Determination of reactions between Criegee intermediates and methanesulfonic acid at the air-water interface

Journal Pre-proof Determination of reactions between Criegee intermediates and methanesulfonic acid at the air-water interface

Xiaohui Ma, Xianwei Zhao, Zixiao Huang, Junjie Wang, Guochun Lv, Fei Xu, Qingzhu Zhang, Wenxing Wang PII:

S0048-9697(19)35799-7

DOI:

https://doi.org/10.1016/j.scitotenv.2019.135804

Reference:

STOTEN 135804

To appear in:

Science of the Total Environment

Received date:

21 October 2019

Revised date:

21 November 2019

Accepted date:

26 November 2019

Please cite this article as: X. Ma, X. Zhao, Z. Huang, et al., Determination of reactions between Criegee intermediates and methanesulfonic acid at the air-water interface, Science of the Total Environment (2019), https://doi.org/10.1016/j.scitotenv.2019.135804

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Determination of Reactions between Criegee Intermediates and Methanesulfonic Acid at the Air-Water Interface Xiaohui Ma, Xianwei Zhao, Zixiao Huang, Junjie Wang, Guochun Lv, Fei Xu, Qingzhu Zhang*, Wenxing Wang

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Environment Research Institute, Shandong University,

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Qingdao 266237, P. R. China

Keywords: Criegee Intermediates; Interface Reaction; BOMD; Addition Reaction; Hydration __________________________________________________________ *

Corresponding authors. E-mail: [email protected]

Fax: 86-531-8836 1990

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Abstract In recent years, Criegee chemistry has become an important research focus due to its relevance in regulating concentrations of tropospheric OH radicals, hydroperoxides, sulfates, nitrates, and aerosols. However, to date, its interface

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behavior remains poorly understood. Thus, in this study, we used the

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Born-Oppenheimer molecular dynamics (BOMD) simulation method to explore the

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reaction mechanisms between Criegee intermediates (CIs) and methylsulfonic acid

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(MSA) at the air-water interface, then compared the observed behaviors with those in the gas phase. The addition of Criegee intermediates to MSA is nearly a

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barrierless reaction and follows a loop-structure mechanism in the gas phase. The

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high rate constants indicate that the Criegee intermediates and MSA reactions are the

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main acid removal channels. At the water’s surface, the interaction of Criegee intermediates with MSA includes three main channels: 1) direct addition reaction, 2) H2O-mediated hydroperoxide formation, and 3) MSA-mediated Criegee hydration. These reaction channels follow a loop-structure or a stepwise mechanism and proceed at the picosecond time-scale. The results of this work broaden our understanding of Criegee atmospheric behaviors in polluted urban and marine areas, which in turn will aid in developing more effective pollution control measures.

1. Introduction The air-water interface is pervasive throughout Earth's atmosphere and 2

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plays a significant role in tropospheric chemistry (Gerber et al., 2015; Huang et al., 2018; Nowakowski et al., 2016; Rossignol et al., 2016; Tinel et al., 2016; Zhong et al., 2018), as water interfaces, such as oceans, lakes, cloud droplets, fogs, and aerosols, provide a good medium for most atmospheric reactions. The unique network structures formed by hydrogen bonds at the interface promote aggregation

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and collision of the adsorbed molecules. As such, the interfacial reaction is often more intense than the gas-phase reaction (Li et al., 2016; Zhu et al., 2016a). For

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instance, the OH radical formation rate from ozone photolysis in cloud droplets is

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104 times faster than that in the gas phase (Anglada et al., 2014). In addition, due to

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the induction of proton transfer pathways by interfacial water molecules, the reaction

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mechanism at the interface may differ from that in the gas phase (Kumar et al.,

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2018a; Zhong et al., 2019; Zhu et al., 2016b). However, heterogeneous reactions at the air-water interface are often overlooked in the current atmospheric models.

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Understanding the unique reaction mechanisms at the air-water interface is necessary for exploring the impact of cloud droplets and aqueous aerosols on climate and human health.

