Atmospheric Aerosols over the Amazon Basin: Composition, Microphysics, Sources, and Sinks (Invited Presentation)

The first large expeditions to study the composition of the atmosphere over Amazonia were undertaken in the 1980s. They were based on the premise that this region was a pristine, self-contained system, where atmosphere-biosphere interactions were completely dominant as drivers of atmospheric chemistry. The NASA-ABLE campaigns of 1985 and 1987 revealed two surprises that contradicted this premise: First, that in the dry season, biomass burning (BB) along the southern edge of the Amazon could impact the atmosphere over the entire Basin, thousands of kilometers from the burning regions. Second, that in the wet season dust and smoke from the Sahara and West Africa was transported all the way across the Atlantic and even reached the central Amazon. Long-range transport of pollutants emerged as an important driver of atmospheric chemistry over Amazonia.

Studies in the subsequent decades showed the presence of a wide range of aerosol types over the Amazon. In the wet season, during the least polluted periods, the submicron mode is dominated by secondary organic aerosol (SOA) produced from biogenic precursors, mostly isoprene and terpenes. The supermicron aerosol consists mostly of primary biogenic aerosol particles (PBAPs), especially fungal spores, which are usually covered with SOA coatings. In addition, the Amazon biota emits small particles composed of a mixture of inorganic salts and organics, which may have an important role as cloud condensation nuclei (CCN).

When the Amazon Basin is under the influence of the northeastern trade winds during the rainy season, frequent episodes of mineral dust and BB aerosol from northern and western Africa are superimposed on this pristine aerosol. In contrast, the dry season is characterized by inflow from the southeastern trades, which brings in BB smoke and fossil-fuel pollution from southern-hemisphere Africa. In the northern part of the Basin, with relatively few regional pollution sources, this accounts for a substantial fraction of the dry-season aerosol. Further south, approaching the arc of deforestation along the southern edge of the rainforest, massive amounts of emissions from fires dominate the atmospheric composition. As a result of increasing development in urban and rural areas, anthropogenic pollutants are added to the mix.

Given the great biological productivity of the Amazon rainforest and the dominance of biogenic SOA in the pristine aerosol, it has been surprising that new particle formation (NPF) has rarely been observed in the Amazon boundary layer (BL). Recent aircraft and tower studies suggest that much of the NPF in Amazonia takes place in the upper troposphere (UT). Dramatically enhanced concentrations of ultrafine particles were found in UT regions that had experienced outflow from deep convection in the preceding 24-48 hours. These findings suggest that aerosol production takes place in the UT from volatile material brought up by deep convection, which is converted to condensable species in the UT. Subsequently, downward mixing and transport of upper tropospheric aerosol are a source of particles to the BL, where they increase in size by the condensation of biogenic volatile organic carbon (BVOC) oxidation products. Generally, this may be an important source of aerosol particles in the remote and pristine troposphere, where aerosol nucleation and new particle formation is not commonly observed. Future studies over the Amazon are planned to investigate the specific chemical and microphysical processes that are responsible for aerosol production from biogenic precursors over the tropical continents.