Fundamental understanding of reactive nitrogen in the global upper troposphere

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The upper troposphere (UT), a severely under-researched part of the atmosphere, has profound impacts on global climate, air quality, major atmospheric oxidants, and the protective ozone layer. Key to its influence on the Earth system are reactive nitrogen compounds (collectively NOy). Models, since their inception, have grossly misrepresented observations of UT NOy, hindering application of these models to accurately estimate the impact of humans on climate, the ozone layer, and air quality. The reasons proposed for discrepancies between models and observations are unsatisfactory, as past studies have been hampered by observations that are limited in space and time. Only now are there unprecedented global, high-resolution measurements of the UT from instruments on aircraft and satellites that can be combined with detailed and advanced modelling tools to at last tackle this issue on a global scale. The ground-breaking UpTrop work programme will innovatively combine observations from the recently launched ESA Sentinel-5P mission and a long record of aircraft campaigns (most crucially the 2016-2018 NASA ATom campaign) to create the first truly global dataset of UT NOy abundance, interpreted with the state-of-the-art GEOS-Chem model. This pioneering multiplatform approach, the bedrock of my previous highly cited work, will deliver game-changing objectives: (i) novel insights into the processes controlling UT NOy, (ii) an unequivocal account of the role of the upper troposphere in altering climate and the chemical composition of the atmosphere, and (iii) interpretation of the disruptive impact of improved understanding of UT NOy on widespread application of satellite observations to constrain global air quality. UpTrop is ambitious, with bold objectives that will conceptually change fundamental understanding of UT NOy and address a challenge that has plagued atmospheric chemists for decades. A cascade of new avenues of cross-disciplinary research is inevitable.