Downward transport and modification of tropospheric ozone through moist convection

This study estimated the largely unstudied downward transport and modification of tropospheric ozone associated with tropical moist convection using a coupled meteorology-chemistry model. High-resolution cloud resolving model simulations were conducted for deep moist convection events over West Africa during August 2006 to estimate vertical transport of ozone due to convection. Model simulations realistically reproduced the characteristics of deep convection as revealed by the estimated spatial distribution of temperature, moisture, cloud reflectivity, and vertical profiles of temperature and moisture. Also, results indicated that vertical transport reduced ozone by 50% (50 parts per billion by volume, ppbv) in the upper atmosphere (12-15 km) and enhanced ozone by 39% (10 ppbv) in the lower atmosphere (<2 km). Field observations confirmed model results and indicated that surface ozone levels abruptly increased by 10-30 ppbv in the area impacted by convection due to transport by downdrafts from the upper troposphere. Once in the lower troposphere, the lifetime of ozone decreased due to enhanced dry deposition and chemical sinks. Ozone removal via dry deposition increased by 100% compared to non-convective conditions. The redistribution of tropospheric ozone substantially changed hydroxyl radical formation in the continental tropical boundary layer. Therefore, an important conclusion of this study is that the redistribution of tropospheric ozone, due to deep convection in non-polluted tropical regions, can simultaneously reduce the atmospheric loading of ozone and substantially impact the oxidation capacity of the lower atmosphere via the enhanced formation of hydroxyl radicals.