The Clean Air Status and Trends Network (CASTNet) was established by the U.S. EPA in response to the requirements of the 1990 Clean Air Act Amendments. To satisfy these requirements CASTNet was designed to assess and report on geographic patterns and long-term, temporal trends in ambient air pollution and acid deposition in order to gauge the effectiveness of current and future mandated emission reductions. This paper presents an analysis of the spatial patterns of deposition of sulfur and nitrogen pollutants for the period 1990-2000. Estimates of deposition are provided for two 4-yr periods: 1990-1993 and 1997-2000. These two periods were selected to contrast deposition before and after the large decrease in SO2 emissions that occurred in 1995. Estimates of dry deposition were obtained from measurements at CASTNet sites combined with deposition velocities that were modeled using the multilayer model, a 20-layer model that simulates the various atmospheric processes that contribute to dry deposition. Estimates of wet deposition were obtained from measurements at sites operated bythe National Atmospheric Deposition Program. The estimates of dry and wet deposition were combined to calculate total deposition of atmospheric sulfur (dry SO2, dry and wet SO4(2-)) and nitrogen (dry HNO3, dry and wet NO3-, dry and wet NH4+). An analysis of the deposition estimates showed a significant decline in sulfur deposition and no change in nitrogen deposition. The highest rates of sulfur deposition were observed in the Ohio River Valley and downwind states. This region also observed the largest decline in sulfur deposition. The highest rates of nitrogen deposition were observed in the Midwest from Illinois to southern New York State. Sulfur and nitrogen deposition fluxes were significantly higher in the eastern United States as compared to the western sites. Dry deposition contributed approximately 38% of total sulfur deposition and 30% of total nitrogen deposition in the eastern United States. Percentages are similar for the two 4-yr periods. Wet sulfate and dry SO2 depositions were the largest contributors to sulfur deposition. Wet nitrate, wet ammonium, and dry HNO3 depositions were the largest contributors to nitrogen deposition.

Cloud water deposition was estimated at three high-elevation sites in the Appalachian Mountains of the eastern United States (Whiteface Mountain, NY; Whitetop Mountain, VA; and Clingman's Dome, TN) from 1994 through 1999 as part of the Mountain Acid Deposition Program (MADPro). This paper provides a summary of cloud water chemistry, cloud liquid water content, cloud frequency, estimates of cloud water deposition of sulfur and nitrogen species, and estimates of total deposition of sulfur and nitrogen at these sites. Other cloud studies in the Appalachians and their comparison to MADPro are also summarized. Whiteface Mountain exhibited the lowest mean and median concentrations of sulfur and nitrogen ions in cloud water, while Clingman's Dome exhibited the highest mean and median concentrations. This geographic gradient is partly an effect of the different meteorological conditions experienced at northern versus southern sites in addition to the difference in pollution content of air masses reaching the sites. All sites measured seasonal cloud water deposition rates of SO 4 2Ϫ greater than 50 kg/ha and NO 3 Ϫ rates of greater than 25 kg/ha. These high-elevation sites experienced additional deposition loading of SO 4 2Ϫ and NO 3

The Clean Air Status and Trends Network (CASTNet) was implemented by the U.S. Environmental Protection Agency (EPA) in 1991 in response to Title IX of the Clean Air Amendments of 1990, which mandated the deployment of a national ambient air monitoring network to track progress of the implementation of emission reduction programs in terms of deposition, air quality, and changes to affected ecosystems. CASTNet evolved from the National Dry Deposition Network (NDDN).CASTNet currently consists of 45 sites in the eastern United States and 28 sites in the West. Each site measures sulfur dioxide (SO 2 ), nitric acid (HNO 3 ), particle sulfate (SO 4 = ), particle nitrate (NO 3 -), and ozone. Nineteen sites collect precipitation samples. NDDN/CASTNet uses a uniform set of site-selection criteria which provides the data user with consistent measures to compare each site. These criteria also ensure that, to the extent possible, CASTNet sites are located away from local emission sources.This paper presents an analysis of SO 2 and SO 4 = concentration data collected from 1987 through 1996 at rural NDDN/CASTNet sites. Annual and seasonal variability IMPLICATIONS Atmospheric concentrations of SO 2 and SO -play major roles in the processes of acidic deposition and visibility degradation. Understanding spatial patterns of these atmospheric constituents provides key information to determine whether the emission reduction program for sulfur will mitigate the environmental effects of acid deposition.is examined. Gradients of SO 2 and SO 4 = are discussed. The variability of the atmospheric mix of SO 2 and SO 4 = is explored spatially and seasonally. Data from CASTNet are also compared to SO 2 and SO 4 = data from concurrent monitoring studies in rural areas.The Midwest and the Northeast have the highest annual averages of SO 2 , greater than 10 µg/m 3 . SO 2 concentrations vary by a factor of 6 across the six eastern subregions. The annual SO 2 concentrations for western sites average less than 1 µg/m 3 . A gradient in SO 2 concentrations in the Ohio River Valley was observed, decreasing in northeasterly, easterly, and southeasterly directions. Also, a decreasing concentration gradient was observed for SO 4 = from the Midwest to the East and Southeast, although the gradient was not as pronounced as for SO 2 . Both SO 2 and SO 4 = exhibited strong seasonal cycles.

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