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.