FACTORS INFLUENCING DRY SEASON OZONE DISTRIBUTIONS OVER THE TROPICAL SOUTH-ATLANTIC

Airborne measurements of trace gas and aerosol species were obtained i n the lower troposphere (<5 km) over the western Atlantic Ocean betwee n 13-degrees-S and 40-degrees-N during the August/September 1990 NASA Chemical Instrument Test and Evaluation (CITE 3) experiment. The large st background O3 Mixing ratios, averaging 35 and 70 ppbv within the mi xed layer (ML) and free troposphere (FT; altitudes >2.4 km), respectiv ely, were found over the tropical South Atlantic. Several competing pr ocesses were observed to regulate O3 budgets in this region. Within th e ML, rapid photochemical destruction produced a diurnal O3 variation of 8 ppbv and an O3/altitude gradient between the surface and 5 km of almost 10 ppbV(O3) km-1. ML O3 concentrations were replenished by atmo spheric downwelling which occurred at rates of up to and exceeding 1 c m s-1. Ozone values within the subsiding FT air were enriched both by long-range transport Of O3 produced photochemically within biomass com bustion plumes and the downward propagation of dry, upper tropospheric air masses. Overall, the tropospheric O3 column below 3.3 km averaged 13.5 Dobson units (DU) over the South Atlantic region, which is 8-9 D U higher than observed during CITE 3 ferry flights over the northern t ropical Atlantic Ocean or measured by ozonesondes over coastal Brazil during the wet season. An examination of simultaneous dew point and co mbustion tracer (e.g., CO) measurements suggests that the dry subsidin g layers and biomass burning layers make approximately equal contribut ions to the observed O3 enhancement.