Article pubs.acs.org/accounts
Cite This: Acc. Chem. Res. 2018, 51, 1229−1237
Insight into Chemistry on Cloud/Aerosol Water Surfaces Jie Zhong, Manoj Kumar, Joseph S. Francisco,* and Xiao Cheng Zeng* Department of Chemistry University of NebraskaLincoln Lincoln, Nebraska 68588, United States S Supporting Information *
CONSPECTUS: Cloud/aerosol water surfaces exert significant influence over atmospheric chemical processes. Atmospheric processes at the water surface are observed to follow mechanisms that are quite different from those in the gas phase. This Account summarizes our recent findings of new reaction pathways on the water surface. We have studied these surface reactions using Born−Oppenheimer molecular dynamics simulations. These studies provide useful information on the reaction time scale, the underlying mechanism of surface reactions, and the dynamic behavior of the product formed on the aqueous surface. According to these studies, the aerosol water surfaces confine the atmospheric species into a specific orientation depending on the hydrophilicity of atmospheric species or the hydrogen-bonding interactions between atmospheric species and interfacial water. As a result, atmospheric species are activated toward a particular reaction on the aerosol water surface. For example, the simplest Criegee intermediate (CH2OO) exhibits high reactivity toward the interfacial water and hydrogen sulfide, with the reaction times being a few picoseconds, 2−3 orders of magnitude faster than that in the gas phase. The presence of interfacial water molecules induces proton-transfer-based stepwise pathways for these reactions, which are not possible in the gas phase. The strong hydrophobicity of methyl substituents in larger Criegee intermediates (>C1), such as CH3CHOO and (CH3)2COO, blocks the formation of the necessary prereaction complexes for the Criegee−water reaction to occur at the water droplet surface, which lowers their proton-transfer ability and hampers the reaction. The aerosol water surface provides a solvent medium for acids (e.g., HNO3 and HCOOH) to participate in reactions via mechanisms that are different from those in the gas and bulk aqueous phases. For example, the anti-CH3CHOO−HNO3 reaction in the gas phase follows a direct reaction between anti-CH3CHOO and HNO3, whereas on a water surface, the HNO3-mediated stepwise hydration of anti-CH3CHOO is dominantly observed. The high surface/volume ratio of interfacial water molecules at the aerosol water surface can significantly lower the energy barriers for the proton transfer reactions in the atmosphere. Such catalysis by the aerosol water surface is shown to cause the barrier-less formation of ammonium bisulfate from hydrated NH3 and SO3 molecules rather than from the reaction of H2SO4 with NH3. Finally, an aerosol water droplet is a polar solvent, which would favorably interact with high polarity substrates. This can accelerate interconversion of different conformers (e.g., anti and syn) of atmospheric species, such as glyoxal, depending on their polarity. The results discussed here enable an improved understanding of atmospheric processes on the aerosol water surface.
1. INTRODUCTION One major issue in atmospheric chemistry that has emerged is understanding the role played by aerosols in chemical changes in the atmosphere. Aerosols are defined as a colloid of solid or liquid droplets, which widely exist in the atmosphere as cloud, fog, mist, and haze. These aerosol droplets exert significant influence on atmospheric chemical processes due to their unique surface properties. This is because (i) the size of the aerosol droplets is generally in the nanometer to micrometer regime, which results in large specific surface area; (ii) most atmospheric species need to overcome an energy barrier to penetrate into the well-formed hydrogen bonding network formed by the aqueous water and as a result, there is a “surface preference”, and (iii) aerosol surfaces can lower energy barriers and enhance the rate of numerous atmospheric reactions or change the existing mechanism of these atmospheric reactions. Understanding how aerosols change chemical mechanisms compared to the well understood and established gas phase in the atmosphere is essential to gain deeper insights into how © 2018 American Chemical Society
aerosols modify chemical composition in the atmosphere, and hence the role that clouds play in influencing chemical composition. Therefore, it is important to understand the reactions that occur on the surface of an aerosol droplet, and their unique mechanisms. The investigation of atmospheric reactions on the aerosol water droplet was previously difficult because the traditional spectroscopic methods could not selectively and differentially probe the interface regions from the bulk aqueous regions. Recently, techniques, such as second-harmonic generation, Xray photoelectron spectroscopy, and sum-frequency generation have begun to address this problem by allowing the study of the adsorption behavior of atmospheric compounds on the aerosol surface. However, the high reactivity of some atmospheric species, such as Criegee intermediate and SO3, on short-time scales can be challenging to detect by current experiment Received: January 31, 2018 Published: April 10, 2018 1229
DOI: 10.1021/acs.accounts.8b00051 Acc. Chem. Res. 2018, 51, 1229−1237
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Figure 1. (A) Schematic diagram of multiple reaction pathways for the CH2OO−water reaction at the air−water interface. (B) Population analysis of the reaction of CH2OO with water monomer (red), water dimer (green), water trimer (blue), and water tetramer (cyan) at air−water interface. The fraction of total product from the stepwise Criegee intermediate reaction is shown in magenta. Reproduced with permission from ref 13. Copyright 2016 American Chemical Society.
