error. Overall the total error in monthly means is estimated to be smaller than 0.07% in refractivity and 0.15 K in temperature at low to mid latitudes, increasing towards higher latitudes. This study focuses on dry atmospheric parameters as retrieved from RO measurements so for context we also quantitatively explain the difference between dry and physical atmospheric parameters, which can be significant at altitudes below about 6 km (high latitudes) to 10 km (low latitudes).

Description: Remote sensing of aerosols by using polarized, directional and spectral measurements within the A-Train: the PARASOL mission Atmospheric Measurement Techniques, 4, 1383-1395, 2011 Author(s): D. Tanré, F. M. Bréon, J. L. Deuzé, O. Dubovik, F. Ducos, P. François, P. Goloub, M. Herman, A. Lifermann, and F. Waquet Instruments dedicated to aerosol monitoring are recently available and the POLDER (POLarization and Directionality of the Earth's Reflectances) instrument on board the PARASOL (Polarization & Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar) mission is one of them. By measuring the spectral, angular and polarization properties of the radiance at the top of the atmosphere, in coordination with the other A-Train instruments, PARASOL provides the aerosol optical depths (AOD) as well as several optical and microphysical aerosol properties. The instrument, the inversion schemes and the list of aerosol parameters are described. Examples of retrieved aerosol parameters are provided as well as innovative approaches and further inversion techniques.

Description: Sensitivity studies for a space-based methane lidar mission Atmospheric Measurement Techniques, 4, 2195-2211, 2011 Author(s): C. Kiemle, M. Quatrevalet, G. Ehret, A. Amediek, A. Fix, and M. Wirth Methane is the third most important greenhouse gas in the atmosphere after water vapour and carbon dioxide. A major handicap to quantify the emissions at the Earth's surface in order to better understand biosphere-atmosphere exchange processes and potential climate feedbacks is the lack of accurate and global observations of methane. Space-based integrated path differential absorption (IPDA) lidar has potential to fill this gap, and a Methane Remote Lidar Mission (MERLIN) on a small satellite in polar orbit was proposed by DLR and CNES in the frame of a German-French climate monitoring initiative. System simulations are used to identify key performance parameters and to find an advantageous instrument configuration, given the environmental, technological, and budget constraints. The sensitivity studies use representative averages of the atmospheric and surface state to estimate the measurement precision, i.e. the random uncertainty due to instrument noise. Key performance parameters for MERLIN are average laser power, telescope size, orbit height, surface reflectance, and detector noise. A modest-size lidar instrument with 0.45 W average laser power and 0.55 m telescope diameter on a 506 km orbit could provide 50-km averaged methane column measurement along the sub-satellite track with a precision of about 1% over vegetation. The use of a methane absorption trough at 1.65 μm improves the near-surface measurement sensitivity and vastly relaxes the wavelength stability requirement that was identified as one of the major technological risks in the pre-phase A studies for A-SCOPE, a space-based IPDA lidar for carbon dioxide at the European Space Agency. Minimal humidity and temperature sensitivity at this wavelength position will enable accurate measurements in tropical wetlands, key regions with largely uncertain methane emissions. In contrast to actual passive remote sensors, measurements in Polar Regions will be possible and biases due to aerosol layers and thin ice clouds will be minimised.

Description: 3-D imaging and quantification of graupel porosity by synchrotron-based micro-tomography Atmospheric Measurement Techniques, 4, 2225-2234, 2011 Author(s): F. Enzmann, M. M. Miedaner, M. Kersten, N. von Blohn, K. Diehl, S. Borrmann, M. Stampanoni, M. Ammann, and T. Huthwelker The air bubble structure is an important parameter to determine the radiation properties of graupel and hailstones. For 3-D imaging of this structure at micron resolution, a cryo-stage was developed. This stage was used at the tomography beamline of the Swiss Light Source (SLS) synchrotron facility. The cryo-stage setup provides for the first time 3-D-data on the individual pore morphology of ice particles down to infrared wavelength resolution. In the present study, both sub-mm size natural and artificial ice particles rimed in a wind tunnel were investigated. In the natural rimed ice particles, Y-shaped air-filled closed pores were found. When kept for half an hour at −8 °C, this morphology transformed into smaller and more rounded voids well known from literature. Therefore, these round structures seem to represent an artificial rather than in situ pore structure, in contrast to the observed y-shaped structures found in the natural ice particles. Hence, for morphological studies on natural ice samples, special care must be taken to minimize any thermal cycling between sampling and measurement, with least artifact production at liquid nitrogen temperatures.

