Measurements of rural sulfur dioxide and particle sulfate: Analysis of CASTNet data, 1987 through 1996

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 (SO2), nitric acid (HNO3), particle sulfate (SO4--), particle nitrate (NO3-), 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 SO2 and SO4-- concentration data collected from 1987 through 1996 at rural NDDN/CASTNet sites. Annual and seasonal variability is examined. Gradients of SO2 and SO4-- are discussed. The variability of the atmospheric mix of SO2 and SO4-- is explored spatially and seasonally. Data from CASTNet are also compared to SO2 and SO4-- data from concurrent monitoring studies in rural areas. The Midwest and the Northeast have the highest annual averages of SO2, greater than 10 mu g/m(3). SO2 concentrations vary by a factor of 6 across the six eastern subregions. The annual SO2 concentrations for western sites average less than 1 mu g/m(3). A gradient in SO2 concentrations in the Ohio River Valley was observed, decreasing in northeasterly, easterly, and southeasterly directions. Also, a decreasing concentration gradient was observed for SO4-- from the Midwest to the East and Southeast, although the gradient was not as pronounced as for SO2. Both SO2 and SO4-- exhibited strong seasonal cycles.