OZONE AND AITKEN NUCLEI OVER EQUATORIAL AFRICA - AIRBORNE OBSERVATIONS DURING DECAFE-88

We determined the distribution of ozone (O3) and Aitken condensation nuclei (CN) over the rain forest in equatorial Africa during February 12-25, 1988. A pronounced O3 maximum with levels up to 70 ppbv was present in a layer between 1 and 3 km altitude throughout the period. It coincided with a CO maximum and with high levels of CO2 and gaseous organic acids. In general, the vertical distribution of CN was similar to that of O3, with number densities ranging up to approximately 3000 cm-3. Dense haze was visible within this layer. O3 and CN decreased sharply above the haze layer to values typical of the remote troposphere. Survey flights showed little change in levels of O3 and CN or in their vertical distribution over distances of hundreds of kilometers. Meteorological observations suggest that this ozone and particulate enriched layer is formed from air masses which originate in northern Africa and subsequently advect over dry tropical regions where biomass burning emits large amounts of aerosols, CO, NO, and hydrocarbons. These air masses then become trapped in the equatorial region between the near-surface monsoon flow from the southeast and the permanent easterly flow above 3-4 km. Differences in the vertical distribution of O3 and CN result from the removal of O3 by surface uptake and reactions with NO and hydrocarbons, leading to surface O3 concentrations near zero and a steep O3 gradient through the subcloud layer at night. During the day this gradient is reduced by convective mixing. CN concentrations showed no pronounced gradients in the subcloud layer, consistent with the absence of a strong sink for CN at the ground. CN gradients near the surface suggest emission of particles from the forest vegetation or from biomass burning.