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As an ubiquitous product of the oxidation of many volatile organic compounds (VOCs), formaldehyde (HCHO) plays a key role as a short-lived and reactive intermediate in the atmospheric photo-oxidation pathways leading to the formation of tropospheric ozone and secondary organic aerosols. In this study, HCHO profiles have been successfully retrieved from ground-based Fourier transform infrared (FTIR) solar spectra and UV-visible Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) scans recorded during the July 2010-December 2012 time period at the Jungfraujoch station (Swiss Alps, 46.5° N, 8.0° E, 3580ma.s.l.). Analysis of the retrieved products has revealed different vertical sensitivity between both remote sensing techniques. Furthermore, HCHO amounts simulated by two state-of-the-art chemical transport models (CTMs), GEOSChem and IMAGES v2, have been compared to FTIR total columns and MAX-DOAS 3.6-8 km partial columns, accounting for the respective vertical resolution of each ground-based instrument. Using the CTM outputs as the intermediate, FTIR and MAX-DOAS retrievals have shown consistent seasonal modulations of HCHO throughout the investigated period, characterized by summertime maximum and wintertime minimum. Such comparisons have also highlighted that FTIR and MAX-DOAS provide complementary products for the HCHO retrieval above the Jungfraujoch station. Finally, tests have revealed that the updated IR parameters from the HITRAN 2012 database have a cumulative effect and significantly decrease the retrieved HCHO columns with respect to the use of the HITRAN 2008 compilation. [ABSTRACT FROM AUTHOR]

An improved nitrogen dioxide (NO2) slant column density retrieval for the Ozone Monitoring Instrument (OMI) in the 405-465 nm spectral region is presented. Since the launch of OMI on board NASA's EOS-Aura satellite in 2004, differential optical absorption spectroscopy (DOAS) retrievals of NO2 slant column densities have been the starting point for the KNMI DOMINO and NASA SP NO2 vertical column data as well as the OMI NO2 data of some other institutes. However, recent intercomparisons between NO2 retrievals from OMI and other UV/Vis and limb spectrometers, as well as ground-based measurements, suggest that OMI stratospheric NO2 is biased high. This study revises and, for the first time, fully documents the OMI NO2 retrieval in detail. The representation of the OMI slit function to convolve high-resolution reference spectra onto the relevant spectral grid is improved. The window used for the wavelength calibration is optimised, leading to much-reduced fitting errors. Ozone and water vapour spectra used in the fit are updated, reflecting the recently improved knowledge of their absorption cross section in the literature. The improved spectral fit also accounts for absorption by the O2-O2 collision complex and by liquid water over clearwater areas. The main changes in the improved spectral fitting result from the updates related to the wavelength calibration: the RMS error of the fit is reduced by 23% and the NO2 slant column by 0.85×1015 molec cm-2, independent of latitude, solar zenith angle and NO2 value. Including O2-O2 and liquid water absorption and updating the O3 and water vapour cross-section spectra further reduces NO2 slant columns on average by 0.35×1015 molec cm-2, accompanied by a further 9% reduction in the RMS error of the fit. The improved OMI NO2 slant columns are consistent with independent NO2 retrievals from other instruments to within a range that can be explained by photochemically driven diurnal increases in stratospheric NO2 and by small differences in fitting window and approach. The revisions indicate that current OMI NO2 slant columns suffered mostly from an additive positive offset, which is removed by the improved wavelength calibration and representation of the OMI slit function. It is therefore anticipated that the improved NO2 slant columns are most important to retrievals of spatially homogeneous stratospheric NO2 rather than to heterogeneous tropospheric NO2. [ABSTRACT FROM AUTHOR]

A four year data set of MAX-DOAS observations in the Beijing area (2008-2012) is analysed with a focus on NO2, HCHO, and aerosols. Two very different retrieval methods are applied. Method A describes the tropospheric profile with 13 layers and makes use of the optimal estimation method. Method B uses 2-4 parameters to describe the tropospheric profile and an inversion based on a least-squares fit. For each constituent (NO2, HCHO and aerosols) the retrieval outcomes are compared in terms of tropospheric columns, surface concentrations, and "characteristic profile heights" (i.e. the height below which 75% of the vertically integrated tropospheric column resides). We find best agreement between the two methods for tropospheric NO2 columns, with a standard deviation of relative differences below 10%, a correlation of 0.99 and a linear regression with a slope of 1.03. For tropospheric HCHO columns we find a similar slope, but also a systematic bias of almost 10 % which is likely related to differences in profile height. Aerosol optical depths (AODs) retrieved with method B are 20% high compared to method A. They are more in agreement with AERONET measurements, which are on average only 5% lower, however with considerable relative differences (standard deviation ~ 25%). With respect to near surface volume mixing ratios and aerosol extinction we find considerably larger relative differences: 10 ± 30%, -23 ± 28% and - 8 ± 33% for aerosols, HCHO and NO2 respectively. The frequency distributions of these near-surface concentrations show however a quite good agreement, and this indicates that near-surface concentrations derived from MAX-DOAS are certainly useful in a climatological sense. A major difference between the two methods is the dynamic range of retrieved characteristic profile heights which is larger for method B than for method A. This effect is most pronounced for HCHO, where retrieved profile shapes with method A are very close to the a priori, and moderate for NO2 and aerosols which on average show quite good agreement for characteristic profile heights below 1.5 km. [ABSTRACT FROM AUTHOR]

The article presents a study which investigated a discrepancy between in-situ and remote measurements of bromine oxide (BrO) during Antarctic enhancements. The study measured BrO using a ground-based remote-sensing spectrometer at Halley in Antarctica. It notes that a large discrepancy between BrO in-situ and remote-sensing measurements existed during the Antarctic enhancements episodes when the gas laid in a surface layer having similar thickness with that of the boundary layer.

