bluuebunny/crossref_metadata_2025 huggingface dataset:
Aerodynamic gradient measurements of the NH3-HNO3-NH4NO3 triad using a wet chemical instrument: an analysis of precision requirements and flux errors
Authors: Wolff V. , Trebs I. , Ammann C. , Meixner F. X.
Published: April 2010

Abstract. The aerodynamic gradient method is widely used for flux measurements of ammonia, nitric acid, particulate ammonium nitrate (the NH3-HNO3-NH4NO3 triad) and other water-soluble reactive trace compounds. The surface exchange flux is derived from a measured concentration difference and micrometeorological quantities (turbulent exchange coefficient). The significance of the measured concentration difference is crucial for the significant determination of surface exchange fluxes. Additionally, measurements of surface exchange fluxes of ammonia, nitric acid and ammonium nitrate are often strongly affected by phase changes between gaseous and particulate compounds of the triad, which make measurements of the four individual species (NH3, HNO3, NH4+, NO3–) necessary for a correct interpretation of the measured concentration differences. We present here a rigorous analysis of results obtained with a multi-component, wet-chemical instrument, able to simultaneously measure gradients of both gaseous and particulate trace substances. Basis for our analysis are two field experiments, conducted above contrasting ecosystems (grassland, forest). Precision requirements of the instrument as well as errors of concentration differences and micrometeorological exchange parameters have been estimated, which, in turn, allows the establishment of thorough error estimates of the derived fluxes of NH3, HNO3, NH4+, and NO3–. Derived median flux errors for the grassland and forest field experiments were: 39 and 50% (NH3), 31 and 38% (HNO3), 62 and 57% (NH4+), and 47 and 68% (NO3–), respectively. Additionally, we provide the basis for using field data to characterize the instrument performance, as well as subsequent quantification of surface exchange fluxes and underlying mechanistic processes under realistic ambient measurement conditions.
A method for improved SCIAMACHY CO2 retrieval in the presence of optically thin clouds
Authors: Reuter M. , Buchwitz M. , Schneising O. , Heymann J. , Bovensmann H. , Burrows J. P.
Published: April 2010

Abstract. An optimal estimation based retrieval scheme for satellite based measurements of XCO2 (the column averaged mixing ratio of atmospheric CO2) is presented enabling accurate retrievals also in the presence of thin clouds. The proposed method is designed to analyze near-infrared nadir measurements of the SCIAMACHY instrument in the CO2 absorption band at 1580 nm and in the O2-A absorption band at around 760 nm. The algorithm accounts for scattering in an optically thin cirrus cloud layer and at aerosols of a default profile. The scattering information is mainly obtained from the O2-A band and a merged fit windows approach enables the transfer of information between the O2-A and the CO2 band. Via the optimal estimation technique, the algorithm is able to account for a priori information to further constrain the inversion. Test scenarios of simulated SCIAMACHY sun-normalized radiance measurements are analyzed in order to specify the quality of the proposed method. In contrast to existing algorithms, the systematic errors due to cirrus clouds with optical thicknesses up to 1.0 are reduced to values typically below 4 ppm. This shows that the proposed method has the potential to reduce uncertainties of SCIAMACHY retrieved XCO2 making this data product useful for surface flux inverse modeling.
Measurements of greenhouse gases and related tracers at Bialystok tall tower station in Poland
Authors: Popa M. E. , Gloor M. , Manning A. C. , Jordan A. , Schultz U. , Haensel F. , Seifert T. , Heimann M.
Published: April 2010

Abstract. Quasi-continuous, in-situ measurements of atmospheric CO2, O2/N2, CH4, CO, N2O, and SF6 have been performed since August 2005 at the tall tower station near Bialystok, in Eastern Poland, from five heights up to 300 m. Besides the in-situ measurements, flask samples are filled approximately weekly and measured at Max-Planck Institute for Biogeochemistry for the same species and, in addition, for H2, Ar/N2 and the stable isotopes 13C and 18O in CO2. The in-situ measurement system was build based on commercially available analysers: a LiCor 7000 for CO2, a Sable Systems "Oxzilla" FC-2 for O2, and an Agilent 6890 gas chromatograph for CH4, CO, N2O and SF6. The system was optimized to run continuously with very little maintenance and to fulfill the precision requirements of the CHIOTTO project. The O2 measurements in particular required special attention in terms of technical setup and quality assurance. The evaluation of the performance after more than three years of operation gave overall satisfactory results, proving that this setup is suitable for long term remote operation with little maintenance. The precision achieved for all species is within or close to the project requirements. The comparison between the in-situ and flask sample results, used to verify the accuracy of the in-situ measurements, showed no significant difference for CO2, O2/N2, CH4 and N2O, and a very small difference for SF6. The same comparison however revealed a statistically significant difference for CO, of about 6.5 ppb, for which the cause could not be fully explained at the moment. From more than three years of data, the main features at Bialystok have been characterized in terms of variability, trends, and seasonal and diurnal variations. CO2 and O2/N2 show large short term variability, and large diurnal signals during the warm seasons, which attenuate with the increase of sampling height. The trends calculated from this dataset, over the period August 2005 to December 2008, are 2.02±0.46 ppm/year for CO2 and −23.2±2.5 per meg/year for O2/N2. CH4, CO and N2O show also higher variability at the lower sampling levels, which in the case of CO is strongly seasonal. Diurnal variations in CH4, CO and N2O mole fractions can be observed during the warm season, due to the periodicity of vertical mixing combined with the diurnal cycle of anthropogenic emissions. We calculated increase rates of 10.1±4.4 ppb/year for CH4, (−8.3)±5.3 ppb/year for CO and 0.67±0.08 ppb/year for N2O. SF6 shows only few events, and generally no vertical gradients, which suggests that there are no significant local sources. A weak SF6 seasonal cycle has been detected, which most probably is due to the seasonality of atmospheric circulation. SF6 increased during the time of our measurement at an average rate of 0.29±0.01 ppt/year.
Extending differential optical absorption spectroscopy for limb measurements in the UV
Authors: Puķīte J. , Kühl S. , Deutschmann T. , Platt U. , Wagner T.
Published: April 2010

