Aerosol samples from South American tropical rain forests near Manaus, Brazil, and Zongo, Bolivia, before and during rainfall periods, collected by using cascade impactors and time sequence filter samplers ('streakers') contained elements that may be classified into two distinct groups in the coarse particle size range (>1-μm aerodynamic diameter). One group, whose atmospheric concentrations are suppressed during rainfall episodes, consists of the elements Al, Si, Ca, and Fe, which may be soil derived. The other group consisting of the elements S, K, and P, shows little, if any, reduction in concentration caused by rainfall. The fact that the latter group is associated with the forest as a potential source and has elemental ratios corresponding to those found in biological materials is evidence of a biogenic source for S, K, and P in coarse aerosol particles over tropical rain forests sufficient to account for the observed concentrations of ~50 ng S/m3, ~100 ng K/m3, and ~50 ng P/m3. These data suggest that organic matter released to the atmosphere contains not only nonmetals but potassium and perhaps additional metals.