Decadal change of summertime ozone and organic nitrates over the Southeastern United States

We use a global chemistry-climate model, combined with extensive aircraft and ground observations, to examine the changes of summertime ozone and organic nitrates that have taken place over the Southeast U.S. as a result of the significant NOx emission reductions during the past decade. We first evaluate our model simulations with aircraft measurements from ICARTT (July-August, 2004), SENEX (June-July, 2013) and SEAC4RS (August-September, 2013). Our model can well reproduce the mean vertical profiles of major reactive nitrogen (NOy) species (NOx, HNO3, PAN, and åANs) and total NOy in both 2004 and 2013, as well as speciated alkyl nitrates in 2013. We find that the large discrepancy between åANs and the measured speciated alkyl nitrates can be explained by a combination of terpene nitrates, dinitrates and nighttime NO3 oxidation products. Comparisons of simulated surface ozone with observations from EPA AQS suggest that our model can capture the decadal change of surface ozone from 2004 to 2013 (-1.1 ppbv yr-1), despite a positive bias of 8-11 ppbv in the boundary layer. Nitrogen species decline proportionally with NOx emission reductions in the model and observations. We find that surface ozone will continue to decline in response to projected further reductions in NOx emissions, leading to improved air quality and, in particular, fewer extreme ozone events.