Abstract. Methods of UV/VIS absorption spectroscopy to determine the constituents in the Earth's atmosphere from measurements of scattered light are often based on the Beer-Lambert law, like e.g. Differential Optical Absorption Spectroscopy (DOAS). Therefore they are strictly valid for weak absorptions and narrow wavelength intervals (strictly only for monochromatic radiation). For medium and strong absorption (e.g. along very long light-paths like in limb geometry) the relation between the optical depth and the concentration of an absorber is not linear anymore. As well, for large wavelength intervals the wavelength dependent differences in the travelled light-paths become important, especially in the UV, where the probability for scattering increases strongly with decreasing wavelength. However, by taking into account these dependencies, the applicability of the DOAS method can be extended also to cases with medium to strong absorptions and for broader wavelength intervals. Common approaches for this correction are the so called air mass factor modified (or extended) DOAS and the weighting function modified DOAS. These approaches take into account the wavelength dependency of the slant column densities (SCDs), but also require a-priori knowledge for the air mass factor or the weighting function calculation by radiative transfer modelling. We describe an approach that considers the fitting results obtained from DOAS, the SCDs, as a function of wavelength and vertical optical depth and expands this function into a Taylor series of both quantities. The Taylor coefficients are then applied as additional fitting parameters in the DOAS analysis. Thus the variability of the SCD in the fit window is determined by the retrieval itself. This new approach gives a description of the SCD that is as close to reality as desired (depending on the order of the Taylor expansion), and is independent from any assumptions or a-priori knowledge of the considered absorbers. In case studies for simulated and measured spectra in the UV (332–357 nm), we demonstrate the improvement by this approach for the retrieval of vertical profiles of BrO from the SCIAMACHY limb observations. Compared to the standard DOAS approach, the results for BrO obtained from the simulated spectra are closer to the true profile, when applying the new method for the SCDs of ozone. Also for the measured spectra the agreement with validation measurements is improved significantly, especially for cases with strong ozone absorption. While the focus of this article is on the improvement of the BrO profile retrieval from the SCIAMACHY limb measurements, the novel approach may be applied for a wide range of DOAS retrievals.
Retrieval of tropospheric column densities of NO2 from combined SCIAMACHY nadir/limb measurements
Authors: Beirle S. , Kühl S. , Puķīte J. , Wagner T.
Published: April 2010

Abstract. The SCIAMACHY instrument onboard the ESA satellite ENVISAT allows the retrieval of column densities of various trace gases, among them NO2. As only instrument of its kind, SCIAMACHY measures in an alternating limb/nadir mode. The limb measurements allow a direct determination of stratospheric column densities, which are needed to extract tropospheric from the total column density measurements performed in (quasi simultaneous) nadir geometry. Here we discuss the potential and limitations of SCIAMACHY limb measurements for estimating stratospheric column densities of NO2 in comparison to a simple reference sector method, and the consequences for the resulting tropospheric column densities. A direct, absolute limb correction scheme improves spatial patterns of tropospheric NO2 column densities at high latitudes compared to the simple reference sector method. However, it results in artificial zonal stripes at low latitudes. Thus, also a relative limb correction scheme was defined, which turned out to successfully reduce stratospheric artefacts in the resulting tropospheric data product without introducing new ones. This relative limb correction scheme is rather simple, robust, and, in essence, based on measurements alone. The effect of the different stratospheric estimation schemes on tropospheric column densities is discussed with respect to zonal and temporal dependencies. In addition, error quantities are defined from the nadir/limb measurements which indicate remaining systematic errors as function of latitude and day. Our new suggested stratospheric estimation scheme, the relative limb correction, improves monthly mean tropospheric slant column densities significantly, e.g. from −1×1015 molec/cm2 (using a simple reference sector method) to ≈0 in the Atlantic ocean, and from +1×1015 molec/cm2 to ≈0 over Siberia, at 50° N in January.
Total peroxy nitrates (ΣPNs) in the atmosphere: the thermal dissociation-laser induced fluorescence (TD-LIF) technique and comparisons to speciated PAN measurements
Authors: Wooldridge P. J. , Perring A. E. , Bertram T. H. , Flocke F. M. , Roberts J. M. , Singh H. B. , Huey L. G. , Thornton J. A. , Murphy J. G. , Fry J. L. , Rollins A. W. , LaFranchi B. W. , Cohen R. C.
Published: April 2010

