A phenomenon of frequent noontime SO concentration peaks wasdiscovered in a detailed analysis of the SO concentrations in theNorth China Plain (NCP). The possible causes and their contributions areanalyzed. The impacts of such a phenomenon on the sulphur cycle were studiedand the implications of the phenomenon for atmospheric chemistry, cloudphysics, and climate were discussed. Different from the more common SOdiurnal patterns with high nighttime concentrations, NCP witnessed highfrequencies of noontime SO peaks, with an occurrence frequency of 50to 72% at four stations. Down mixing of elevated pollution layers, plumetransport processes, mountain-valley winds, and fog/high RH haze events werethe possible causes. The contribution of each process varies from day to dayand from station to station, however, none of those four processes can beneglected. SO peaks occurring during noontime instead of nighttimewill 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 phaseconversions, which will increase the hygroscopicity and the light scatteringof aerosols, thus having important impacts on atmospheric chemistry, cloudphysics, and climate.
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