In recent years, a variety of technologies—such as second-harmonic generation (SHG) spectroscopy (Nowakowski et al., 2016; Sagar et al., 2010), X-ray reflectometry (XR) (Fujii et al., 2017; Srivastava et al., 2014), and vibrational sum-frequency generation (VSFG) spectroscopy (Hsieh et al., 2014; Ishiyama et al., 2014; Medders and Paesani, 2016), have been employed to examine the properties associated with the water interface and adsorption behavior of gas molecules. 3

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However, some high-activity, short-lived atmospheric materials, such as Criegee intermediates (CIs) and SO3, cannot be detected by currently available methods. Given the rapid development in computer capabilities and the improvements in quantum chemistry methods, theoretical calculations can fill in for some of the experimental gaps and provide atomistic level insights. Born-Oppenheimer

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molecular dynamics (BOMD) is a powerful simulation tool coupled with density functional computations that use plane-wave basis sets. BOMD has demonstrated

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impressive performance in previous studies with respect to directly describing the

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bond breakage and formation by the systems’ interaction potentials (Kumar and

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Francisco, 2019; Kumar et al., 2018c; Kumar et al., 2017; Zhong et al., 2017b). By

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using ab initio molecular dynamics (AIMD), Li et al. assessed whether the air-water

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interface plays an important role in the HONO formation from NO2 and NH3 (Li et al., 2018). Furthermore, with the aid of the BOMD simulations, the loop-structure

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mechanism was proposed to explain the formation of NH4HSO4 from SO3 and NH4 at the water droplet interface (Li et al., 2016). Therefore, the quantum chemistry approach can broaden our understanding of air-water interface reactions. CIs are a class of carbonyl oxides that are derived from ozonolysis of unsaturated hydrocarbons. CIs' interfacial reactions have attracted extensive attention from the scientific community due to their influence on concentrations of OH radicals, hydroperoxides, sulfates, nitrates, and aerosols. The ab initio kinetic simulations show that the reaction of CH2OO with water at the gas-water interface proceeds on the picosecond time-scale, which is 100-1000 times faster than that in 4

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the gas phase (Zhu et al., 2016b). At the air-water interface, the larger CIs, such as CH3CHOO and (CH3)2COO, remain inert during the simulated time in response to the addition of a hydrophobic substituent (-CH3), which allows CIs to react with other gases (Zhong et al., 2017a). For example, at the air-water surface, H2S and HNO3 react with CIs and contribute to the formation of additional substituted

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hydroperoxides (Kumar et al., 2017; Kumar et al., 2018d). Furthermore, HNO3 serves as a trigger to promote CI hydration (Kumar et al., 2018d). Experimental

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studies indicate that oligomers containing CIs as chain units contribute to secondary

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organic aerosol formation (Inomata et al., 2014; Sadezky et al., 2008; Sakamoto et

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al., 2013). Methylsulfonic acid (MSA) is the simplest sulfonic acid in the gas phase,

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and results from the oxidation of organosulfur compounds (Barnes et al., 2006;

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Hoffmann et al., 2016). The MSA atmospheric concentration is 105-107 molecules cm-3, which equates to ~10-100% of the sulfuric acid concentration (Berresheim et

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al., 2002; Eisele and Tanner, 1993). Due to its acidic properties, MSA is also a potential CI scavenger. However, the reaction mechanisms between CIs and MSA are not well understood. Because MSA is a typical atmospheric pollutant in marine areas, it is important to study the reactions between MSA and CIs in order to expand our comprehension of air pollution in coastal areas. In this study, we explored the differences in chemical reaction mechanisms between CIs and MSA at the air-water interface and in the gas phase using BOMD simulation methods. Density functional theory (DFT) calculations were performed to investigate the reactions between CIs and MSA in the gas phase; and the rate 5

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constants of CIs+MSA were evaluated by the hard-sphere collision theory. The outcomes of this study are pivotal to building a more comprehensive understanding of aerosol formation and modeling the fate of CIs in the atmosphere, which can subsequently be used to perform more targeted air pollution mitigation.

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2. Methods

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All electronic structure calculations were carried out using Gaussian 09

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(Frisch et al., 2009). C2 CIs were selected due to differences in the H atom and

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terminal O atom orientation i.e., the syn- and anti- structures. The structure optimization and transition state searching were performed using the M06-2X

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functional (Zhao and Truhlar, 2008) coupled with a split valence polarized

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6-311++G(d,p) basis set. The intrinsic reaction coordinates (IRC) were calculated to

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confirm that every transition state structure was connected to the corresponding reactant and product. Single point energy calculations were carried out via a high-level CCSD(T) method (Pople et al., 1987; Purvis III and Bartlett, 1982) with the aug-cc-pVTZ basis set. In addition, kinetic simulation was used to further explore the reaction mechanism between CIs and MSA in the gas phase. The BOMD simulation was based on the DFT method and performed with the CP2K program (VandeVondele et al., 2005). Becke-Lee-Yang-Par (BLYP) functional method (Becke, 1988; Lee et al., 1988) was implemented to treat the exchange and correlation interactions. The correction method for Grimme’s dispersion (BLYP-D3) (Grimme et al., 2010) was adopted to resolve the weak 6