Abstract. Peroxyacetyl nitrate (PAN) and its chemical analogues are increasingly being quantified in the ambient atmosphere by thermal dissociation (TD) followed by detection of either the peroxyacyl radical or the NO2 product. Here we present details of the technique developed at University of California, Berkeley which detects the sum of all peroxynitrates (ΣPNs) via laser-induced fluorescence (LIF) of the NO2 product. We review the various deployments and compare the Berkeley ΣPNs measurements with the sums of PAN and its homologue species detected individually by other instruments. The observed TD-LIF ΣPNs usually agree to within 10% with the summed individual species, thus arguing against the presence of significant concentrations of unmeasured PAN-type compounds in the atmosphere, as suggested by some photochemical mechanisms. Examples of poorer agreement are attributed to a sampling inlet design that is shown to be inappropriate for high NOx conditions. Interferences to the TD-LIF measurements are described along with strategies to minimize their effects.
Ground-based observations for the validation of contrails and cirrus detection in satellite imagery
Authors: Mannstein H. , Brömser A. , Bugliaro L.
Published: April 2010

Abstract. Contrails and additional cirrus clouds caused by air traffic have a potential warming effect due to their optical properties and their location in the upper troposphere. The effect of contrails is directly related to their coverage and optical properties, which can be derived from satellite observations, but considerable local and global uncertainties remain, as detection limits and efficiency are still unknown. A 6 months time series of the occurrence of high-level clouds and contrails was analysed visually from an all-sky camera situated at Oberpfaffenhofen (Southern Germany). It shows a contrail occurrence (fraction of time with visible contrails during one hour) of 21% being nearly constant over daytime and a cirrus occurrence that increases from 27% in the morning to 48% in the evening, suggesting a possible influence of air traffic or, more probably, convection. Furthermore, we compared selected all-sky camera images with data of the satellite instruments NOAA/AVHRR and MSG/SEVIRI. As expected, the fraction of contrails visible and detectable in satellite images depends highly on their width. Of the contrails observed with the all-sky camera being 1–5 km wide, 60–65% are visually detectable in AVHRR data, while only 17% are identified by an automated contrail detection algorithm (CDA). However, the CDA detects approx. 28% of the visually detected contrails. As far as SEVIRI is concerned, visual inspection yields 48% of the contrails of 1–5 km width, the CDA 19%. This value rises to 40% when comparing to the visually detected contrails. As far as cirrus detection with SEVIRI is concerned, an automated algorithm tends to overestimate cirrus occurrence but correctly appraises cirrus changes during the day.
A new technique for the selective measurement of atmospheric peroxy radical concentrations of HO2 and RO2 using denuding method
Authors: Miyazaki K. , Parker A. E. , Fittschen C. , Monks P. S. , Kajii Y.
Published: April 2010

Abstract. A technique for the selective measurement of atmospheric HO2 and RO2 using peroxy radical chemical amplification coupled to laser-induced fluorescence NO2 detection (PERCA-LIF) technique is proposed. By pulling the air through a filled pre-inlet advantage can be taken of the higher heterogeneous loss rate of HO2 relative to CH3O2. Pre-inlet conditions have been found where ca. 90% of HO2 was removed whereas the comparable CH3O2 loss was 15%. The dependence of loss rate on humidity and peroxy radicals' concentration has also been investigated. When using glass beads as the surface for peroxy radical remove, the influence of the relative humidity on the removal efficiency becomes negligible. It may therefore be possible to apply this technique to the measurement of absolute concentrations of solely RO2 as well as the sum of HO2 and RO2. The application of this technique to atmospheric measurements is suggested.
Characterizing a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of atmospheric ammonia
Authors: Ellis R. A. , Murphy J. G. , Pattey E. , van Haarlem R. , O'Brien J. M. , Herndon S. C.
Published: April 2010

