Atmospheric Nitrogen Deposition to the Chesapeake Bay Watershed: A History of Change
Abstract
Pioneering studies during 1980s and 90s in the Chesapeake Bay watershed showed that atmospheric deposition was a major contributor of nitrogen (N) loads, providing 1/3 or more among all sources and a principal driver of hypoxia in the Bay. Lowering levels of atmospheric N deposition later became part of efforts to reduce N loads to the Bay in a previously unprecedented merging of goals under the Clean Air and Clean Water Acts. Atmospheric N deposition has shown sharp declines as the 1990 Amendments to the Clean Air Act were implemented. Wet and dry deposition measurements at sites in the Chesapeake Bay airshed indicate decreases in oxidized N deposition of 57% to 73% synchronous with declines in oxidized N emissions during 1995-2019. Reduced N, however, shows contrasting patterns; NH3 emissions declined by 21% during 1995-2019, wet NH4+ deposition showed no trend, and dry NH4+ deposition declined by 73%. Dry NH4+ deposition is controlled in part by formation of NO3- and SO42- aerosols resulting in some disconnection with NH3 emissions. A century-long atmospheric deposition N time series was developed for 1950-2050 using measurement-modeling fusion and projections based on clean air regulations. This time series suggests a peak of 15 kg N/ha/yr in 1979, a slight decline by the mid-1990s, sharp decreases of 40% by 2019, and further decreases of 21% by 2050. These deposition estimates were compared with century-long watershed estimates of N loads from manure, fertilizer, and wastewater. Atmospheric N deposition was estimated as 30% of total N load to the watershed in 1950, a peak of 40% by 1973, declining to 28% by 2015, and decreasing to 24% by 2050. These data indicate the importance of atmospheric N deposition in the Chesapeake Bay watershed and highlight that Clean Air Act implementation has contributed to reducing nutrient loads and improving estuarine trophic status.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA074...08B
- Keywords:
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- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 0402 Agricultural systems;
- BIOGEOSCIENCES;
- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0426 Biosphere/atmosphere interactions;
- BIOGEOSCIENCES