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dispersion interaction. In addition, the system valence electrons were addressed using the double-ζ Gaussian basis set coupled with an auxiliary basis set (VandeVondele and Hutter, 2007), and core electrons were treated using Goedecker-Teter-Hutter (GTH) norm-conserved pseudopotentials (Goedecker et al., 1996; Hartwigsen et al., 1998). Cut-off energies of 280 Ry and 40 Ry were selected

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for the plane wave basis set and the Gaussian basis set, respectively. The constant volume and temperature (300 K) NVT ensemble were used in the simulation. For the

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gas-phase small molecule system, we selected a supercell (20×20×20 Å3) with

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periodic boundary conditions to avoid the effect of neighboring replicas. A droplet

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system with 191 water molecules was constructed for the chemical pathway at the

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interface. The water box cubicle measured 35×35×35 Å3, as previous research has

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demonstrated that specific size is sufficient for eliminating the interaction of adjacent water droplet systems (Kumar and Francisco, 2017; Kumar et al., 2018a; Li et al.,

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2016). The simulation step size was set to 1 fs, which based on previous studies, is suitable for simulating water droplet systems (Kumar et al., 2018b; Kumar et al., 2017; Li et al., 2016; Zhong et al., 2015; Zhu et al., 2016b). The water droplet structure was re-optimized for 6 ps by the BOMD simulation in order to stabilize the system. Subsequently, a CI and an MSA molecule were placed at the air-water interface. For syn-CH3CHOO, the distance between the terminal O atom and the MSA hydroxyl H atom is 2.5~4.0 Å, while, for anti-CH3CHOO, the value is 2.8~4.0 Å. Twenty simulations were carried out to avoid effects of the initial conformations at the interface. 7

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3. Results and Discussion 3.1 Gas-phase Reactions In this study, only the reaction barrier between syn-CH3CHOO and MSA is reported and no transition state is observed for the reaction between the

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anti-CH3CHOO and MSA. As shown in Figure 1, the low free energy barrier (0.69

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kcal/mol) implies that the reaction between syn-CH3CHOO and MSA occurs easily

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under atmospheric conditions. The reactions between CIs and MSA yielded a substituted hydroperoxide, which is able to participant in aerosol formation. The

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gas-phase reaction mechanisms are shown in Figure S1. The reaction pathways for

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the CIs and MSA follow a loop-structure mechanism. The proton transfer and C-O

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bond formation are almost simultaneously completed. Furthermore, with respect to reaction time, anti-CH3CHOO (0.51 ps) has greater reactivity with MSA in the gas

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phase, followed by CH2OO (1.35 ps) and syn-CH3CHOO (5.51 ps). This reaction trend is consistent with the reactivity between these CIs and water molecules (Zhong et al., 2017a).

3.2 Reactions at the Air-water Interface The reaction pathways between MSA and CIs at the air-water interface remains elusive. On one hand, water molecules may act as catalysts to promote or inhibit the combination of MSA and CIs. On the other hand, MSA molecules may also act as catalysts to accelerate or impede the reaction of H2O and CIs. In addition, 8

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the water environment significantly impacts the reaction mechanisms. For example, the reaction between CIs and H2O follows the loop-structure mechanisms in the gas phase, while the CI hydration at the air-water interface also includes a stepwise mechanism (Zhu et al., 2016b). Moreover, in the gas phase, the organic acids can promote SO3 hydration (Hazra and Sinha, 2011; Long et al., 2012; Lv et al., 2019;

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Torrent-Sucarrat et al., 2012); while at the water interface, reactions between acids and SO3 will form acid anhydrides (Zhong et al., 2019). The air-water interface

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typically includes clouds, rivers, oceans, and aerosols, all of which play a critical

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role in atmospheric chemistry (Gerber et al., 2015; Napari et al., 2006; Tobias et al.,

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2013; Zhao et al., 2013). Under water-restricting conditions, the adsorbed molecules

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are concentrated and aligned, which increases the reaction likelihood. The reaction

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mechanisms between C2 CIs and MSA at the air-water interface were evaluated using BOMD simulations (ten sets for anti-CH3CHOO and ten sets for

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syn-CH3CHOO). Three main reaction channels were identified: 1) direct reaction, 2) H2O-mediated hydroperoxide formation, and 3) MSA-mediated CI hydration.

