Atmospheric nitrogen deposition in the Chesapeake Bay watershed: A history of change

Douglas A Burns; Gopal Bhatt; Lewis C Linker; Jesse O Bash; Paul D Capel; Gary W Shenk

doi:10.1016/j.atmosenv.2021.118277

Atmospheric nitrogen deposition in the Chesapeake Bay watershed: A history of change

Atmos Environ (1994). 2021 Apr 15;251(15):1-118277. doi: 10.1016/j.atmosenv.2021.118277.

Authors

Douglas A Burns¹, Gopal Bhatt², Lewis C Linker³, Jesse O Bash⁴, Paul D Capel⁵, Gary W Shenk⁶

Affiliations

¹ U.S. Geological Survey, Troy, NY, USA.
² Pennsylvania State University, Annapolis, MD, USA.
³ U.S. Environmental Protection Agency, Chesapeake Bay Program Office, Annapolis, MD, USA.
⁴ National Exposure Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA.
⁵ Department of Civil, Environmental, and Geo-Engineering, University of Minnesota, Minneapolis, MN, USA.
⁶ U.S. Geological Survey, Annapolis, MD, USA.

Abstract

The Chesapeake Bay watershed has been the focus of pioneering studies of the role of atmospheric nitrogen (N) deposition as a nutrient source and driver of estuarine trophic status. Here, we review the history and evolution of scientific investigations of the role of atmospheric N deposition, examine trends from wet and dry deposition networks, and present century-long (1950-2050) atmospheric N deposition estimates. Early investigations demonstrated the importance of atmospheric deposition as an N source to the Bay, providing 25%-40% among all major N sources. These early studies led to the unprecedented inclusion of targeted decreases in atmospheric N deposition as part of the multi-stakeholder effort to reduce N loads to the Bay. Emissions of nitrogen oxides (NO_x) and deposition of wet nitrate, oxidized dry N, and dry ammonium $({NH}_{4}^{+})$ sharply and synchronously declined by 60%-73% during 1995-2019. These decreases largely resulted from implementation of Title IV of the 1990 Clean Air Act Amendments, which began in 1995. Wet ${NH}_{4}^{+}$ deposition shows no significant trend during this period. The century-long atmospheric N deposition estimates indicate an increase in total atmospheric N deposition in the Chesapeake watershed from 1950 to a peak of ~15 kg N/ha/yr in 1979, trailed by a slight decline of <10% through the mid-1990s, and followed by a sharp decline of about 40% thereafter through 2019. An additional 21% decline in atmospheric N deposition is projected from 2015 to 2050. A comparison of the Potomac River and James River watersheds indicates higher atmospheric N deposition in the Potomac, likely resulting from greater emissions from higher proportions of agricultural and urban land in this basin. Atmospheric N deposition rose from 30% among all N sources to the Chesapeake Bay watershed in 1950 to a peak of 40% in 1973, and a decline to 28% by 2015. These data highlight the important role of atmospheric N deposition in the Chesapeake Bay watershed and present a potential opportunity for decreases in deposition to contribute to further reducing N loads and improving the trophic status of tidal waters.

Keywords: Atmospheric deposition; Chesapeake Bay; Nitrogen; Trends.

Grants and funding

EPA999999/ImEPA/Intramural EPA/United States