Constraining tropospheric ozone column through data assimilation

Global distributions of the tropospheric ozone column (TOC) and its error are estimated by assimilating satellite observations of total ozone column and stratospheric ozone profiles in a global chemistry‐transport model. A quantitative comparison of the calculated TOC with collocated ozonesondes observations shows good agreement over a wide range of latitudes for the period studied (July–September 1992), even at the daily timescale. In most cases, the difference with the ozonesonde‐based TOC is within the calculated error estimate. Comparison with the TOC distribution by Thompson and Hudson [1999] does not lead to any quantitative conclusion due to a mix of similarities and differences, even in the geographical distribution of the TOC. A positive TOC bias with respect to ozonesondes is found in this study over Hilo and Irene, probably due to a misrepresentation of the transport of ozone in a region of strong TOC gradients. In September 1992 the LIDAR observations from the TRACE‐A campaign are also assimilated. This is shown to provide a constraint on the TOC that significantly propagates across the Atlantic Ocean, indicating the importance of point measurements of ozone for constraining its large‐scale distribution.