On the origin of tropospheric O3 over the Indian Ocean during the winter monsoon: African biomass burning vs. stratosphere-troposphere exchange
1Space Research Organization Netherlands (SRON), the Netherlands
Abstract. A comparison and analysis of modeled and measured O3 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 in the O3 profiles: mid tropospheric O3 maxima (300--500 hPa) over the tropical Indian Ocean, and the upper-tropospheric O3 laminae that occur above approximately 14 km (>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.
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.
An analysis of the model simulations shows that the model cannot exactly reproduce the measured upper-tropospheric O3 maxima. However, modeled O3 mixing 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.