Tropical Deep Convection and Ozone Formation

Abstract

Theoretical studies, aircraft, and space-borne measurements show that deep convection can be an effective conduit for introducing reactive surface pollutants into the free troposphere. The chemical consequences of convective systems are complex. For example, sensitivity studies show potential for both enhancement and diminution of ozone formation. Field observations of cloud and mesoscale phenomena have been investigated with the Goddard Cumulus Ensemble and Tropospheric Chemistry models. Case studies from the tropical ABLE 2, STEP, and TRACE-A experiments show that free tropospheric ozone formation should increase when deep convection and urban or biomass burning pollution coincide, and decrease slightly in regions relatively free of ozone precursors (often marine). Confirmation of post-convective ozone enhancement in the free troposphere over Brazil, the Atlantic, and southern Africa was a major accomplishment of the September–October 1992 TRACE-A (Transport and Atmospheric Chemistry near the Equator—Atlantic) aircraft mission. A flight dedicated to cloud outflow showed that deep convection led to a factor of 3–4 increase in upper tropospheric ozone formation downwind. Analysis of ozonesondes during TRACE-A was consistent with 20%–30% of seasonally enhanced ozone over the South Atlantic being supplied by a combination of biomass burning emissions, lightning, and deep convection over South America. With the Tropics the critical region for troposphere-to-stratosphere transfer of pollutants, these results have implications for the total ozone budget. Cloud-scale analyses will guide the development of more realistic regional and global chemical-transport models to assess the full impact of deep convection on atmospheric chemical composition.