Seasonal budgets of reactive nitrogen species and ozone over the United States, and export fluxes to the global atmosphere

Abstract

A three-dimensional continental-scale photochemical model is used to investigate seasonal budgets of O3 and NOy species (including NOx and its oxidation products) in the boundary layer over the United States and to estimate the export of these species from the US boundary layer to the global atmosphere. Model results are evaluated with year-round observations for O3, CO and NOy species at nonurban sites. A seasonal transition from NOx to hydrocarbon-limited conditions for O3 production over the eastern United States is found to take place in the fall, with the reverse transition taking place in the spring. The mean NOx/NOy molar ratio in the US boundary layer in the model ranges from 0.2 in summer to 0.6 in winter, in accord with observations, and reflecting largely the seasonal variation in the chemical lifetime of NOx. Formation of hydroxy organic nitrates during oxidation of isoprene, followed by decomposition of these nitrates to HNO3, is estimated to account for 30% of the chemical sink of NOx in the US boundary layer in summer. Model results indicate that the peroxyacylnitrates (PANs) are most abundant in the US boundary layer in spring (25% of total NOy), reflecting a combination of active photochemistry and low temperatures. About 20% of the NOx emitted from fossil fuel combustion in the United States in the model is exported out of the US boundary layer as NOx or PANs (15% in summer, 25% in winter). This export responds less than proportionally to changes in NOx emissions in summer, but more than proportionally in winter. The annual mean export of NOx and PANs from the US boundary layer is estimated to be 1.4 Tg N yr-1, representing an important source of NOX on the scale of the northern hemisphere troposphere. The eventual O3 production in the global troposphere due to the exported NOx and PANs is estimated to be twice as large, on an annual basis, as the direct export of O3 pollution from the US boundary layer. Fossil fuel combustion in the United States is estimated to account for about 10% of the total source of O3 in the northern hemisphere troposphere on an annual basis.