Abstract. A compact, fast-response Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS) for measurements of ammonia has been evaluated under both laboratory and field conditions. Absorption of radiation from a pulsed, thermoelectrically cooled QC laser occurs at reduced pressure in a 0.5 L multiple pass absorption cell with an effective path length of 76 m. Detection is achieved using a thermoelectrically cooled Mercury Cadmium Telluride (HgCdTe) infrared detector. A novel sampling inlet was used, consisting of a short, heated, quartz tube with a hydrophobic coating to minimize the adsorption of ammonia to surfaces. The inlet contains a critical orifice that reduces the pressure, a virtual impactor for separation of particles, and additional ports for delivering ammonia-free background air and calibration gas standards. This instrument has been found to have a detection limit of 0.23 ppb at 1 Hz. The sampling technique has been compared to the results of a conventional lead salt Tunable Diode Laser Absorption Spectrometer (TDLAS) during a laboratory intercomparison. The effect of humidity and heat on the surface interaction of ammonia with sample tubing was investigated at mixing ratios ranging from 30–1000 ppb. Humidity was seen to worsen the ammonia time response and considerable improvement was observed when using a heated sampling line. A field intercomparison of the QC-TILDAS with a modified Thermo 42CTL chemiluminescence based analyzer was also performed at Environment Canada's Centre for Atmospheric Research Experiments (CARE) in the rural town of Egbert, ON between May–July 2008. Background tests and calibrations using two different permeation tube sources and an ammonia gas cylinder were regularly carried out throughout the study. Results indicate a very good correlation with 1 min time resolution (R2=0.93) between the two instruments at the beginning of the study, when regular background subtraction was applied to the QC-TILDAS. An overall good correlation of R2=0.83 was obtained over the entire two month data set, where the majority of the spread can be attributed to differences in inlet design and background subtraction methods.
Measurement of ozone production sensor
Authors: Cazorla M. , Brune W. H.
Published: April 2010

Abstract. A new ambient air monitor, the Measurement of Ozone Production Sensor (MOPS), measures directly the rate of ozone production in the atmosphere. The sensor consists of two 11.3 L environmental chambers made of UV-transmitting Teflon film, a unit to convert NO2 to O3, and a modified ozone monitor. In the sample chamber, flowing ambient air is exposed to the sunlight so that ozone is produced just as it is in the atmosphere. In the second chamber, called the reference chamber, a UV-blocking film over the Teflon film prevents ozone formation but allows other processes to occur as they do in the sample chamber. The air flows that exit the two chambers are sampled by an ozone monitor operating in differential mode so that the difference between the two ozone signals, divided by the residence time in the chambers, gives the ozone production rate. High-efficiency conversion of NO2 to O3 prior to detection in the ozone monitor accounts for differences in the NOx photostationary state that can occur in the two chambers. The MOPS measures the ozone production rate, but with the addition of NO to the sampled air flow, the MOPS can be used to study the sensitivity of ozone production to NO. Preliminary studies with the MOPS on the campus of the Pennsylvania State University show the potential of this new technique.
The inter-comparison of major satellite aerosol retrieval algorithms using simulated intensity and polarization characteristics of reflected light
Authors: Kokhanovsky A. A. , Deuzé J. L. , Diner D. J. , Dubovik O. , Ducos F. , Emde C. , Garay M. J. , Grainger R. G. , Heckel A. , Herman M. , Katsev I. L. , Keller J. , Levy R. , North P. R. J. , Prikhach A. S. , Rozanov V. V. , Sayer A. M. , Ota Y. , Tanré D. , Thomas G. E. , Zege E. P.
Published: April 2010

Abstract. Remote sensing of aerosol from space is a challenging and typically underdetermined retrieval task, requiring many assumptions to be made with respect to the aerosol and surface models. Therefore, the quality of a priori information plays a central role in any retrieval process (apart from the cloud screening procedure and the forward radiative transfer model, which to be most accurate should include the treatment of light polarization and molecular-aerosol coupling). In this paper the performance of various algorithms with respect to the of spectral aerosol optical thickness determination from optical spaceborne measurements is studied. The algorithms are based on various types of measurements (spectral, angular, polarization, or some combination of these). It is confirmed that multiangular spectropolarimetric measurements provide more powerful constraints compared to spectral intensity measurements alone, particularly those acquired at a single view angle and which rely on a priori assumptions regarding the particle phase function in the retrieval process.
MIPAS reduced spectral resolution UTLS-1 mode measurements of temperature, O3, HNO3, N2O, H2O and relative humidity over ice: retrievals and comparison to MLS
Authors: Chauhan S. , Höpfner M. , Stiller G. P. , von Clarmann T. , Funke B. , Glatthor N. , Grabowski U. , Linden A. , Kellmann S. , Milz M. , Steck T. , Fischer H. , Froidevaux L. , Lambert A. , Santee M. L. , Schwartz M. , Read W. G. , Livesey N. J.
Published: April 2010

