Abstract
Abstract. A phenomenon of frequent noontime SO2 concentration peaks was discovered in a detailed analysis of the SO2 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 SO2 diurnal patterns with high nighttime concentrations, NCP witnessed high frequencies of noontime SO2 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. SO2 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.
Highlights
High emissions and concentrations of sulphur dioxide (SO2) have been observed in the North China Plain (NCP) (Lin et al, 2012; Xu et al, 2011b; Zhao et al, 2013), which exerts great impacts on aerosol hygroscopicity and cloud physics (Liu et al, 2011)
The special topography, emission distribution, and meteorological conditions in the NCP have complicated the variation of SO2 concentrations
The diurnal variation of primary gas pollutants in polluted regions is typically characterized by morning and evening peaks, due to both the diurnal variations of the planetary boundary layer height (PBLH), emissions, and the diurnal variation in photochemistry (Jacobson, 2002)
Summary
High emissions and concentrations of sulphur dioxide (SO2) have been observed in the North China Plain (NCP) (Lin et al, 2012; Xu et al, 2011b; Zhao et al, 2013), which exerts great impacts on aerosol hygroscopicity and cloud physics (Liu et al, 2011). Xu et al.: SO2 noontime-peak phenomenon in the North China Plain attributed to down-mixing from elevated sources While this phenomenon seems to be a rare case all over world, several sites in the NCP shows long-term averaged diurnal variation patterns with such variation characteristics (Lin et al, 2011, 2009, 2012, 2008). Long-term measurements at rural sites in the Yangtze River delta region displayed pre-noon SO2 peaks that were not in accordance with the diurnal variation pattern of other primary gas pollutants (Ding et al, 2013; Qi et al, 2012), observations during shorter cam‘paigns seem to disagree with these results (Wang et al, 2004). The representativeness of such averaged profiles and how common such events are is uncertain Their cause or the possible impacts such diurnal variation patterns might have is still not clear. The possible impact of such events on the sulphur cycle is investigated and its influence on atmospheric chemistry and climate are pointed out
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