Spatial trends of precipitation chemistry in the Central Plains region of the United States

In the late 1970s, historically high concentrations of anthropogenic air pollutants in the United States prompted the development of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). While much has been learned since program inception, many long-term data sets have yet to be interpreted, the results of which will help guide changing policies. Given the lack of studies in the Central Plains of the Midwestern United States, spatial trends of the past 30 years are presented from ten NADP sites surrounding, and including, Missouri, USA. Concentrations of calcium were highest (0.40 mg/L) and lowest (0.14 mg/L) at north and southeast sites, respectively, with a westward trend of increasing concentration. Ammonium concentrations were highest (0.67 mg/L) at northern NADP locations, with all sites showing an average increase in concentration of 65 % from 1984 to 2014, including south central Iowa which showed an increase of 226 %. Sulfate concentrations were highest for eastern sites (1.70 mg/L), but showed an overall decrease of at least 50 % for all locations since the early 1980s. Nitrate concentrations decreased between 5 and 30 % for each site over the 30-year period. Alkalinity increased between 10 and 25 % from southern to northern sites, respectively. When grouped into north–south, and east–west regions, significant differences (p ≤ 0.05) were observed for all constituents, except potassium, between each region. Data syntheses, such as that presented here, advances understanding of long-term precipitation chemistry, thereby improving management strategies intended to mitigate atmospheric chemical deposition.