Description: Modeling the ascent of sounding balloons: derivation of the vertical air motion Atmospheric Measurement Techniques, 4, 2235-2253, 2011 Author(s): A. Gallice, F. G. Wienhold, C. R. Hoyle, F. Immler, and T. Peter A new model to describe the ascent of sounding balloons in the troposphere and lower stratosphere (up to ∼30–35 km altitude) is presented. Contrary to previous models, detailed account is taken of both the variation of the drag coefficient with altitude and the heat imbalance between the balloon and the atmosphere. To compensate for the lack of data on the drag coefficient of sounding balloons, a reference curve for the relationship between drag coefficient and Reynolds number is derived from a dataset of flights launched during the Lindenberg Upper Air Methods Intercomparisons (LUAMI) campaign. The transfer of heat from the surrounding air into the balloon is accounted for by solving the radial heat diffusion equation inside the balloon. In its present state, the model does not account for solar radiation, i.e. it is only able to describe the ascent of balloons during the night. It could however be adapted to also represent daytime soundings, with solar radiation modeled as a diffusive process. The potential applications of the model include the forecast of the trajectory of sounding balloons, which can be used to increase the accuracy of the match technique, and the derivation of the air vertical velocity. The latter is obtained by subtracting the ascent rate of the balloon in still air calculated by the model from the actual ascent rate. This technique is shown to provide an approximation for the vertical air motion with an uncertainty error of 0.5 m s −1 in the troposphere and 0.2 m s −1 in the stratosphere. An example of extraction of the air vertical velocity is provided in this paper. We show that the air vertical velocities derived from the balloon soundings in this paper are in general agreement with small-scale atmospheric velocity fluctuations related to gravity waves, mechanical turbulence, or other small-scale air motions measured during the SUCCESS campaign (Subsonic Aircraft: Contrail and Cloud Effects Special Study) in the orographically unperturbed mid-latitude middle troposphere.

Description: Analysis of GPS radio occultation data from the FORMOSAT-3/COSMIC and Metop/GRAS missions at CDAAC Atmospheric Measurement Techniques, 4, 2255-2272, 2011 Author(s): W. Schreiner, S. Sokolovskiy, D. Hunt, C. Rocken, and Y.-H. Kuo This study investigates the noise level and mission-to-mission stability of Global Positioning System (GPS) radio occultation (RO) neutral atmospheric bending angle data at the UCAR COSMIC Data Analysis and Archive Center (CDAAC). Data are used from two independently developed RO instruments currently flying in orbit on the FORMOSAT-3/COSMIC (F3C) and Metop/GRAS (GNSS Receiver for Atmospheric Sounding) missions. The F3C 50 Hz RO data are post-processed with a single-difference excess atmospheric phase algorithm, and the Metop/GRAS 50 Hz closed loop and raw sampling (down-sampled from 1000 Hz to 50 Hz) data are processed with a zero-difference algorithm. The standard deviations of the F3C and Metop/GRAS bending angles from climatology between 60 and 80 km altitude from June–December 2009 are approximately 1.78 and 1.13 μrad, respectively. The F3C standard deviation reduces significantly to 1.44 μrad when single-difference processing uses GPS satellites on the same side of the spacecraft. The higher noise level for F3C bending angles can be explained by additional noise from the reference link phase data that are required with single-difference processing. The F3C and Metop/GRAS mean bending angles differences relative to climatology during the same six month period are statistically significant and have values of −0.05 and −0.02 μrad, respectively. A comparison of ~13 500 collocated F3C and Metop/GRAS bending angle profiles over this six month period shows a similar mean difference of ~0.02 ± 0.02 μrad between 30 and 60 km impact heights that is marginally significant. The observed mean difference between the F3C and Metop/GRAS bending angles of ~0.02–0.03 μrad is quite small and illustrates the high degree of re-produceability and mission independence of the GPS RO data at high altitudes. Collocated bending angles between two F3C satellites from early in the mission differ on average by up to 0.5% near the surface due to systematically lower signal-to-noise ratio for one of the satellites. Results from F3C and Metop/GRAS differences in the lower troposphere suggest the Metop/GRAS bending angles are negatively biased compared to F3C with a maximum of several percents near the surface in tropical regions. This bias is related to different tracking depths (deeper in F3C) and data gaps in Metop/GRAS which make it impossible to process the data from both missions in exactly the same way.

Description: The "Lung": a software-controlled air accumulator for quasi-continuous multi-point measurement of agricultural greenhouse gases Atmospheric Measurement Techniques, 4, 2293-2303, 2011 Author(s): R. J. Martin, A. M. Bromley, M. J. Harvey, R. C. Moss, E. Pattey, and D. Dow We describe the design and testing of a flexible bag ("Lung") accumulator attached to a gas chromatographic (GC) analyzer capable of measuring surface-atmosphere greenhouse gas exchange fluxes in a wide range of environmental/agricultural settings. In the design presented here, the Lung can collect up to three gas samples concurrently, each accumulated into a Tedlar bag over a period of 20 min or longer. Toggling collection between 2 sets of 3 bags enables quasi-continuous collection with sequential analysis and discarding of sample residues. The Lung thus provides a flexible "front end" collection system for interfacing to a GC or alternative analyzer and has been used in 2 main types of application. Firstly, it has been applied to micrometeorological assessment of paddock-scale N 2 O fluxes, discussed here. Secondly, it has been used for the automation of concurrent emission assessment from three sheep housed in metabolic crates with gas tracer addition and sampling multiplexed to a single GC. The Lung allows the same GC equipment used in laboratory discrete sample analysis to be deployed for continuous field measurement. Continuity of measurement enables spatially-averaged N 2 O fluxes in particular to be determined with greater accuracy, given the highly heterogeneous and episodic nature of N 2 O emissions. We present a detailed evaluation of the micrometeorological flux estimation alongside an independent tuneable diode laser system, reporting excellent agreement between flux estimates based on downwind vertical concentration differences. Whilst the current design is based around triplet bag sets, the basic design could be scaled up to a larger number of inlets or bags and less frequent analysis (longer accumulation times) where a greater number of sampling points are required.

