Impact of present aircraft NO_<i>x</i> and aerosol emissions on atmospheric composition and climate: results from a model intercomparison

<p>Aircraft emissions of nitrogen oxides (NO<span class="inline-formula">_<i>x</i>=</span> NO <span class="inline-formula">+</span> NO<span class="inline-formula">₂</span>), 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 NO<span class="inline-formula">_<i>x</i></span> 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<span class="inline-formula">_<i>x</i></span>, ozone, and hydroxyl radical (OH) responses. The NO<span class="inline-formula">_<i>x</i></span> net ERF is systematically positive with a model mean of 18.3 mW m<span class="inline-formula">⁻²</span>, ranging from 9.4 to 24.5 mW m<span class="inline-formula">⁻²</span> among the different models. This net NO<span class="inline-formula">_<i>x</i></span> 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 <span class="inline-formula">−</span>6.5 and <span class="inline-formula">−</span>17.8 mW m<span class="inline-formula">⁻²</span>, compensating most of the net NO<span class="inline-formula">_<i>x</i></span> ERF albeit with noticeable intermodel differences arising from the diversity in aerosol parameterizations. This work shows encouraging results regarding our confidence in aviation NO<span class="inline-formula">_<i>x</i></span>-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.</p>