ALBERT

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

Description: Retrieval and satellite intercomparison of O 3 measurements from ground-based FTIR Spectrometer at Equatorial Station: Addis Ababa, Ethiopia Atmospheric Measurement Techniques, 6, 495-509, 2013 Author(s): S. Takele Kenea, G. Mengistu Tsidu, T. Blumenstock, F. Hase, T. von Clarmann, and G. P. Stiller Since May 2009, high-resolution Fourier Transform Infrared (FTIR) solar absorption spectra have been recorded at Addis Ababa (9.01° N latitude, 38.76° E longitude, 2443 m altitude above sea level), Ethiopia. The vertical profiles and total column amounts of ozone (O 3 ) are deduced from the spectra by using the retrieval code PROFFIT (V9.5) and regularly determined instrumental line shape (ILS). A detailed error analysis of the O 3 retrieval is performed. Averaging kernels of the target gas shows that the major contribution to the retrieved information comes from the measurement. The degrees of freedom for signals is found to be 2.1 on average for the retrieval of O 3 from the observed FTIR spectra. The ozone Volume Mixing Ratio (VMR) profiles and column amounts retrieved from FTIR spectra are compared with the coincident satellite observations of Microwave Limb Sounding (MLS), Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Tropospheric Emission Spectrometer (TES), Ozone Monitoring Instrument (OMI), Atmospheric Infrared Sounding (AIRS) and Global Ozone Monitoring Experiment (GOME-2) instruments. The mean relative differences in ozone profiles of FTIR from MLS and MIPAS are generally lower than 15% within the altitude range of 27 to 36 km, whereas difference from TES is lower than 1%. Comparisons of measurements of column amounts from the satellite and the ground-based FTIR show very good agreement as exhibited by relative differences within +0.2% to +2.8% for FTIR versus MLS and GOME-2; and −0.9 to −9.0% for FTIR versus OMI, TES and AIRS. The corresponding standard deviations are within 2.0 to 2.8% for FTIR versus MLS and GOME-2 comparisons whereas that of FTIR versus OMI, TES and AIRS are within 3.5 to 7.3%. Thus, the retrieved O 3 VMR and column amounts from a tropical site, Addis Ababa, is found to exhibit very good agreement with all coincident satellite observations over an approximate 3-yr period.

Description: Influence of spatial heterogeneity of local surface albedo on the area-averaged surface albedo retrieved from airborne irradiance measurements Atmospheric Measurement Techniques, 6, 527-537, 2013 Author(s): E. Jäkel, M. Wendisch, and B. Mayer Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. Real surfaces are not homogeneous in their reflectivity. Therefore, this work studies the effects of the heterogeneity of surface reflectivity on the area-averaged surface albedo to quantify how well aircraft measurements can resolve the small-scale variability of the local surface albedo. For that purpose spatially heterogeneous surface albedo maps were input into a 3-dimensional (3-D) Monte Carlo radiative transfer model to simulate 3-D irradiance fields. The calculated up- and downward irradiances in altitudes between 0.1 and 5 km are used to derive the area-averaged surface albedo using an iterative retrieval method that removes the effects due to atmospheric scattering and absorption within the layer beneath the considered level. For the case of adjacent land and sea surfaces, parametrizations are presented which quantify the horizontal distance from the coastline that is required to reduce surface heterogeneity effects on the area-averaged surface albedo to a given limit. The parametrization which is a function of altitude, aerosol optical depth, single scattering albedo, and the ratio of local land and sea albedo was applied for airborne spectral measurements. In addition, the deviation between area-averaged and local surface albedo is determined for more complex surface albedo maps. For moderate aerosol conditions (optical depth less than 0.4) and a wavelength range between 400 and 1000 nm, the altitude and the heterogeneity of the surface albedo are the dominant factors determining the mean deviation between local and area-averaged surface albedo. A parametrization of the mean deviation is applied to an albedo map that was derived from a Landsat image of an area in East Anglia (UK). Parametrization and direct comparison of local and area-averaged surface albedo show similar mean deviations (20% vs. 25%) over land.

Description: Thermodynamic phase retrieval of convective clouds: impact of sensor viewing geometry and vertical distribution of cloud properties Atmospheric Measurement Techniques, 6, 539-547, 2013 Author(s): E. Jäkel, J. Walter, and M. Wendisch The sensitivity of passive remote sensing measurements to retrieve microphysical parameters of convective clouds, in particular their thermodynamic phase, is investigated by three-dimensional (3-D) radiative transfer simulations. The effects of different viewing geometries and vertical distributions of the cloud microphysical properties are investigated. Measurement examples of spectral solar radiance reflected by cloud sides (passive) in the near-infrared (NIR) spectral range are performed together with collocated lidar observations (active). The retrieval method to distinguish the cloud thermodynamic phase (liquid water or ice) exploits different slopes of cloud side reflectivity spectra of water and ice clouds in the NIR. The concurrent depolarization backscattering lidar provides geometry information about the cloud distance and height as well as the depolarization.

Description: Improvements to the retrieval of tropospheric NO 2 from satellite – stratospheric correction using SCIAMACHY limb/nadir matching and comparison to Oslo CTM2 simulations Atmospheric Measurement Techniques, 6, 565-584, 2013 Author(s): A. Hilboll, A. Richter, A. Rozanov, Ø. Hodnebrog, A. Heckel, S. Solberg, F. Stordal, and J. P. Burrows Satellite measurements of atmospheric trace gases have proved to be an invaluable tool for monitoring the Earth system. When these measurements are to be used for assessing tropospheric emissions and pollution, as for example in the case of nadir measurements of nitrogen dioxide (NO 2 ), it is necessary to separate the stratospheric from the tropospheric signal. The SCIAMACHY instrument offers the unique opportunity to combine its measurements in limb- and nadir-viewing geometries into a tropospheric data product, using the limb measurements of the stratospheric NO 2 abundances to correct the nadir measurements' total columns. In this manuscript, we present a novel approach to limb/nadir matching, calculating one stratospheric NO 2 value from limb measurements for every single nadir measurement, abandoning global coverage for the sake of spatial accuracy. For comparison, modelled stratospheric NO 2 columns from the Oslo CTM2 are also evaluated for stratospheric correction. Our study shows that stratospheric NO 2 columns from SCIAMACHY limb measurements very well reflect stratospheric conditions. The zonal variability of the stratospheric NO 2 field is captured by our matching algorithm, and the quality of the resulting tropospheric NO 2 columns improves considerably. Both stratospheric datasets need to be adjusted to the level of the nadir measurements, because a time- and latitude-dependent bias to the measured nadir columns can be observed over clean regions. After this offset is removed, the two datasets agree remarkably well, and both stratospheric correction methods provide a significant improvement to the retrieval of tropospheric NO 2 columns from the SCIAMACHY instrument.

