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- Title
RH and O<sub>3</sub> concentration as two prerequisites for sulfate formation.
- Authors
Yanhua Fang; Chunxiang Ye; Junxia Wang; Yusheng Wu; Min Hu; Weili Lin; Fanfan Xu; Tong Zhu
- 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 ~35ppb, 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.
- Subjects
BEIJING (China); CHINA; TRANSITION metal ions; SULFATES; PARTICULATE matter
- Publication
Atmospheric Chemistry & Physics Discussions, 2019, p1
- ISSN
1680-7367
- Publication type
Article
- DOI
10.5194/acp-2019-284