Impact of present and future aircraft NOx and aerosol emissions on atmospheric composition and associated direct radiative forcing of climate

Aviation NOx emissions not only have an impact on global climate by changing ozone and methane levels but also contribute to the deterioration of local air quality. A new version of the LMDZ-INCA global model, including chemistry of both the troposphere and the stratosphere and the sulfate-nitrate-ammonium cycle, is applied to re-evaluate the impact of aircraft NOx and aerosol emissions on climate. The results confirm that the efficiency of NOx to produce ozone is very much dependent on the injection height; it increases with the background methane and NOx concentrations and with decreasing aircraft NOx emissions. The methane lifetime variation is less sensitive to the location of aircraft NOx emissions than the ozone change. The net NOx radiative forcing (RF) (O-3 + CH4) is largely affected by the revised CH4 RF formula. The ozone positive forcing and the methane negative forcing largely offset each other, resulting in a slightly positive forcing for the present day. However, in the future, the net forcing turns to negative, essentially due to higher methane background concentrations. Additional RFs involving particle formation arise from aircraft NOx emissions since the increased hydroxyl radical (OH) concentrations are responsible for an enhanced conversion of SO2 to sulfate particles. Aircraft NOx emissions also increase the formation of nitrate particles in the lower troposphere. However, in the upper troposphere, increased sulfate concentrations favour the titration of ammonia leading to lower ammonium nitrate concentrations. The climate forcing of aircraft NOx emissions is likely to be small or even switch to negative (cooling), depending on atmospheric NOx or CH4 future background concentrations, or when the NOx impact on sulfate and nitrate particles is considered. However, large uncertainties remain for the NOx net impact on climate and in particular on the indirect forcings associated with aerosols, which are even more uncertain than the other forcings from gaseous species. Hence, additional studies with a range of models are needed to provide a more consolidated view. Nevertheless, our results suggest that reducing aircraft NOx emissions is primarily beneficial for improving air quality.