Description: Droplet activation, separation, and compositional analysis: laboratory studies and atmospheric measurements Atmospheric Measurement Techniques, 4, 2333-2343, 2011 Author(s): N. Hiranuma, M. Kohn, M. S. Pekour, D. A. Nelson, J. E. Shilling, and D. J. Cziczo Droplets produced in a cloud condensation nuclei chamber (CCNC) as a function of supersaturation have been separated from unactivated aerosol particles using counterflow virtual impaction. Residual material after droplets were evaporated was chemically analyzed with an Aerodyne Aerosol Mass Spectrometer (AMS) and the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. Experiments were initially conducted to verify activation conditions for monodisperse ammonium sulfate particles and to determine the resulting droplet size distribution as a function of supersaturation. Based on the observed droplet size, the counterflow virtual impactor cut-size was set to differentiate droplets from unactivated interstitial particles. Validation experiments were then performed to verify that only droplets with sufficient size passed through the counterflow virtual impactor for subsequent analysis. A two-component external mixture of monodisperse particles was also exposed to a supersaturation which would activate one of the types (hygroscopic salts) but not the other (polystyrene latex spheres or adipic acid). The mass spectrum observed after separation indicated only the former, validating separation of droplets from unactivated particles. Results from ambient measurements using this technique and AMS analysis were inconclusive, showing little chemical differentiation between ambient aerosol and activated droplet residuals, largely due to low signal levels. When employing as single particle mass spectrometer for compositional analysis, however, we observed enhancement of sulfate in droplet residuals.

Description: Retrieval of stratospheric aerosol density profiles from SCIAMACHY limb radiance measurements in the O 2 A-band Atmospheric Measurement Techniques, 4, 2359-2373, 2011 Author(s): B. Ovigneur, J. Landgraf, R. Snel, and I. Aben In this paper we present an approach to retrieve stratospheric aerosol number densities in the altitude range 10–40 km from SCIAMACHY limb radiance measurements in the spectral range of the O 2 A absorption band, near 760 nm. Here, the characteristic light paths differ for the measured light in the O 2 A-band and in the spectral continuum next to the absorption band. This difference is used to distinguish the effect of stratospheric aerosol scattering and ground reflection on the limb measurement. The capability to disentangle both effects is illustrated for SCIAMACHY limb observations over the Libyan desert, where the measurements are not affected by tropospheric clouds. Comparison of the SCIAMACHY retrieval and the SAGE II aerosol extinction product between 75° southern and northern latitude shows the clear need for prior knowledge of the mean size of the stratospheric aerosol for the SCIAMACHY retrieval. We found best agreement between SCIAMACHY and SAGE II aerosol extinction for the period 2003–2005 for a prior choice of the mean aerosol size radius of 0.2 μm. The overall agreement between both data sets is in the range

Description: Improved identification of clouds and ice/snow covered surfaces in SCIAMACHY observations Atmospheric Measurement Techniques, 4, 2213-2224, 2011 Author(s): J. M. Krijger, P. Tol, L. G. Istomina, C. Schlundt, H. Schrijver, and I. Aben In the ultra-violet, visible and near infra-red wavelength range the presence of clouds can strongly affect the satellite-based passive remote sensing observation of constituents in the troposphere, because clouds effectively shield the lower part of the atmosphere. Therefore, cloud detection algorithms are of crucial importance in satellite remote sensing. However, the detection of clouds over snow/ice surfaces is particularly difficult in the visible wavelengths as both clouds an snow/ice are both white and highly reflective. The SCIAMACHY Polarisation Measurement Devices (PMD) Identification of Clouds and Ice/snow method (SPICI) uses the SCIAMACHY measurements in the wavelength range between 450 nm and 1.6 μm to make a distinction between clouds and ice/snow covered surfaces, specifically developed to identify cloud-free SCIAMACHY observations. For this purpose the on-board SCIAMACHY PMDs are used because they provide higher spatial resolution compared to the main spectrometer measurements. In this paper we expand on the original SPICI algorithm (Krijger et al., 2005a) to also adequately detect clouds over snow-covered forests which is inherently difficult because of the similar spectral characteristics. Furthermore the SCIAMACHY measurements suffer from degradation with time. This must be corrected for adequate performance of SPICI over the full SCIAMACHY time range. Such a correction is described here. Finally the performance of the new SPICI algorithm is compared with various other datasets, such as from FRESCO, MICROS and AATSR, focusing on the algorithm improvements.

