ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-15-973-2015 Influence of aerosol chemical composition on N<sub>2</sub>O<sub>5</sub> uptake: airborne regional measurements in northwestern Europe Morgan W. T. 1 Ouyang B. 2 Allan J. D. https://orcid.org/0000-0001-6492-4876 1 3 Aruffo E. https://orcid.org/0000-0002-9164-7293 4 Di Carlo P. https://orcid.org/0000-0003-4971-4509 4 Kennedy O. J. 2 Lowe D. https://orcid.org/0000-0002-1248-3594 1 Flynn M. J. 1 Rosenberg P. D. https://orcid.org/0000-0002-6920-0559 5 Williams P. I. 1 3 Jones R. 2 McFiggans G. B. 1 Coe H. https://orcid.org/0000-0002-3264-1713 1 School of Earth, Atmospheric & Environmental Sciences, University of Manchester, Manchester, UK Department of Chemistry, University of Cambridge, Cambridge, UK National Centre for Atmospheric Science, University of Manchester, Manchester, UK CETEMPS – Dipartimento di Fisica, Universita di L'Aquila, L'Aquila, Italy School of Earth & Environment, University of Leeds, Leeds, UK 28 01 2015 15 2 973 990 Copyright: © 2015 W. T. Morgan et al. 2015 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/15/973/2015/acp-15-973-2015.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/15/973/2015/acp-15-973-2015.pdf

Aerosol chemical composition was found to influence nighttime atmospheric chemistry during a series of airborne measurements in northwestern Europe in summer conditions, which has implications for regional air quality and climate. The uptake of dinitrogen pentoxide, &gamma; (N<sub>2</sub>O<sub>5</sub>), to particle surfaces was found to be modulated by the amount of water content and ammonium nitrate present in the aerosol. The conditions prevalent in this study suggest that the net uptake rate of N<sub>2</sub>O<sub>5</sub> to atmospheric aerosols was relatively efficient compared to previous studies, with &gamma; (N<sub>2</sub>O<sub>5</sub>) values in the range 0.01–0.03. This is likely a consequence of the elevated relative humidity in the region, which promotes greater aerosol water content. Increased nitrate concentrations relative to particulate water were found to suppress N<sub>2</sub>O<sub>5</sub> uptake. The results presented here contrast with previous ambient studies of N<sub>2</sub>O<sub>5</sub> uptake, which have generally taken place in low-nitrate environments in the USA. Comparison of the N<sub>2</sub>O<sub>5</sub> uptake derived from the measurements with a parameterised scheme that is based on the ratio of particulate water to nitrate yielded reasonably good agreement in terms of the magnitude and variation in uptake, provided the effect of chloride was neglected. An additional suppression of the parameterised uptake is likely required to fully capture the variation in N<sub>2</sub>O<sub>5</sub> uptake, which could be achieved via the known suppression by organic aerosol. However, existing parameterisations representing the suppression by organic aerosol were unable to fully represent the variation in N<sub>2</sub>O<sub>5</sub> uptake. These results provide important ambient measurement constraint on our ability to predict N<sub>2</sub>O<sub>5</sub> uptake in regional and global aerosol models. N<sub>2</sub>O<sub>5</sub> uptake is a potentially important source of nitrate aerosol and a sink of the nitrate radical, which is the main nocturnal oxidant in the atmosphere. The results further highlight the importance of ammonium nitrate in northwestern Europe as a key component of atmospheric composition in the region.

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