Characteristics and major influencing factors of sulfate production via heterogeneous transition-metal-catalyzed oxidation during haze evolution in China
Introduction
Sulfate (SO42−) is one of the main constituents of atmospheric aerosols and has a significant impact on environmental chemistry and the global climate. The assessment from Haywood and Boucher (2000) showed that the direct radiative forcing of sulfate aerosols ranged from −0.26 to −0.82 W m−2, indicating a net cooling effect on the Earth's climate. Sulfate aerosols are mainly produced through the oxidation of SO2 in the atmosphere. Sources of SO2 include direct emissions of fossil fuel combustion, industrial production, volcanic eruption, or oxidation of reduced sulfur such as dimethyl sulfur (DMS) emitted from marine phytoplankton. As a major component of secondary inorganic aerosols (SIA), the SO42− content often increases rapidly during the evolution of haze in China. Taking the haze in January 2013 for example, mass percentages of SO42− in PM2.5 around Beijing increased from 10.3% to 13.4% on non-haze days to 25.1% on haze days (Quan et al., 2014; Zhang et al., 2014). Many other studies also showed that SO42− mass fractions of PM2.5 clearly increased during haze pollution periods (Cheng et al., 2016; Li et al., 2017; Yue et al., 2019). These observations indicate that the formation of SO42− is an important driver of the rapid growth of PM2.5.
The transformation processes from SO2 to SO42− include three major pathways: gas-phase reactions, liquid-phase reactions in clouds, and heterogeneous reactions on the surface and/or within the bulk pre-existing aerosols (Sofen et al., 2014). The gas phase reactions of SO2 are mainly carried out through OH⋅ radical oxidation (Stockwell and Calvert, 1983), and about 25% of the secondary SO42− is produced by this pathway globally (Faloona, 2009). During haze periods in China, the sharp decline of solar radiation can weaken photochemical activities, which may greatly reduce the importance of the SO2 + OH⋅ gas phase reactions (Zheng et al., 2015). The liquid phase reactions in clouds are considered the primary pathway of SO42− production at the global scale, accounting for about 60% of SO42− formation (Alexander et al., 2009, 2012bib_Alexander_et_al_2012bib_Alexander_et_al_2009). The reactions include the oxidation of S(IV) (total soluble phase SO2·H2O, HSO3,− and SO32− mass) through H2O2, O3, and O2 catalyzed through transition-metal ions (TMIs) in cloud droplets (Harris et al., 2013; Hoffmann and Calvert, 1985; McArdle and Hoffmann, 1983). Vogt et al. (1996) proposed hypochlorous acid (HOX = HOCl + HOBr) as a potentially important oxidant of S(IV) in the marine boundary layer. Chen et al. (2016) added this reaction to the global model GEOS-Chem and used SO42− oxygen isotopes observed in the marine boundary layer as constraints, and found that 33%–50% of the SO42− in the marine boundary layer could be formed through this pathway. However, estimates of the contribution from this pathway are limited by the uncertainty of the HOX concentration, its corresponding reaction rate, and the Henry's Law constant. Different from liquid phase reactions in clouds, the contribution of heterogeneous SO42− production is usually thought to be minor because of the low aerosol liquid water content (ALWC) compared with clouds (Jacob, 2000). However, recent studies suggested that the traditional gas- and aqueous-phase pathways in cloud droplets were insufficient to explain the rapid SO42− production observed during haze periods (Cheng et al., 2016; Huang et al., 2014; Shao et al., 2019; Wang et al., 2014). This underestimation may be due to the fact that some SO42− formation mechanisms such as heterogeneous oxidation of SO2 by NO2 (Cheng et al., 2016; Gao et al., 2016; Zhang et al., 2016a) and TMI-catalyzed oxidation by O2 (Li et al., 2017; Shao et al., 2019) are overlooked.
