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 (NOx= NO + NO2), aerosols, and aerosol precursors provide a non-negligible contribution to the climate impact of air traffic, and the uncertainty in their climate Effective Radiative Forcing (ERF) 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 NOx 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 NOx , ozone, and hydroxyl radical (OH) responses. The NOx net ERF is systematically positive with a model mean of 18.3 mW m−2, ranging from 9.4 to 24.5 mW m−2 among the different models. This net NOx forcing is reduced by 35 % and 43 % accounting for the negative forcing arising from the formation of nitrate and sulfate particles, respectively. Estimates of the aerosol direct ERF are systematically negative and range between -6.5 and -17.8 mW m−2, compensating most of the net NOx ERF albeit with noticeable intermodel differences arising from the diversity in aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NOx -induced ozone response because of a good model agreement. To a lesser extent, some similarities in the results regarding aerosols are also encouraging, given the few existing model intercomparisons on this topic. However, the 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.