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

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

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

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

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

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

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

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

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

Description: Quantification of uncertainties of water vapour column retrievals using future instruments Atmospheric Measurement Techniques, 6, 359-370, 2013 Author(s): H. Diedrich, R. Preusker, R. Lindstrot, and J. Fischer This study presents a quantification of uncertainties of total column water vapour retrievals based on simulated near-infrared measurements of upcoming instruments. The concepts of three scheduled spectrometers were taken into account: OLCI (Ocean and Land Color Instrument) on Sentinel-3, METimage on an EPS-SG (EUMETSAT Polar System – Second Generation) satellite and FCI (Flexible Combined Imager) on MTG (Meteosat Third Generation). Optimal estimation theory was used to estimate the error of a hypothetical total water vapour column retrieval for 27 different atmospheric cases. The errors range from 100% in very dry cases to 2% in humid cases with a very high surface albedo. Generally, the absolute uncertainties increase with higher water vapour column content due to H 2 O-saturation and decrease with a brighter surface albedo. Uncertainties increase with higher aerosol optical thickness, apart from very dark cases. Overall, the METimage channel setting enables the most accurate retrievals. The retrieval using the MTG-FCI build-up has the highest uncertainties apart from very bright cases. On the one hand, a retrieval using two absorption channels increases the accuracy, in some cases by one order of magnitude, in comparison to a retrieval using just one absorption channel. On the other hand, a retrieval using three absorption channels has no significant advantage over a two-absorption channel retrieval. Furthermore, the optimal position of the absorption channels was determined using the concept of the "information content". For a single channel retrieval, a channel at 900 or 915 nm has the highest mean information content over all cases. The second absorption channel is ideally weakly correlated with the first one, and therefore positioned at 935 nm, in a region with stronger water vapour absorption.

Description: Characterisation of GOME-2 formaldehyde retrieval sensitivity Atmospheric Measurement Techniques, 6, 371-386, 2013 Author(s): W. Hewson, H. Bösch, M. P. Barkley, and I. De Smedt Formaldehyde (CH 2 O) is an important tracer of tropospheric photochemistry, whose slant column abundance can be retrieved from satellite measurements of solar backscattered UV radiation, using differential absorption retrieval techniques. In this work a spectral fitting sensitivity analysis is conducted on CH 2 O slant columns retrieved from the Global Ozone Monitoring Experiment 2 (GOME-2) instrument. Despite quite different spectral fitting approaches, the retrieved CH 2 O slant columns have geographic distributions that generally match expected CH 2 O sources, though the slant column magnitudes and corresponding uncertainties are particularly sensitive to the retrieval set-up. The choice of spectral fitting window, polynomial order, I 0 correction, and inclusion of minor absorbers tend to result in the largest modulations of retrieved slant column magnitude and fit quality. However, application of a reference sector correction using observations over the remote Pacific Ocean is shown to largely homogenise the resulting CH 2 O vertical columns obtained with different retrieval settings, thereby largely reducing any systematic error sources from spectral fitting.

Description: Retrieval interval mapping: a tool to visualize the impact of the spectral retrieval range on differential optical absorption spectroscopy evaluations Atmospheric Measurement Techniques, 6, 275-299, 2013 Author(s): L. Vogel, H. Sihler, J. Lampel, T. Wagner, and U. Platt Remote sensing via differential optical absorption spectroscopy (DOAS) has become a standard technique to identify and quantify trace gases in the atmosphere. Due to the wide range of measurement conditions, atmospheric compositions and instruments used, a specific challenge of a DOAS retrieval is to optimize the retrieval parameters for each specific case and particular trace gas of interest. Of these parameters, the retrieval wavelength range is one of the most important ones. Although for many trace gases the overall dependence of common DOAS retrieval on the evaluation wavelength interval is known, a systematic approach for finding the optimal retrieval wavelength range and quantitative assessment is missing. Here we present a novel tool to visualize the effect of different evaluation wavelength ranges. It is based on mapping retrieved column densities in the retrieval wavelength space and thus visualizing the consequences of different choices of spectral retrieval ranges caused by slightly erroneous absorption cross sections, cross correlations and instrumental features. Based on the information gathered, an optimal retrieval wavelength range may be determined systematically. The technique is demonstrated using examples of a theoretical study of BrO retrievals for stratospheric BrO and BrO measurements in volcanic plumes. However, due to the general nature of the tool, it is applicable to any type of DOAS retrieval (active or passive).

Description: Water vapor isotopologue retrievals from high-resolution GOSAT shortwave infrared spectra Atmospheric Measurement Techniques, 6, 263-274, 2013 Author(s): C. Frankenberg, D. Wunch, G. Toon, C. Risi, R. Scheepmaker, J.-E. Lee, P. Wennberg, and J. Worden Remote sensing of the isotopic composition of water vapor can provide valuable information on the hydrological cycle. Here, we demonstrate the feasibility of retrievals of the relative abundance of HDO (the HDO/H 2 O ratio) from the Japanese GOSAT satellite. For this purpose, we use high spectral resolution nadir radiances around 6400 cm −1 (1.56 μm) to retrieve vertical column amounts of H 2 O and HDO. Retrievals of H 2 O correlate well with ECMWF (European Centre for Medium-Range Weather Forecasts) integrated profiles ( r 2 = 0.96). Typical precision errors in the retrieved column-averaged deuterium depletion (δD) are 20–40‰. We compare δD against a TCCON (Total Carbon Column Observing Network) ground-based station in Lamont, Oklahoma. Using retrievals in very dry areas over Antarctica, we detect a small systematic offset in retrieved H 2 O and HDO column amounts and take this into account for a bias correction of δD. Monthly averages of δD in the June 2009 to September 2011 time frame are well correlated with TCCON ( r 2 = 0.79) and exhibit a slope of 0.98 (1.23 if not bias corrected). We also compare seasonal averages on the global scale with results from the SCIAMACHY instrument in the 2003–2005 time frame. Despite the lack of temporal overlap, seasonal averages in general agree well, with spatial correlations ( r 2 ) ranging from 0.62 in September through November to 0.83 in June through August. However, we observe higher variability in GOSAT δD, indicated by fitted slopes between 1.2 and 1.46. The discrepancies are likely related to differences in vertical sensitivities but warrant further validation of both GOSAT and SCIAMACHY and an extension of the validation dataset.

Description: An improved cirrus detection algorithm MeCiDA2 for SEVIRI and its evaluation with MODIS Atmospheric Measurement Techniques, 6, 309-322, 2013 Author(s): F. Ewald, L. Bugliaro, H. Mannstein, and B. Mayer In this study, a substantially improved version of the Meteosat cirrus detection algorithm (MeCiDA2) will be presented, which now allows application to the full earth disc visible by the Meteosat satellite. As cirrus clouds have an influence on the radiation budget of the earth, their optical properties and their global coverage has to be monitored at the global scale using instruments aboard geostationary satellites. Since MeCiDA was optimised for the area of Europe only, various changes were necessary to handle the variable conditions found over the full Meteosat disc. Required changes include the consideration of the viewing angle dependency and of the sensitivity of the 9.7 μm channel to the ozone column. To this end, a correction is implemented that minimises the influence of the variability of the stratospheric ozone. The evaluation of the proposed improvements is carried out by using MeCiDA applied to MODIS (moderate resolution imaging spectrometer) data to address viewing angle-dependent cirrus detection, and by additionally comparing it to the cloud optical properties MOD06 cirrus product. The new MeCiDA version detects less cirrus than the original one for latitudes larger than 40°, but almost the same amount elsewhere. MeCiDA's version for MODIS is more sensitive than that for SEVIRI (spinning enhanced visible and infrared imager) with cirrus occurrences higher by 10%, and the new MeCiDA provides almost the same cirrus coverage (±0.1) as given by the cloud phase optical properties from MODIS for latitudes smaller than 50°. Finally, the influence of sub-pixel clouds on the SEVIRI cirrus detection has been examined: more than 60% of the undetected SEVIRI cirrus pixels have a cirrus coverage smaller than 0.5.