Description: Tropospheric ozone column retrieval at northern mid-latitudes from the Ozone Monitoring Instrument by means of a neural network algorithm Atmospheric Measurement Techniques, 4, 2375-2388, 2011 Author(s): P. Sellitto, B. R. Bojkov, X. Liu, K. Chance, and F. Del Frate Monitoring tropospheric ozone from space is of critical importance in order to gain more thorough knowledge on phenomena affecting air quality and the greenhouse effect. Deriving information on tropospheric ozone from UV/VIS nadir satellite spectrometers is difficult owing to the weak sensitivity of the measured radiance spectra to variations of ozone in the troposphere. Here we propose an alternative method of analysis to retrieve tropospheric ozone columns from Ozone Monitoring Instrument radiances by means of a neural network algorithm. An extended set of ozone sonde measurements at northern mid-latitudes for the years 2004–2008 has been considered as the training and test data set. The design of the algorithm is extensively discussed. Our retrievals are compared to both tropospheric ozone residuals and optimal estimation retrievals over a similar independent test data set. Results show that our algorithm has comparable accuracy with respect to both correlative methods and its performance is slightly better over a subset containing only European ozone sonde stations. Possible sources of errors are analyzed. Finally, the capabilities of our algorithm to derive information on boundary layer ozone are studied and the results critically discussed.

Description: Characterization of aerosol photooxidation flow reactors: heterogeneous oxidation, secondary organic aerosol formation and cloud condensation nuclei activity measurements Atmospheric Measurement Techniques, 4, 445-461, 2011 Author(s): A. T. Lambe, A. T. Ahern, L. R. Williams, J. G. Slowik, J. P. S. Wong, J. P. D. Abbatt, W. H. Brune, N. L. Ng, J. P. Wright, D. R. Croasdale, D. R. Worsnop, P. Davidovits, and T. B. Onasch Motivated by the need to develop instrumental techniques for characterizing organic aerosol aging, we report on the performance of the Toronto Photo-Oxidation Tube (TPOT) and Potential Aerosol Mass (PAM) flow tube reactors under a variety of experimental conditions. The PAM system was designed with lower surface-area-to-volume (SA/V) ratio to minimize wall effects; the TPOT reactor was designed to study heterogeneous aerosol chemistry where wall loss can be independently measured. The following studies were performed: (1) transmission efficiency measurements for CO 2 , SO 2 , and bis(2-ethylhexyl) sebacate (BES) particles, (2) H 2 SO 4 yield measurements from the oxidation of SO 2 , (3) residence time distribution (RTD) measurements for CO 2 , SO 2 , and BES particles, (4) aerosol mass spectra, O/C and H/C ratios, and cloud condensation nuclei (CCN) activity measurements of BES particles exposed to OH radicals, and (5) aerosol mass spectra, O/C and H/C ratios, CCN activity, and yield measurements of secondary organic aerosol (SOA) generated from gas-phase OH oxidation of m -xylene and α-pinene. OH exposures ranged from (2.0 ± 1.0) × 10 10 to (1.8 ± 0.3) × 10 12 molec cm −3 s. Where applicable, data from the flow tube reactors are compared with published results from the Caltech smog chamber. The TPOT yielded narrower RTDs. However, its transmission efficiency for SO 2 was lower than that for the PAM. Transmission efficiency for BES and H 2 SO 4 particles was size-dependent and was similar for the two flow tube designs. Oxidized BES particles had similar O/C and H/C ratios and CCN activity at OH exposures greater than 10 11 molec cm −3 s, but different CCN activity at lower OH exposures. The O/C ratio, H/C ratio, and yield of m -xylene and α-pinene SOA was strongly affected by reactor design and operating conditions, with wall interactions seemingly having the strongest influence on SOA yield. At comparable OH exposures, flow tube SOA was more oxidized than smog chamber SOA, possibly because of faster gas-phase oxidation relative to particle nucleation. SOA yields were lower in the TPOT than in the PAM, but CCN activity of flow-tube-generated SOA particles was similar. For comparable OH exposures, α-pinene SOA yields were similar in the PAM and Caltech chambers, but m -xylene SOA yields were much lower in the PAM compared to the Caltech chamber.

Description: A comparison of light backscattering and particle size distribution measurements in tropical cirrus clouds Atmospheric Measurement Techniques, 4, 557-570, 2011 Author(s): F. Cairo, G. Di Donfrancesco, M. Snels, F. Fierli, M. Viterbini, S. Borrmann, and W. Frey An FSSP-100 Optical Particle Counter designed to count and size particles in the micron range and a backscattersonde that measures in-situ particle optical properties such as backscatter and depolarization ratio, are part of the payload of the high altitude research aircraft M55 Geophysica. This aircraft was deployed in tropical field campaigns in Bauru, Brasil (TROCCINOX, 2004) Darwin, Australia (SCOUT-Darwin, 2005) and Ouagadougou, Burkina Faso (SCOUT-AMMA, 2006). In those occasions, measurements of particle size distributions and optical properties within cirrus cloud were performed. Scope of the present work is to assess and discuss the consistency between the particle volume backscatter coefficient observed by the backscattersonde and the same parameter retrieved by optical scattering theory applied to particle size distributions as measured by the FSSP-100. In addition, empirical relationships linking the optical properties measured in-situ by the backscattersonde, which generally can be obtained by remote sensing techniques (LIDAR), and microphysical bulk properties like total particle number, surface and volume density will be presented and discussed.

