OMI tropospheric NO2 air mass factors over South America: effects of biomass burning aerosols

[摘要] Biomass burning is an important and uncertain source of aerosols andNO_x (NO + NO₂) to the atmosphere. Satellite observations oftropospheric NO₂ are essential for characterizing this emissionssource, but inaccuracies in the retrieval of NO₂ tropospheric columnsdue to the radiative effects of aerosols, especially light-absorbingcarbonaceous aerosols, are not well understood. It has been shown that theO₂–O₂ effective cloud fraction and pressure retrieval is sensitiveto aerosol optical and physical properties, including aerosol optical depth(AOD). Aerosols implicitly influence the tropospheric air mass factor (AMF)calculations used in the NO₂ retrieval through the effective cloudparameters used in the independent pixel approximation. In this work, weexplicitly account for the effects of biomass burning aerosols in the OzoneMonitoring Instrument (OMI) tropospheric NO₂ AMF calculation forcloud-free scenes. We do so by including collocated aerosol extinctionvertical profile observations from the CALIOP instrument, and aerosoloptical depth (AOD) and single scattering albedo (SSA) retrieved by the OMInear-UV aerosol algorithm (OMAERUV) in the DISAMAR radiative transfer model.Tropospheric AMFs calculated with DISAMAR were benchmarked against AMFsreported in the Dutch OMI NO₂ (DOMINO) retrieval; the mean and standarddeviation of the difference was 0.6 ± 8 %. Averaged over threesuccessive South American biomass burning seasons (2006–2008), the spatialcorrelation in the 500 nm AOD retrieved by OMI and the 532 nm AOD retrievedby CALIOP was 0.6, and 68 % of the daily OMAERUV AOD observations werewithin 30 % of the CALIOP observations. Overall, tropospheric AMFscalculated with observed aerosol parameters were on average 10 % higherthan AMFs calculated with effective cloud parameters. For effective cloudradiance fractions less than 30 %, or effective cloud pressures greaterthan 800 hPa, the difference between tropospheric AMFs based on implicit andexplicit aerosol parameters is on average 6 and 3 %, respectively,which was the case for the majority of the pixels considered in our study;70 % had cloud radiance fraction below 30 %, and 50 % had effectivecloud pressure greater than 800 hPa. Pixels with effective cloud radiancefraction greater than 30 % or effective cloud pressure less than 800 hPacorresponded with stronger shielding in the implicit aerosol correctionapproach because the assumption of an opaque effective cloud underestimatesthe altitude-resolved AMF; tropospheric AMFs were on average 30–50 %larger when aerosol parameters were included, and for individual pixelstropospheric AMFs can differ by more than a factor of 2. Theobservation-based approach to correcting tropospheric AMF calculations foraerosol effects presented in this paper depicts a promising strategy for aglobally consistent aerosol correction scheme for clear-sky pixels.

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