3.2.1 CH3C(H)(OOH)(SO3CH3) Direct Formation

By experimentally measuring the reaction rates, Welz et al. demonstrated that reactions between CIs and acid make a significant contribution to acid removal in the equatorial and northern high latitudes (Welz et al., 2014). Our BOMD simulation shows that both syn-CH3CHOO and anti-CH3CHOO react quickly with MSA at the air-water interface. Similar to the reaction pathways in the gas phase, the

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direct reactions of CIs with MSA at the air-water interface also follow a loop-structure mechanism. The interaction (adduct) of anti-CH3CHOO and MSA is shown in Figure 2. The additional reaction pathway consists of CI protonation and C-OSO2CH3 bond formation. When an anti-CH3CHOO and an MSA are placed at the air-water interface, the distance between the reaction molecules rapidly shortens

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and the system reaches relative stability at 0.55 ps. At this stable state, the H1-O3 length shortens to ~1.50 Å, whereas the O1-H1 length only fluctuates at ~1.10 Å.

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Subsequently, at 0.74 ps, CI is protonated and the transition state-like structure is

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formed, where the lengths of O1-H1, H1-O3, and C-O2 are 1.28, 1.29, and 2.85 Å，

binds

to

the

protonated

CI

active

C

atom,

leading

to

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fragment

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respectively. The MSA H atom adds to the CI terminal O atom, while its remnant

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CH3C(H)(OOH)(SO3CH3) formation. The entire reaction is completed in 1.00 ps. Similar to the direct reaction of anti-CH3CHOO with MSA, syn-CH3CHOO and

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MSA form a temporary compound by H1-O3 hydrogen bond interaction at 0.25 ps. The proton transfer occurs at 0.72 ps, followed by C-OSO2CH3 bond formation, which marks the reaction completion at 0.92 ps (Figure S2). Compared with the reaction in the gas phase, the syn-CH3CHOO+MSA is faster at the air-water interface. These results indicate that the air-water interface serves as a support media to promote the combination of CIs and MSA.

3.2.2 H2O-Mediated CH3C(H)(OOH)(SO3CH3) Formation

The air-water interface not only provides binding sites for reaction

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molecules during CH3C(H)(OOH)(SO3CH3) formation, but also participates in the reactions between CIs and MSA. Figures 3a and 3b show the reaction mechanism between anti-CH3CHOO and MSA with the aid of a water monomer and dimer, respectively. Unlike the loop-structure mechanism in the gas phase, the reaction pathways between CI and MSA that involve water molecules follow a stepwise

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mechanism. The adduct formation includes three steps: 1) MSA ionization, 2) proton transfer, and 3) C-OSO2CH3 bond formation. As shown in Figure 3a, a temporary

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complex is generated containing MSA, H2O, and anti-CH3CHOO. The constituents

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in this temporary complex are connected via hydrogen bond interactions of H1-O3

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and H2-O4 at 0.25 ps. For MSA ionization, the transition state-like structure

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formation occurs at 0.84 ps, where the O2-H1 length prolongs to 1.19 Å and the

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H1-O3 distance shortens to 1.20 Å. The lengths of H1-O3 (1.05 Å) and O2-H1 (1.50 Å) stabilize at 0.90 ps, indicating MSA-∙∙H3O+ ion pair formation. Beyond 1.40 ps,

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the O3-H2 length in H3O+ is enlarged, while the H2-O4 length is concertedly shortened. Finally, at 1.57 ps, the MSA-∙∙H3O+ ion pair falls apart and the CI is protonated. The water molecule serves as a bridge for proton transfer. Subsequently, the MSA- ion O1 atom adds to the protonated CI active C atom, indicating CH3(H)(OOH)(SO3CH3) formation. The entire process is completed in 1.70 ps. Overall, the additional product formation mechanism mediated by the water dimer at the air-water interface is analogous to that mediated by the water monomer; however, the reaction involving the water dimer is faster. Initially, MSA, the water dimer, and the anti-CH3CHOO were connected via rapid formation of H1-O3, H2-O4, and 11

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H3-O5 hydrogen bonds. At 0.71 ps, O2-H1 bond cleavage (the distance between H1 and O2 atoms is 1.41 Å) and H1-O3 bond formation (the distance between H1 and O3 atoms is 1.01 Å) demonstrate the MSA-∙∙H3O+ ion pair formation. During the proton transfer process, the H3O+ H2 atom migrates to the adjacent water molecule. Concurrently, the water molecule gives its H atom to the CI terminal O atom.

