Atmospheric concentrations of NH3, HNO3, an SO2 have been measured at a variety of sites in the southwestern United States over the last few years. Sampling was by condensation collection. Over much of the region, NH3 is associated primarily with agricultural activity, HNO3 mostly with urban emissions, and SO2 with industrial effluent. However, relatively high SO2 concentrations (1 ppb) were observed at a remote coastal site, indicating substantial natural release of sulfur compounds in this area. Data from a relatively remote site in southern Arizona showed that copper smelting in the region increased SO2 levels from 0.3 to 2 ppb and probably causesd a small increase in HNO2 levels. There were large diurnal variations of both components, though perhaps with different causes. Urban concentrations of SO2 and HNO3 were unaffected by smelting. Levels of NH3 varied with proximity to agricultural sources. Average values of 10 ppb were observed close to a recently fertilized field. The most frequency concentration observed at the more remote sites (taken to be background for the area) for 0.9 ppb NH3. Diurnal and seasonal variations of NH3 were small.