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 O 3 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 U.S. boundary layer to the global atmosphere. Model results are evaluated with year‐round observations for O 3, CO, and NOy species at nonurban sites. A seasonal transition from NOx to hydrocarbon‐limited conditions for O 3 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 U.S. 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 HNO 3, is estimated to account for 30% of the chemical sink of NOx in the U.S. boundary layer in summer. Model results indicate that peroxyacylnitrates (PANs) are most abundant in the U.S. 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 U.S. 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 U.S. 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 O 3 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 O 3 pollution from the U.S. boundary layer. Fossil fuel combustion in the United States is estimated to account for about 10% of the total source of O 3 in the northern hemisphere troposphere on an annual basis.