A profiling algorithm based on the optimal estimation method is applied to ground-based zenith-sky UV-visible measurements from Harestua, Southern Norway (60°N, 11°E) in order to retrieve BrO vertical profiles. The sensitivity of the zenith-sky observations to the tropospheric BrO detection is increased by using for the spectral analysis a fixed reference spectrum corresponding to clear-sky noon summer conditions. The information content and retrieval errors are characterized and it is shown that the retrieved stratospheric profiles and total columns are consistent with correlative balloon and satellite observations, respectively. Tropospheric BrO columns are derived from profiles retrieved at 80° solar zenith angle during sunrise and sunset for the 2000-2006 period. They show a marked seasonality with mean column value ranging from 1.52±0.62x1013 molec/cm² in late winter/early spring to 0.92±0.38x1013 molec/cm² in summer, which corresponds to 1.0±0.4 and 0.6±0.2 pptv, respectively, if we assume that BrO is uniformly mixed in the troposphere. These column values are also consistent with previous estimates made from balloon, satellite, and other ground-based observations. Daytime (10:30 LT) tropospheric BrO columns are compared to the p-TOMCAT 3-D tropospheric chemical transport model (CTM) for the 2002-2003 period. p-TOMCAT shows a good agreement with the retrieved columns except in late winter/early spring where an underestimation by the model is obtained. This finding could be explained by the non-inclusion of sea-ice bromine sources in the current version of p-TOMCAT. Therefore the model cannot reproduce the possible transport of air-masses with enhanced BrO concentration due to bromine explosion [ABSTRACT FROM AUTHOR]

Spectral measurements of BrO using zenithsky and off-axis viewing geometries are combined in a linear multiple regression retrieval algorithm to provide stratospheric and tropospheric BrO vertical columns. One year of measurement data are investigated over Reunion-Island (20.9° S, 55.5° E), from August 2004 to June 2005. A comparison between the stratospheric columns retrieved at 45°, 80°, 85°, 87.5° and 92.5° solar zenith angles and photochemical simulations initialized by chemical fields from the 3-D-CTM SLIMCAT and further constrained by observed NO2 profiles shows a good agreement only by considering a contribution from the very short-lived organic bromine substances to the stratospheric inorganic bromine budget, of 6 to 8 pptv. Furthermore, stratospheric BrO profiles retrieved from late twilight zenith-sky observations are consistent with a total inorganic bromine (Bry) loading of approximately 23 pptv. This represents 6 to 7 pptv more than can be supplied by long-lived organic bromine sources, and therefore supports an added contribution from very short-lived organic bromine substances as recently suggested in several other studies. Moreover strong evidences are presented for the existence of a substantial amount of BrO in the tropical free-troposphere, around 6 km altitude, possibly supplied by the decomposition of short-lived biogenic bromine organic compounds. Tropospheric BrO vertical columns of 1.1±0.45x1013 molec/cm² are derived for the entire observation period. Comparisons between ground-based BrO vertical columns and total BrO columns derived from SCIAMACHY (onboard the ENVISAT satellite) nadir observations in a latitudinal band centered around 21° S present a good level of consistency, which further strengthens the conclusions of our study. [ABSTRACT FROM AUTHOR]

Abstract: Tropospheric BrO was measured by a ground-based remote-sensing spectrometer at Halley in Antarctica in spring 2007, and BrO was measured by satellite-borne remote-sensing spectrometers using similar spectral regions and similar Differential Optical Absorption Spectroscopy (DOAS) analyses. Near-surface BrO was simultaneously measured in situ at Halley by Chemical Ionisation Mass Spectrometer (CIMS), and in an earlier year near-surface BrO was measured at Halley over a long path by a ground-based DOAS spectrometer. During enhancement episodes, total amounts of tropospheric BrO from the ground-based remote-sensor were similar to those from space, but if we assume that the BrO was confined to the mixed layer they were very much larger than values measured by either near-surface technique. This large apparent discrepancy can be resolved if substantial amounts of BrO were in the free troposphere during most enhancement episodes. Amounts observed by the ground-based remote sensor at different elevation angles, and their formal inversions to vertical profiles, demonstrate that much of the BrO was indeed often in the free troposphere. This is consistent with the ~5 day lifetime of Bry and with the enhanced BrO observed during some Antarctic blizzards. [Copyright &y& Elsevier]