Abstract. During several periods since 2005 the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat has performed observations dedicated to the region of the upper troposphere/lower stratosphere (UTLS). For the duration of November/December 2005 global distributions of temperature and several trace gases from MIPAS UTLS-1 mode measurements have been retrieved using the IMK/IAA (Institut für Meteorologie und Klimaforschung/Instituto de Astrofísica de Andalucía) scientific processor. In the UTLS region a vertical resolution of 2.5 to 3 km has been achieved. The retrieved temperature, H2O, O3, HNO3, N2O, and relative humidity over ice are intercompared with the Microwave Limb Sounder (MLS/Aura) v2.2 data. In general, MIPAS and MLS temperatures agree within ±4 K over the whole pressure range of 316–0.68 hPa. Systematic, latitude-independent differences of −2 to −4 K (MIPAS-MLS) at 121 hPa are explained by previously observed biases in the MLS v2.2 temperature retrievals. Temperature differences of −4 K up to 12 K above 10.0 hPa are present similarly in MIPAS and MLS with respect to ECMWF (European Centre for Medium-Range Weather Forecasts) and are likely due to deficiencies of the ECMWF analysis data. MIPAS and MLS stratospheric volume mixing ratios (vmr) of H2O agree within ±1 ppmv, with indication of oscillations between 146 and 26 hPa in the MLS dataset. Tropical upper tropospheric values of relative humidity over ice measured by the two instruments differ by ±20% in the pressure range ~146 to 68 hPa. These differences are mainly caused by the MLS temperature biases. Ozone mixing ratios agree within 0.5 ppmv (10 to 20%) between 68 and 14 hPa. At pressures smaller than 10 hPa, MIPAS O3 vmr are higher than MLS by an average of 0.5 ppmv (10%). General agreement between MIPAS and MLS HNO3 is within the range of −1.0 (−10%) to 1.0 ppbv (20%). MIPAS HNO3 is 1.0 ppbv (10%) higher compared to MLS in the height range of 46 to 10 hPa over the Northern Hemisphere. Over the tropics at 31.6 hPa MLS shows a low bias of more than 1 ppbv (>50%). In general, MIPAS and MLS N2O vmr agree within 20 to 40 ppbv (20 to 40%). Differences in the height range between 100 to 21 hPa are attributed to a known 20% positive bias in MIPAS N2O data.
An improved tropospheric NO2 retrieval for satellite observations in the vicinity of mountainous terrain
Authors: Zhou Y. , Brunner D. , Boersma K. F. , Dirksen R. , Wang P.
Published: April 2010

Abstract. We present an approach to reduce topography-related errors of vertical tropospheric columns (VTC) of NO2 retrieved from the Ozone Monitoring Instrument (OMI) in the vicinity of mountainous terrain. This is crucial for reliable estimates of air pollution levels over our particular area of interest, the Alpine region and the adjacent planes, where the operational OMI products exhibit significant biases due to the coarse resolution of surface parameters used in the retrieval. Our approach replaces the coarse-gridded surface pressures by accurate pixel-average values using a high-resolution topography data set, and scales the a priori NO2 profiles accordingly. NO2 VTC reprocessed in this way for the period 2006–2007 suggest that the current Dutch OMI NO2 product (DOMINO) underestimates NO2 over the Po Valley in Italy and over the Swiss plateau by about 20% in winter and 5% in summer under clear-sky conditions (cloud radiance fraction <0.5). A sensitivity analysis shows that these seasonal differences are mainly due to the different a priori NO2 profile shapes and solar zenith angles in winter and summer. The comparison of NO2 columns from the original and the enhanced retrieval with corresponding columns deduced from ground-based in situ observations over the Swiss plateau and the Po Valley illustrates the promise of our new retrieval. It partially reduces the underestimation of the OMI VTCs at polluted sites in winter and fall and generally improves the agreement in terms of slope and correlation at rural stations. It does not solve, however, the issue that the OMI DOMINO product tends to overestimate very low columns observed at rural sites in spring and summer.
New perspectives on gravity wave remote sensing by spaceborne infrared limb imaging
Authors: Preusse P. , Schroeder S. , Hoffmann L. , Ern M. , Friedl-Vallon F. , Oelhaf H. , Fischer H. , Riese M.
Published: April 2010

Abstract. Gravity wave (GW) remote sensing from space now has reached a stage of maturity that some first confinements for GW modeling can be deduced. This is in particular due to global distributions of absolute values of GW momentum flux from infrared limb sounders and due to 2-D maps of the horizontal wave field provided by nadir viewing instruments. The logical step forward is an infrared limb imager (ILI) which combines the good vertical resolution of limb sounding with horizontal mapping capabilities and provides 3-D images of the GW temperature structures. In this paper we investigate 1) how an ILI advances measurements of GW momentum flux, 2) which additional benefits are achieved by limb imaging of GWs, and 3) how an ILI compares to other GW momentum flux measurements, in-situ, ground-based, and from space. In particular, the large advance made by gaining regular 3-D sampling is demonstrated.
Multiple wavelength retrieval of tropospheric aerosol optical properties from MAXDOAS measurements in Beijing
Authors: Clémer K. , Van Roozendael M. , Fayt C. , Hendrick F. , Hermans C. , Pinardi G. , Spurr R. , Wang P. , De Mazière M.
Published: April 2010

