http://www.atmos-meas-tech.net/ Atmospheric Measurement Techniques Latest Articlesenhttp://www.atmos-meas-tech.net/3/1185/2010/<b>Accounting for surface reflectance anisotropy in satellite retrievals of tropospheric NO₂</b><br /><br />Atmospheric Measurement Techniques, 3, 1185-1203, 2010<br /><br />Author(s): Y. Zhou, D. Brunner, R. J. D. Spurr, K. F. Boersma, M. Sneep, C. Popp, and B. Buchmann<br /><br />Surface reflectance is a key parameter in satellite trace gas retrievals in the UV/visible range and in particular for the retrieval of nitrogen dioxide (NO₂) vertical tropospheric columns (VTCs). Current operational retrievals rely on coarse-resolution reflectance data and do not account for the generally anisotropic properties of surface reflectance. Here we present a NO₂ VTC retrieval that uses MODIS bi-directional reflectance distribution function (BRDF) data at high temporal (8 days) and spatial (1 km &times; 1 km) resolution in combination with the LIDORT radiative transfer model to account for the dependence of surface reflectance on viewing and illumination geometry. The method was applied to two years of NO₂ observations from the Ozone Monitoring Instrument (OMI) over Europe. Due to its wide swath, OMI is particularly sensitive to BRDF effects. Using representative BRDF parameters for various land surfaces, we found that in July (low solar zenith angles) and November (high solar zenith angles) and for typical viewing geometries of OMI, differences between MODIS black-sky albedos and surface bi-directional reflectances are of the order of 0–10% and 0–40%, respectively, depending on the position of the OMI pixel within the swath. In the retrieval, black-sky albedo was treated as a Lambertian (isotropic) reflectance, while for BRDF effects we used the kernel-based approach in the MODIS BRDF product. Air Mass Factors were computed using the LIDORT radiative transfer model based on these surface reflectance conditions. Differences in NO₂ VTCs based on the Lambertian and BRDF approaches were found to be of the order of 0–3% in July and 0–20% in November with the extreme values found at large viewing angles. The much larger differences in November are mainly due to stronger BRDF effects at higher solar zenith angles. To a smaller extent, they are also caused by the typically more pronounced maximum of the NO₂ a priori profiles in the boundary layer during the cold season, which make the retrieval more sensitive to radiation changes near the surface. However, BRDF impacts vary considerably across Europe due to differences in land surface type and increasing solar zenith angles at higher latitude. Finally, we compare BRDF-based NO₂ VTCs with those retrieved using the GOME/TOMS Lambertian equivalent reflectance (LER) data set. The relative differences are mostly below 15% in July but in November the NO₂ VTCs from TOMS/GOME are lower by 20–60%. Our results indicate that the specific choice of albedo data set is even more important than accounting for surface BRDF effects, and this again demonstrates the strong requirement for more accurate surface reflectance data sets.Wed, 01 Sep 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1175/2010/<b>Near-infrared laser desorption/ionization aerosol mass spectrometry for measuring organic aerosol at atmospherically relevant aerosol mass loadings</b><br /><br />Atmospheric Measurement Techniques, 3, 1175-1183, 2010<br /><br />Author(s): S. Geddes, B. Nichols, K. Todd, J. Zahardis, and G. A. Petrucci<br /><br />A new method, near-infrared laser desorption/ionization aerosol mass spectrometry (NIR-LDI-AMS), is described for the real time analysis of organic aerosols at atmospherically relevant total mass loadings. Particles are sampled with an aerodynamic lens onto an aluminum probe. A moderate energy NIR laser pulse at 1064 nm is directed onto the probe to vaporize and ionize particle components. Delayed pulse extraction is then used to sample the ions into a reflectron time of flight mass spectrometer for chemical analysis. The soft ionization afforded by the NIR photons results in minimal fragmentation (loss of a hydrogen atom) producing intact pseudo-molecular anions at [M-H]^&minus;. The limit of detection measured for pure oleic acid particles (geometric mean diameter and standard deviation of 180 nm and 1.3, respectively) was 140 fg (or 1.7 ng m⁻³ per minute sampling time). As an example of the utility of NIR-LDI-AMS to measurements of atmospheric importance, the method was applied to laboratory chamber measurements of the secondary organic aerosol formation from ozonolysis of α-pinene. High quality mass spectra were recorded with a 2-min time resolution for total aerosol mass loadings ranging from 1.5 to 8.7 μg m⁻³. These results demonstrate the potential of NIR-LDI-AMS to allow for more accurate measurements of the organic fraction of atmospheric particulate at realistic mass loadings. Measurements