Description: A centrifugal ice microtome for measurements of atmospheric CO 2 on air trapped in polar ice cores Atmospheric Measurement Techniques, 6, 251-262, 2013 Author(s): B. Bereiter, T. F. Stocker, and H. Fischer For atmospheric CO 2 reconstructions using ice cores, the technique to release the trapped air from the ice samples is essential for the precision and accuracy of the measurements. We present here a new dry extraction technique in combination with a new gas analytical system that together show significant improvements with respect to current systems. Ice samples (3–15 g) are pulverised using a novel centrifugal ice microtome (CIM) by shaving the ice in a cooled vacuum chamber (−27 °C) in which no friction occurs due to the use of magnetic bearings. Both, the shaving principle of the CIM and the use of magnetic bearings have not been applied so far in this field. Shaving the ice samples produces finer ice powder and releases a minimum of 90% of the trapped air compared to 50%–70% when needle crushing is employed. In addition, the friction-free motion with an optimized design to reduce contaminations of the inner surfaces of the device result in a reduced system offset of about 2.0 ppmv compared to 4.9 ppmv. The gas analytical part shows a higher precision than the corresponding part of our previous system by a factor of two, and all processes except the loading and cleaning of the CIM now run automatically. Compared to our previous system, the complete system shows a 3 times better measurement reproducibility of about 1.1 ppmv (1 σ) which is similar to the best reproducibility of other systems applied in this field. With this high reproducibility, no replicate measurements are required anymore for most future measurement campaigns resulting in a possible output of 12–20 measurements per day compared to a maximum of 6 with other systems.

Description: Evaluation of a cavity ring-down spectrometer for in situ observations of 13 CO 2 Atmospheric Measurement Techniques, 6, 301-308, 2013 Author(s): F. R. Vogel, L. Huang, D. Ernst, L. Giroux, S. Racki, and D. E. J. Worthy With the emergence of wide-spread application of new optical techniques to monitor δ 13 C in atmospheric CO 2 there is a growing need to ensure well-calibrated measurements. We characterized one commonly available instrument, a cavity ring-down spectrometer (CRDS) system used for continuous in situ monitoring of atmospheric 13 CO 2 . We found no dependency of δ 13 C on the CO 2 concentration in the range of 303–437 ppm. We designed a calibration scheme according to the diagnosed instrumental drifts and established a quality assurance protocol. We find that the repeatability (1-σ) of measurements is 0.25‰ for 10 min and 0.15‰ for 20 min integrated averages, respectively. Due to a spectral overlap, our instrument displays a cross-sensitivity to CH 4 of 0.42 ± 0.024‰ ppm −1 . Our ongoing target measurements yield standard deviations of δ 13 C from 0.22‰ to 0.28‰ for 10 min averages. We furthermore estimate the reproducibility of our system for ambient air samples from weekly measurements of a long-term target gas to be 0.18‰. We find only a minuscule offset of 0.002 ± 0.025‰ between the CRDS and Environment Canada's isotope ratio mass spectrometer (IRMS) results for four target gases used over the course of one year.

Description: Continuous stand-alone controllable aerosol/cloud droplet dryer for atmospheric sampling Atmospheric Measurement Techniques, 6, 349-357, 2013 Author(s): S. Sjogren, G. P. Frank, M. I. A. Berghof, and B. G. Martinsson We describe a general-purpose dryer designed for continuous sampling of atmospheric aerosol, where a specified relative humidity (RH) of the sample flow (lower than the atmospheric humidity) is required. It is often prescribed to measure the properties of dried aerosol, for instance for monitoring networks. The specific purpose of our dryer is to dry cloud droplets (maximum diameter approximately 25 μm, highly charged, up to 5 × 10 2 charges). One criterion is to minimise losses from the droplet size distribution entering the dryer as well as on the residual dry particle size distribution exiting the dryer. This is achieved by using a straight vertical downwards path from the aerosol inlet mounted above the dryer, and removing humidity to a dry, closed loop airflow on the other side of a semi-permeable GORE-TEX membrane (total area 0.134 m 2 ). The water vapour transfer coefficient, k , was measured to be 4.6 × 10 -7 kg m −2 s −1 % RH −1 in the laboratory (temperature 294 K) and is used for design purposes. A net water vapour transfer rate of up to 1.2 × 10 -6 kg s −1 was achieved in the field. This corresponds to drying a 5.7 L min −1 (0.35 m 3 h −1 ) aerosol sample flow from 100% RH to 27% RH at 293 K (with a drying air total flow of 8.7 L min −1 ). The system was used outdoors from 9 May until 20 October 2010, on the mountain Brocken (51.80° N, 10.67° E, 1142 m a.s.l.) in the Harz region in central Germany. Sample air relative humidity of less than 30% was obtained 72% of the time period. The total availability of the measurement system was 〉94% during these five months.

Description: The effect of hygroscopicity on eddy covariance estimates of sea-spray aerosol fluxes: a comparison of high-rate and bulk correction methods Atmospheric Measurement Techniques, 6, 323-335, 2013 Author(s): D. A. J. Sproson, I. M. Brooks, and S. J. Norris The eddy covariance technique is the most direct of the methods that have been used to measure the flux of sea-spray aerosol between the ocean and atmosphere, but has been applied in only a handful of studies. However, unless the aerosol is dried before the eddy covariance measurements are made, the hygroscopic nature of sea-spray may combine with a relative humidity flux to result in a bias in the calculated aerosol flux. "Bulk" methods have been presented to account for this bias, however, they rely on assumptions of the shape of the aerosol spectra which may not be valid for near-surface measurements of sea-spray. Here we describe a method of correcting aerosol spectra for relative humidity induced size variations at the high frequency (10 Hz) measurement timescale, where counting statistics are poor and the spectral shape cannot be well represented by a simple power law. Such a correction allows the effects of hygroscopicity and relative humidity flux on the aerosol flux to be explicitly evaluated and compared to the bulk corrections, both in their original form and once reformulated to better represent the measured mean aerosol spectra. In general, the bulk corrections – particularly when reformulated for the measured mean aerosol spectra – perform relatively well, producing flux corrections of the right sign and approximate magnitude. However, there are times when the bulk methods either significantly over- or underestimate the required flux correction. We conclude that, where possible, relative humidity corrections should be made at the measurement frequency.

