Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 2 4 2002 10.5194/acpd-2-943-2002 http://www.atmos-chem-phys-discuss.net/2/943/2002/ http://www.atmos-chem-phys-discuss.net/2/943/2002/acpd-2-943-2002.html http://www.atmos-chem-phys-discuss.net/2/943/2002/acpd-2-943-2002.pdf 943 981 2002-07-10 On the origin of tropospheric O₃ over the Indian Ocean during the winter monsoon: African biomass burning vs. stratosphere-troposphere exchange A. T. J. de Laat Space Research Organization Netherlands (SRON), the Netherlands A comparison and analysis of modeled and measured O₃ profiles from the INDOEX campaign is presented. European Centre for Medium-Range Weather Forecast&nbsp; (ECMWF) meteorological analyses have been assimilated into the model to represent actual meteorology. The focus of this study is on two commonly observed features in the O₃ profiles: mid tropospheric O₃maxima (300--500 hPa) over the tropical Indian Ocean, and the upper-tropospheric O_3&nbsp; laminae that occur above approximately 14 km (&gt;150 hPa) altitude. A comparison of model simulated O₃ profiles with measured O₃ profiles indicates that the model realistically simulates the observed mid-tropospheric O₃ maxima. An analysis of the model simulations shows that the major source of the mid-tropospheric O₃ maxima is advection of polluted air masses from continental biomass burning areas over Africa, with generally only a small contribution of stratospheric O₃. Previous studies hinted at Stratosphere-Troposphere exchange (STE) along the subtropical jet (STJ) as the primary source of the mid-tropospheric O₃ maxima over the Indian Ocean.<br> <br> Analysis of the model simulations shows that the mechanism causing the mid-tropospheric transport of African biomass burning pollution and stratospheric air masses are frontal zones or waves passing along the subtropical jets, causing advection of tropical air masses in the prefrontal zone. Furthermore, these frontal zones or waves also cause STE at the mid-latitudinal side of the STJ. The model simulations also indicate that the contribution of STE in general is minor compared to advection and in situ tropospheric production of O₃ for the mid-tropospheric O₃ budget over the Indian Ocean region.<br> <br> An analysis of the model simulations shows that the model cannot exactly reproduce the measured upper-tropospheric O₃maxima. However, modeled O₃mixing ratios at 14 and 16 km altitude are significantly higher than at 8 to 12 km altitude, indicating that the model does simulate an upper-tropospheric layer. According to the model simulations, the sources of O₃ at 14 and 16 km altitude are advection of both tropospheric and stratospheric O₃ as well as in situ O₃ formation.