at ambient-levels of SOA mass loading are important to improve parameterizations of chamber-based SOA formation for modeling regional and global SOA fluxes and to aid in remediating the discrepancy between modeled and observed atmospheric total SOA production rates and concentrations.Tue, 31 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1155/2010/<b>The effect of horizontal gradients and spatial measurement resolution on the retrieval of global vertical NO₂ distributions from SCIAMACHY measurements in limb only mode</b><br /><br />Atmospheric Measurement Techniques, 3, 1155-1174, 2010<br /><br />Author(s): J. Puķīte, S. Kühl, T. Deutschmann, S. Dörner, P. Jöckel, U. Platt, and T. Wagner<br /><br />Limb measurements provided by the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) on the ENVISAT satellite allow retrieving stratospheric profiles of various trace gases on a global scale. Combining measurements of the same air volume from different viewing positions along the orbit, a tomographic approach can be applied and 2-D distribution fields of stratospheric trace gases can be acquired in one inversion. With this approach, it is possible to improve the accounting for the effect of horizontal gradients in the trace gas distribution on the profile retrieval. This was shown in a previous study for the retrieval of NO₂ and OClO profiles in the Arctic region near the polar vortex boundary. <br></br> In this study, the tomographic retrieval is applied on measurements during special limb-only orbits performed on 14 December 2008. For these orbits the distance between consecutive limb scanning sequences was reduced to ~3.3° of the orbital circle (i.e. more than two times with respect to the nominal operational mode). Thus, the same air volumes are scanned successively by more than one scanning sequence also for midlatitudes and the tropics. It is found that the profiles obtained by the tomographic 2-D approach show significant differences to those obtained by the 1-D approach. In particular, for regions close to stratospheric transport barriers (i.e. near to the edge of the polar vortex and subtropical transport barrier) up to 50% larger or smaller NO₂ number densities (depending on the sign of the gradient along the line of sight) for altitudes below the peak of the profile (around 20 km) are obtained. <br></br> The limb-only measurements allow examining the systematic error if the horizontal gradient is not accounted for, and studying the impact of the gradient strength on the profile retrieval on a global scale. The findings for the actual SCIAMACHY observations are verified by sensitivity studies for simulated data for which the NO₂ distributions to be retrieved are known in advance. In addition, the impact of the horizontal distance between consecutive limb scanning sequences on the quality of the tomographic 2-D retrieval is investigated and a possibility to take into account the horizontal gradients by an interpolation approach is studied.Mon, 30 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1143/2010/<b>A performance assessment of the World Wide Lightning Location Network (WWLLN) via comparison with the Canadian Lightning Detection Network (CLDN)</b><br /><br />Atmospheric Measurement Techniques, 3, 1143-1153, 2010<br /><br />Author(s): D. Abreu, D. Chandan, R. H. Holzworth, and K. Strong<br /><br />The World Wide Lightning Location Network (WWLLN) uses globally-distributed Very Low Frequency (VLF) receivers in order to observe lightning around the globe. Its objective is to locate as many global lightning strokes as possible, with high temporal and spatial (&lt; 10 km) accuracy. Since detection is done in the VLF range, signals from high peak current lightning strokes are able to propagate up to ~10⁴ km before being detected by the WWLLN sensors, allowing for receiving stations to be sparsely spaced. <br><br> Through a comparison with measurements made by the Canadian Lightning Detection Network (CLDN) between May and August 2008 over a 4° latitude by 4° longitude region centered on Toronto, Canada, this study found that WWLLN detection was most sensitive to high peak current lightning strokes. Events were considered shared between the two networks if they fell within 0.5 ms of each other. Using this criterion, 19 128 WWLLN strokes (analyzed using the Stroke_B algorithm) were shared with CLDN lightning strokes, producing a detection efficiency of 2.8%. The peak current threshold for WWLLN detection is found to be ~20 kA, with its detection efficiency increasing from 11.3% for peak currents greater than 20 kA to 75.8% for peak currents greater than 120 kA. The detection efficiency is seen to have a clear diurnal dependence, with a higher detection efficiency at local midnight than at local noon; this is attributed to the difference in the thickness of the ionospheric D-region between night and day. The mean time difference (WWLLN &minus; CLDN) between shared events was −6.44 μs with a standard deviation of 35 μs, and the mean absolute location accuracy was 7.24 km with a standard deviation of 6.34 km. These results are generally consistent with previous comparison studies of the WWLLN with other regional networks around the world. Additional receiver stations are continuously being added to the network, acting to improve this detection efficiency.Fri, 27 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1129/2010/<b>Fast and simple model for atmospheric radiative transfer</b><br /><br />Atmospheric Measurement Techniques, 3, 1129-1141, 2010<br /><br />Author(s): F. C. Seidel, A. A. Kokhanovsky, and M. E. Schaepman<br /><br />Radiative transfer models (RTMs) are of utmost importance for quantitative remote sensing, especially for compensating atmospheric perturbation. A persistent trade-off exists between approaches that prefer accuracy at the cost of computational complexity, versus those favouring simplicity at the cost of reduced accuracy. We propose an approach in the latter category, using analytical equations, parameterizations and a correction factor to efficiently estimate the effect of molecular multiple scattering. We discuss the approximations together with an analysis of the resulting performance and accuracy. The proposed Simple Model for Atmospheric Radiative Transfer (SMART) decreases the calculation time by a factor of more than 25 in comparison to the benchmark RTM 6S on the same infrastructure. The relative difference between SMART and 6S is about 5% for spaceborne and about 10% for airborne computations of the atmospheric reflectance function. The combination of a large solar zenith angle (SZA) with high aerosol optical depth (AOD) at low wavelengths lead to relative differences of up to 15%. SMART can be used to simulate the hemispherical conical reflectance factor (HCRF) for spaceborne and airborne sensors, as well as for the retrieval of columnar AOD.Wed, 25 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1113/2010/<b>Continuous low-maintenance CO₂/CH₄/H₂O measurements at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia</b><br /><br />Atmospheric Measurement Techniques, 3, 1113-1128, 2010<br /><br />Author(s): J. Winderlich, H. Chen, C. Gerbig, T. Seifert, O. Kolle, J. V. Lavrič, C. Kaiser, A. Höfer, and M. Heimann<br /><br />To monitor the continental carbon cycle, a fully automated low maintenance measurement system is installed at the Zotino Tall Tower Observatory in Central Siberia (ZOTTO, 60°48' N, 89°21' E) since April 2009. A cavity ring-down spectroscopy (CRDS) analyzer continuously measures carbon dioxide (CO₂) and methane (CH₄) from six heights up to 301 m a.g.l. Buffer volumes in each air line remove short term CO₂ and CH₄ mixing ratio fluctuations associated with turbulence, and allow continuous, near-concurrent measurements from all tower levels. Instead of drying the air sample, the simultaneously measured water vapor is used to correct the dilution and pressure-broadening effects for the accurate determination of dry air CO₂ and CH₄ mixing ratios. The stability of the water vapor correction was demonstrated by repeated laboratory and field tests. The effect of molecular adsorption in the wet air lines was shown to be negligible. The low consumption of four calibration tanks that need recalibration only on decadal timescale further reduces maintenance. The measurement precision (accuracy) of 0.04 ppm (0.09 ppm) for CO₂ and 0.3 ppb (1.5 ppb) for CH₄ is compliant with the WMO recommendations. The data collected so far (until April 2010) reveals a seasonal cycle amplitude for CO₂ of 30.4 ppm at the 301 m level.Tue, 24 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1103/2010/<b>Remotely operable compact instruments for measuring atmospheric CO₂ and CH₄ column densities at surface monitoring sites</b><br /><br />Atmospheric Measurement Techniques, 3, 1103-1112, 2010<br /><br />Author(s): N. Kobayashi, G. Inoue, M. Kawasaki, H. Yoshioka, M. Minomura, I. Murata, T. Nagahama, Y. Matsumi, T. Tanaka, I. Morino, and T. Ibuki<br /><br />Remotely operable compact instruments for measuring atmospheric CO₂ and CH₄ column densities were developed in two independent systems: one utilizing a grating-based desktop optical spectrum analyzer (OSA) with a resolution enough to resolve rotational lines of CO₂ and CH₄ in the regions of 1565–1585 and 1674–1682 nm, respectively; the other is an application of an optical fiber Fabry-Perot interferometer (FFPI) to obtain the CO₂ column density. Direct sunlight was collimated via a small telescope installed on a portable sun tracker and then transmitted through an optical fiber into the OSA or the FFPI for optical analysis. The near infrared spectra of the OSA were retrieved by a least squares spectral fitting algorithm. The CO₂ and CH₄ column densities deduced were in excellent agreement