Description: Cluster analysis of WIBS single-particle bioaerosol data Atmospheric Measurement Techniques, 6, 337-347, 2013 Author(s): N. H. Robinson, J. D. Allan, J. A. Huffman, P. H. Kaye, V. E. Foot, and M. Gallagher Hierarchical agglomerative cluster analysis was performed on single-particle multi-spatial data sets comprising optical diameter, asymmetry and three different fluorescence measurements, gathered using two dual Wideband Integrated Bioaerosol Sensors (WIBSs). The technique is demonstrated on measurements of various fluorescent and non-fluorescent polystyrene latex spheres (PSL) before being applied to two separate contemporaneous ambient WIBS data sets recorded in a forest site in Colorado, USA, as part of the BEACHON-RoMBAS project. Cluster analysis results between both data sets are consistent. Clusters are tentatively interpreted by comparison of concentration time series and cluster average measurement values to the published literature (of which there is a paucity) to represent the following: non-fluorescent accumulation mode aerosol; bacterial agglomerates; and fungal spores. To our knowledge, this is the first time cluster analysis has been applied to long-term online primary biological aerosol particle (PBAP) measurements. The novel application of this clustering technique provides a means for routinely reducing WIBS data to discrete concentration time series which are more easily interpretable, without the need for any a priori assumptions concerning the expected aerosol types. It can reduce the level of subjectivity compared to the more standard analysis approaches, which are typically performed by simple inspection of various ensemble data products. It also has the advantage of potentially resolving less populous or subtly different particle types. This technique is likely to become more robust in the future as fluorescence-based aerosol instrumentation measurement precision, dynamic range and the number of available metrics are improved.

Description: An airborne amplitude-modulated 1.57 μm differential laser absorption spectrometer: simultaneous measurement of partial column-averaged dry air mixing ratio of CO 2 and target range Atmospheric Measurement Techniques, 6, 387-396, 2013 Author(s): D. Sakaizawa, S. Kawakami, M. Nakajima, T. Tanaka, I. Morino, and O. Uchino Simultaneous measurements of the partial column-averaged dry air mixing ratio of CO 2 (XCO 2 ) and target range were demonstrated using airborne amplitude-modulated 1.57 μm differential laser absorption spectrometer (LAS). The LAS system is useful for discriminating between ground and cloud return signals and has a demonstrated ability to suppress the impact of integrated aerosol signals on atmospheric CO 2 measurements. A high correlation coefficient ( R ) of 0.987 between XCO 2 observed by LAS and XCO 2 calculated from in situ measurements was obtained. The averaged difference in XCO 2 obtained from LAS and validation data was within 1.5 ppm for all spiral measurements. An interesting vertical profile was observed for both XCO 2LAS and XCO 2val , in which lower altitude CO 2 decreases compared to higher altitude CO 2 attributed to the photosynthesis over grassland in the summer. In the case of an urban area where there are boundary-layer enhanced CO 2 and aerosol in the winter, the difference of XCO 2LAS to XCO 2val is a negative bias of 1.5 ppm, and XCO 2LAS is in agreement with XCO 2val within the measurement precision of 2.4 ppm (1 SD).

Description: Note on the application of planar-fit rotation for non-omnidirectional sonic anemometers Atmospheric Measurement Techniques, 6, 221-229, 2013 Author(s): M. Li, W. Babel, K. Tanaka, and T. Foken For non-omnidirectional sonic anemometers like the Kaijo-Denki DAT 600 TR61A probe, it is shown that separate planar-fit rotations must be used for the undisturbed (open part of the sonic anemometer) and the disturbed sector. This increases the friction velocity while no effect on the scalar fluxes was found. In the disturbed sector, irregular values of − u′ w′ 〈 0 were detected for low wind velocities. Up to a certain extent these results can be transferred to the CSAT3 sonic anemometer (Campbell Scientific Ltd). This study was done for data sets from the Naqu-BJ site on the Tibetan Plateau.

Description: Evaluation of ozone profile and tropospheric ozone retrievals from GEMS and OMI spectra Atmospheric Measurement Techniques, 6, 239-249, 2013 Author(s): J. Bak, J. H. Kim, X. Liu, K. Chance, and J. Kim South Korea is planning to launch the GEMS (Geostationary Environment Monitoring Spectrometer) instrument into the GeoKOMPSAT (Geostationary Korea Multi-Purpose SATellite) platform in 2018 to monitor tropospheric air pollutants on an hourly basis over East Asia. GEMS will measure backscattered UV radiances covering the 300–500 nm wavelength range with a spectral resolution of 0.6 nm. The main objective of this study is to evaluate ozone profiles and stratospheric column ozone amounts retrieved from simulated GEMS measurements. Ozone Monitoring Instrument (OMI) Level 1B radiances, which have the spectral range 270–500 nm at spectral resolution of 0.42–0.63 nm, are used to simulate the GEMS radiances. An optimal estimation-based ozone profile algorithm is used to retrieve ozone profiles from simulated GEMS radiances. Firstly, we compare the retrieval characteristics (including averaging kernels, degrees of freedom for signal, and retrieval error) derived from the 270–330 nm (OMI) and 300–330 nm (GEMS) wavelength ranges. This comparison shows that the effect of not using measurements below 300 nm on retrieval characteristics in the troposphere is insignificant. However, the stratospheric ozone information in terms of DFS decreases greatly from OMI to GEMS, by a factor of ∼2. The number of the independent pieces of information available from GEMS measurements is estimated to 3 on average in the stratosphere, with associated retrieval errors of ~1% in stratospheric column ozone. The difference between OMI and GEMS retrieval characteristics is apparent for retrieving ozone layers above ~20 km, with a reduction in the sensitivity and an increase in the retrieval errors for GEMS. We further investigate whether GEMS can resolve the stratospheric ozone variation observed from high vertical resolution Earth Observing System (EOS) Microwave Limb Sounder (MLS). The differences in stratospheric ozone profiles between GEMS and MLS are comparable to those between OMI and MLS below ~3 hPa (~40 km), except with slightly larger biases and larger standard deviations by up to 5%. At pressure altitudes above ~3 hPa, GEMS retrievals show strong influence of a priori and large differences with MLS, which, however, can be sufficiently improved by using better a priori information. The GEMS-MLS differences show negative biases of less than 4% for stratospheric column ozone, with standard deviations of 1–3%, while OMI retrievals show similar agreements with MLS except for 1% smaller biases at middle and high latitudes. Based on the comparisons, we conclude that GEMS will measure tropospheric ozone and stratospheric ozone columns with accuracy comparable to that of OMI and ozone profiles with slightly worse performance than that of OMI below ~3 hPa.

Description: The detection of nocturnal N 2 O 5 as HNO 3 by alkali- and aqueous-denuder techniques Atmospheric Measurement Techniques, 6, 231-237, 2013 Author(s): G. J. Phillips, U. Makkonen, G. Schuster, N. Sobanski, H. Hakola, and J. N. Crowley The almost total anthropogenic control of the nitrogen cycle has led to wide ranging trans-national and national efforts to quantify the effects of reactive nitrogen on the environment. A number of monitoring techniques have been developed for the measurement of nitric acid and subsequent estimation of nitrogen deposition within large networks and for process studies on shorter measurement campaigns. We discuss the likelihood that many of these techniques are sensitive to another important gas-phase component of oxidized nitrogen: dinitrogen pentoxide (N 2 O 5 ). We present measurements using a MARGA wet annular denuder device alongside measurements of N 2 O 5 with a discussion of evidence from the laboratory and the field which suggests that alkali- and aqueous-denuder measurements are sensitive to the sum of HNO 3 + 2N 2 O 5 . Nocturnal data from these denuder devices should be treated with care before using HNO 3 concentrations derived from these data. This is a systematic error which is highly dependent on ambient conditions and is likely to cause systematic misinterpretation of datasets in periods where N 2 O 5 is significant proportion of NO y . It is also likely that deposition estimates of HNO 3 via data obtained with these methods is compromised to greater and lesser extents depending on the season and environment of the sampling location.

