Retrieval of vertical columns of sulfur dioxide from SCIAMACHY andOMI: Air mass factor algorithm development, validation, and erroranalysis

摘要

We develop an improved retrieval of sulfur dioxide (SO2) vertical columns from twosatellite instruments (SCIAMACHY and OMI) that measure ultraviolet solar backscatter.For each SCIAMACHY and OMI observation, a local air mass factor (AMF) algorithmconverts line-of-sight "slant" columns to vertical columns using altitude-dependent scattering weights computed with a radiative transfer model (LIDORT), weighted byrelative vertical SO2 profile (shape factor) determined locally with a global atmosphericchemistry model (GEOS-Chem). The scattering weights account for viewing geometry,surface albedo, cloud scattering, absorption by ozone, and scattering and absorption by aerosols. Absorption of radiation by mineral dust can reduce seasonal mean instrumentsensitivity by 50%. Mean SO2 shape factors simulated with GEOS-Chem and used in theAMF calculation are highly consistent with airborne in situ measurements (INTEX-Aand INTEX-B); differences would affect the retrieved SO2 columns by 10%. The retrievedvertical columns are validated with coincident airborne in situ measurements (INTEX-A,INTEX-B, and a campaign over east China). The annual mean AMF errors are estimatedto be 35-70% in polluted regions (e.g., East Asia and the eastern United States) andless than 10% over clear ocean regions. The overall SO2 error assessment is 45-80% foryearly averages over polluted regions. Seasonal mean SO2 columns retrieved fromSCIAMACHY and OMI for 2006 are significantly spatially correlated with those fromGEOS-Chem, in particular over the United States (r = 0.85 for SCIAMACHY and 0.82 forOMI). A sensitivity study confirms the sensitivity of SCIAMACHY and OMI toanthropogenic SO2 emissions.