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Subsequently, at 1.28 ps, the C-OSO2CH3 bond is fully formed, indicating reaction completion. The reaction mechanism between syn-CH3CHOO and MSA mediated by

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a water molecule is similar to that of anti-CH3CHOO and MSA. As shown in Figure

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S3, the CH3C(H)(OOH)(OH) formation completes at 3.60 ps, showing that the

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reaction between syn-CH3CHOO and MSA mediated by water molecules at the

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air-water interface is slightly slower than that of anti-CH3CHOO with MSA.

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3.2.3 MSA-Mediated CH3C(H)(OOH)(OH) Formation

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After using dynamic simulation, Zhong (Zhong et al., 2017a) suggested that CH3CHOO exhibits surprising stability and that no hydration occurs at the air-water interface; which is obviously different from the simplest CI (CH2OO) reaction. In this study, CI hydration mediated by MSA is observed, as shown in Figure 4. Moreover, we propose that the reaction of anti-CH3CHOO with MSA includes CI protonation, C-OH2 bond formation, and the subsequent proton transfer in the water molecules. For CI protonation at the air-water interface, the transition state-like structure proceeds at 0.78 ps, where the O1-H1 length extends to 1.18 Å and the H1-O2 length shortens to 1.19 Å. At 0.80 ps, the O1-H1 covalent bond is

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broken, and the H1-O2 bond between reaction molecules morphs from a hydrogen bond to a covalent bond, indicating protonation completion. MSA- ions are stabilized by hydrogen bond interaction with interfacial water molecules and the protonated CI. At 2.97 ps, a water molecule is added to the protonated CI C atom, and a CH3C(H)(OOH)(OH2)+ ion is formed. The proton transfer in the water phase follows

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a stepwise mechanism. At 3.75 ps, surplus H atoms in the CH3C(H)(OOH)(OH2)+ ion are transferred to the adjacent water molecule, forming a stable hydroperoxide.

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Subsequently, the proton again migrates to another water molecule. Throughout the

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reaction, we surmise that the protonated CI has higher activity than the parent and

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will react with the water molecule under suitable conditions. MSA molecules play an

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inducer role in CI hydration. Compared with the MSA addition mediated by water

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molecules, CI hydration, in the presence of MSA, requires more time. As with the anti-CH3CHOO, syn-CH3CHOO hydration also includes an

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indirect CI protonation step, as shown in Figure 5. At the reaction start, the H1 atom in MSA migrates to the surrounding water molecule instead of the CI terminal O atom. After a round of reversible proton transfer, and 2.79 ps, the MSA-∙∙H3O+ ion pair stabilizes. At 4.06 ps, the hydronium is transferred, where the H2 atom binds to another water molecule near the CI. After a stable period, at 9.76 ps, the H3 atom in H3O+ migrates to the terminal O atom in the CI. The CI protonation is similar to the proton transfer during CH3C(H)(OOH)(SO3CH3) formation mediated by a water dimer. At 9.80 ps, the third water molecule’s OH fragment binds to the active C atom in the CI. Meanwhile, the hydronium ion is formed. During the entire reaction, MSA 13

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plays an inducer role, while water molecules act as both proton transfer shuttles and hydration reactants.

4. Atmospheric Implications The reaction rate constants of syn- and anti-CH3CHOO with MSA were

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obtained according to the hard-sphere collision theory (Smith, 1982). The calculation

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details are depicted in the Supporting Information section. As shown in Table 1, the

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rate constants of syn-CH3CHOO+MSA vary from 3.24×10-10 cm3 molecule-1 s-1 (230 K) to 4.07×10-10 cm3 molecule-1 s-1 (300 K), which is close to the upper limits for

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bimolecular reactions (10-10 cm3 molecule-1 s-1). The rate constants for

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anti-CH3CHOO+MSA fluctuate within a small range — 5.26×10-10 – 6.01×10-10 cm3

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molecule-1 s-1 at 230~300 K. The high rate constants indicate that the reaction

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between CIs and MSA could plays a significant role in the MSA removal process. The calculations performed in this study also permitted us to delineate the CI hydration mechanism when mediated by MSA. Previous studies on CIs and trace gases have focused on the reaction of additive acids, bases, or alcohols (Kumar and Francisco, 2019; Liu et al., 2018; Raghunath et al., 2017; Taatjes et al., 2019; Tadayon et al., 2018; Zhao et al., 2018). Yet, the acid-mediated CI hydration process remains poorly understood (Kumar et al., 2018d). Thus, understanding the reaction of CH3CHOO with MSA will facilitate better modeling of the CIs’ atmospheric behaviors in polluted urban and marine areas. Reacting with CIs is another route for MSA to participate in nucleation, given that adduct formation exhibits low volatility. 14

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With the limitation of SO2 emissions, aerosol formation that depends on sulfuric acid (related to SO2) availability has been inhibited. Therefore, the nucleation involved in MSA should be seriously considered in future studies.

