Impact of aircraft NOx and aerosol emissions on atmospheric composition: a model intercomparison, and a multimodel assessment using the airborne IAGOS data
Authors: Cohen Yann , Hauglustaine Didier , Bellouin Nicolas , Eastham Sebastian , Lund Marianne Tronstad , Matthes Sigrun , Righi Mattia , Skowron Agnieszka , Thor Robin 
Published: March 2024

Aircraft emissions consist of carbon dioxide (CO2), nitrogen oxides (NOx), aerosols (black carbon and sulfate) and water vapour. The non-CO2 effects have been recently evaluated as twice the CO2 effects regarding their radiative forcing of climate in 2018 [1]. Among the non-CO2 effects, nitrogen oxides emissions impact several greenhouse gases concentrations. Through tropospheric ozone production and subsequent increased OH concentrations, it enhances the methane chemical destruction, thus decreasing the stratospheric water vapour content and the methane-linked background ozone levels in the troposphere. The net radiative forcing caused by the aircraft NOx emissions is evaluated as a net positive forcing but still shows important uncertainties.In order to investigate representation of key mechanisms involved for climate forcing, in the framework of the ACACIA (Advancing the Science for Aviation and Climate) EU project, six global chemistry-climate models have been used to reevaluate the climate effects of NOx and aerosol aircraft emissions on atmospheric composition following a common protocol. As a first step, the standard runs have been assessed regarding ozone, carbon monoxide (CO), water vapour and reactive nitrogen (NOy) against the IAGOS measurements during 1994- 2018, separately in the upper troposphere and in the lower stratosphere.As a second step, the models have been used to assess the impact of NOx and aerosol emissions on atmospheric composition. The subsonic aircraft perturbations are calculated based on the CEDS aircraft emission inventories [2] for the present-day conditions and based on different socioeconomic scenarios [3] for future (2050) conditions. Several sensitivity simulations will be presented in order to investigate the sensitivity of the results to background atmospheric conditions (present, future) and to lightning emissions. Changes in atmospheric composition will be presented and compared for the different models and scenarios.&#160;Acknowledgement:This study was supported by the European Union&#8217;s Horizon 2020 research and innovation programme under grant agreement No 875036 within the Aeronautics project ACACIA, and by the French Minist&#232;re de la Transition &#233;cologique et Solidaire (grant no. DGAC 382 N2021-39), with support from France&#8217;s Plan National de Relance et de R&#233;silience (PNRR) and the European Union&#8217;s NextGenerationEU.&#160;&#160;References:[1] S. Lee, D.W. Fahey, A. Skowron, M.R. Allen, U. Burkhardt, Q. Chen, S.J. Doherty, S. Freeman, P.M. Forster, J. Fuglestvedt, A. Gettelman, R.R. De Le&#243;n, L.L. Lim, M. T. Lund, R.J. Millar, B. Owen, J.E. Penner, G. Pitari, M.J. Prather, R. Sausen, and L. J. Wilcox, Atmospheric Environment 244, 117834 (2021)
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