Deriving tropospheric ozone from assimilated profiles

Abstract. To derive global tropospheric O 3 columns from satellite observations, O 3 profiles retrieved from GOME-2A and OMI measurements were simultaneously assimilated into the TM5 global chemistry transport model for the year 2008. The horizontal model resolution has been increased by a factor of 6 for more accurate results, but to reduce computational cost, the number of model layers has been reduced from 44 to 31. The model ozone fields are used to derive tropospheric ozone, which is defined here as the partial column between mean sea level and 6 km altitude. Two methods for calculating the tropospheric columns from the free model run and assimilate O 3 fields are compared. In the first method, we calculate the residual between assimilated total columns and the partial model column between 6 km and the top of atmosphere. In the second method, we perform a direct integration of the assimilated O 3 fields between the surface and 6 km. The results are validated against tropospheric columns derived from ozone sonde measurements. It turned out that the residual method has a too large variation to be used reliably for the determination of tropospheric ozone, so the direct integration method has been used instead. The median global bias is smaller for the assimilated O 3 fields than for the free model run, but the large variation makes it difficult to make definitive statements on a regional or local scale. The monthly mean ozone fields show significant improvements and more detail when comparing the assimilated O 3 fields with the free model run, especially for features such as biomass burning enhanced O 3 concentrations and outflow of O 3 rich air from Asia over the Pacific.