An overview of changes in wet deposition across the U.S. over the past 25 years

Joseph Pinto1, Adam Benson2 and Alan Talhelm3

Large differences in wet deposition of nitrate (NO3-), ammonium (NH4+) and sulfate (SO42-) between two three-year periods (1989 to 1991 and 2012 to 2014) are seen across the United States based on data from the NADP/NTN. The magnitude and sign of differences varies regionally and with species. As expected, based on controls mandated by the Clean Air Act Amendments of 1990, the eastern U.S. is characterized by large decreases in wet fluxes of SO42- and NO3-. Changes in wet fluxes of SO42- and NO3- in the central and western U.S. are typically not as large with some areas experiencing increases and others decreases. The most notable changes in these regions are associated with increases in wet deposition of SO42- and NO3- in the North Central U.S. and in southern Idaho. Several studies in the literature have indicated increases in emissions from oil and gas exploration and production in the region, transport from oil and gas fields in Alberta and also expansion of motor vehicles and EGUs to meet the demands of population growth. All of these sources may have contributed to the observed increases in deposition. Concentrated animal feeding and wildfires are other sources of deposition, which might be particularly important in southern Idaho/ northern Utah. Wet deposition at sites in the area is characterized by a high degree of interannual variability. Wildfires are also characterized by a high degree of interannual variability and are known to be increasing in the Intermountain West and may contribute to this observation.    

In contrast to SO42- and NO3-, the largest area-wide increases in wet deposition of NH4+ are found in the Central U.S., with notable hot spots in eastern North Carolina, and in the Intermountain West. Increases in wet deposition of NH4+ are especially notable in areas characterized by concentrated animal feeding operations. Interestingly, wet deposition of NH4+ appears to have decreased in the Gulf States. In general, these results suggest that the ratio of NH4+ to NO3- in rainwater has increased and in many areas, NH4+ is the dominant form of reactive nitrogen.  Analyses of trends and interannual variability in deposition at selected individual sites in distinct ecological regions will also be presented.

The views expressed are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.