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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Apr 2019

Research article | 15 Apr 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

RH and O3 concentration as two prerequisites for sulfate formation

Yanhua Fang1,*, Chunxiang Ye1,*, Junxia Wang1, Yusheng Wu1, Min Hu1, Weili Lin2, Fanfan Xu1, and Tong Zhu1 Yanhua Fang et al.
  • 1BIC-ESAT and SKL-ESPC, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
  • 2College of Life and Environmental Sciences, Minzu University of China, Beijing, 100081, China
  • *These authors contributed equally to the paper.

Abstract. Sulfate formation mechanisms have been discussed extensively but are still disputed. In this work, a year-long particulate matter (PM2.5) sampling campaign was conducted together with measurements of gaseous pollutant concentrations and meteorological parameters in Beijing, China, from March 2012 to February 2013. The sulfur oxidation ratio (SOR), an indicator of secondary sulfate formation, displayed a clear summer peak and winter valley, even though no obvious seasonal variations in sulfate mass concentration were observed. A rapid rise in the SOR was found at a RH threshold of ~ 45 % or an O3 concentration threshold of ~ 35 ppb, suggesting that RH and O3 concentrations were two prerequisites for rapid sulfate formation, which likely occurred via multiphase reactions. H2O2 oxidation was proposed to be the major route of sulfate formation, since the O3 oxidation route has previously been shown to be unimportant. The seasonal variations in sulfate formation could be accounted for by variations in the RH and O3 prerequisites. For example, over the year-long study, the fastest SO2-to-sulfate conversion occurred in summer, which was associated with the highest values of both O3 concentration and RH. The SOR also displayed variations with pollution levels, i.e., the SOR increased as pollution evolved in all seasons. Such variations were primarily associated with a transition from the slow gas phase formation of sulfate to rapid multiphase reactions, since RH increased as pollution evolved. In addition, the self-catalytic nature of sulfate formation (i.e., increasing aerosol water content with simultaneous increases in sulfate mass concentrations and RH) also contributed to variations among the pollution scenarios. Overall, our observations validated the two prerequisites for fast sulfate formation and revealed the seasonal and pollution level variations in sulfate formation in Beijing. H2O2 oxidation was the major route of sulfate formation, although reactions involving transition metal ions (TMIs) and NO2 might have also competed.

Yanhua Fang et al.
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Yanhua Fang et al.
Yanhua Fang et al.
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Short summary
Year-long observations of PM2.5, gaseous pollutants, and meteorological parameters in Beijing were analysed to investigate sulfate formation. RH and O3 concentrations above thresholds of 45 % and 35 ppb, respectively, greatly accelerated sulfate formation. Ambient changes in RH and O3 contributed to variations in sulfate formation among different seasons and pollution levels. A shift from gas phase to multiphase SO2 oxidation contributed to fast sulfate formation under polluted conditions.
Year-long observations of PM2.5, gaseous pollutants, and meteorological parameters in Beijing...