Impact of present aircraft NO_x and aerosol emissions on atmospheric composition and climate: results from a model intercomparison

Abstract. Aircraft emissions of nitrogen oxides (NO_x=NO+NO₂), aerosols, and aerosol precursors provide a non-negligible contribution to the climate impact of air traffic, and the uncertainty on their Effective Radiative Forcing (ERF) of climate remains significant. This study presents results from a new model intercomparison of the impact of aircraft emissions involving five state-of-the-art global models including both tropospheric and stratospheric chemistry. Aircraft NO_x increases ozone photochemical production in the free troposphere throughout the year and decreases ozone chemical loss in the high-latitude lowermost stratosphere during spring–early summer. The models generally agree on the spatial pattern of NO_x, ozone, and hydroxyl radical (OH) responses. The NO_x net ERF is systematically positive and ranges from 7.3 to 22.1 mW m^-2 among the different models (14.1–22.1 mW m^-2 without the least sensitive model). Estimates of the aerosol direct ERF are systematically negative and range between -6.5 and -17.8 mW m^-2, with differences arising from the diversity in model aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NO_x-induced ozone response because of a better model agreement. However, results also highlight areas where further modeling experiments are needed, both with more models and with dedicated sensitivity simulations to further understand the factors giving rise to the spread in model estimates of aviation emission impacts on atmospheric composition and climate.