Nitrogen deposition onto the United States and western Europe: Synthesis of observations and models

The documented acceleration of NH3 and NOx (NO + NO2) emissions over the last 150 years has accelerated N deposition, compromising air and water quality and altering the functioning of terrestrial and aquatic ecosystems worldwide. To construct continental-scale N budgets, we produced maps of N deposition fluxes from site-network observations for the United States and Western Europe. Increases in the rates of N cycling for these two regions of the world are large, and they have undergone profound modification of biospheric-atmospheric N exchanges, and ecosystem function. The maps are necessarily restricted to the network measured quantities and consist of statistically interpolated fields of aqueous NO3- and NH4+, gaseous HNO3 and NO2 (in Europe), and particulate NO3- and NH4+. There remain a number of gaps in the budgets, including organic N and NH3 deposition. The interpolated spatially continuous fields allow estimation of regionally integrated budget terms. Dry-deposition fluxes were the most problematic because of low station density and uncertainties associated with exchange mechanisms. We estimated dry N deposition fluxes by multiplying interpolated surface-air concentrations for each chemical species by model-calculated, spatially explicit deposition velocities. Deposition of the oxidized N species, by-products of fossil-fuel combustion, dominate. the U.S. N deposition budget with 2.5 Tg of NOy-N out of a total of 3.7-4.5 Tg of N deposited annually onto the conterminous United States. Deposition of the reduced species, which are by-products of farming and animal husbandry, dominate the Western European N-deposition budget with a total of 4.3-6.3 Tg N deposited each year out of a total of 8.4-10.8 Tg N. Western Europe receives five times more N in precipitation than does the conterminous United States. Estimated N emissions exceed measured deposition in the United States by 5.3-7.81 Tg N, suggesting significant N export or under-sampling of urban influence. In Europe, estimated emissions better balance measured deposition, with an imbalance of between -0.63 and 2.88 Tg N, suggesting that much of the N emitted in Europe is deposited there, with possible N import from the United States. The sampling network in Europe includes urban influences because of the greater population density of Western Europe. Our analysis of N deposition for both regions was limited by sampling density. The framework we present for quantification of patterns of N deposition provides a constraint on our understanding of continental biospheric-atmospheric N cycles. These spatially explicit wet and dry N fluxes also provide a tool for verifying regional and global models of atmospheric chemistry and transport, and they represent critical inputs into terrestrial models of biogeochemistry.