Description: Quality assessment of Automatic Dependent Surveillance Contract (ADS-C) wind and temperature observation from commercial aircraft Atmospheric Measurement Techniques, 6, 199-206, 2013 Author(s): S. de Haan, L. J. Bailey, and J. E. Können Aircraft observations of wind and temperature are very important for upper air meteorology. In this article, the quality of the meteorological information of an Automatic Dependent Surveillance-Contract (ADS-C) message is assessed. The ADS-C messages broadcast by the aircraft are received at air traffic control centres for surveillance and airline control centres for general aircraft and dispatch management. A comparison is performed against a global numerical prediction (NWP) model and wind and temperature observations derived from Enhanced Surveillance (EHS) air-traffic control radar which interrogates all aircraft in selective mode (Mode-S EHS). Almost 16 000 ADS-C reports with meteorological information were compiled from the Royal Dutch Airlines (KLM) database. The length of the data set is 76 consecutive days and started on 1 January 2011. The wind and temperature observations are of good quality when compared to the global NWP forecast fields from the European Centre for Medium-Range Weather Forecasts (ECMWF). Comparison of ADS-C wind and temperature observations against Mode-S EHS derived observations in the vicinity of Amsterdam Airport Schiphol shows that the wind observations are of similar quality and the temperature observations of ADS-C are of better quality than those from Mode-S EHS. However, the current ADS-C data set has a lower vertical resolution than Mode-S EHS. High vertical resolution can be achieved by requesting more ADS-C when aircraft are ascending or descending, but could result in increased data communication costs.

Description: Organic particle types by single-particle measurements using a time-of-flight aerosol mass spectrometer coupled with a light scattering module Atmospheric Measurement Techniques, 6, 187-197, 2013 Author(s): S. Liu, L. M. Russell, D. T. Sueper, and T. B. Onasch Chemical and physical properties of individual ambient aerosol particles can vary greatly, so measuring the chemical composition at the single-particle level is essential for understanding atmospheric sources and transformations. Here we describe 46 days of single-particle measurements of atmospheric particles using a time-of-flight aerosol mass spectrometer coupled with a light scattering module (LS-ToF-AMS). The light scattering module optically detects particles larger than 180 nm vacuum aerodynamic diameter (130 nm geometric diameter) before they arrive at the chemical mass spectrometer and then triggers the saving of single-particle mass spectra. 271 641 particles were detected and sampled during 237 h of sampling in single-particle mode. By comparing timing of the predicted chemical ion signals from the light scattering measurement with the measured chemical ion signals by the mass spectrometer for each particle, particle types were classified and their number fractions determined as follows: prompt vaporization (46%), delayed vaporization (6%), and null (48%), where null was operationally defined as less than 6 ions per particle. Prompt and delayed vaporization particles with sufficient chemical information (i.e., more than 40 ions per particle) were clustered based on similarity of organic mass spectra (using k -means algorithm) to result in three major clusters: highly oxidized particles (dominated by m/z 44), relatively less oxidized particles (dominated by m/z 43), and particles associated with fresh urban emissions. Each of the three organic clusters had limited chemical properties of other clusters, suggesting that all of the sampled organic particle types were internally mixed to some degree; however, the internal mixing was never uniform and distinct particle types existed throughout the study. Furthermore, the single-particle mass spectra and time series of these clusters agreed well with mass-based components identified (using factor analysis) from simultaneous ensemble-averaged measurements, supporting the connection between ensemble-based factors and atmospheric particle sources and processes. Measurements in this study illustrate that LS-ToF-AMS provides unique information about organic particle types by number as well as mass.

Description: Corrigendum to "MAX-DOAS formaldehyde slant column measurements during CINDI: intercomparison and analysis improvement" published in Atmos. Meas. Tech., 6, 167–185, 2013 Atmospheric Measurement Techniques, 6, 219-219, 2013 Author(s): G. Pinardi, M. Van Roozendael, N. Abuhassan, C. Adams, A. Cede, K. Clémer, C. Fayt, U. Frieß, M. Gil, J. Herman, C. Hermans, F. Hendrick, H. Irie, A. Merlaud, M. Navarro Comas, E. Peters, A. J. M. Piters, O. Puentedura, A. Richter, A. Schönhardt, R. Shaiganfar, E. Spinei, K. Strong, H. Takashima, M. Vrekoussis, T. Wagner, F. Wittrock, and S. Yilmaz No abstract available.

Description: Improved mixing height monitoring through a combination of lidar and radon measurements Atmospheric Measurement Techniques, 6, 207-218, 2013 Author(s): A. D. Griffiths, S. D. Parkes, S. D. Chambers, M. F. McCabe, and A. G. Williams Surface-based radon ( 222 Rn) measurements can be combined with lidar backscatter to obtain a higher quality time series of mixing height within the planetary boundary layer (PBL) than is possible from lidar alone, and a more quantitative measure of mixing height than is possible from only radon. The reason why lidar measurements are improved is that there are times when lidar signals are ambiguous, and reliably attributing the mixing height to the correct aerosol layer presents a challenge. By combining lidar with a mixing length scale derived from a time series of radon concentration, automated and robust attribution is possible during the morning transition. Radon measurements provide mixing information during the night, but concentrations also depend on the strength of surface emissions. After processing radon in combination with lidar, we obtain nightly measurements of radon emissions and are able to normalise the mixing length scale for changing emissions. After calibration with lidar, the radon-derived equivalent mixing height agrees with other measures of mixing on daily and hourly timescales and is a potential method for studying intermittent mixing in nocturnal boundary layers.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Description: Improved cloud and snow screening in MAIAC aerosol retrievals using spectral and spatial analysis Atmospheric Measurement Techniques, 5, 843-850, 2012 Author(s): A. Lyapustin, Y. Wang, I. Laszlo, and S. Korkin An improved cloud/snow screening technique in the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm is described. It is implemented as part of MAIAC aerosol retrievals based on analysis of spectral residuals and spatial variability. Comparisons with AERONET aerosol observations and a large-scale MODIS data analysis show strong suppression of aerosol optical thickness outliers due to unresolved clouds and snow. At the same time, the developed filter does not reduce the aerosol retrieval capability at high 1 km resolution in strongly inhomogeneous environments, such as near centers of the active fires. Despite significant improvement, the optical depth outliers in high spatial resolution data are and will remain the problem to be addressed by the application-dependent specialized filtering techniques.

