Presentation Information
[AAS08-P08]Clay-Mineral-Promoted Oxidation of Sulfite in Aqueous Aerosol Systems
*Ou Langying1, Yoshio Takahashi1 (1.The University of Tokyo)
Keywords:
Sulfite Oxidation,Clay Minerals,Environmental Conditions
Aerosols are small solid and/or liquid particles suspended in the atmosphere. At present, aerosols are considered to be significant contributors to radiative forcing and have an overall cooling effect. However, limited studies, together with the wide variety of precursor species and formation processes, make aerosol effects difficult to estimate, resulting in large uncertainties (IPCC, 2021).
Sulfate aerosol is one of the most common aerosol species in the atmosphere and is formed from the oxidation of sulfur dioxide (SO2). The traditional pathways of sulfate formation are considered to be gas-phase photochemistry involving OH radicals, and cloud or haze chemistry involving oxidants such as H2O2, O3, or NOx (Cheng et al., 2016). In the gas phase, SO2 oxidation by OH proceeds through radical intermediates and does not involve stable sulfite species. In contrast, once SO2 is dissolved into aqueous or interfacial water, it forms S(IV) species such as HSO3- or SO32-, whose oxidation pathways are strongly influenced by environmental conditions such as pH and salinity.
While liquid-phase oxidation promoted by transition metal ions such as Fe3+, Cu2+, and Mn2+ has been extensively studied (Zhou et al., 2018), heterogeneous oxidation occurring on particle surfaces remains unclear. Ma et al. proposed a possible pathway for such processes, involving adsorption followed by reaction (Ma et al., 2023). Clay minerals, which are common solid particles in the atmosphere and are composed of layered structures, provide large surface areas and therefore have high potential to promote heterogeneous reactions.
In this study, we focus on iron-bearing smectite-group minerals, including nontronite and montmorillonite, which are both 2:1 layer-type clay minerals with large surface areas. One of the most important differences between them is their iron content: nontronite contains a relatively high iron concentration (~22%), whereas montmorillonite contains only ~2%, which may lead to different behaviors in sulfite oxidation.
Our current results showed that nontronite exhibits a stronger promoting effect on sulfite oxidation than montmorillonite, suggesting a potential role of structural Fe in clay minerals in sulfite oxidation. In addition, UV-Vis and XAFS measurements were conducted under pH = 3 and 5 conditions, with and without added salinity, in order to further elucidate the effects of environmental conditions on sulfate formation processes.
Sulfate aerosol is one of the most common aerosol species in the atmosphere and is formed from the oxidation of sulfur dioxide (SO2). The traditional pathways of sulfate formation are considered to be gas-phase photochemistry involving OH radicals, and cloud or haze chemistry involving oxidants such as H2O2, O3, or NOx (Cheng et al., 2016). In the gas phase, SO2 oxidation by OH proceeds through radical intermediates and does not involve stable sulfite species. In contrast, once SO2 is dissolved into aqueous or interfacial water, it forms S(IV) species such as HSO3- or SO32-, whose oxidation pathways are strongly influenced by environmental conditions such as pH and salinity.
While liquid-phase oxidation promoted by transition metal ions such as Fe3+, Cu2+, and Mn2+ has been extensively studied (Zhou et al., 2018), heterogeneous oxidation occurring on particle surfaces remains unclear. Ma et al. proposed a possible pathway for such processes, involving adsorption followed by reaction (Ma et al., 2023). Clay minerals, which are common solid particles in the atmosphere and are composed of layered structures, provide large surface areas and therefore have high potential to promote heterogeneous reactions.
In this study, we focus on iron-bearing smectite-group minerals, including nontronite and montmorillonite, which are both 2:1 layer-type clay minerals with large surface areas. One of the most important differences between them is their iron content: nontronite contains a relatively high iron concentration (~22%), whereas montmorillonite contains only ~2%, which may lead to different behaviors in sulfite oxidation.
Our current results showed that nontronite exhibits a stronger promoting effect on sulfite oxidation than montmorillonite, suggesting a potential role of structural Fe in clay minerals in sulfite oxidation. In addition, UV-Vis and XAFS measurements were conducted under pH = 3 and 5 conditions, with and without added salinity, in order to further elucidate the effects of environmental conditions on sulfate formation processes.