with those measured by a Fourier transform spectrometer with high resolution. The rovibronic lines in the wavelength region of 1570–1575 nm were analyzed by the FFPI. The <i>I</i>₀ and <i>I</i> values in the Beer-Lambert law equation to obtain CO₂ column density were deduced by modulating temperature of the FFPI, which offered column CO₂ with the statistical error less than 0.2% for six hours measurement.Tue, 24 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1089/2010/<b>An automatic contrail tracking algorithm</b><br /><br />Atmospheric Measurement Techniques, 3, 1089-1101, 2010<br /><br />Author(s): M. Vazquez-Navarro, H. Mannstein, and B. Mayer<br /><br />A method designed to track the life cycle of contrail-cirrus using satellite data with high temporal and spatial resolution, from its formation to the final dissolution of the aviation-induced cirrus cloud is presented. The method follows the evolution of contrails from their linear stage until they are undistinguishable from natural cirrus clouds. Therefore, the study of the effect of aircraft-induced clouds in the atmosphere is no longer restricted to linear contrails and can include contrail-cirrus. The method takes advantage of the high spatial resolution of polar orbiting satellites and the high temporal resolution of geostationary satellites to identify the pixels that belong to an aviation induced cloud. The high spatial resolution data of the MODIS sensor is used for contrail detection, and the high temporal resolution of the SEVIRI sensor in the Rapid Scan mode is used for contrail tracking. An example is included in which the method is applied to the study of a long lived contrail over the bay of Biscay.Thu, 19 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1075/2010/<b>A Chemical Ionization Mass Spectrometer for ambient measurements of Ammonia</b><br /><br />Atmospheric Measurement Techniques, 3, 1075-1087, 2010<br /><br />Author(s): D. R. Benson, A. Markovich, M. Al-Refai, and S.-H. Lee<br /><br />This study presents a chemical ionization mass spectrometer (CIMS) for fast response, in-situ measurements of gas phase ammonia (NH₃). The NH₃ background level detected with the CIMS ranged between 0.3–1 ppbv, with an uncertainty of 30 pptv under optimized conditions. The instrument sensitivity varied from 4–25 Hz/pptv for >1 MHz of reagent ion signals (protonated ethanol ions), with a 30% uncertainty estimated based on variability in calibration signals. The CIMS detection limit for NH₃ was ~60 pptv at a 1 min integration time (3 sigma). The CIMS time response was <30 s. This new NH₃-CIMS has been used for ambient measurements in Kent, Ohio, for several weeks throughout three seasons. The measured NH₃ mixing ratios were usually at the sub-ppbv level and higher in spring (200 &plusmn; 120 pptv) than in winter (60 &plusmn; 75 pptv) and fall (150 &plusmn; 80 pptv). High emissions of SO₂ from power plants in this region, and thus possible high acidity of aerosol particles, may explain these low NH₃ mixing ratios in general.Thu, 19 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1063/2010/<b>High time-resolution chemical characterization of the water-soluble fraction of ambient aerosols with PILS-TOC-IC and AMS</b><br /><br />Atmospheric Measurement Techniques, 3, 1063-1074, 2010<br /><br />Author(s): H. Timonen, M. Aurela, S. Carbone, K. Saarnio, S. Saarikoski, T. Mäkelä, M. Kulmala, V.-M. Kerminen, D. R. Worsnop, and R. Hillamo<br /><br />A particle-into-liquid sampler (PILS) was coupled with a total organic carbon analyzer (TOC) and two ion chromatographs (IC) to enable high time-resolution measurements of water-soluble ions and water-soluble organic carbon (WSOC) by a single sampling and analytical set-up. The new high time-resolution measurement system, the PILS-TOC-IC, was able to provide essential chemical and physical information about fast changes in composition, concentrations and likely sources of the water-soluble fraction of atmospheric aerosol. The concentrations of major water-soluble ions and WSOC were measured by the PILS-TOC-IC system from 25 April to 28 May 2009. <br><br> The data of the PILS-TOC-IC setup was compared with the data from the High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) data measured from 25 April to 8 May 2009. The measured water-soluble particulate organic matter (WSPOM) concentration varied typically from 0.10 to 8.8 μg m⁻³ (on average 1.5 μg m⁻³). The WSPOM contributed on average 51% to particulate organic matter (POM) measured with the AMS. The correlation between the data of all the online measurement devices (AMS, PILS-TOC-IC, semicontinuous EC/OC carbon analyzer and TEOM) was excellent. For sulfate, nitrate and ammonium the correlations between the PILS-TOC-IC and AMS were 0.93, 0.96 and 0.96, respectively. The correlation between WSPOM and POM was also strong (<I>r</I> = 0.88). The identified sources of WSPOM were