Description: Validation of routine continuous airborne CO 2 observations near the Bialystok Tall Tower Atmospheric Measurement Techniques, 5, 873-889, 2012 Author(s): H. Chen, J. Winderlich, C. Gerbig, K. Katrynski, A. Jordan, and M. Heimann Since 2002 in situ airborne measurements of atmospheric CO 2 mixing ratios have been performed regularly aboard a rental aircraft near Bialystok (53°08´ N, 23°09´ E), a city in northeastern Poland. Since August 2008, the in situ CO 2 measurements have been made by a modified commercially available and fully automated non-dispersive infrared (NDIR) analyzer system. The response of the analyzer has been characterized and the CO 2 mixing ratio stability of the associated calibration system has been fully tested, which results in an optimal calibration strategy and allows for an accuracy of the CO 2 measurements within 0.2 ppm. Besides the in situ measurements, air samples have been collected in glass flasks and analyzed in the laboratory for CO 2 and other trace gases. To validate the in situ CO 2 measurements against reliable discrete flask measurements, we developed weighting functions that mimic the temporal averaging of the flask sampling process. Comparisons between in situ and flask CO 2 measurements demonstrate that these weighting functions can compensate for atmospheric variability, and provide an effective method for validating airborne in situ CO 2 measurements. In addition, we show the nine-year records of flask CO 2 measurements. The new system, automated since August 2008, has eliminated the need for manual in-flight calibrations, and thus enables an additional vertical profile, 20 km away, to be sampled at no additional cost in terms of flight hours. This sampling strategy provides an opportunity to investigate both temporal and spatial variability on a regular basis.

Description: Determination of optical and microphysical properties of thin warm clouds using ground based hyper-spectral analysis Atmospheric Measurement Techniques, 5, 851-871, 2012 Author(s): E. Hirsch, E. Agassi, and I. Koren Clouds play a critical role in the Earth's radiative budget as they modulate the atmosphere by reflecting shortwave solar radiation and absorbing long wave IR radiation emitted by the Earth's surface. Although extensively studied for decades, cloud modelling in global circulation models is far from adequate, mostly due to insufficient spatial resolution of the circulation models. In addition, measurements of cloud properties still need improvement, since the vast majority of remote sensing techniques are focused in relatively large, thick clouds. In this study, we utilize ground based hyperspectral measurements and analysis to explore very thin water clouds. These clouds are characterized by liquid water path (LWP) that spans from as high as ~50g m −2 and down to 65 mg m −2 with a minimum of about 0.01 visible optical depth. The retrieval methodology relies on three elements: a detailed radiative transfer calculations in the longwave IR regime, signal enhancement by subtraction of a clear sky reference, and spectral matching method which exploits fine spectral differences between water droplets of different radii. A detailed description of the theoretical basis for the retrieval technique is provided along with a comprehensive discussion regarding its limitations. The proposed methodology was validated in a controlled experiment where artificial clouds were sprayed and their effective radii were both measured and retrieved simultaneously. This methodology can be used in several ways: (1) the frequency and optical properties of very thin water clouds can be studied more precisely in order to evaluate their total radiative forcing on the Earth's radiation budget. (2) The unique optical properties of the inter-region between clouds (clouds' "twilight zone") can be studied in order to more rigorously understanding of the governing physical processes which dominate this region. (3) Since the optical thickness of a developed cloud gradually decreases towards its edges, the proposed methodology can be used to study the spatial microphysical behaviour of these edges. (4) A spatial-temporal analysis can be used to study mixing processes in clouds' entrainment zone.

Description: A modeling approach to evaluate the uncertainty in estimating the evaporation behaviour and volatility of organic aerosols Atmospheric Measurement Techniques, 5, 735-757, 2012 Author(s): E. Fuentes and G. McFiggans The uncertainty in determining the volatility behaviour of organic particles from thermograms using calibration curves and a kinetic model has been evaluated. In the analysis, factors such as re-condensation, departure from equilibrium and analysis methodology were considered as potential sources of uncertainty in deriving volatility distribution from thermograms obtained with currently used thermodenuder designs. The previously found empirical relationship between C * (saturation concentration) and T 50 (temperature at which 50% of aerosol mass evaporates) was theoretically interpreted and tested to infer volatility distributions from experimental thermograms. The presented theoretical analysis shows that this empirical equation is in fact an equilibrium formulation, whose applicability is lessened as measurements deviate from equilibrium. While using a calibration curve between C * and T 50 to estimate volatility properties was found to hold at equilibrium, significant underestimation was obtained under kinetically-controlled evaporation conditions. Because thermograms obtained at ambient aerosol loading levels are most likely to show departure from equilibrium, the application of a kinetic evaporation model is more suitable for inferring volatility properties of atmospheric samples than the calibration curve approach; however, the kinetic model analysis implies significant uncertainty, due to its sensitivity to the assumption of "effective" net kinetic evaporation and condensation coefficients. The influence of re-condensation on thermograms from the thermodenuder designs under study was found to be highly dependent on the particular experimental condition, with a significant potential to affect volatility estimations for aerosol mass loadings 〉50 μg m −3 and with increasing effective kinetic coefficient for condensation and decreasing particle size. These results show that the geometry of current thermodenuder systems should be modified to prevent re-condensation.

Description: Multi-sensor Aerosol Products Sampling System (MAPSS) Atmospheric Measurement Techniques, 5, 913-926, 2012 Author(s): M. Petrenko, C. Ichoku, and G. Leptoukh Global and local properties of atmospheric aerosols have been extensively observed and measured using both spaceborne and ground-based instruments, especially during the last decade. Unique properties retrieved by the different instruments contribute to an unprecedented availability of the most complete set of complimentary aerosol measurements ever acquired. However, some of these measurements remain underutilized, largely due to the complexities involved in analyzing them synergistically. To characterize the inconsistencies and bridge the gap that exists between the sensors, we have established a Multi-sensor Aerosol Products Sampling System (MAPSS), which consistently samples and generates the spatial statistics (mean, standard deviation, direction and rate of spatial variation, and spatial correlation coefficient) of aerosol products from multiple spaceborne sensors, including MODIS (on Terra and Aqua), MISR, OMI, POLDER, CALIOP, and SeaWiFS. Samples of satellite aerosol products are extracted over Aerosol Robotic Network (AERONET) locations as well as over other locations of interest such as those with available ground-based aerosol observations. In this way, MAPSS enables a direct cross-characterization and data integration between Level-2 aerosol observations from multiple sensors. In addition, the available well-characterized co-located ground-based data provides the basis for the integrated validation of these products. This paper explains the sampling methodology and concepts used in MAPSS, and demonstrates specific examples of using MAPSS for an integrated analysis of multiple aerosol products.

Description: NO 2 measurements in Hong Kong using LED based long path differential optical absorption spectroscopy Atmospheric Measurement Techniques, 5, 901-912, 2012 Author(s): K. L. Chan, D. Pöhler, G. Kuhlmann, A. Hartl, U. Platt, and M. O. Wenig In this study we present the first long term measurements of atmospheric nitrogen dioxide (NO 2 ) using a LED based Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument. This instrument is measuring continuously in Hong Kong since December 2009, first in a setup with a 550 m absorption path and then with a 3820 m path at about 30 m to 50 m above street level. The instrument is using a high power blue light LED with peak intensity at 450 nm coupled into the telescope using a Y-fibre bundle. The LP-DOAS instrument measures NO 2 levels in the Kowloon Tong and Mongkok district of Hong Kong and we compare the measurement results to mixing ratios reported by monitoring stations operated by the Hong Kong Environmental Protection Department in that area. Hourly averages of coinciding measurements are in reasonable agreement ( R = 0.74). Furthermore, we used the long-term data set to validate the Ozone Monitoring Instrument (OMI) NO 2 data product. Monthly averaged LP-DOAS and OMI measurements correlate well ( R = 0.84) when comparing the data for the OMI overpass time. We analyzed weekly patterns in both data sets and found that the LP-DOAS detects a clear weekly cycle with a reduction on weekends during rush hour peaks, whereas OMI is not able to observe this weekly cycle due to its fix overpass time (13:30–14:30 LT – local time).

