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

Open Access
Publisher Website
Google Scholar
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 (ΔY/ΔX, where Δ indicates the enhancement of a compound in the plume above the local background mixing ratio,Yare individual hydrocarbons, CO, O3, N2O, HNO3, peroxyacetyl nitrate (PAN), CH2O, acetone, H2O2, CH3OOH, HCOOH, CH3COOH or aerosols andXis CO or CO2) 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. O3is produced in plumes over at least a 1 week period with mean ΔO3/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 HNO3. Ozone production in the plume was limited by the availability of NOx, and because of the short lifetime of O3at 4‐km, net ozone production in the plume was negligible. Within the region, the majority of O3production takes place in air above median CO concentration, indicating that most O3production occurs in plumes. Scaling up from the mean observed ΔO3/ΔCO in old plumes, we estimate a minimum regional O3production of 17×1010molecules O3cm−2s−1. This O3production rate is sufficient to fully explain the observed enhancement in tropospheric O3over the tropical South Atlantic during the dry season.

This publication has 59 references indexed in Scilit: