Atmospheric Deposition is Elevated in Cities Relative to Rural Areas Across the United States and this Pattern is Strengthening Over Time

Background/Question/Methods

Human activities, including burning of fossil fuels and agricultural practices such as planting of leguminous crops and synthesis and application of fertilizers, have led to elevated rates of atmospheric nitrogen (N) and sulfur (S) deposition. As a result of the U.S. Clean Air Act and its Amendments, rates of sulfate and nitrate deposition have declined in much of the U.S. A majority of research on atmospheric deposition has focused on rural areas, but a growing body of literature indicates that urban areas are hotspots for atmospheric deposition. In this study we utilized long-term atmospheric deposition data for sites across the U.S. from the National Atmospheric Deposition Program (NADP)’s National Trends Network. The NADP has publicly available data for site specific deposition, as well as interpolated annual gradient maps that are created with data from non-urban sites only. Our goal was to determine whether rates of atmospheric deposition of N (nitrate and ammonium), S (sulfate), cations (calcium, magnesium, potassium, and sodium), and chloride are higher in urban than non-urban sites and whether this difference is increasing over time. We compared empirical measurements of atmospheric deposition in urban to estimates from non-urban sites.

Results/Conclusions

In urban areas from 1979-2020, atmospheric deposition of nitrate, total inorganic nitrogen, sulfate, and hydrogen ions decreased over time, while deposition of ammonium, magnesium, potassium, sodium, and chloride significantly increased over this period. Deposition of sulfate and potassium were higher in urban than rural sites. However, the difference between urban and rural deposition is increasing over time for ammonium, nitrate, and calcium. These results indicate that atmospheric deposition remains higher in urban compared to rural areas and that this difference continues to increase. This paper broadens our understanding of atmospheric deposition and demonstrates the need for expanded monitoring of atmospheric deposition in cities.