Description: Validation of SCIAMACHY limb NO 2 profiles using solar occultation measurements Atmospheric Measurement Techniques, 5, 1059-1084, 2012 Author(s): R. Bauer, A. Rozanov, C. A. McLinden, L. L. Gordley, W. Lotz, J. M. Russell III, K. A. Walker, J. M. Zawodny, A. Ladstätter-Weißenmayer, H. Bovensmann, and J. P. Burrows The increasing amounts of reactive nitrogen in the stratosphere necessitate accurate global measurements of stratospheric nitrogen dioxide (NO 2 ). Over the past decade, the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument on ENVISAT (European Environmental Satellite) has been providing global coverage of stratospheric NO 2 every 6 days. In this study, the vertical distributions of NO 2 retrieved from SCIAMACHY limb measurements of the scattered solar light are validated by comparison with NO 2 products from three different satellite instruments (SAGE II, HALOE and ACE-FTS). The retrieval algorithm based on the information operator approach is discussed, and the sensitivity of the SCIAMACHY NO 2 limb retrievals is investigated. The photochemical corrections needed to make this validation feasible, and the chosen collocation criteria are described. For each instrument, a time period of two years is analyzed with several hundreds of collocation pairs for each year. As NO 2 is highly variable, the comparisons are performed for five latitudinal bins and four seasons. In the 20 to 40 km altitude range, mean relative differences between SCIAMACHY and other instruments are found to be typically within 20 to 30%. The mean partial NO 2 columns in this altitude range agree typically within 15% (both global monthly and zonal annual means). Larger differences are seen for SAGE II comparisons, which is consistent with the results presented by other authors. For SAGE II and ACE-FTS, the observed differences can be partially attributed to the diurnal effect error.

Description: Validation of ACE and OSIRIS ozone and NO 2 measurements using ground-based instruments at 80° N Atmospheric Measurement Techniques, 5, 927-953, 2012 Author(s): C. Adams, K. Strong, R. L. Batchelor, P. F. Bernath, S. Brohede, C. Boone, D. Degenstein, W. H. Daffer, J. R. Drummond, P. F. Fogal, E. Farahani, C. Fayt, A. Fraser, F. Goutail, F. Hendrick, F. Kolonjari, R. Lindenmaier, G. Manney, C. T. McElroy, C. A. McLinden, J. Mendonca, J.-H. Park, B. Pavlovic, A. Pazmino, C. Roth, V. Savastiouk, K. A. Walker, D. Weaver, and X. Zhao The Optical Spectrograph and Infra-Red Imager System (OSIRIS) and the Atmospheric Chemistry Experiment (ACE) have been taking measurements from space since 2001 and 2003, respectively. This paper presents intercomparisons between ozone and NO 2 measured by the ACE and OSIRIS satellite instruments and by ground-based instruments at the Polar Environment Atmospheric Research Laboratory (PEARL), which is located at Eureka, Canada (80° N, 86° W) and is operated by the Canadian Network for the Detection of Atmospheric Change (CANDAC). The ground-based instruments included in this study are four zenith-sky differential optical absorption spectroscopy (DOAS) instruments, one Bruker Fourier transform infrared spectrometer (FTIR) and four Brewer spectrophotometers. Ozone total columns measured by the DOAS instruments were retrieved using new Network for the Detection of Atmospheric Composition Change (NDACC) guidelines and agree to within 3.2%. The DOAS ozone columns agree with the Brewer spectrophotometers with mean relative differences that are smaller than 1.5%. This suggests that for these instruments the new NDACC data guidelines were successful in producing a homogenous and accurate ozone dataset at 80° N. Satellite 14–52 km ozone and 17–40 km NO 2 partial columns within 500 km of PEARL were calculated for ACE-FTS Version 2.2 (v2.2) plus updates, ACE-FTS v3.0, ACE-MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) v1.2 and OSIRIS SaskMART v5.0x ozone and Optimal Estimation v3.0 NO 2 data products. The new ACE-FTS v3.0 and the validated ACE-FTS v2.2 partial columns are nearly identical, with mean relative differences of 0.0 ± 0.2% and −0.2 ± 0.1% for v2.2 minus v3.0 ozone and NO 2 , respectively. Ozone columns were constructed from 14–52 km satellite and 0–14 km ozonesonde partial columns and compared with the ground-based total column measurements. The satellite-plus-sonde measurements agree with the ground-based ozone total columns with mean relative differences of 0.1–7.3%. For NO 2 , partial columns from 17 km upward were scaled to noon using a photochemical model. Mean relative differences between OSIRIS, ACE-FTS and ground-based NO 2 measurements do not exceed 20%. ACE-MAESTRO measures more NO 2 than the other instruments, with mean relative differences of 25–52%. Seasonal variation in the differences between NO 2 partial columns is observed, suggesting that there are systematic errors in the measurements and/or the photochemical model corrections. For ozone spring-time measurements, additional coincidence criteria based on stratospheric temperature and the location of the polar vortex were found to improve agreement between some of the instruments. For ACE-FTS v2.2 minus Bruker FTIR, the 2007–2009 spring-time mean relative difference improved from −5.0 ± 0.4% to −3.1 ± 0.8% with the dynamical selection criteria. This was the largest improvement, likely because both instruments measure direct sunlight and therefore have well-characterized lines-of-sight compared with scattered sunlight measurements. For NO 2 , the addition of a ±1° latitude coincidence criterion improved spring-time intercomparison results, likely due to the sharp latitudinal gradient of NO 2 during polar sunrise. The differences between satellite and ground-based measurements do not show any obvious trends over the missions, indicating that both the ACE and OSIRIS instruments continue to perform well.

Description: Continuous measurements of methane mixing ratios from ice cores Atmospheric Measurement Techniques, 5, 999-1013, 2012 Author(s): C. Stowasser, C. Buizert, V. Gkinis, J. Chappellaz, S. Schüpbach, M. Bigler, X. Faïn, P. Sperlich, M. Baumgartner, A. Schilt, and T. Blunier This work presents a new, field-deployable technique for continuous, high-resolution measurements of methane mixing ratios from ice cores. The technique is based on a continuous flow analysis system, where ice core samples cut along the long axis of an ice core are melted continuously. The past atmospheric air contained in the ice is separated from the melt water stream via a system for continuous gas extraction. The extracted gas is dehumidified and then analyzed by a Wavelength Scanned-Cavity Ring Down Spectrometer for methane mixing ratios. We assess the performance of the new measurement technique in terms of precision (±0.8 ppbv, 1σ), accuracy (±8 ppbv), temporal (ca. 100 s), and spatial resolution (ca. 5 cm). Using a firn air transport model, we compare the resolution of the measurement technique to the resolution of the atmospheric methane signal as preserved in ice cores in Greenland. We conclude that our measurement technique can resolve all climatically relevant variations as preserved in the ice down to an ice depth of at least 1980 m (66 000 yr before present) in the North Greenland Eemian Ice Drilling ice core. Furthermore, we describe the modifications, which are necessary to make a commercially available spectrometer suitable for continuous methane mixing ratio measurements from ice cores.

