Export of reactive nitrogen from North America during summertime: sensitivity to hydrocarbon chemistry

Abstract

The global influence on tropospheric chemistry of nitrogen oxides (NOx=NO+NO2) emitted by fossil fuel combustion may be strongly modulated by nonmethane hydrocarbon (NMHC) chemistry taking place in the continental boundary layer. This effect is investigated using a three-dimensional, continental-scale model of tropospheric O3-NOx-NMHC chemistry over North America in summer. The model includes detailed representation of NMHC chemistry including in particular isoprene. Model results are evaluated with observations for ozone, reactive nitrogen species, and photochemical tracers at a number of sites in North America. Sensitivity analyses are conducted to explore the effects of assumptions regarding the aerosol chemistry of peroxy radicals and the fate of hydroxy organic nitrates produced from the oxidation of isoprene. A budget analysis for the United States in the model indicates that 9% of NOX emitted from fossil fuel combustion is exported out of the continental boundary layer as NOx, 3.5% is exported as peroxyacetyl nitrate (PAN), and 3.7% is exported as other organic nitrates. Isoprene is the principal NMHC responsible for the formation and export of organic nitrates, which eventually decompose to provide a source of NOx in the remote troposphere. Export of NOx and organic nitrates for the US boundary layer is found to be a major source of NOx on the scale of the northern hemisphere troposphere and on the scale of the upper troposphere at northern midlatitudes. Proper representation of isoprene chemistry in the continental boundary layer is important for simulation of NOx in global tropospheric chemistry models.