ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-7757-2014 SO<sub>2</sub> noontime-peak phenomenon in the North China Plain Xu W. Y. 1 3 Zhao C. S. 1 Ran L. 2 Lin W. L. https://orcid.org/0000-0002-0711-6378 3 4 Yan P. 3 4 Xu X. B. https://orcid.org/0000-0003-4321-9267 3 Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China Key Laboratory of Middle Atmosphere and Global Environment Observation, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Key Laboratory for Atmospheric Chemistry, Institute of Atmospheric Composition, Chinese Academy of Meteorological Sciences, Beijing, China Meteorological Observation Center, China Meteorological Administration, Beijing, China 04 08 2014 14 15 7757 7768 Copyright: © 2014 W. Y. Xu et al. 2014 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/14/7757/2014/acp-14-7757-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/7757/2014/acp-14-7757-2014.pdf

A phenomenon of frequent noontime SO<sub>2</sub> concentration peaks was discovered in a detailed analysis of the SO<sub>2</sub> concentrations in the North China Plain (NCP). The possible causes and their contributions are analyzed. The impacts of such a phenomenon on the sulphur cycle were studied and the implications of the phenomenon for atmospheric chemistry, cloud physics, and climate were discussed. Different from the more common SO<sub>2</sub> diurnal patterns with high nighttime concentrations, NCP witnessed high frequencies of noontime SO<sub>2</sub> peaks, with an occurrence frequency of 50 to 72% at four stations. Down mixing of elevated pollution layers, plume transport processes, mountain-valley winds, and fog/high RH haze events were the possible causes. The contribution of each process varies from day to day and from station to station, however, none of those four processes can be neglected. SO<sub>2</sub> peaks occurring during noontime instead of nighttime will lead to a 13 to 35% increase in sulphur dry deposition, a 9 to 23% increase in gas phase oxidation, and an 8 to 33% increase in aqueous phase conversions, which will increase the hygroscopicity and the light scattering of aerosols, thus having important impacts on atmospheric chemistry, cloud physics, and climate.

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