Pickering, K. E., et al., Convective transport of biomass burning emissions over Brazil during TRACE-A, J. Geophys. Res., 101, 23,993-24,012, 1996.


A series of large mesoscale convective systems that occurred during the Brazilian phase of GTE/TRACE-A (Transport and Atmospheric Chemistry near the Equator - Atlantic) provided an opportunity to observe deep convective transport of trace gases from biomass burning. This paper reports a detailed analysis of Flight 6, on 27 September 1992, which sampled cloud- and biomass burning-perturbed regions north of Brasilia. High-frequency sampling of cloud outflow at 9-12 km from the NASA DC-8 showed enhancement of CO mixing ratios typically a factor of 3 above background (200-300 ppbv vs 90 ppbv) and significant increases in NOx and hydrocarbons. Clear signals of lightning-generated NO were detected; we estimate that at least 40% of NOx at the 9.5 km level and 32% at 11.3 km originated from lightning. Four 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 carbon monoxide, initialized with observations. Mid-upper tropospheric enhancements of a factor of 3 above background are reproduced. (2) a cloud-resolving model (GCE, the Goddard Cumulus Ensemble model) has been run for one representative convective cell during the 26-27 September episode; (3) photochemical calculations (the Goddard tropospheric chemical model), initialized with trace gas observations (e.g. CO, NOx, hydrocarbons, O3) observed in cloud outflow show appreciable O3 formation post-convection - initially up to 7-8 ppbv O3/day. (4) Forward trajectories from cloud outflow levels (post convective conditions) put the ozone-producing air masses in eastern Brazil and the tropical Atlantic 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 (30 September 1992) upper tropospheric levels in the Natal ozone sounding show an average increase of ~30 ppbv (3 Dobson units integrated) compared to the 28 September sounding. Our simulated net O3 production rates in cloud outflow are a factor of 3 or more greater than those in air undisturbed by the storms. Integrated over the 8-16 km cloud outflow layer, the post-convection net O3 production (~5-6 DU over 8 days) accounts for ~25% of the excess O3 (15-25 DU) over the South Atlantic. Comparison of TRACE-A Brazilian ozonesondes and the frequency of deep convection with climatology [Kirchhoff et al, 1995] suggests that the late September 1992 conditions represented an unusually active period for both convection and upper tropospheric ozone formation.