PHOTOCHEMISTRY IN BIOMASS BURNING PLUMES AND IMPLICATIONS FOR TROPOSPHERIC OZONE OVER THE TROPICAL SOUTH ATLANTIC

Photochemistry occuring in biomass burning plumes over the tropical so uth Atlantic is analyzed using data collected during the Transport and Atmospheric Chemistry Near the Equator-Atlantic aircraft expedition c onducted during the tropical dry season in September 1992 and a photoc hemical point model. Enhancement ratios (Delta Y/Delta X, where Delta indicates the enhancement of a compound in the plume above the local b ackground mixing ratio, Y are individual hydrocarbons, CO, O-3, N2O, H NO3, peroxyacetyl nitrate (PAN), CH2O, acetone, H2O2, CH3OOH, HCOOH, C H3COOH or aerosols and X is CO or CO2) are reported as a function of p lume age inferred from the progression of Delta non-methane hydrocarbo ns/Delta CO enhancement ratios. Emission, formation, and loss of speci es in plumes can be diagnosed from progression of enhancement ratios f rom fresh to old plumes. O-3 is produced in plumes over at least a 1 w eek period with mean Delta O-3/Delta CO = 0.7 in old plumes. However, enhancement ratios in plumes can be influenced by changing background mixing ratios and by photochemical loss of CO. We estimate a downward correction of similar to 20% in enhancement ratios in old plumes relat ive to Delta CO to correct for CO loss. In a case study of a large per sistent biomass burning plume at 4 -km we found elevated concentration s of PAN in the fresh plume. The degradation of PAN helped maintain NO x mixing ratios in the plume where, over the course of a week, PAN was converted to HNO3. Ozone production in the plume was limited by the a vailability of NOx, and because of the short lifetime of O-3 at 4 -km, net ozone production in the plume was negligible. Within the region, the majority of O-3 production takes place in air above median CO conc entration, indicating that most O-3 production occurs in plumes. Scali ng up from the mean observed Delta O-3/Delta CO in old plumes, we esti mate a minimum regional O-3 production of 17 x 10(10) molecules O-3 cm (-2)s(-1). This O-3 production rate is sufficient to fully explain the observed enhancement in tropospheric 03 over the tropical South Atlan tic during the dry season.