The USEPA National Dry Deposition Network (NDDN) is designed to provide long-term estimates of acidic gas and aerosol concentrations, and associated fluxes, across the continental United States. Inspection of data collected since 1988 shows species-dependent variability in atmospheric concentrations from site to site, season to season and year to year. In general, gas and aerosol concentrations were much higher (factor of 2-10) at eastern sites than western sites. Data for 25 eastern sites operational from 1988 through 1991 suggest that SO4(-2) concentrations have been essentially constant. In contrast, SO2 and HNO3 appear to have decreased, on average, by about 20 percent and 15 percent, respectively. Examination of sub-regional concentration patterns shows marked variability in areas of complex terrain. Data from a ridgetop site and a nearby base elevation site in southwestern North Carolina show that reactive gas concentrations, but not aerosol concentrations, are 2-3 times higher at ridgetop than at base elevation. Elevational gradients thus need to be accounted for in analysis of large-scale concentration patterns.