Abstract. We report on the retrieval of aerosol extinction profiles at four wavelengths from ground-based multi-axis differential absorption spectroscopy (MAXDOAS) measurements performed in Beijing (N 39°58'37" E 116°22'51"), China. Measurements were made over a 10-month time period (June 2008 to April 2009) using a newly developed MAXDOAS instrument. A retrieval algorithm, based on an on-line implementation of the radiative transfer code LIDORT and the optimal estimation technique, has been designed to provide near real time information on aerosol extinction vertical profiles. The algorithm was applied to O4 measurements at four wavelengths (360, 477, 577, and 630 nm). The total aerosol optical depths (AODs) calculated from the retrieved profiles exhibit higher values in spring and summer and lower values in autumn and winter. Comparison of the retrieved total AODs with values from a co-located CIMEL sunphotometer revealed a good correlation. The best results are obtained for the UV region with a correlation coefficient (R) of 0.91 and a slope of the linear regression fit of 1.1. At the longest wavelength, R drops down to 0.67 and the slope increases to 1.5. The results indicate that good quality O4 slant column measurements are essential for the success of the retrievals. It is demonstrated that the algorithm is capable of reliably retrieving aerosol extinction profiles for a wide range of atmospheric conditions (total AODs at 360 nm ranging from about 0.1 to 3). The results open up new perspectives for the extension of the algorithm for the near real time retrieval of trace gas vertical profiles.
Chemical ionization mass spectrometer (CIMS) for ambient measurements of ammonia
Authors: Benson D. R. , Al-Refai M. , Lee S.-H.
Published: April 2010

Abstract. This study describes a chemical ionization mass spectrometer (CIMS) for fast response, in-situ measurements for gas phase ammonia. Protonated ethanol ions were used as the ion-molecule reaction reagent. The CIMS sensitivity was estimated to be between 4–25 Hz/pptv with 30% uncertainty. The instrument background was below 1 ppbv and at lowest was 300 pptv. The uncertainty associated with the instrumental background was less than 30 pptv under the optimized experimental conditions. The time response was less than 30 s, and the detection limit was approximately 60 pptv. This CIMS was used to measure the ambient NH3 in Kent, Ohio, for several weeks throughout three seasons. The measured ammonia mixing ratios were usually at the sub-ppbv level, and higher during the spring (200±120 pptv) than in the winter (60±75 pptv) and fall (150±80 pptv).
Carbon monoxide mixing ratios over Oklahoma between 2002 and 2009 retrieved from Atmospheric Emitted Radiance Interferometer spectra
Authors: Yurganov L. , McMillan W. , Wilson C. , Fischer M. , Biraud S.
Published: April 2010

Abstract. CO mixing ratios weighted over the bottom 2-km thick atmospheric layer between 2002 and 2009 were retrieved from downwelling infrared (IR) radiance spectra of the clear sky measured by a zenith-viewing Atmospheric Emitted Radiance Interferometer (AERI) deployed at the Southern Great Plains (SGP) observatory of the Atmospheric Radiation Measurements (ARM) Program near Lamont, Oklahoma. A version of the algorithm proposed by He at al. (2001) was significantly improved and validated. Essentially, the new algorithm retrieves a CO mixing ratio that is determined by the convolution of the a priori profile (assumed to be constant with altitude), the true profile, and the averaging kernel which maximizes near the surface. Approximately 70% of the CO signal comes from the boundary layer and the remaining 30% come from the lower part of the free troposphere. Archived temperature and water vapor profiles retrieved from the same AERI spectra through automated ARM processing were used as input data for the CO retrievals. We found the archived water vapor profiles required additional constraint using SGP Microwave Radiometer retrievals of total precipitable water vapor. Additionally, a correction for scattered solar light was developed. The retrieved CO was validated using simultaneous independently measured CO profiles. An aircraft supplied in situ CO measurements at altitudes up to 4572 m above sea level once or twice a week between March 2006 and December 2008. The aircraft measurements were supplemented with ground-based CO measurements at the SGP and retrievals from the Atmospheric IR Sounder (AIRS) above 5 km to create full tropospheric CO profiles. Comparison of the convolved profiles to the AERI CO retrievals found a squared correlation coefficient of 0.57, a standard deviation of ±11.7 ppbv, a bias of 16 ppbv, and a slope of 0.92. Averaged seasonal and diurnal cycles measured by AERI are compared with those measured continuously in situ at the SGP in the boundary layer. Monthly mean CO values measured by AERI between 2002 and 2009 are compared with those measured by AIRS over North America, the Northern Hemisphere mid-latitudes, and over the tropics.
Speeding up the AOT retrieval procedure using RTT analytical solutions: FAR code
Authors: Katsev I. L. , Prikhach A. S. , Zege E. P. , Grudo J. O. , Kokhanovsky A. A.
Published: April 2010

