5:15 PM - 7:15 PM
[AAS11-P04] Consideration of Sulfate Aerosol Formation Processes via Sulfite Oxidation Enhanced by Clay Minerals
Keywords:Aerosol, Sulfur Cycles, Atmospheric Chemistry
Anthropogenic sulfur dioxide (SO2) is oxidized in the atmosphere to sulfuric acid (H2SO4), forming sulfuric acid aerosols. These aerosols exhibit high hygroscopicity and act as cloud condensation nuclei (CCN), increasing albedo and lowering surface temperatures. The oxidation process occurs through both gas- and aqueous-phase reactions. In the gas phase, SO2(g) is primarily oxidized by hydroxyl radicals, producing sulfuric acid gas (H2SO4(g)). In the aqueous phase, SO2(g) dissolves into liquid droplets and is oxidized by oxidants to form sulfate ions (SO42-). The SO42- produced through aqueous-phase reactions can be released into the atmosphere via droplet evaporation and added to CCN particle, leading to particle growth (Ishino, 2024).
Oxidants involved in aqueous-phase reactions have been considered to include ozone, hydrogen peroxide, hydroxyl radicals, nitrogen dioxide, and dissolved oxygen catalyzed by transition metal ions (Fe3+, Mn2+) (Seinfeld and Pandis, 2006). However, few studies have focused on oxidation facilitated by solid-phase substances present in droplets.
This study investigates the probability for sulfite oxidation promoted by clay minerals as solid-phase catalysts. Clay minerals are abundant in dust originating from deserts and arid regions (Jeong, 2024). Previous research suggests that clay minerals can oxidize various coexisting ions (Gorski et al., 2013; Ilgen et al., 2017). Furthermore, aerosol samples from marine and inland environments have revealed the coexistence of clay minerals and sulfates, indicating that sulfite oxidation by clay minerals may actually occur in the atmosphere (Posfai et al., 1995; Jeong, 2024).
In this study, we focused on the smectite group minerals montmorillonite, nontronite, and saponite, as well as kaolinite from the kaolin mineral group. Montmorillonite and nontronite contain iron (2 wt% and 22 wt%, respectively), whereas synthetic saponite we used in this study does not. Smectites are 2:1 type clay minerals with large surface areas, while kaolinite is a 1:1 type clay mineral with a smaller surface area.
Oxidation enhancement was observed in both iron-containing and iron-free smectites but not in kaolinite, suggesting that iron in clay minerals is not directly involved in oxidation and that surface area plays a crucial role. Additionally, the oxidation rate of montmorillonite was measured under conditions simulating actual liquid droplets (with an added salt concentration of 0.1 M). A comparison with oxidation rates induced by ozone and hydrogen peroxide was conducted, leading to a discussion on the potential enhancement of sulfite oxidation by clay minerals in real atmospheric environments.
Oxidants involved in aqueous-phase reactions have been considered to include ozone, hydrogen peroxide, hydroxyl radicals, nitrogen dioxide, and dissolved oxygen catalyzed by transition metal ions (Fe3+, Mn2+) (Seinfeld and Pandis, 2006). However, few studies have focused on oxidation facilitated by solid-phase substances present in droplets.
This study investigates the probability for sulfite oxidation promoted by clay minerals as solid-phase catalysts. Clay minerals are abundant in dust originating from deserts and arid regions (Jeong, 2024). Previous research suggests that clay minerals can oxidize various coexisting ions (Gorski et al., 2013; Ilgen et al., 2017). Furthermore, aerosol samples from marine and inland environments have revealed the coexistence of clay minerals and sulfates, indicating that sulfite oxidation by clay minerals may actually occur in the atmosphere (Posfai et al., 1995; Jeong, 2024).
In this study, we focused on the smectite group minerals montmorillonite, nontronite, and saponite, as well as kaolinite from the kaolin mineral group. Montmorillonite and nontronite contain iron (2 wt% and 22 wt%, respectively), whereas synthetic saponite we used in this study does not. Smectites are 2:1 type clay minerals with large surface areas, while kaolinite is a 1:1 type clay mineral with a smaller surface area.
Oxidation enhancement was observed in both iron-containing and iron-free smectites but not in kaolinite, suggesting that iron in clay minerals is not directly involved in oxidation and that surface area plays a crucial role. Additionally, the oxidation rate of montmorillonite was measured under conditions simulating actual liquid droplets (with an added salt concentration of 0.1 M). A comparison with oxidation rates induced by ozone and hydrogen peroxide was conducted, leading to a discussion on the potential enhancement of sulfite oxidation by clay minerals in real atmospheric environments.