Regional aerosol chemistry of the Amazon Basin during the dry season

Abstract

We report here a set of measurements which describe the distribution and chemical composition of the atmospheric aerosol over the tropical rain forest of the Amazon Basin, in Brazil, during the 1985 July–August dry season. Our data suggest that there is remarkable compositional and spatial homogeneity of the atmospheric aerosol on an extensive regional scale. We describe the aerosol regime associated with emissions from the moist tropical forest during early to middle dry season conditions. The moist forest ecosystem is a source of aerosols, composed of material directly released by vegetation, together with species originating from biogenic emissions of various reduced atmospheric gases. Particulate organic carbon is the dominant component of the atmospheric aerosol, exhibiting an average concentration of 740±190 nmol C m−3 in the mixed layer and 220±60 nmol C m−3 in free tropospheric air. On the average the atmospheric concentration of aerosol constituents was enhanced in mixed layer air fivefold over free tropospheric levels. Oxalate and SO42− exhibited the greatest enrichment in the mixed layer, while Cl− showed essentially no enrichment in mixed layer aerosols. For many constituents their enhancement in mixed layer air was greatest in the aerosol coarse fraction, exceeding a factor of 10 in some cases. Approximately 75% of total aerosol NO3− in the mixed layer was contained in the coarse atmospheric aerosol. The atmospheric aerosol in the Amazonian atmosphere is essentially acid‐base neutral, primarily as a result of incorporation Of NH4+, presumably derived from NH3 released by the forest ecosystem. Ammonium ion is the principal ionic constituent of the atmospheric aerosol, being present at a concentration typically more than twice that of other major ionic species. The biospheric‐atmospheric coupling of emissions of NH3 and reduced sulfur gases by the forest ecosystem, and subsequent incorporation of atmospheric NH3 into acidic aerosols produced by oxidation of reduced atmospheric sulfur gases, is an important aspect of biogeochemical cycling to be pursued in future studies.