Abstract. We present here the aerosol retrieval technique that uses radiative transfer computations in the process of retrieval rather than look-up tables (LUT). This approach provides operational satellite data processing due to the use of the accurate and extremely fast radiative transfer code RAY previously developed by authors along with approximate analytical solutions of the radiative transfer theory. The aerosol optical thickness (AOT) and Angström exponent are optimized in the iteration process using the least-squares technique with fast computations of the derivatives of radiative characteristics in respect to retrieved values. The developed technique can be adapted for processing data of various satellite instruments (including any spectral multi-angle polarization-sensitive sensors). Beside, two important problems that determine the accuracy of the AOT retrieval are considered. The first one is the effect of the preliminary choice of the aerosol model, particularly for retrieval from satellite instrument providing only spectral data (MERIS, MODIS). The second problem is the influence of clouds in adjacent pixels. As for our knowledge, this problem has not been given required attention up to now and it should be properly accounted in the AOT retrieval algorithms.
High time-resolution chemical characterization of the water-soluble fraction of ambient aerosols with PILS-TOC-IC and AMS
Authors: Timonen H. , Aurela M. , Carbone S. , Saarnio K. , Saarikoski S. , Mäkelä T. , Worsnop D. R. , Kulmala M. , Kerminen V.-M. , Hillamo R.
Published: April 2010

Abstract. 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. The data of the PILS-TOC-IC setup was completed 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, semicontinous 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 (r=0.88). The identified sources of WSPOM were long-range transported biomass burning and secondary organic aerosol (SOA) formation. The WSPOM/POM-ratio followed the trends of the ambient daytime temperature. The temperature dependency of the WSPOM/POM-ratio suggest that in the absence of emissions from biomass burning, the SOA formation was the prevailing source for WSPOM. WSPOM produced in biomass burning was clearly correlated with carbon monoxide, confirming that biomass burning was producing primary WSPOM. In addition, elevated oxalate and potassium concentrations were measured during the biomass burning episode.
Retrieval of aerosol optical depth in vicinity of broken clouds from reflectance ratios: case study
Authors: Kassianov E. , Ovchinnikov M. , Berg L. K. , McFarlane S. A. , Flynn C. , Ferrare R. , Hostetler C. , Alexandrov M.
Published: April 2010

Abstract. A recently developed reflectance ratio (RR) method for the retrieval of aerosol optical depth (AOD) is evaluated using extensive airborne and ground-based data sets collected during the Cloud and Land Surface Interaction Campaign (CLASIC) and the Cumulus Humilis Aerosol Processing Study (CHAPS), which took place in June 2007 over the US Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains site. A detailed case study is performed for a field of single-layer shallow cumuli observed on 12 June 2007. The RR method is applied to retrieve the spectral values of AOD from the reflectance ratios measured by the MODIS Airborne Simulator (MAS) for two pairs of wavelengths (660 and 470 nm and 870 and 470 nm) collected at a spatial resolution of 0.05 km. The retrieval is compared with an independent AOD estimate from three ground-based Multi-filter Rotating Shadowband Radiometers (MFRSRs). The interpolation algorithm that is used to project MFRSR point measurements onto the aircraft flight tracks is tested using AOD derived from NASA Langley High Spectral Resolution Lidar (HSRL). The RR AOD estimates are in a good agreement (within 5%) with the MFRSR-derived AOD values for the 660-nm wavelength. The AODs obtained from MAS reflectance ratios overestimate those derived from MFRSR measurements by 15–30% for the 470-nm wavelength and underestimate the 870-nm AOD by the same amount.
Water vapour profiles from SCIAMACHY solar occultation measurements derived with an onion peeling approach
Authors: Noël S. , Bramstedt K. , Rozanov A. , Bovensmann H. , Burrows J. P.
Published: April 2010

Abstract. A new retrieval method has been developed to derive water vapour number density profiles from solar occultation measurements of the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY). This method is intentionally kept simple and based on a combination of an onion peeling approach with a modified DOAS (Differential Optical Absorption Spectroscopy) fit in the wavelength region around 940 nm. Reasonable resulting water vapour profiles are currently obtained in the altitude range 15–45 km. Comparisons of the SCIAMACHY profiles with water vapour data provided by the Atmospheric Chemistry Explorer Fourier Transform Spectrometer (ACE-FTS) show an average agreement within about 5% between 20 and 45 km. SCIAMACHY water vapour data tend to be systematically higher than ACE-FTS. These results are in principal confirmed by comparisons with water vapour profiles derived from model data of the European Centre for Medium Range Weather Forecasts (ECMWF), although ECMWF concentrations are systematicly lower than both corresponding SCIAMACHY and ACE-FTS data at all altitudes.
Automatic cloud classification of whole sky images
Authors: Heinle A. , Macke A. , Srivastav A.
Published: April 2010

