The distribution and chemistry of the atmospheric aerosol over the Amazon Basin during the April-May segment of the 1987 wet season are described using ground- and aircraft-based data. Wet season aerosol concentrations and composition are variable in contrast to the remarkably uniform distribution and composition of the predominantly biogenic aerosol that we observed during the 1985 dry season. Four distinct intervals of enhanced aerosol concentration coincided with 3- to 5-day periods of extensive rainfall over central Amazonia. It is hypothesized that a major source of aerosols to the basin was the direct input of northern hemispheric air laden with variable mixtures of Saharan dust, marine aerosol, and possibly biomass combustion products. The enhanced aerosol concentrations over Amazonia were reduced in 1–3 days to 5–10% of their peak levels by large-scale changes in the circulation field with subsequent decoupling from the source region, frequent precipitation, and intermixing of northern and southern hemispheric air masses. The intrusion of northern hemispheric air into the Amazon Basin is linked to the establishment and persistence of the West African Subtropical High (WASH) in a limited region over west Africa. Marine aerosols may be intermixed with the soil dust during transit across the Atlantic or within the sea breeze regime along the northeast coast of South America. It is proposed that a principal source of NO3− and SO42− associated with the dust is biomass burning south of the Sahara in western Africa.