Tropospheric O3 distribution over the Indian Ocean during spring 1995 evaluated with a chemistry-climate model

An analysis of tropospheric O 3 over the Indian Ocean during spring 1995 is presented based on O 3 soundings and results from the chemistry-general circulation model ECHAM (European Centre Hamburg Model). The ECHAM model is nudged towards actual meteorology using ECMWF analyses, to enable a direct comparison between model results and in situ observations. The model reproduces observed CO levels in different air mass categories. The model also reproduces the general tendencies and the diurnal variation in the observed surface pressure, although the amplitude of the diurnal variation in the amplitude is underestimated. The model simulates the general O 3 tendencies as seen in the sonde observations. Tropospheric O 3 profiles were characterized by low surface concentrations (< 10 ppbv), mid-tropospheric maxima (60-100 ppbv, between 700-250 hPa) and upper-tropospheric minima (< 20 ppbv, between 250-100 hPa). Large-scale upper tropospheric O 3 minima were caused by convective transport of O 3 -depleted boundary layer air in the Inter Tropical Convergence Zone (ITCZ). Similarly, an upper tropospheric O 3 minimum was caused by cyclone Marlene south of the ITCZ. The mid-tropospheric O 3 maxima were caused by transport of polluted African air. The ECHAM model appears to overestimate surface O 3 levels, and does not reproduce the diurnal variations very well This could be related to unaccounted multiphase O 3 destruction mechanisms involving low level clouds and aerosols, and missing halogen chemistr