long-range transported biomass burning and secondary organic aerosol (SOA) formation. WSPOM and oxalate produced in biomass burning were clearly correlated with carbon monoxide.Wed, 18 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1055/2010/<b>Formaldehyde measurements by Proton transfer reaction – Mass Spectrometry (PTR-MS): correction for humidity effects</b><br /><br />Atmospheric Measurement Techniques, 3, 1055-1062, 2010<br /><br />Author(s): A. Vlasenko, A .M. Macdonald, S. J. Sjostedt, and J. P. D. Abbatt<br /><br />Formaldehyde measurements can provide useful information about photochemical activity in ambient air, given that HCHO is formed via numerous oxidation processes. Proton transfer reaction mass spectrometry (PTR-MS) is an online technique that allows measurement of VOCs at the sub-ppbv level with good time resolution. PTR-MS quantification of HCHO is hampered by the humidity dependence of the instrument sensitivity, with higher humidity leading to loss of PTR-MS signal. In this study we present an analytical, first principles approach to correct the PTR-MS HCHO signal according to the concentration of water vapor in sampled air. The results of the correction are validated by comparison of the PTR-MS results to those from a Hantzsch fluorescence monitor which does not have the same humidity dependence. Results are presented for an intercomparison made during a field campaign in rural Ontario at Environment Canada's Centre for Atmospheric Research Experiments.Tue, 17 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1039/2010/<b>A high-resolution mass spectrometer to measure atmospheric ion composition</b><br /><br />Atmospheric Measurement Techniques, 3, 1039-1053, 2010<br /><br />Author(s): H. Junninen, M. Ehn, T. Petäjä, L. Luosujärvi, T. Kotiaho, R. Kostiainen, U. Rohner, M. Gonin, K. Fuhrer, M. Kulmala, and D. R. Worsnop<br /><br />In this paper we present recent achievements on developing and testing a tool to detect the composition of ambient ions in the mass/charge range up to 2000 Th. The instrument is an Atmospheric Pressure Interface Time-of-Flight Mass Spectrometer (APi-TOF, Tofwerk AG). Its mass accuracy is better than 0.002%, and the mass resolving power is 3000 Th/Th. In the data analysis, a new efficient Matlab based set of programs (tofTools) were developed, tested and used. The APi-TOF was tested both in laboratory conditions and applied to outdoor air sampling in Helsinki at the SMEAR III station. Transmission efficiency calibrations showed a throughput of 0.1–0.5% in the range 100–1300 Th for positive ions, and linearity over 3 orders of magnitude in concentration was determined. In the laboratory tests the APi-TOF detected sulphuric acid-ammonia clusters in high concentration from a nebulised sample illustrating the potential of the instrument in revealing the role of sulphuric acid clusters in atmospheric new particle formation. The APi-TOF features a high enough accuracy, resolution and sensitivity for the determination of the composition of atmospheric small ions although the total concentration of those ions is typically only 400–2000 cm⁻³. The atmospheric ions were identified based on their exact masses, utilizing Kendrick analysis and correlograms as well as narrowing down the potential candidates based on their proton affinities as well isotopic patterns. In Helsinki during day-time the main negative ambient small ions were inorganic acids and their clusters. The positive ions were more complex, the main compounds were (poly)alkyl pyridines and – amines. The APi-TOF provides a near universal interface for atmospheric pressure sampling, and this key feature will be utilized in future laboratory and field studies.Tue, 17 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1029/2010/<b>The effect of the global UV irradiance measurement accuracy on the single scattering albedo retrieval</b><br /><br />Atmospheric Measurement Techniques, 3, 1029-1037, 2010<br /><br />Author(s): S. Kazadzis, J. Gröbner, A. Arola, and V. Amiridis<br /><br />The possibility of measuring aerosol optical absorption properties in the UV spectral range such as single scattering albedo (SSA), using remote sensing techniques, is currently an open scientific issue. We investigate the limitations on calculating column average SSA using a combination of global UV spectral measurements (that are comon in various UV monitoring stations worldwide) with radiative transfer modeling. To point out the difficulties in such a retrieval we have used the travelling reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) results from 27 visits to UV monitoring stations around Europe. We have used the QASUME instrument as relative reference, analyzing absolute differences and also temporal and spectral deviations of UV irraidances, that are used as basic input for the SSA retrieval. <br><br> The results comparing the mean SSA derived by all