Abstract
Abstract. It was formerly demonstrated that O2SOO− forms at collisions rate in the gas phase as a result of SO2 reaction with O2-. Here, we present a theoretical investigation of the chemical fate of O2SOO− by reaction with O3 in the gas phase, based on ab initio calculations. Two main mechanisms were found for the title reaction, with fundamentally different products: (i) formation of a van der Waals complex followed by electron transfer and further decomposition to O2 + SO2 + O3- and (ii) formation of a molecular complex from O2 switching by O3, followed by SO2 oxidation to SO3- within the complex. Both reactions are exergonic, but separated by relatively low energy barriers. The products in the former mechanism would likely initiate other SO2 oxidations as shown in previous studies, whereas the latter mechanism closes a path wherein SO2 is oxidized to SO3-. The latter reaction is atmospherically relevant since it forms the SO3- ion, hereby closing the SO2 oxidation path initiated by O2-. The main atmospheric fate of SO3- is nothing but sulfate formation. Exploration of the reactions kinetics indicates that the path of reaction (ii) is highly facilitated by humidity. For this path, we found an overall rate constant of 4.0×10-11 cm3 molecule−1 s−1 at 298 K and 50 % relative humidity. The title reaction provides a new mechanism for sulfate formation from ion-induced SO2 oxidation in the gas phase and highlights the importance of including such a mechanism in modeling sulfate-based aerosol formation rates.
Highlights
The chemistry of sulfur is highly important in the atmosphere
Two main mechanisms were found for the title reaction, with fundamentally different products: (i) formation of a van der Waals complex followed by electron transfer and further decomposition to O2 + SO2 + O−3 and (ii) formation of a molecular complex from O2 switching by O3, followed by SO2 oxidation to SO−3 within the complex
Starting with optimized structures of O2SOO− · · · (H2O)0−1 and O3 shown in Fig. S1 in the Supplement, a series of geometry optimizations was performed on the O2SOO− · · · (H2O)0−1 + O3 system, taking into account different spatial orientations of the reactants at impact
Summary
Sulfur participates in the formation of secondary atmospheric aerosols, clouds, and acid rain. Sulfur dioxide (SO2), the most abundant sulfurcontaining molecule in the atmosphere, is known to react both in the gas phase and in multiphase oxidation processes following different mechanisms to form sulfate as the final oxidation species. The main SO2 oxidizers in the gas phase include the hydroxyl radical (OH) (Seinfeld and Pandis, 2016), stabilized Criegee intermediates (Welz et al, 2012; Mauldin III et al, 2012; Vereecken et al, 2012), and atmospheric ions (Fehsenfeld and Ferguson, 1974; Enghoff et al, 2012; Tsona et al, 2015). The role of ions in this formation has been well established (Yu, 2006; Yu and Turco, 2000, 2001; Enghoff and Svensmark, 2008; Kirkby et al, 2011; Wagner et al, 2017; Yan et al, 2018), relatively minor compared to the mechanism involving neutral particles exclusively (Eisele et al, 2006; Manninen et al, 2010; Kirkby et al, 2011; Hirsikko et al, 2011; Wagner et al, 2017)
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