Ozone production and transports in the tropical Atlantic region during the biomass burning season

Tropospheric O₃ distributions over the Atlantic Ocean have been calculated with a coupled chemistry-general circulation model. Photochemically produced O₃, mainly from biomass burning emissions, dominates O₃ abundances over the tropical Atlantic Ocean in the southern hemisphere (SH) during September/October. On the other hand, O₃ of stratospheric origin strongly contributes to the tropospheric O₃ column at latitudes poleward of 30°S. Simulated tropospheric flow patterns are in good agreement with European Centre for Medium-Range Weather Forecasts analyses and trajectory studies. Tracer transports over the tropical South Atlantic Ocean are strongly influenced by wind shear between the boundary layer and the free troposphere, leading to a stratification of O₃. At the surface relatively O₃-poor air is transported from SH middle latitudes. At higher altitudes, relatively O₃-rich air is transported from the African biomass burning regions between 2 and 5 km and from South American biomass burning regions in the mid-to-upper troposphere. The model simulates O₃ production rates of 10-50 parts per billion by volume (ppbv) O₃ d^-1 in the lower troposphere over the biomass burning regions and 2-6 ppbv O₃ d^-1 in the middle and upper troposphere. Photochemical destruction of O₃ prevails in the lower troposphere over the ocean, maximizing in the African and South American outflow regions. In the northern hemisphere, in situ photochemically produced O₃ dominates throughout most of the troposphere. Calculated O₃ volume mixing ratios are compared with a latitude-altitude O₃ distribution measured during an Atlantic ship cruise along 30°W, between 55°N and 30°S, in September/October 1988, and with ozone sonde measurements from Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry Near the Equator—Atlantic (SAFARI/TRACE A) in September/October 1992. Calculated O₃ levels agree reasonably well with the ship cruise data, except for the tropical SH lower troposphere where the model significantly underestimates O₃. However, modeled vertical O₃ distributions are underpredicted compared to TRACE A sonde measurements. Simulated O₃ columns over the ocean are somewhat lower compared to values retrieved from satellite data, in particular over the tropical Atlantic. The underestimation is probably due to the neglect of higher hydrocarbon chemistry in the model.