TROPICAL DEEP CONVECTION AND OZONE FORMATION

Theoretical studies, aircraft, and space-borne measurements show that deep convection can be an effective conduit for introducing reactive s urface pollutants into the free troposphere. The chemical consequences of convective systems are complex. For example, sensitivity studies s how potential for both enhancement and diminution of ozone formation. Field observations of cloud and mesoscale phenomena have been investig ated with the Goddard Cumulus Ensemble and Tropospheric Chemistry mode ls. Case studies from the tropical ABLE 2, STEP, and TRACE-A experimen ts show that foe tropospheric ozone formation should increase when dee p convection and urban or biomass burning pollution coincide, and decr ease slightly in regions relatively free of ozone precursors (often ma rine). Confirmation of post-convective ozone enhancement in the free t roposphere over Brazil, the Atlantic, and southern Africa was a major accomplishment of the September-October 1992 TRACE-A (Transport and At mospheric Chemistry near the Equator - Atlantic) aircraft mission. A f light 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% o f seasonally enhanced ozone over the South Atlantic being supplied by a combination of biomass burning emissions, lightning, and deep convec tion over South America. With the Tropics the critical region for trop osphere-to-stratosphere transfer of pollutants, these results have imp lications for the total ozone budget. Cloud-scale analyses will guide the development of more realistic regional and global chemical-transpo rt models to assess the full impact of deep convection on atmospheric chemical composition.