An ambient particle monitoring study was conducted in Washington, D.C. during the summer of 1994 as part of the Metropolitan Acid Aerosol Characterization Study (MAACS). Acid aerosol and inhalable (particulate matter with aerodynamic diameters < 10 micron; PM10) and fine (particulate matter with aerodynamic diameters < 2.5 micron; PM2.5) particle samples were collected for 24-hr periods (9 A.M.-9 A.M. EDT) on alternate days at six monitoring sites located throughout the greater Washington, D.C. area. Monitoring sites were located in both urban and rural areas and were generally situated along a southwest to northeast line due to the prevailing winds. Information on site characteristics, including population density and distance from the city center, was also obtained, as were data on meteorological parameters. Results from this study show strong correlations among the particulate measures, PM10, PM2.5, SO4(2-), and H+. These strong correlations resulted from the fact that PM2.5 comprised 77% of PM10, with SO4(2-)-related species accounting for 49% of total PM2.5. PM10, PM2.5, SO4(2-), and H+ concentrations were found to be uniform across the metropolitan Washington area. Spatial variation was found, however, for coarse particles (PM2.5-10) and NH3 concentrations. In our previous Philadelphia study, population density was an important determinant of spatial variation in coarse particles and NH3 concentrations, however, in Washington, D.C., population density was not associated with observed spatial patterns in coarse particle concentrations, but was an important determinant of NH3 concentrations. When data from one site (Reservoir) was excluded from the analysis, population density explained larger percentage of the variability in NH3 levels and became an important determinant of the H+/SO4(2-) ratio as well. Ambient H+ models developed from Philadelphia data were found to predict H+ concentrations in Washington, D.C. reasonably well, representing an improvement over measurements made at a single stationary ambient monitoring site.