This report presents data summaries from three networks. Contour maps present NTN data. Concentration and deposition maps present MDN data. The bar charts present AIRMoN concentrations over time. The NTN maps portray spatial variability in the concentration and deposition of selected acids, nutrients, and base cations on regional and national scales. Only sites that meet prescribed data completeness criteria are included. In 2003, 193 sites met these criteria. Annual concentration or deposition values are printed next to each site. The concentrations are volume-weighted averages. The MDN maps show the concentration and deposition of total mercury in precipitation. Only sites meeting prescribed data completeness criteria are included. In 2003, 65 sites met these criteria. Annual concentration or deposition is printed next to each site. The concentrations are volume-weighted averages. The AIRMoN data shown by bar charts show the fraction of samples with ammonium to sulfate ratios in five classes for the nine AIRMoN sites active at the end of 2003. Percentage frequency-of-occurrence bars are shown for 1993, 1998, and 2003. A missing bar indicates a site not yet operational. Chemical equivalent concentrations were used to compute the ratios. Ammonium hydrogen sulfate, an aerosol commonly observed in eastern states, has a ratio of 0.50 and is referred to as an acidic aerosol (ammonium equivalents balance only one of two sulfate equivalents). Ammonium sulfate, another common aerosol, has a ratio of 1.00 and is referred to as a neutral aerosol (ammonium and sulfate equivalents are equal). The percentage of high ratios generally increased from 1993 to 2003.