Atmospheric wet deposition in urban and suburban sites across the United States

Compared to rural areas, there is a much poorer understanding of trends in wet atmospheric deposition across urban and suburban areas of the U.S. and how they compare to rural sites over time. The goal of this study was to investigate long-term trends in atmospheric wet deposition of nitrogen (ammonium and nitrate; NH 4 + and NO 3 − ), sulfate (SO 4 2− ), cations, and chloride (Cl − ) for urban and suburban sites in the U.S. using data from the National Atmospheric Deposition Program (NADP) and to determine how long-term deposition trends relate to urbanization. For urban and suburban sites, we found that from the late 1970s to 2020, atmospheric deposition of NO 3 −, dissolved inorganic N (DIN), SO 4 2−, hydrogen (H + ), and magnesium (Mg 2+ ) decreased over time, while deposition of NH 4 + and potassium (K + ) significantly increased over this period. When sites were categorized based on population density, we found no significant differences in rates of wet deposition between urban and background rural values, but the difference between urban and background rural values increased significantly over time for NO 3 − and DIN. When sites were categorized based on impervious surface area (ISA), we found that rates of NH 4 +, DIN, SO 4 2, and calcium (Ca 2+ ) deposition were significantly higher in urban sites (that had greater than 25% ISA) compared to background rural values. There were statistically significant positive relationships between population density and deposition of Mg 2+, K +, sodium (Na + ), and Cl − and significant positive relationships between % ISA and deposition of NO 3 − and Mg 2+. Together, the results of this study demonstrate the urban enhancement of many forms of atmospheric deposition and the need for increased long-term monitoring of atmospheric deposition in cities and suburban sites throughout the U.S., especially in the western U.S.