Abstract. The recently increasing development of whole sky imagers enables temporal and spatial high-resolution sky observations. One application already performed in most cases is the estimation of fractional sky cover. A distinction between different cloud types, however, is still in progress. Here, an automatic cloud classification algorithm is presented, based on a set of mainly statistical features describing the color as well as the texture of an image. The k-nearest-neighbour classifier is used due to its high performance in solving complex issues, simplicity of implementation and low computational complexity. Seven different sky conditions are distinguished: high thin clouds (cirrus and cirrostratus), high patched cumuliform clouds (cirrocumulus and altocumulus), stratocumulus clouds, low cumuliform clouds, thick clouds (cumulonimbus and nimbostratus), stratiform clouds and clear sky. Based on the Leave-One-Out Cross-Validation the algorithm achieves an accuracy of about 97%, outperforming previous algorithms with accuracies of at most 62%. An additional test run of random images is presented, still yielding a success rate of about 75%, or up to 88% if only "serious" errors with respect to radiation impact are considered. Reasons for the decrement in accuracy are discussed, and ideas to further improve the classification results, especially in problematic cases, are investigated.
Development and validation of a portable gas phase standard generation and calibration system for volatile organic compounds
Authors: Veres P. , Gilman J. B. , Roberts J. M. , Kuster W. C. , Warneke C. , Burling I. R. , de Gouw J.
Published: April 2010

Abstract. We report on the development of an accurate, portable, dynamic calibration system for volatile organic compounds (VOCs). The Mobile Organic Carbon Calibration System (MOCCS) combines the production of gas-phase VOC standards using permeation or diffusion sources with quantitative total organic carbon (TOC) conversion on a palladium surface to CO2 in the presence of oxygen, and the subsequent CO2 measurement. MOCCS was validated using three different comparisons: (1) TOC of high accuracy methane standards compared well to expected concentrations (3% relative error), (2) a gas-phase benzene standard was generated using a permeation source and measured by TOC and gas chromatography mass spectrometry (GC-MS) with excellent agreement (<4% relative difference), and (3) total carbon measurement of 4 known gas phase mixtures were performed and compared to a calculated carbon content to agreement within the stated uncertainties of the standards. Measurements from laboratory biomass burning experiments of formic acid by negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS) and formaldehyde by proton transfer reaction-mass spectrometry (PTR-MS), both calibrated using MOCCS, were compared to open path Fourier transform infrared spectroscopy (OP-FTIR) to validate the MOCCS calibration and were found to compare well (R2 of 0.91 and 0.99 respectively).
Balloon-borne limb measurements of the diurnal variation of UV/vis absorbing radicals – a case study on NO2 and O3
Authors: Kritten L. , Butz A. , Dorf M. , Deutschmann T. , Kühl S. , Prados-Roman C. , Puķīte J. , Rozanov A. , Schofield R. , Pfeilsticker K.
Published: April 2010

Abstract. 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.
Theoretical description of functionality, applications, and limitations of SO2 cameras for the remote sensing of volcanic plumes
Authors: Kern C. , Kick F. , Lübcke P. , Vogel L. , Wöhrbach M. , Platt U.
Published: April 2010

Abstract. The SO2 camera is a novel technique for the remote sensing of volcanic emissions using solar radiation scattered in the atmosphere as a light source for the measurements. The method is based on measuring the ultra-violet absorption of SO2 in a narrow wavelength window around 310 nm by employing a band-pass interference filter and a 2-D UV-sensitive CCD detector. The effect of aerosol scattering can be eliminated by additionally measuring the incident radiation around 325 nm where the absorption of SO2 is no longer significant, thus rendering the method applicable to optically opaque plumes. The ability to deliver spatially resolved images of volcanic SO2 distributions at a frame rate on the order of 1 Hz makes the SO2 camera a very promising technique for volcanic monitoring and for studying the dynamics of volcanic plumes in the atmosphere. This study gives a theoretical basis for the pertinent aspects of working with SO2 camera systems, including the measurement principle, instrument design, data evaluation and technical applicability. Several issues are identified that influence camera calibration and performance. For one, changes in the solar zenith angle lead to a variable light path length in the stratospheric ozone layer and therefore change the spectral distribution of scattered solar radiation incident at the Earth's surface. The thus varying spectral illumination causes a shift in the calibration of the SO2 camera's results. Secondly, the lack of spectral resolution inherent in the measurement technique leads to a non-linear relationship between measured weighted average optical density and the SO2 column density. In addition, as is the case with all remote sensing techniques that use scattered solar radiation as a light source, the radiative transfer between the sun and the instrument is variable, with both radiative dilution as well as multiple scattering occurring. These effects can lead to both, over or underestimation of the SO2 column density by more than an order of magnitude. As the accurate assessment of volcanic emissions depends on our ability to correct for these issues, recommendations for correcting the individual effects during data analysis are given. Aside from the above mentioned intrinsic effects, the particular technical design of the SO2 camera can also greatly influence its performance, depending on the chosen setup. A general description of the instrument setup is given, and the advantages and disadvantages of certain specific instrument designs are discussed. Finally, several measurement examples are shown and possibilities to combine SO2 camera measurements with other remote sensing techniques are explored.