Sensitivity of the NOy budget over the United States to anthropogenic and lightning NOx in summer

We examine the implications of new estimates of the anthropogenic and lightning nitrogen oxide (NOx) source for the budget of oxidized nitrogen (NOy) over the United States in summer using a 3-D global chemical transport model (Model of Ozone and Related Tracers-4). As a result of the Environmental Protection Agency (EPA) State Implementation call, power plant NOx emissions over the eastern United States decreased significantly, as reflected by a 23% decrease in summer surface emissions from the 1999 U.S. EPA National Emissions Inventory to our 2004 inventory. We increase the model lightning NOx source over northern midlatitude continents (by a factor of 10) and the fraction emitted into the free troposphere (FT, from 80% to 98%) to better match the recent observation-based estimates. While these NOx source updates improve the simulation of NOx and O3 compared to the Intercontinental Chemical Transport Experiment-North America aircraft observations, a bias in the partitioning between nitric acid (HNO3) and peroxyacetylnitrate (PAN) remains especially above 8 km, suggesting gaps in the current understanding of upper tropospheric processes. We estimate a model NOy export efficiency of 4%−14% to the North Atlantic in the FT, within the range of previous plume-based estimates (3%−20%) and lower than the 30% exported directly from the continental boundary layer. Lightning NOx contributes 24%−43% of the FT NOy export from the U.S. to the North Atlantic and 28%−34% to the NOy wet deposition over the United States, with the ranges reflecting different assumptions. Increasing lightning NOx decreases the fractional contribution of PAN to total NOy export, increases the O3 production in the northern extratropical FT by 33%, and decreases the regional mean ozone production efficiency per unit NOx (OPE) by 30%. If models underestimate the lightning NOx source, they would overestimate the background OPE in the FT and the fractional contribution of PAN to NOy export. Therefore, a model underestimate of lightning NOx would likely lead to an overestimate of the downwind O3 production due to anthropogenic NOx export. Better constraints on the lightning NOx source are required to more confidently assess the impacts of anthropogenic emissions and their changes on air quality over downwind regions