Description: On the accuracy of integrated water vapor observations and the potential for mitigating electromagnetic path delay error in InSAR Atmospheric Measurement Techniques, 5, 1015-1030, 2012 Author(s): D. Cimini, N. Pierdicca, E. Pichelli, R. Ferretti, V. Mattioli, S. Bonafoni, M. Montopoli, and D. Perissin A field campaign was carried out in the framework of the Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects (METAWAVE) project sponsored by the European Space Agency (ESA) to investigate the accuracy of currently available sources of atmospheric columnar integrated water vapor measurements. The METAWAVE campaign took place in Rome, Italy, for the 2-week period from 19 September to 4 October 2008. The collected dataset includes observations from ground-based microwave radiometers and Global Positioning System (GPS) receivers, from meteorological numerical model analysis and predictions, from balloon-borne in-situ radiosoundings, as well as from spaceborne infrared radiometers. These different sources of integrated water vapor (IWV) observations have been analyzed and compared to quantify the accuracy and investigate the potential for mitigating IWV-related electromagnetic path delay errors in Interferometric Synthetic Aperture Radar (InSAR) imaging. The results, which include a triple collocation analysis accounting for errors inherently present in every IWV measurements, are valid not only to InSAR but also to any other application involving water vapor sensing. The present analysis concludes that the requirements for mitigating the effects of turbulent water vapor component into InSAR are significantly higher than the accuracy of the instruments analyzed here. Nonetheless, information on the IWV vertical stratification from satellite observations, numerical models, and GPS receivers may provide valuable aid to suppress the long spatial wavelength (〉20 km) component of the atmospheric delay, and thus significantly improve the performances of InSAR phase unwrapping techniques.

Description: Sensitivity of the Single Particle Soot Photometer to different black carbon types Atmospheric Measurement Techniques, 5, 1031-1043, 2012 Author(s): M. Laborde, P. Mertes, P. Zieger, J. Dommen, U. Baltensperger, and M. Gysel Black carbon (BC) is now mainly of anthropogenic origin. It is the dominant light absorbing component of atmospheric aerosols, playing an important role in the earth's radiative balance and therefore relevant to climate change studies. In addition, BC is known to be harmful to human beings making it relevant to policy makers. Nevertheless, the measurement of BC remains biased by the instrument-based definition of BC. The Single Particle Soot Photometer (SP2), allows the measurement of the refractory BC (rBC) mass of individual particles using laser-induced incandescence. However, the SP2 needs an empirical calibration to retrieve the rBC mass from the incandescence signal and the sensitivity of the SP2 differs between different BC types. Ideally, for atmospheric studies, the SP2 should be calibrated using ambient particles containing a known mass of ambient rBC. However, such "ambient BC" calibration particles cannot easily be obtained and thus commercially available BC particles are commonly used for SP2 calibration instead. In this study we tested the sensitivity of the SP2 to different BC types in order to characterize the potential error introduced by using non-ambient BC for calibration. The sensitivity of the SP2 was determined, using an aerosol particle mass analyzer, for rBC from thermodenuded diesel exhaust, wood burning exhaust and ambient particles as well as for commercially available products: Aquadag ® and fullerene soot. Thermodenuded, fresh diesel exhaust has been found to be ideal for SP2 calibration for two reasons. First, the small amount of non-BC matter upon emission reduces the risk of bias due to incomplete removal of non-BC matter and second, it is considered to represent atmospheric rBC in urban locations where diesel exhaust is the main source of BC. The SP2 was found to be up to 16% less sensitive to rBC from thermodenuded ambient particles (≤15 fg) than rBC from diesel exhaust, however, at least part of this difference can be explained by incomplete removal of non-refractory components in the thermodenuder. The amount of remaining non-refractory matter was estimated to be below 30% by mass, according to a comparison of the scattering cross sections of the whole particles with that of the pure BC cores. The SP2 sensitivity to rBC from wood burning exhaust agrees with the SP2 sensitivity to rBC from diesel exhaust within an error of less than 14% (≤40 fg). If, due to experimental restrictions, diesel exhaust cannot be used, untreated fullerene soot was found to give an SP2 calibration curve similar to diesel exhaust and ambient rBC (within ±10% for a rBC mass ≤15 fg) and is therefore recommended although two different batches differed by ~14% between themselves. In addition, the SP2 was found to be up to 40% more sensitive to Aquadag ® than to diesel exhaust rBC. Therefore Aquadag ® cannot be recommended for atmospheric application without accounting for the sensitivity difference. These findings for fullerene soot and Aquadag ® confirm results from previous literature.

Description: TRANC – a novel fast-response converter to measure total reactive atmospheric nitrogen Atmospheric Measurement Techniques, 5, 1045-1057, 2012 Author(s): O. Marx, C. Brümmer, C. Ammann, V. Wolff, and A. Freibauer The input and loss of plant available nitrogen (reactive nitrogen: N r ) from/to the atmosphere can be an important factor for the productivity of ecosystems and thus for its carbon and greenhouse gas exchange. We present a novel converter for reactive nitrogen (TRANC: Total Reactive Atmospheric Nitrogen Converter), which offers the opportunity to quantify the sum of all airborne reactive nitrogen compounds (∑N r ) in high time resolution. The basic concept of the TRANC is the full conversion of all N r to nitrogen monoxide (NO) within two reaction steps. Initially, reduced N r compounds are being oxidised, and oxidised N r compounds are thermally converted to lower oxidation states. Particulate N r is being sublimated and oxidised or reduced afterwards. In a second step, remaining higher nitrogen oxides or those generated in the first step are catalytically converted to NO with carbon monoxide used as reduction gas. The converter is combined with a fast response chemiluminescence detector (CLD) for NO analysis and its performance was tested for the most relevant gaseous and particulate N r species under both laboratory and field conditions. Recovery rates during laboratory tests for NH 3 and NO 2 were found to be 95 and 99%, respectively, and 97% when the two gases were combined. In-field longterm stability over an 11-month period was approved by a value of 91% for NO 2 . Effective conversion was also found for ammonium and nitrate containing particles. The recovery rate of total ambient N r was tested against the sum of individual measurements of NH 3 , HNO 3 , HONO, NH 4 + , NO 3 − , and NO x using a combination of different well-established devices. The results show that the TRANC-CLD system precisely captures fluctuations in ∑N r concentrations and also matches the sum of all individual N r compounds measured by the different single techniques. The TRANC features a specific design with very short distance between the sample air inlet and the place where the thermal and catalytic conversions to NO occur. This assures a short residence time of the sample air inside the instrument, and minimises wall sorption problems of water soluble compounds. The fast response time (e-folding times of 0.30 to 0.35 s were found during concentration step changes) and high accuracy in capturing the dominant N r species enables the converter to be used in an eddy covariance setup. Although a source attribution of specific N r compounds is not possible, the TRANC is a new reliable tool for permanent measurements of the net ∑N r flux between ecosystem and atmosphere at a relatively low maintenance and reasonable cost level allowing for diurnal, seasonal and annual investigations.