This report presents data summaries from three networks. Contour maps present NTN data. Concentration and deposition maps present MDN data. The line graphs 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 2004, 201 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 2004, 73 sites met these criteria. Annual concentration or deposition is printed next to each site. The concentrations or deposition values appear next to each site. The AIRMoN data shown by line graphs show sulfate and ammonium concentrations and sulfate-minusammonium- concentration differences for the eight AIRMoN sites active at the end of 2004. A locally weighted least squares smoothing (LOWESS) function was applied to individual data points. The LOWESS method is especially suited for smoothing unequally spaced highly scattered data, such as AIRMoN measurements. For the sulfate and ammonium graphs, the LOWESS smoothing window was set for 180 days. This filtered out short-duration variations and accentuated seasonal cycles. For the concentration difference line, the smoothing window was widened to filter out all but long-term changes. Sulfate and ammonium concentrations exhibit seasonal cycles that vary by site and over time. Except for the Illinois site, the sulfate line is always above the ammonium line. Sulfate-minus-ammonium differences tend to decrease over time at many sites, suggesting long-term trends in concentrations of sulfate, ammonium, or both.