On the origin of tropospheric O3 over the Indian Ocean

A comparison and analysis of modeled and measured O 3 profiles from the INDOEX campaign is presented. European Centre for Medium-Range Weather Forecast (ECMWF) meteorological analyses have been assimilated into the model to represent actual meteorology. The focus of this study is on two commonly observed features 5 in the O 3 profiles: mid tropospheric O 3 maxima (300–500 hPa) over the tropical In-dian Ocean, and the upper-tropospheric O 3 laminae that occur above approximately 14 km (> 150 hPa) altitude. A comparison of model simulated O 3 profiles with measured O 3 profiles indicates that the model realistically simulates the observed mid-tropospheric O 3 maxima. An analysis of the model simulations shows that the major 10 source of the mid-tropospheric O 3 maxima is advection of polluted air masses from continental biomass burning areas over Africa, with generally only a small contribution of stratospheric O 3. Previous studies hinted at Stratosphere-Troposphere exchange (STE) along the subtropical jet (STJ) as the primary source of the mid-tropospheric O 3 maxima over the Indian Ocean. 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 ad-vection 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 20 also indicate that the contribution of STE in general is minor compared to advection and in situ tropospheric production of O 3 for the mid-tropospheric O 3 budget over the Indian Ocean region. An analysis of the model simulations shows that the model cannot exactly reproduce the measured upper-tropospheric O 3 maxima. However, modeled O 3 mixing ratios at 25 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 O 3 at 14 and 16 km altitude are advection of both tropospheric 944 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Print Version Interactive Discussion c EGS 2002 and stratospheric O 3 as well as in situ O 3 formation.