UC Irvine

Photochemistry occurring in biomass burning plumes over the tropical south Atlantic is analyzed using data collected during the Transport and Atmospheric Chemistry Near the Equator-Atlantic aircraft expedition conducted during the tropical dry season in September 1993 and a photochemical point model. Enhancement ratios (ΔY/ΔX, where Δ indicates the enhancement of a compound in the plume above the local background mixing ratio, Y are individual hydrocarbons CO, O , N O, HNO , peroxyacetyl nitrate (PAN), CH O, acetone, H O , CH OOH, HCOOH, CH OOH or aerosols and X is CO or CO ) are reported as a function of plume age inferred from the progression of Δnon-methane hydrocarbons/ΔCO enhancement ratios. Emission, formation, and loss of species in plumes can be diagnosed from progression of enhancement ratios from fresh to old plumes. O is produced in plumes over at least a 1 week period with mean ΔO /ΔCO = 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 ~20% in enhancement ratios in old plumes relative to ΔCO to correct for CO loss. In a case study of a large persistent biomass burning plume at 4-km we found elevated concentrations of PAN in the fresh plume. The degradation of PAN helped maintain NO mixing ratios in the plume where, over the course of a week, PAN was converted to HNO . Ozone production in the plume was limited by the availability of NO , and because of the short lifetime of O at 4-km, net ozone production in the plume was negligible. Within the region, the majority of O production takes place in air above meadian CO concentration, indicating that most O production occurs in plumes. Scaling up from the mean observed ΔO /ΔCO in old plumes, we estimate a minimum regional O production of 17 x 10 molecules O cm s . This O production rate is sufficient to fully explain the observed enhancement in tropospheric O over the tropical South Atlantic during the dry season. 3 2 3 2 2 2 3 3 2 3 3 x 3 x 3 3 3 3 3 3 3 3 10 -2 -1