instruments, measuring synchronous UV spectra, showed that 5 were within ± 0.02 difference from the SSA calculated from the QASUME instrument, while 17 were within ± 0.04, for the Solar zenith angle of 60 degrees. As for the uncertainty that has been calculated using the 2σ standard deviation of the spectral measurements, a mean 0.072 and 0.10 (2σ) uncertainties have been calculated for 60° and 30°, respectively. Based on the fact that additional uncertainties would be introduced in the SSA retrieval from AOD model input accuracy, assymetry parameter assumptions, we show that only very few instrumnents could be able to detect long term SSA changes. However, such measurements/results ar useful in order to retrieve SSA at UV wavelengths, a product needed for various applications such as, inputs for modeling radiative forcing studies and satellite retrieval algorithms.Thu, 12 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1019/2010/<b>Retrievals from GOMOS stellar occultation measurements using characterization of modeling errors</b><br /><br />Atmospheric Measurement Techniques, 3, 1019-1027, 2010<br /><br />Author(s): V. F. Sofieva, J. Vira, E. Kyrölä, J. Tamminen, V. Kan, F. Dalaudier, A. Hauchecorne, J.-L. Bertaux, D. Fussen, F. Vanhellemont, G. Barrot, and O. Fanton d'Andon<br /><br />In this paper, we discuss the development of the inversion algorithm for the GOMOS (Global Ozone Monitoring by Occultation of Star) instrument on board the Envisat satellite. The proposed algorithm takes accurately into account the wavelength-dependent modeling errors, which are mainly due to the incomplete scintillation correction in the stratosphere. The special attention is paid to numerical efficiency of the algorithm. The developed method is tested on a large data set and its advantages are demonstrated. Its main advantage is a proper characterization of the uncertainties of the retrieved profiles of atmospheric constituents, which is of high importance for data assimilation, trend analyses and validation.Wed, 11 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/1005/2010/<b>The detection of cloud-free snow-covered areas using AATSR measurements</b><br /><br />Atmospheric Measurement Techniques, 3, 1005-1017, 2010<br /><br />Author(s): L. G. Istomina, W. von Hoyningen-Huene, A. A. Kokhanovsky, and J. P. Burrows<br /><br />A new method to detect cloud-free snow-covered areas has been developed using the measurements by the Advanced Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite in order to discriminate clear snow fields for the retrieval of aerosol optical thickness or snow properties. The algorithm uses seven AATSR channels from visible (VIS) to thermal infrared (TIR) and analyses the spectral behaviour of each pixel in order to recognize the spectral signature of snow. The algorithm includes a set of relative thresholds and combines all seven channels into one flexible criterion, which allows us to filter out all the pixels with spectral behaviour similar to that of snow. The algorithm does not use any kind of morphological criteria and does not require the studied surface to have any special structure. The snow spectral shape criterion was determined by a comprehensive theoretical study, which included radiative transfer simulations for various atmospheric conditions as well as studying existing models and measurements of optical and physical properties of snow in different spectral bands. The method has been optimized to detect cloud-free snow-covered areas, and does not produce cloud/land/ocean/snow mask. However, the algorithm can be extended and able to discriminate various kinds of surfaces. <br><br> The presented method has been validated against Micro Pulse Lidar data and compared to Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask over snow-covered areas, showing quite good correspondence to each other. <br><br> Comparison of both MODIS cloud mask and presented snow mask to AATSR operational cloud mask showed that in some cases of snow surface the accuracy of AATSR operational cloud mask is questionable.Tue, 03 Aug 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/991/2010/<b>Potential for the use of reconstructed IASI radiances in the detection of atmospheric trace gases</b><br /><br />Atmospheric Measurement Techniques, 3, 991-1003, 2010<br /><br />Author(s): N. C. Atkinson, F. I. Hilton, S. M. Illingworth, J. R. Eyre, and T. Hultberg<br /><br />Principal component (PC) analysis has received considerable attention as a technique for the extraction of meteorological signals from hyperspectral infra-red sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder (AIRS). In addition to achieving substantial bit-volume reductions for dissemination purposes, the technique can also be used to generate reconstructed radiances in which random instrument noise has been reduced. Studies on PC analysis of hyperspectral infrared sounder