Photochemistry in biomass burning plumes and implications for tropospheric ozone over the tropical South Atlantic

Abstract

Photochemistry occuring 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 1992 and a photochemical point model. Enhancement ratios (Δ Y/Δ X, where Δ indicates the enhancement of a compound in the plume above the local background mixing ratio,Y are individual hydrocarbons, CO, O 3, N 2 O, HNO 3, peroxyacetyl nitrate (PAN), CH 2 O, acetone, H 2 O 2, CH 3 OOH, HCOOH, CH 3 COOH or aerosols and X is CO or CO 2) 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 3 is produced in plumes over at least a 1 week period with mean ΔO 3/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 NOxmixing ratios in the plume where, over the course of a week, PAN was converted to HNO 3. Ozone production in the plume was limited by the availability 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 concentration, indicating that most O 3 production occurs in plumes. Scaling up from the mean observed ΔO 3/ΔCO in old plumes, we estimate a minimum regional O 3 production of 17×10 10 molecules O 3 cm−2 s−1. This O 3 production rate is sufficient to fully explain the observed enhancement in tropospheric O 3 over the tropical South Atlantic during the dry season.