Continuous sampling of atmospheric aerosols was carried out at two different sites in the Amazon Basin: Cuiab¿ (16 ¿S, 56 ¿W), since July 1990, and Alta Floresta (09 ¿S, 56 ¿W), since August 1992. Aerosols were collected on polycarbonate filters mounted in stacked filter units (SFU). Particle-induced X ray emission (PIXE) was used to measure concentrations of up to 26 elements (Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Zr, Au, and Pb). Black carbon (BC) and gravimetric mass analyses were also performed. Inhalable particle ground concentrations showed a marked seasonality, with maxima of more than 100 μg m-3 in the dry season. The high aerosol optical thickness (AOT) values in this period (up to 4.0 at λ=0.440 μm) indicate that the whole air column has a significant aerosol load. Three main types of aerosol sources were identified: biomass burning, natural biogenic emissions, and soil dust resuspension. During the dry season the fine mode aerosol originated predominantly from biomass burning emissions (they were responsible for about 73% of fine aerosol mass), while the coarse mode was dominated by soil dust particles (50 to 60% of the aerosol mass). Crustal elements exhibited a unimodal coarse mode size distribution, while the pyrogenic elements showed a clear submicrometer mode. Black carbon had a submicrometer mode centered at 0.175 μm. During the wet season, biogenic aerosols were the dominant particle type and also the main source of atmospheric P in the region. ¿ 1998 American Geophysical Union