In previous studies, the TMI-catalyzed pathway was mainly considered within the cloud environment because of its high liquid water content. Previous observations of oxygen and sulfur isotopes indicated that such liquid phase reactions in clouds are more important in continental regions with strong anthropogenic and/or natural transition-metal emission sources (Alexander et al., 2009; Harris et al., 2013). Observations and corresponding simulations of SO42− oxygen isotopes suggested that in high-latitude regions of the northern hemisphere, where OH⋅ radicals and H2O2 concentrations in winter are sufficiently low, the TMI-catalyzed pathway in clouds is the main SO42− formation pathway (Alexander et al., 2009). Observations on Mountain Tai showed that the TMI catalyzation was the fastest SO42− generation pathway during 12% of the sampling period (Shen et al., 2012). However, this pathway has gained little attention in studies of heterogeneous reactions that occur on aerosol surfaces (e.g., in aerosol liquid water). By incorporating different heterogeneous SO42− formation mechanisms into the GEOS-Chem model and combining with measurements of oxygen isotopes (Δ17O (SO42−)) (He et al., 2018), Shao et al. (2019) found that the TMI-catalyzed pathway was dominant among heterogeneous SO42− production in both heavily-polluted periods (69%) and clean periods (67%), suggesting a considerable contribution of this pathway to atmospheric SO42− formation.
A model sensitivity analysis by Cheng et al. (2016) showed that SO42− production rates of the TMI-catalyzed pathway are highly sensitive to aerosol pH in heterogeneous reactions, with substantially higher production rates under lower pH conditions. However, their analysis ignored the effect of high ionic strength (IS) in aerosol water, which would vary considerably during haze evolution and cause non-trivial inhibition to this pathway (Zhang et al., 2015). In addition, soluble Fe(III) and Mn(II) concentrations may vary significantly in different haze stages because of the change in air masses, which may carry sufficient crustal particles from northwest regions to Beijing (Yue et al., 2019). On account of limited research on the SO42− heterogeneous TMI-catalyzed pathway and its importance on SO42− production, we explored the characteristics of this pathway during haze evolution and considered the main influencing factors of this pathway in different haze stages. The results of this study can be used to better understand SO42− formation during haze periods.
Section snippets
Sample collection, auxiliary data, ion measurement, and pH calculation
Detailed information on sample collection, ion measurements, corresponding auxiliary data (e.g., hourly meteorological, PM2.5 and SO2 concentration data), and aerosol pH calculations can be found in Yue et al. (2019). Briefly, our sampling site is located on the rooftop of the First Teaching Building in the Yanqi campus of the University of Chinese Academy of Sciences (40.41°N, 116.68°E, elevation 20 m above ground level), approximately 60 km northeast of central Beijing. The sampling
General characteristics of PM2.5 and SO42− concentrations
Several haze events occurred during our sampling period, during which PM2.5 reached 361.06 μg⋅m3 (Fig. 1a), accompanied with inversely co-varied visibility. The accumulation of atmospheric pollutants under stable synoptic meteorological conditions is conducive to the formation of severe air pollution in fall and winter over a large area of northeastern China (Zhang et al., 2016b; Zheng et al., 2015). The concentration of SO42− ranged from 1.55 to 56.43 μg⋅m−3, with a mean of 15.62 ± 14.20 μg⋅m−3
Conclusions and implications
In this study, we examined the characteristics and major influencing factors of the SO42− heterogeneous TMI-catalyzed pathway during haze evolution. There was no clear temporal pattern in the production rates of the TMI-catalyzed pathway in different haze stages. Although aerosol acidity increased during the haze aggravation stage, corresponding production rates did not rise as expected, suggesting that this pathway does not solely depend on aerosol pH in actual haze events. Further
CRediT authorship contribution statement
Fange Yue: Conceptualization, Methodology, Writing - original draft. Pengzhen He: Methodology, Writing - review & editing. Xiyuan Chi: Investigation. Longquan Wang: Investigation. Xiawei Yu: Visualization. Pengfei Zhang: Writing - review & editing. Zhouqing Xie: Conceptualization, Supervision, Project administration.
Declaration of competing interest
The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Project of Ministry of Science and Technology of China (MOST) (2016YFC0203302), National Natural Science of China (NSFC) (91544103) and the Key Project of Chinese Academy of Sciences (CAS) (KJZD-EW-TZ-G06-01).
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