5. Conclusion

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In this study, we employed DFT calculations and BOMD simulation to

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characterize the interactions between CIs and MSA in both the gas phase and at the

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air-water interface. Results demonstrated that the C2 CIs+MSA reaction in the gas

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phase is barrierless or nearly barrierless. The CIs+MSA reaction rate constants indicate that the reaction with CH3CHOO plays an active role in the MSA removal

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channel. At the air-water interface, CIs react with MSA directly or are mediated by

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water molecules, resulting in additional product formation. Furthermore, MSA acts

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as a trigger to promote CI hydration. These reactions follow the stepwise mechanism observed at the air-water interface and proceed on picosecond time-scales. This study is the first of its kind, in that it illustrates the mechanism of CIs interacting with MSA; and thus, expands our understanding of Criegee chemistry in the atmosphere.

Acknowledgments We thank PhD student Jinfeng Chen in Westlake University for helpful discussion. This work was supported by NSFC (National Natural Science Foundation of China, Project No. 21677089) and Taishan Scholars (No. 15

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ts201712003)

Supporting Information Calculation details of reaction rate constants in the gas phase; Snapshots and time evolutions of key bond distances for the reaction of CIs with MSA in the

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gas phase; Snapshots and time evolutions of key bond distances for the reactions of

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syn-CH3CHOO with MSA at the air-water interface; The hydration reaction

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mechanism of anti-CH3CHOO mediated by MSA in the gas phase. The changes of

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electronic energy, enthalpy and free energy for the reactions of CH3CHOO with MSA; The cartesian coordinates of configurations. The kinetic trajectories for the

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reactions of CIs with MSA in the gas phase and at the air-water interface.

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Figure Caption Figure

1

Gibbs

potential

energy

profile

for

syn-CH3CHOO+MSA,

anti-CH3CHOO+MSA, and CH2OO+MSA at 298.15 K and 1 atm. Figure 2 The direct reaction mechanism of anti-CH3CHOO with MSA at the air-water interface.

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Figure 3 The reaction mechanism of anti-CH3CHOO with MSA mediated by a (a) water monomer; (b) water dimer at the air-water interface.

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Figure 4 The hydration reaction mechanism of anti-CH3CHOO mediated by MSA at

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the air-water interface.

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Figure 5 The hydration reaction mechanism of syn-CH3CHOO mediated by MSA at

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the air-water interface.

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1

Gibbs

potential

energy

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Figure

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profile

for

syn-CH3CHOO+MSA,

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anti-CH3CHOO+MSA, and CH2OO+MSA at 298.15 K and 1 atm.

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Figure 2 The direct reaction mechanism of anti-CH3CHOO with MSA at the air-water interface.

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Figure 3 The reaction mechanism of anti-CH3CHOO with MSA mediated by a (a) water monomer; (b) water dimer at the air-water interface.

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Figure 4 The hydration reaction mechanism of anti-CH3CHOO mediated by MSA at the air-water interface.

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Figure 5 The hydration reaction mechanism of syn-CH3CHOO mediated by MSA at the air-water interface.

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Table 1 The rate constants of syn-CH3CHOO+MSA and anti-CH3CHOO+MSA at different temperatures and 1 atm.

T (K)

230

250

270

290

297

298

300 4.07×10-10

anti-CH3CHOO+MSA 5.26×10-10 5.48×10-10 5.70×10-10 5.91×10-10 5.98×10-10 5.99×10-10

6.01×10-10

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syn-CH3CHOO+MSA 3.24×10-10 3.49×10-10 3.73×10-10 3.96×10-10 4.04×10-10 4.05×10-10

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Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights: ·The ab initio dynamics simulation was adopted to simulate the reactions at the air-water interface. ·The reaction of MSA with CH3CHOO plays an active role in the MSA removal channel. · At the water’s surface, the reaction channels follow a loop-structure or a stepwise mechanism and proceed at the picosecond time-scale.

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·MSA also acts as a trigger to promote Criegee hydration at the air-water interface.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5