Seasonal Chemical and Dynamical Responses in a Chemistry-Climate model to Aircraft NOx Emissions: Simulations with the GEOS CCM and Comparisons with GMI

We have run multi-year ensembles of one-year simulations with GEOS CCM to study both annual average and seasonal effects of NOx emissions by aviation. GEOS CCM is an atmospheric GCM with interactive stratospheric and tropospheric chemistry. Hourly aircraft NOx emissions over the globe for the year 2006 were provided by courtesy of the US Department of Transportation. All simulations were forced by historical SSTs and sea ice with constant greenhouse gases for 2005, and climatological lightning NOx, aerosols and dust were prescribed in monthly fields. On average, the perturbation response in NOx is positive and predominantly in the upper troposphere and lower stratosphere of the Northern Hemisphere where the bulk of the emissions lie, reaching values over 50 pptv at ~10 km on an annual average basis. The concomitant ozone response extends over a deeper layer in the troposphere with statistically significant mid-tropospheric increases of more than 5% and 8% in mid-latitudes and at the northern polar regions. Seasonal differences in the responses are striking, with the largest perturbations of NOx during January at the tropopause near the core of the emissions at 40°N, while in July perturbations over 50 pptv spread throughout the Arctic. These differences are reflected in ozone, which shows perturbations of 50 ppbv and greater in July at the summer polar tropopause while in January the tropospheric response throughout the middle and polar latitudes is limited to ~10 ppbv. We compare these seasonal variations in response to aviation NOx emissions in GEOS CCM with those from GMI, a chemical transport model that shares the COMBO chemistry scheme with GEOS CCM . Finally, we examine the radiative impacts of these seasonal perturbations in ozone and methane.