data have been undertaken in the context of numerical weather prediction, instrument monitoring and geophysical variable retrieval, as well as data compression. This study examines the potential of PC analysis for chemistry applications. <br><br> A major concern in the use of PC analysis for chemistry is that the spectral features associated with trace gases may not be well represented in the reconstructed spectra, either due to deficiencies in the training set or due to the limited number of PC scores used in the radiance reconstruction. In this paper we show examples of reconstructed IASI radiances for several trace gases: ammonia, sulphur dioxide, methane and carbon monoxide. It is shown that care must be taken in the selection of spectra for the initial training set: an iterative technique, in which outlier spectra are added to a base training set, gives the best results. For the four trace gases examined, key features of the chemical signatures are retained in the reconstructed radiances, whilst achieving a substantial reduction in instrument noise. <br><br> A new regional re-transmission service for IASI is scheduled to start in 2010, as part of the EUMETSAT Advanced Retransmission Service (EARS). For this EARS-IASI service it is intended to include PC scores as part of the data stream. The paper describes the generation of the reference eigenvectors for this new service.Fri, 30 Jul 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/981/2010/<b>Measurement of HONO, HNCO, and other inorganic acids by negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS): application to biomass burning emissions</b><br /><br />Atmospheric Measurement Techniques, 3, 981-990, 2010<br /><br />Author(s): J. M. Roberts, P. Veres, C. Warneke, J. A. Neuman, R. A. Washenfelder, S. S. Brown, M. Baasandorj, J. B. Burkholder, I. R. Burling, T. J. Johnson, R. J. Yokelson, and J. de Gouw<br /><br />A negative-ion proton-transfer chemical ionization mass spectrometric technique (NI-PT-CIMS), using acetate as the reagent ion, was applied to the measurement of volatile inorganic acids of atmospheric interest: hydrochloric (HCl), nitrous (HONO), nitric (HNO₃), and isocyanic (HNCO) acids. Gas phase calibrations through the sampling inlet showed the method to be intrinsically sensitive (6–16 cts/pptv), but prone to inlet effects for HNO₃ and HCl. The ion chemistry was found to be insensitive to water vapor concentrations, in agreement with previous studies of carboxylic acids. The inlet equilibration times for HNCO and HONO were 2 to 4 s, allowing for measurement in biomass burning studies. Several potential interferences in HONO measurements were examined: decomposition of HNO₃·NO₃^&minus; clusters within the CIMS, and NO₂-water production on inlet surfaces, and were quite minor (≤1%, 3.3%, respectively). The detection limits of the method were limited by the instrument backgrounds in the ion source and flow tube, and were estimated to range between 16 and 50 pptv (parts per trillion by volume) for a 1 min average. The comparison of HONO measured by CIMS and by in situ FTIR showed good correlation and agreement to within 17%. The method provided rapid and accurate measurements of HNCO and HONO in controlled biomass burning studies, in which both acids were seen to be important products.Fri, 23 Jul 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/959/2010/<b>A comparison of GC-FID and PTR-MS toluene measurements in ambient air under conditions of enhanced monoterpene loading</b><br /><br />Atmospheric Measurement Techniques, 3, 959-980, 2010<br /><br />Author(s): J. L. Ambrose, K. Haase, R. S. Russo, Y. Zhou, M. L. White, E. K. Frinak, C. Jordan, H. R. Mayne, R. Talbot, and B. C. Sive<br /><br />Toluene was measured using both a gas chromatographic system (GC), with a flame ionization detector (FID), and a proton transfer reaction-mass spectrometer (PTR-MS) at the AIRMAP atmospheric monitoring station Thompson Farm (THF) in rural Durham, NH during the summer of 2004. Simultaneous measurements of monoterpenes, including &alpha;- and &beta;-pinene, camphene, &Delta; ³-carene, and <i>d</i>-limonene, by GC-FID demonstrated large enhancements in monoterpene mixing ratios relative to toluene, with median and maximum enhancement ratios of ~2 and ~30, respectively. A detailed comparison between the GC-FID and PTR-MS toluene measurements was conducted to test the specificity of PTR-MS for atmospheric toluene measurements under conditions often dominated by biogenic emissions. We derived quantitative estimates of potential interferences in the PTR-MS toluene measurements related to sampling and analysis of monoterpenes, including fragmentation of the monoterpenes and some of their primary carbonyl oxidation products via reactions with H₃O⁺, O₂⁺ and NO⁺ in the PTR-MS drift tube. The PTR-MS and GC-FID toluene measurements were in good quantitative agreement and the two