CONVECTIVE-TRANSPORT OF BIOMASS BURNING EMISSIONS OVER BRAZIL DURING TRACE-A

A series of large mesoscale convective systems that occurred during th e Brazilian phase of GTE/TRACE A (Transport and Atmospheric Chemistry near the Equator-Atlantic) provided an opportunity to observe deep con vective transport of trace gases from biomass burning. This paper repo rts a detailed analysis uf flight 6, on September 27, 1992, which samp led cloud- and biomass-burning-perturbed regions north of Brasilia. Hi gh-frequency sampling of cloud outflow at 9-12 km from the NASA DC-8 s howed enhancement of CO mixing ratios typically a factor of 3 above ba ckground (200-300 parts per billion by volume (ppbv) versus 90 ppbv) a nd significant increases in NO, and hydrocarbons, Clear signals of lig htning-generated NO were detected; we estimate that at least 40% of NO x at the 9.5-km level and 32% at 11.3 km originated from lightning, Fo ur types of model studies have been performed to analyze the dynamical and photochemical characteristics of the series of convective events. (1) Regional simulations for the period have been performed with the NCAR/Penn State mesoscale model (MM5), including tracer transport of c arbon monoxide, initialized with observations. Middle-upper tropospher ic enhancements of a factor of 3 above background are reproduced. (2) A cloud-resolving model (the Goddard cumulus ensemble (GCE) model) has been run for one representative convective cell during the September 26-27 episode. (3) Photochemical calculations (the Goddard tropospheri c chemical model), initialized with trace gas observations (e.g., CO, NOx, hydrocarbons, O-3) observed in cloud outflow, show appreciable O- 3 formation postconvection, initially up to 7-8 ppbv O-3/d. (4) Forwar d trajectories from cloud outflow levels (postconvective conditions) p ut the ozone-producing air masses in eastern Brazil and the tropical A tlantic within 2-4 days and over the Atlantic, Africa, and the Indian Ocean in 6-8 days, Indeed, 3-4 days after the convective episode (Sept ember 30, 1992), upper tropospheric levels in the Natal ozone sounding show an average increase of similar to 30 ppbv (3 Dobson units (DU) i ntegrated) compared to the September 28 sounding. Our simulated net O- 3 production rates in cloud outflow are a factor of 3 or more greater than those in air undisturbed by the storms, Integrated over the 8- to 16-km cloud outflow layer, the postconvection net O-3 production (sim ilar to 5-6 DU over 8 days) accounts for similar to 25% of the excess O-3 (15-25 DU) over the South Atlantic. Comparison of TRACE A Brazilia n ozonesondes and the frequency of deep convection with climatology [K irchhoff et al., this issue] suggests that the late September 1992 con ditions represented an unusually active period for both convection and upper tropospheric ozone formation.