OZONE PRODUCTION AND TRANSPORTS IN THE TROPICAL ATLANTIC REGION DURING THE BIOMASS BURNING SEASON

Tropospheric O-3 distributions over the Atlantic Ocean have been calcu lated with a coupled chemistry-general circulation model. Photochemica lly produced O-3, mainly from biomass burning emissions, dominates O-3 abundances over the tropical Atlantic Ocean in the southern hemispher e (SH) during September/October. On the other hand, O-3 of stratospher ic origin strongly contributes to the tropospheric O-3 column at latit udes poleward of 30 degrees S. Simulated tropospheric flow patterns ar e in good agreement with European Centre for Medium-Range Weather Fore casts analyses and trajectory studies. Tracer transports over the trop ical South Atlantic Ocean are strongly influenced by wind shear betwee n the boundary layer and the free troposphere, leading to a stratifica tion of O-3. At the surface relatively O-3-poor air is transported fro m SH middle latitudes. At higher altitudes, relatively O-3-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 O-3 production rates of 10-50 parts per billion by volume (ppbv) O-3 d(-1) in the lower troposphere over t he biomass burning regions and 2-6 ppbv O-3 d(-1) in the middle and up per troposphere. Photochemical destruction of O-3 prevails in the lowe r troposphere over the ocean, maximizing in the African and South Amer ican outflow regions. In the northern hemisphere, in situ photochemica lly produced O-3 dominates throughout most of the troposphere. Calcula ted O-3 volume mixing ratios are compared with a latitude-altitude O-3 distribution measured during an Atlantic ship cruise along 30 degrees W, between 55 degrees N and 30 degrees S, in September/October 1988, and with ozone sonde measurements from Southern African Fire Atmospher ic Research Initiative/Transport and Atmospheric Chemistry Near the Eq uator-Atlantic (SAFARI/TRACE A) in September/October 1992. Calculated O-3 levels agree reasonably well with the ship cruise data, except for the tropical SH lower troposphere where the model significantly under estimates O-3. However, modeled vertical O-3 distributions are underpr edicted compared to TRACE A sonde measurements. Simulated O-3 columns over the ocean are somewhat lower compared to values retrieved from sa tellite data, in particular over the tropical Atlantic. The underestim ation is probably due to the neglect of higher hydrocarbon chemistry i n the model.