The aerosol in a rural area in eastern Arizona was sampled in the spring and fall of 1980 and subsequently studied by scanning electron microscopy. The most abundant microparticles were ammonium sulfate, soil components and other salts (sulfates and chlorides). In May the geometric volume mean diameter (DGV) of the ammonium sulfates was 0.43 μm and the sulfate concentration 5.8 μg m−3. In October the DGV was 0.28 μm and the sulfate concentration 4.5 μg m−3. The salts (except ammonium sulfates) were probably derived from salt flats.