Description: On transport phenomena and equilibration time scales in thermodenuders Atmospheric Measurement Techniques, 4, 571-581, 2011 Author(s): R. Saleh, A. Shihadeh, and A. Khlystov This paper presents a theoretical and experimental investigation of thermodenuders that addresses two controversial issues: (1) equilibration time scales and (2) the need for an activated carbon (AC) denuder in the cooling section. We describe a plug flow model for transport phenomena in a TD, which can be used to simulate the rate of vapor build-up in the gas phase and the corresponding change in particle size distribution. Model simulations were found to have excellent agreement with experiments performed with pure and mixed dicarboxylic acid aerosols. Both simulations and experiments showed that the aerosols approached equilibrium within reasonable residence times (15 s–30 s) for aerosol concentrations and size distributions typical for laboratory measurements, and that volatility studies at sufficiently high aerosol loadings, therefore, need not resort to kinetic models for inference of thermodynamic properties. However, for size distributions relevant for ambient aerosols, equilibration time scales were much larger than residence times available with current TD designs. We have also performed dimensional analysis on the problem of equilibration in TDs, and derived a dimensionless equilibration parameter which can be used to determine the residence time needed for an aerosol of given size distribution and kinetic properties to approach equilibrium. It is also shown theoretically and empirically that aerosol volatility has no effect on the equilibration time scales. Model simulations and experiments showed that with aerosol size distributions relevant to both ambient and laboratory measurements re-condensation in the cooling section, with and without an AC denuder, was negligible. Thus, there is no significant benefit in using an AC denuder in the cooling section. Due to the risk of stripping volatile material from the aerosol, the use of AC denuders in the cooling section should be avoided. Finally, we present a rationale for why Δ C is the proper measure of volatility, while using mass fraction remaining (MFR) can be misleading.

Description: Greenhouse gas observations from Cabauw Tall Tower (1992–2010) Atmospheric Measurement Techniques, 4, 617-644, 2011 Author(s): A. T. Vermeulen, A. Hensen, M. E. Popa, W. C. M. van den Bulk, and P. A. C. Jongejan Since 1992 semi-continuous in-situ observations of greenhouse gas concentrations have been performed at the tall tower of Cabauw (4.927° E, 51.971° N, −0.7 m a.s.l.). Through 1992 up to now, the measurement system has been gradually extended and improved in precision, starting with CO 2 and CH 4 concentrations from 200 m a.g.l. in 1992 to vertical gradients at 4 levels of the gases CO 2 , CH 4 , SF 6 , N 2 O, H 2 , CO and gradients at 2 levels for 222 Rn. In this paper the measurement systems and measurement results are described for the main greenhouse gases and CO, for the whole period. The automatic measurement system now provides half-hourly concentration gradients with a precision better than or close to the WMO recommendations. The observations at Cabauw show a complex pattern caused by the influence of sources and sinks from a large area around the tower with significant contributions of sources and sinks at distances up to 500–700 km. The concentration footprint area of Cabauw is one the most intensive and complex source areas of greenhouse gases in the world. Despite this, annual mean trends for the most important greenhouse gases, compatible with the values derived using the global network, can be reproduced from the measured concentrations at Cabauw over the entire measurement period, with a measured increase in the period 2000–2009 for CO 2 of 1.90 ± 0.1 ppm yr −1 , for CH 4 of 4.4 ± 0.6 ppb yr −1 , for N 2 O of 0.86 ± 0.04 ppb yr −1 , and for SF 6 of 0.27 ± 0.01 ppt yr −1 ; for CO no significant trend could be detected. The influences of strong local sources and sinks are reflected in the amplitude of the mean seasonal cycles observed at Cabauw, that are larger than the mean Northern Hemisphere average; Cabauw mean seasonal amplitude for CO 2 is 25–30 ppm (higher value for lower sampling levels). The observed CH 4 seasonal amplitude is 50–110 ppb. All gases except N 2 O show highest concentrations in winter and lower concentrations in summer, N 2 O observations show two additional concentration maxima in early summer and in autumn. Seasonal cycles of the day-time mean concentrations show that surface concentrations or high elevation concentrations alone do not give a representative value for the boundary layer concentrations, especially in winter time, but that the vertical profile data along the mast can be used to construct a useful boundary layer mean value. The variability at Cabauw in the atmospheric concentrations of CO 2 on time scales of minutes to hours is several ppm and is much larger than the precision of the measurements (0.1 ppm). The diurnal and synoptical variability of the concentrations at Cabauw carry information on the sources and sinks in the footprint area of the mast, that will be useful in combination with inverse atmospheric transport model to verify emission estimates and improve ecosystem models. For this purpose a network of tall tower stations like Cabauw forms a very useful addition to the existing global observing network for greenhouse gases.

