Huge amounts of nitrogen are deposited on the watershed as the result of air pollution. Much of it is taken up by plants, but a sizable amount still runs off the land and enters the Bay. Estimates vary, but somewhere between a quarter and a third of all the nitrogen entering the Bay stems from air pollution.

It arrives in two main forms. About two-thirds is the result of nitrogen oxide emissions from fossil fuel combustion, including such sources as cars, power plants, industries, trucks, boats, tractors, lawn mowers and almost anything that burns oil or coal.

The remaining third results from ammonia emissions. These overwhelmingly come from agriculture, primarily animal farm operations. But significant amounts also originate from car exhaust, refrigerants and some industries.

Comprehensive information about emissions, sources and the ultimate fate of airborne nitrogen is not available. But the best estimates suggest that, overall, airborne contributions of nitrogen to the Bay have changed little, if at all, over the past two decades.

Power plant emissions have begun to decline, but overall emissions from other sources have remained stable. Ammonia emissions seem to be increasing. One note of concern is that while decreases have been seen in northern parts of the watershed—the area farthest from the Bay—ammonia and NOx emissions seem to be increasing in areas closest to the Chesapeake, making it more likely they will reach the Bay.

Regulations imposed over the past half-dozen years should gradually result in reduced NOx emissions over the coming decades. The 2010 nitrogen reduction goals anticipate an 8 million pound per year reduction stemming from the implementation of proposed “Clear Skies” regulations.