Relative humidity and O<sub>3</sub> concentration as two prerequisites for sulfate formation

Abstract

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 relative humidity (RH) threshold of ∼45 % or an O3 concentration threshold of ∼35 ppb, allowing us to first introduce the idea that RH and O3 concentrations are two prerequisites for rapid sulfate formation via multiphase reactions. In the case of the RH threshold, this is consistent with current understanding of the multiphase formation of sulfate, since it relates to the semisolid-to-liquid phase transition of atmospheric aerosols. Correlation analysis between SOR and aerosol water content (AWC) further backed this up. In the case of the O3 concentration threshold, this is consistent with the consumption of liquid oxidants in multiphase sulfate formation. The thresholds introduced here lead us to a better understanding of the sulfate formation mechanisms and sulfate formation variations. H2O2 might be the major oxidant of sulfate formation, since another liquid-phase oxidant, O3, 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 O3 (and also H2O2) concentration and RH. The SOR also displayed variations with pollution levels; i.e. the SOR increased with PM2.5 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 higher than its prerequisite value of around 45 % as pollution evolved. In addition, the self-catalytic nature of sulfate formation (i.e. the formation of hydrophilic sulfate aerosols under high RH conditions results in an increase in aerosol water content, which results in greater particle volume for further multiphase sulfate formation) also contributed to variations among the pollution scenarios.

Enhancement of secondary aerosol formation by reduced anthropogenic emissions during Spring Festival 2019 and enlightenment for regional PM<sub>2.5</sub> control in Beijing

“…This suggests that sulfate formation during the holiday was likely enhanced by nocturnal aqueous-phase chemical reactions between SO2 and O3. This is consistent with the study of Fang et al (2019), which found that ambient RH and the O3 concentration are two prerequisites for rapid sulfate formation via aqueous-phase oxidation reactions. This result highlights that controlling O3 formation under current emission conditions in winter in Beijing is key to further reducing the formation of sulfate and implies that the high underestimation of sulfate at night in models (Miao et al, 2020) could be caused by the inaccurate simulation of [O3].…”

The impact of stay-home policies during Coronavirus-19 pandemic on the chemical and toxicological characteristics of ambient PM2.5 in the metropolitan area of Milan, Italy

“…S2). Ambient temperature and relative humidity are not expected to induce significant variations in the rate of secondary aerosols formation from 2019 to 2020 based on the comparable temperature and relative humidity during the investigation periods of these consecutive years ( Fang et al, 2019 ; Pun et al, 2006 ). As shown in Fig.…”

Impacts of primary emissions and secondary aerosol formation on air pollution in an urban area of China during the COVID-19 lockdown

“…In contrast, sulfate/ΔCO did not increase with O x mixing ratio during either the normal or lockdown periods. One explanation for the lack of a relationship is that ~96% of hourly O 3 mixing ratios were < 35 ppb, which may have been too low for efficient sulfate formation via photochemical oxidation ( Fang et al, 2019 ).…”