Description: Eddy covariance flux measurements of ammonia by high temperature chemical ionisation mass spectrometry Atmospheric Measurement Techniques, 4, 599-616, 2011 Author(s): J. Sintermann, C. Spirig, A. Jordan, U. Kuhn, C. Ammann, and A. Neftel A system for fast ammonia (NH 3 ) measurements with chemical ionisation mass spectrometry (CIMS) based on a commercial Proton Transfer Reaction-Mass Spectrometer (PTR-MS) is presented. It uses electron transfer reaction as ionisation pathway and features a drift tube of polyetheretherketone (PEEK) and silica-coated steel. Heating the instrumental inlet and the drift tube to 180 °C enabled an effective time resolution of ~1 s and made it possible to apply the instrument for eddy covariance (EC) measurements. EC fluxes of NH 3 were measured over two agricultural fields in Oensingen, Switzerland, following fertilisations with cattle slurry. Air was aspirated close to a sonic anemometer at a flow of 100 STP L min −1 and was directed through a 23 m long 1/2" PFA tube heated to 150 °C to an air-conditioned trailer where the gas was sub-sampled from the large bypass stream. This setup minimised damping of fast NH 3 concentration changes between the sampling point and the actual measurement. High-frequency attenuation loss of the NH 3 fluxes of 20 to 40% was quantified and corrected for using an empirical ogive method. The instrumental NH 3 background signal showed a minor interference with H 2 O which was characterised in the laboratory. The resulting correction of the NH 3 flux after slurry spreading was less than 1‰. The flux detection limit of the EC system was about 5 ng m −2 s −1 while the accuracy of individual flux measurements was estimated 16% for the high-flux regime during these experiments. The NH 3 emissions after broad spreading of the slurry showed an initial maximum of 150 μg m −2 s −1 with a fast decline in the following hours.

Description: Synergetic cloud fraction determination for SCIAMACHY using MERIS Atmospheric Measurement Techniques, 4, 319-337, 2011 Author(s): C. Schlundt, A. A. Kokhanovsky, W. von Hoyningen-Huene, T. Dinter, L. Istomina, and J. P. Burrows Since clouds play an essential role in the Earth's climate system, it is important to understand the cloud characteristics as well as their distribution on a global scale using satellite observations. The main scientific objective of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) onboard the ENVISAT satellite is the retrieval of vertical columns of trace gases. On the one hand, SCIAMACHY has to be sensitive to low variations in trace gas concentrations which means the ground pixel size has to be large enough. On the other hand, such a large pixel size leads to the problem that SCIAMACHY spectra are often contaminated by clouds. SCIAMACHY spectral measurements are not well suitable to derive a reliable sub-pixel cloud fraction that can be used as input parameter for subsequent retrievals of cloud properties or vertical trace gas columns. Therefore, we use MERIS/ENVISAT spectral measurements with its high spatial resolution as sub-pixel information for the determination of MerIs Cloud fRation fOr Sciamachy (MICROS). Since MERIS covers an even broader swath width than SCIAMACHY, no problems in spatial and temporal collocation of measurements occur. This enables the derivation of a SCIAMACHY cloud fraction with an accuracy much higher as compared with other current cloud fractions that are based on SCIAMACHY's PMD (Polarization Measurement Device) data. We present our new developed MICROS algorithm, based on the threshold approach, as well as a qualitative validation of our results with MERIS satellite images for different locations, especially with respect to bright surfaces such as snow/ice and sands. In addition, the SCIAMACHY cloud fractions derived from MICROS are intercompared with other current SCIAMACHY cloud fractions based on different approaches demonstrating a considerable improvement regarding geometric cloud fraction determination using the MICROS algorithm.

Description: Observation of the exhaust plume from the space shuttle main engines using the microwave limb sounder Atmospheric Measurement Techniques, 4, 89-95, 2011 Author(s): H. C. Pumphrey, A. Lambert, and N. J. Livesey A space shuttle launch deposits 700 tonnes of water in the atmosphere. Some of this water is released into the upper mesosphere and lower thermosphere where it may be directly detected by a limb sounding satellite instrument. We report measurements of water vapour plumes from shuttle launches made by the Microwave Limb Sounder (MLS) on the Aura satellite. Approximately 50%–65% of shuttle launches are detected by MLS. The signal appears at a similar level across the upper 10 km of the MLS limb scan, suggesting that the bulk of the observed water is above the top of the scan. Only a small fraction at best of smaller launches (Ariane 5, Proton) are detected. We conclude that the sensitivity of MLS is only just great enough to detect a shuttle sized launch, but that a suitably designed instrument of the same general type could detect the exhausts from a large proportion of heavy-lift launches.