systems tracked one another well from the instrumental limits of detection to maximum mixing ratios of ~0.5 ppbv. A correlation plot of the PTR-MS versus GC-FID toluene measurements was described by the least squares regression equation <i>y</i>=(1.13&plusmn; 0.02)<i>x</i>&minus;(0.008&plusmn;0.003) ppbv, suggesting a small ~13% positive bias in the PTR-MS measurements. The bias corresponded with a ~0.055 ppbv difference at the highest measured toluene level. The two systems agreed quantitatively within the combined 1&sigma; measurement precisions for 60% of the measurements. Discrepancies in the measured mixing ratios were not well correlated with enhancements in the monoterpenes. Better quantitative agreement between the two systems was obtained by correcting the PTR-MS measurements for contributions from monoterpene fragmentation in the PTR-MS drift tube; however, the improvement was minor (<10%). Interferences in the PTR-MS measurements from fragmentation of the monoterpene oxidation products pinonaldehyde, caronaldehyde and &alpha;-pinene oxide were also likely negligible. A relatively large and variable toluene background in the PTR-MS instrument likely drove the measurement bias; however, the precise contribution was difficult to accurately quantify and thus was not corrected for in this analysis. The results from THF suggest that toluene can be reliably quantified by PTR-MS using our operating conditions (drift tube pressure, temperature and voltage of 2.0 mbar, 45 °C and 600 V, respectively) under the ambient compositions probed. This work extends the range of field conditions under which PTR-MS validation studies have been conducted.Fri, 23 Jul 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/947/2010/<b>Total column CO₂ measurements at Darwin, Australia &ndash; site description and calibration against in situ aircraft profiles</b><br /><br />Atmospheric Measurement Techniques, 3, 947-958, 2010<br /><br />Author(s): N. M. Deutscher, D. W. T. Griffith, G. W. Bryant, P. O. Wennberg, G. C. Toon, R. A. Washenfelder, G. Keppel-Aleks, D. Wunch, Y. Yavin, N. T. Allen, J.-F. Blavier, R. Jiménez, B. C. Daube, A. V. Bright, D. M. Matross, S. C. Wofsy, and S. Park<br /><br />An automated Fourier Transform Spectroscopic (FTS) solar observatory was established in Darwin, Australia in August 2005. The laboratory is part of the Total Carbon Column Observing Network, and measures atmospheric column abundances of CO₂ and O₂ and other gases. Measured CO₂ columns were calibrated against integrated aircraft profiles obtained during the TWP-ICE campaign in January–February 2006, and show good agreement with calibrations for a similar instrument in Park Falls, Wisconsin. A clear-sky low airmass relative precision of 0.1% is demonstrated in the CO₂ and O₂ retrieved column-averaged volume mixing ratios. The 1% negative bias in the FTS <i>X</i>_CO₂ relative to the World Meteorological Organization (WMO) calibrated in situ scale is within the uncertainties of the NIR spectroscopy and analysis.Mon, 19 Jul 2010 00:00:00 +0200http://www.atmos-meas-tech.net/3/933/2010/<b>Time dependent profile retrieval of UV/vis absorbing radicals from balloon-borne limb measurements – a case study on NO₂ and O₃</b><br /><br />Atmospheric Measurement Techniques, 3, 933-946, 2010<br /><br />Author(s): L. Kritten, A. Butz, M. Dorf, T. Deutschmann, S. Kühl, C. Prados-Roman, J. Puķīte, A. Rozanov, R. Schofield, and K. Pfeilsticker<br /><br />A new "Bayesian" minimization algorithm for the retrieval of the diurnal variation of UV/vis absorbing radicals (O₃, NO₂, BrO, OClO and HONO) from balloon-borne limb scattered skylight observations is discussed. The method evaluates spectroscopic measurements in combination with radiative transfer calculations to drive a mathematical inversion on a discrete time and height grid. Here, the proposed method is applied to data obtained during two deployments of the mini-DOAS instrument on different balloon payloads in northern Brazil in June 2005. The retrieval is tested by comparing the inferred profiles to in-situ ozone sounding data and to measurements of the ENVISAT/SCIAMACHY satellite instrument performed during a collocated overpass. The comparison demonstrates the strength and validity of our approach. In particular for time-varying radical concentrations, photochemical corrections due to temporal mismatch of the corresponding observations are rendered dispensable. Thus, limb scanning UV/vis spectrometry from balloon platforms offers a more direct and concise approach for satellite validation of radical measurements than solar occultation measurements. Furthermore, monitoring of the diurnal variation of stratospheric radicals allows us to constrain photochemical parameters which are critical for stratospheric ozone chemistry, such as the photolysis frequency of N₂O₅ by observations of the diurnal variation of NO₂.Fri, 16 Jul 2010 00:00:00 +0200