That work also indicated that about 75 percent of the nitrogen oxides that land on the Bay and its watershed are emitted from outside the Bay's watershed. In the coming months, Bay Program officials are planning to further refine their information on air pollution and the Bay. Much of the nitrogen deposition on the watershed is taken up by plants or reduced by other biological processes and never reaches the Chesapeake. Current estimates place the amount of nitrogen deposition that actually reaches the Bay from deposition on the land at about 10 percent. Nonetheless, that would still mean that air pollution contributes about one-quarter of all the nitrogen entering the Bay.
Officials hope to refine that estimate in the next few months.
Also, because the pattern of deposition varies throughout the watershed, the impact on some tributaries may be greater than others. The highest deposition rates, about 12 pounds per acre per year, occur in the northernmost portions of the basin. The southernmost portions of the basin, by contrast, get about 7.5 pounds of nitrogen deposition per acre per year. In addition, some officials believe that current calculations underestimate atmospheric contributions to the Bay because they do not factor in air pollution that lands on coastal waters outside the Chesapeake's mouth. Some of that, in turn, is washed into the Bay as well, but no one knows how much.