Description: CO 2 -gradient measurements using a parallel multi-analyzer setup Atmospheric Measurement Techniques, 4, 409-423, 2011 Author(s): L. Siebicke, G. Steinfeld, and T. Foken Accurate CO 2 concentration gradient measurements are needed for the computation of advective flux terms, which are part of the full Net Ecosystem Exchange (NEE) budget equation. A typical draw back of current gradient measurement designs in advection research is the inadequate sampling of complex flow phenomena using too few observation points in space and time. To overcome this draw back, a new measurement design is presented which allows the parallel measurement of several sampling points at a high frequency. Due to the multi-analyzer nature of the design, inter-instrument bias becomes more of a concern compared to conventional setups. Therefore a statistical approach is presented which allows for accurate observations of concentration gradients, which are typically small in relation to analyzer accuracy, to be obtained. This bias correction approach applies a conditional, time dependent signal correction. The correction depends on a mixing index based on cross correlation analysis, which characterizes the degree of mixing of the atmosphere between individual sample points. The approach assumes statistical properties of probability density functions (pdf) of concentration differences between a sample point and the field average which are common to the pdf's from several sample points. The applicability of the assumptions made was tested by Large Eddy Simulation (LES) using the model PALM and could be verified for a test case of well mixed conditions. The study presents concentration time series before and after correction, measured at a 2 m height in the sub-canopy at the FLUXNET spruce forest site Waldstein-Weiden\-brunnen (DE-Bay), analyzes the dependence of statistical parameters of pdf's from atmospheric parameters such as stratification, quantifies the errors and evaluates the performance of the bias correction approach. The improvements that are achieved by applying the bias correction approach are one order of magnitude larger than possible errors associated with it, which is a strong incentive to use the correction approach. In conclusion, the presented bias correction approach is well suited for – but not limited to – horizontal gradient measurements in a multi-analyzer setup, which would not have been reliable without this approach. Finally, possible future improvements of the bias correction approach are outlined and further fields of application indicated.

Description: Performance of a corona ion source for measurement of sulfuric acid by chemical ionization mass spectrometry Atmospheric Measurement Techniques, 4, 437-443, 2011 Author(s): A. Kürten, L. Rondo, S. Ehrhart, and J. Curtius The performance of an ion source based on corona discharge has been studied. This source is used for the detection of gaseous sulfuric acid by chemical ionization mass spectrometry (CIMS) through the reaction of NO 3 – ions with H 2 SO 4 . The ion source is operated under atmospheric pressure and its design is similar to the one of a radioactive (americium-241) ion source which has been used previously. The results show that the detection limit for the corona ion source is sufficiently good for most applications. For an integration time of 1 min it is ~6 × 10 4 molecule cm −3 of H 2 SO 4 . In addition, only a small cross-sensitivity to SO 2 has been observed for concentrations as high as 1 ppmv in the sample gas. This low sensitivity to SO 2 is achieved even without the addition of an OH scavenger. When comparing the new corona ion source with the americium ion source for the same provided H 2 SO 4 concentration, both ion sources yield almost identical values. These features make the corona ion source investigated here favorable over the more commonly used radioactive ion sources for most applications where H 2 SO 4 is measured by CIMS.

Description: An over-land aerosol optical depth data set for data assimilation by filtering, correction, and aggregation of MODIS Collection 5 optical depth retrievals Atmospheric Measurement Techniques, 4, 379-408, 2011 Author(s): E. J. Hyer, J. S. Reid, and J. Zhang MODIS Collection 5 retrieved aerosol optical depth (AOD) over land (MOD04/MYD04) was evaluated using 4 years of matching AERONET observations, to assess its suitability for aerosol data assimilation in numerical weather prediction models. Examination of errors revealed important sources of variation in random errors (e.g., atmospheric path length, scattering angle "hot spot"), and systematic biases (e.g., snow and cloud contamination, surface albedo bias). A set of quality assurance (QA) filters was developed to avoid conditions with potential for significant AOD error. An empirical correction for surface boundary condition using the MODIS 16-day albedo product captured 25% of the variability in the site mean bias at low AOD. A correction for regional microphysical bias using the AERONET fine/coarse partitioning information increased the global correlation between MODIS and AERONET from r 2 = 0.62–0.65 to r 2 = 0.71–0.73. Application of these filters and corrections improved the global fraction of MODIS AOD within (0.05 ± 20%) of AERONET to 77%, up from 67% using only built-in MODIS QA. The compliant fraction in individual regions was improved by as much as 20% (South America). An aggregated Level 3 product for use in a data assimilation system is described, along with a prognostic error model to estimate uncertainties on a per-observation basis. The new filtered and corrected Level 3 product has improved performance over built-in MODIS QA with less than a 15% reduction in overall data available for data assimilation.

