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. A critical step in satellite retrievals of trace gas columns is thecalculation of the air mass factor (AMF) used to convert observed slantcolumns to vertical columns. This calculation requires a priori information on theshape of the vertical profile. As a result, comparisons betweensatellite-retrieved and model-simulated column abundances are influenced bythe a priori profile shape. We examine how differences between the shape of the simulated and a priori profiles can impact the interpretation of satellite retrievalsby performing an adjoint-based four-dimensionalvariational (4D-Var) assimilation of synthetic NO2observations for constraining NOx emissions. We use the GEOS-Chemadjoint model to perform assimilations using a variety of AMFs to examinehow a posteriori emission estimates are affected if the AMF is calculated using an a priori shapefactor that is inconsistent with the simulated profile. In these tests, aninconsistent a priori shape factor increased root mean square errors in a posteriori emissionestimates by up to 30 % for realistic conditions over polluted regions. Asthe difference between the simulated profile shape and the a priori profile shapeincreases, so do the corresponding assimilated emission errors. This revealsthe importance of using simulated profile information for AMF calculationswhen comparing that simulated output to satellite-retrieved columns.

atmospheric chemistry and physics

Effects of a priori profile shape assumptions on comparisons between satellite NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; columns and model simulations