In the atmosphere, hydrogen atom transfer (HAT) reactions play a significant role in many processes. The radical reaction (R-H+OH→R+H2O), as the most traditional HAT reaction, can be widely found in the atmosphere (Alvarez-Idaboy et al., 2001; Cameron et al., 2002; Steckler et al., 1997; Atkinson et al., 2006). The HAT process can also be found in some addition (Steudel, 1995; Williams et al., 1983; Courmier et al., 2005; Zhang and Zhang, 2002), decomposition (Rayez et al., 2002; Kumar and Francisco, 2015; Gutbrod et al., 1996), isomerization (Zheng and Truhlar, 2010; Atkinson, 2007) and abstraction reactions (Ji et al., 2013, 2017). These atmospheric HAT reactions display a main feature that the two-point hydrogen bond can occur and thus facilitates HAT (Kumar et al., 2016). Water molecules, acids and other catalysts, acting as hydrogen donors and acceptors, can contribute to the formation of two-point hydrogen bond (Vöhringer-Martinez et al., 2007; da Silva, 2010; Gonzalez et al., 2010; Bandyopadhyay et al., 2017). Thus, the effect of catalysts on promoting atmospheric HAT reactions has attracted more attention from atmospheric scientists.
The hydration of SO3 to form sulfuric acid (H2SO4) is a typical addition reaction involving the HAT. In the atmosphere, this hydration reaction is regarded as the main source of gas-phase sulfuric acid. For the reaction SO3 + H2O → H2SO4, the pre-reactive SO3… H2O complex is firstly formed, and the complex is then rearranged to produce H2SO4 (Holland and Castleman, 1978; Hofmann-Sievert and Castleman, 1984). But subsequent research found that this hydration reaction involving a single water molecule cannot take place in the atmosphere due to its high energy barrier (Hofmann and Schleyer, 1994; Morokuma and Muguruma, 1994; Steudel, 1995). The inclusion of a second water molecule in the above reaction has been proven to significantly reduce the hydration energy barrier (Morokuma and Muguruma, 1994; Loerting and Liedl, 2000; Larson et al., 2000). The promoting effect can be mainly attributed to the formation of the two-point hydrogen bond, which reduces the ring strain occurring in the pre-reactive complex, and facilitates the rearrangement of the pre-reactive complex via double HAT. It has also been shown that some other atmospheric molecules can serve as a catalyst to promote the hydration of SO3. To the best of our knowledge, the hydroperoxy radical (Gonzalez et al., 2010), formic acid (Hazra and Sinha, 2011; Long et al., 2012), sulfuric acid (Torrent-Sucarrat et al., 2012), nitric acid (Long et al., 2013) and ammonia (Bandyopadhyay et al., 2017) have been reported to replace the second water molecule to catalyze the hydration reaction of SO3.
Oxalic acid (OA), the most prevalent dicarboxylic acid in the atmosphere (Ho et al., 2015; Kawamura and Ikushima, 1993), is a water-soluble organic acid, so it has a high concentration in aerosols (Kawamura et al., 2013; van Pinxteren et al., 2014; Deshmukh et al., 2016; Wang et al., 2016). In addition to its accumulation in aerosols, OA, as an organic acid in the gas phase, has been found to enhance new particle formation (NPF; Xu et al., 2010, 2017; Weber et al., 2012, 2014; Xu and Zhang, 2012; Peng et al., 2015; Miao et al., 2015; Zhao et al., 2016; Chen et al., 2017; Arquero et al., 2017; Zhang, 2010). Theoretical studies about the effect of OA on atmospheric particle nucleation and growth have shown that it can generate stable complexes with water (Weber et al., 2012), sulfuric acid (Xu et al., 2010; Xu and Zhang, 2012; Miao et al., 2015; Zhao et al., 2016), ammonia (Weber et al., 2014; Peng et al., 2015) and amines (Chen et al., 2017; Xu et al., 2017; Arquero et al., 2017) via an intermolecular hydrogen bond. For OA, its potential to promote NPF is mainly attributed to its capability of forming hydrogen bonds with hydroxyl and/or carbonyl-type functional groups. Conversely to monocarboxylic acids, dicarboxylic acids such as OA have been proved to enhance nucleation in two directions because of its two acid moieties (Xu and Zhang, 2012). Thus, it can be believed that OA is a good candidate for catalyzing the HAT reaction in the atmosphere.
In this paper, we report the hydration reaction of SO3 in the presence of OA, aiming to study the catalytic effect and importance of OA in the hydration of SO3. It is known that OA can exist in several conformational forms (Buemi, 2009), which can be identified through the nomenclature used by Nieminen et al. (1992). Thus, five stable conformers of OA were considered in this work. The rate constants of OA-catalyzed SO3 hydration were calculated using kinetic analysis, and compared with that of the water-catalyzed hydration reaction. Finally, combining concentrations of reactants with the rate constants, we evaluated the importance of the hydration process involving the OA relative to the hydration of SO3 with the second water molecule as a catalyst to form sulfuric acid.