Description: A broadband optical cavity spectrometer for measuring weak near-ultraviolet absorption spectra of gases Atmospheric Measurement Techniques, 4, 425-436, 2011 Author(s): J. Chen and D. S. Venables Accurate absorption spectra of gases in the near–ultraviolet (300 to 400 nm) are essential in atmospheric observations and laboratory studies. This paper describes a novel incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for measuring very weak absorption spectra from 335 to 375 nm. The instrument performance was validated against the 3 B 1 -X 1 A 1 transition of SO 2 . The measured absorption varied linearly with SO 2 column density and the resulting spectrum agrees well with published spectra. Using the instrument, we report new absorption cross-sections of O 3 , acetone, 2-butanone, and 2-pentanone in this spectral region, where literature data diverge considerably. In the absorption minimum between the Huggins and Chappuis bands, our absorption spectra fall at the lower range of reported ozone absorption cross-sections. The spectra of the ketones agree with prior spectra at moderate absorptions, but differ significantly at the limits of other instruments' sensitivity. The collision-induced absorption of the O 4 dimer at 360.5 nm was also measured and found to have a maximum cross-section of ca. 4.0×10 −46 cm 5 molecule −2 . We demonstrate the application of the instrument to quantifying low concentrations of the short-lived radical, BrO, in the presence of stronger absorptions from Br 2 and O 3 .

Description: MERIS albedo climatology for FRESCO+ O 2 A-band cloud retrieval Atmospheric Measurement Techniques, 4, 463-483, 2011 Author(s): C. Popp, P. Wang, D. Brunner, P. Stammes, Y. Zhou, and M. Grzegorski A new global albedo climatology for Oxygen A-band cloud retrievals is presented. The climatology is based on MEdium Resolution Imaging Spectrometer (MERIS) Albedomap data and its favourable impact on the derivation of cloud fraction is demonstrated for the FRESCO+ (Fast Retrieval Scheme for Clouds from the Oxygen A-band) algorithm. To date, a relatively coarse resolution (1° × 1°) surface reflectance dataset from GOME (Global Ozone Monitoring Experiment) Lambert-equivalent reflectivity (LER) is used in FRESCO+. The GOME LER climatology does not account for the usually higher spatial resolution of UV/VIS instruments designed for trace gas remote sensing which introduces several artefacts, e.g. in regions with sharp spectral contrasts like coastlines or over bright surface targets. Therefore, MERIS black-sky albedo (BSA) data from the period October 2002 to October 2006 were aggregated to a grid of 0.25° × 0.25° for each month of the year and for different spectral channels. In contrary to other available surface reflectivity datasets, MERIS includes channels at 754 nm and 775 nm which are located close to the spectral windows required for O 2 A-band cloud retrievals. The MERIS BSA in the near-infrared compares well to Moderate Resolution Imaging Spectroradiometer (MODIS) derived BSA with an average difference lower than 1% and a correlation coefficient of 0.98. However, when relating MERIS BSA to GOME LER a distinctly lower correlation (0.80) and enhanced scatter is found. Effective cloud fractions from two exemplary months (January and July 2006) of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) data were subsequently derived with FRESCO+ and compared to those from the Heidelberg Iterative Cloud Retrieval Utilities (HICRU) algorithm. The MERIS climatology generally improves FRESCO+ effective cloud fractions. In particular small cloud fractions are in better agreement with HICRU. This is of importance for atmospheric trace gas retrieval which relies on accurate cloud information at small cloud fractions. In addition, overestimates along coastlines and underestimates in the Intertropical Convergence Zone introduced by the GOME LER were eliminated. While effective cloud fractions over the Saharan desert and the Arabian peninsula are successfully reduced in January, they are still too high in July relative to HICRU due to FRESCO+'s large sensitivity to albedo inaccuracies of highly reflecting targets and inappropriate aerosol information which hampers an accurate albedo retrieval. Finally, NO 2 tropospheric vertical column densities and O 3 total columns were derived with the FRESCO+ cloud parameters from the new dataset and it is found that the MERIS BSA climatology has a pronounced and beneficial effect on regional scale. Apart from FRESCO+, the new MERIS albedo dataset is applicable to any cloud retrieval algorithms using the O 2 A-band or the O 2 -O 2 absorption band around 477 nm. Moreover, the by-product of BSA at 442 nm can be used in NO 2 remote sensing and the BSA at 620 nm, 665 nm, and 681 nm could be integrated in current H 2 O retrievals.

Description: Calibration of atmospheric hydrogen measurements Atmospheric Measurement Techniques, 4, 509-521, 2011 Author(s): A. Jordan and B. Steinberg Interest in atmospheric hydrogen (H 2 ) has been growing in recent years with the prospect of H 2 being a potential alternative to fossil fuels as an energy carrier. This has intensified research for a quantitative understanding of the atmospheric hydrogen cycle and its total budget, including the expansion of the global atmospheric measurement network. However, inconsistencies in published observational data constitute a major limitation in exploring such data sets. The discrepancies can be mainly attributed to difficulties in the calibration of the measurements. In this study various factors that may interfere with accurate quantification of atmospheric H 2 were investigated including drifts of standard gases in high pressure cylinders. As an experimental basis a procedure to generate precise mixtures of H 2 within the atmospheric concentration range was established. Application of this method has enabled a thorough linearity characterization of the commonly used GC-HgO reduction detector. We discovered that the detector response was sensitive to the composition of the matrix gas. Addressing these systematic errors, a new calibration scale has been generated defined by thirteen standards with dry air mole fractions ranging from 139–1226 nmol mol −1 . This new scale has been accepted as the official World Meteorological Organisation's (WMO) Global Atmospheric Watch (GAW) H 2 mole fraction scale.