Tropospheric ozone and aerosol distributions across the Amazon Basin

Abstract

Ozone and aerosol distributions were measured during July–August 1985 over the tropical rain forest of Brazil as part of the NASA Global Tropospheric Experiment to study the Amazon boundary layer. Remote and in situ measurements of O3 and aerosols were made from a NASA Electra aircraft on several long‐range flights spanning different areas between Tabatinga and Belem, Brazil. Continuous O3 distributions were obtained between the aircraft altitude and the ground with an airborne differential absorption lidar (DIAL) system. Aerosol distributions were also continuously measured above and below the aircraft with the DIAL system. In situ O3 measurements were made on the aircraft and from ground‐launched ozonesondes at Manaus and Natal. Large‐scale variations in the vertical and horizontal distributions of O3 and aerosols were observed on nearly all flights over the Amazon Basin, with O3 exceeding 50 parts per billion by volume (ppbv) in some regions. Both positive and negative correlations were observed between O3 mixing ratios and aerosol concentrations. In nearly all cases, when O3 and aerosols were negatively correlated, this represented clean midtropospheric air, and when they were positively correlated, the air mass had undergone photochemical O3 production as a result of biomass burning. A 59% increase in the planetary boundary layer (PBL) O3 level was observed between the initial Manaus‐Belem flights on July 23–24, 1985 and the later flights on August 8–9, 1985. This was attributed to the increase in biomass burning near the Rio Amazonas and its tributaries and in savannah regions south of the Amazon Basin. Flights to the west of Manaus measured enhanced O3 and aerosol levels near the Rio Solimões, while the area sampled upwind of the river exhibited lower O3 levels. This was explained by the increased incidence of biomass burning near the river. This paper also examines the variability of the trade wind inversion (TWI) height across the Amazon Basin and the influence of O3 levels above the TWI on the budget of O3 in the PBL. Measurements reported for the early dry season characterize the background distribution of O3 and aerosols in the PBL prior to the onset of extensive biomass burning. These data provide the basis for a basin‐scale estimate of photochemical O3 production from biomass‐burning emissions.