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SO2 Measurements from GOME-2: An Optimal Estimation Approach

Caroline R. Nowlan(1), Xiong Liu(2), Kelly Chance(1) and Thomas P. Kurosu(1)

(1) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-50, Cambridge, MA, 02138, United States
(2) University of Maryland Baltimore County, NASA GSFC, Code 613.3, Greenbelt, MD 20771, United States

Abstract

Space-based measurements of sulphur dioxide can be made from observations of backscattered solar radiation in the ultraviolet. In a traditional SO2 retrieval, the slant column amount is first retrieved by spectral fitting and then converted to a vertical column using an independently-determined airmass factor. The strong ozone absorption in the SO2 measurement region (~310-327 nm) means that the airmass factor is strongly dependent on wavelength, but also that ozone must be well-determined to improve the retrieval of SO2. We apply an optimal approach originally developed for tropospheric ozone retrievals for the GOME and OMI instruments to SO2 retrievals from GOME-2. The optimal estimation approach retrieves SO2 vertical column densities by using a radiative transfer model and GEOS-Chem SO2 model profiles to implicitly include the effects of albedo, clouds, viewing geometry and ozone absorption in the retrieval. GOME-2 results from the optimal estimation approach are compared with those derived from traditional slant column fitting for both volcanic and anthropogenic emissions.