Figure 2. (A) The probability of forming loop structures with different numbers of water molecules. (B) An illustration of the interaction of OC (O atom in water interacting with a C1 atom) with other Ow atoms in the water droplet, and computed radial distribution function of Ow atoms in water droplet around the OC atom.
concentrations of volatile organic compounds, oxidation by Criegee intermediates provides a more efficient pathway.8,9 However, these oxidation processes could be strongly affected by the Criegee−water reaction due to the high concentration of water in the troposphere.10,11 The gas-phase reactions of Criegee intermediates with water have been widely viewed as the most plausible Criegee reactions in the troposphere. Specifically, the reaction of Criegee intermediates with a water dimer is their most efficient sink in the atmosphere.12 The water surface, which is the characteristic of the surfaces of oceans, lakes, and atmospheric aerosols, has been supposed to be another important source for the uptake of Criegee intermediates. Zhu et al. used adaptive buffered force quantum mechanics/molecular mechanics (QM/MM) dynamics simulations to investigate the possible reactions of CH2OO with the water droplet.13 The simulated CH2OO is quite reactive at the air−water interface. The time scale of the CH2OO hydration at the air−water interface is on the order of a few picoseconds, 2− 3 orders of magnitude shorter than that in the gas phase. The high reactivity of CH2OO at the air−water interface is attributed to the presence of interfacial water molecules that induce the concerted and stepwise reactions of CH2OO. Figure 1A shows that the CH2OO reaction on the aerosol water surface follows both concerted and stepwise pathways. In the loop-structure-mediated reaction mechanism, the CH2OO can react with water monomer, dimer, trimer, and tetramer, respectively. A hydroxyl fragment in water binds to the carbon atom, and a proton from water transfers to the terminal oxygen of the Criegee intermediate, while the other water molecules in the loop structure act as a “bridge” to promote proton transfer. More interestingly, a new mechanism that involves a two-step reaction was observed at the air−water interface. In this
methods. Complementary to the experimental methods, Born− Oppenheimer molecular dynamics simulations (BOMD) have the advantage of recording reactions on short time scales (see Supporting Information for more details of BOMD method used in our research),1,2 that is, picosecond reactions, and providing the mechanistic details of the reaction, which helps us to better understand aerosol effects on atmospheric chemical processes. A number of atmospheric reactions on the water surface have been theoretically investigated by several research groups,3,4 such as the reaction of (NO+)(NO3−) ion pairs,5 the reaction of HCl with N2O4−5,6 and ionization of acids.7 Our work is built upon these important studies, employing the well-developed BOMD simulations to investigate the chemical processes on the water surface, especially for those that were recently recognized as essential in the atmosphere. This Account summarizes our recent simulation studies to explain how the aerosol water surfaces affect the reaction rates and mechanisms in the atmosphere, which includes bimolecular reactions of Criegee intermediates, SO3−NH3 reaction, and the isomerization of glyoxal on the aerosol surfaces. These recent studies reveal new atmospheric processes and new reaction pathways at the molecular level and provides new and deeper insights into the fundamental interactions of atmospheric species at aerosol interfaces.
2. CRIEGEE INTERMEDIATES ON WATER SURFACES 2.1. CH2OO Criegee Intermediate
Ozone, OH radical, and NO3 are typically believed to be the major oxidants in the atmosphere. Recently, Pierce et al. and Beames et al. found that in an environment with high 1230
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Accounts of Chemical Research mechanism, one water molecule first interacts with CH2OO, giving rise to a proton to a neighboring water molecule, to form (H3O)+ and (HO)CH2(OO)−. A hydrogen bond is then formed between (H3O)+ and the terminal oxygen atom of (HO)CH2(OO)−, promoting a proton transfer and leading to the formation of HOCH2OOH. Figure 1B presents the probability distribution of different reaction pathways. The reaction of CH2OO with a water dimer is the most probable, which is similar to the mechanism predicted from the gas-phase calculations. The stepwise reaction and the loop-structuremediated reaction with the water trimer are also highly probable. To understand the probability distribution of different reaction pathways, the loop structures formed by CH2OO with different numbers of interfacial water molecules are analyzed. It can be seen from Figure 2A that the CH2OO has the highest probability to form a loop structure with the water dimer followed by the water trimer. Thus, in the loop-structuremediated mechanism, CH2OO can easily form the prereaction complex with the water dimer and trimer, which leads to its high reaction probability. Furthermore, CH2OO was aligned parallel to the water droplet surface, as a result of which the water molecules connected to the C atom of the CH2OO (OC in Figure 2B) are inside the water droplet and well hydrated by the surrounding water molecules (seen in radial distribution function in Figure 2B). As a result, one proton in the OC water molecule can easily transfer to the surrounding water molecules and lead to the formation of (H3O)+ and (HO)CH2(OO)− and, thus, promote the stepwise reaction on the water surface. Overall, insights into the interfacial hydration mechanism of CH2OO will inspire future experimental studies and have important implications for tropospheric chemistry.
to study the conformer-dependent reactivity of larger Criegee intermediates (>C1) on the water droplet.16 The simplest Criegee intermediate, CH2OO, is quite reactive toward the interfacial water. However, both syn- and anti-CH3CHOO are found to be inert toward the interfacial water over a simulated time scale of 70 ps. Compared to the CH2OO, the unexpected higher stability (longer lifetime) of C2 Criegee intermediates on the water droplet is largely due to the presence of a hydrophobic methyl substituent on the Criegee carbon, which lowers their proton-transfer ability and hinders the formation of a prereaction complex. The proton bonded to the OC atom (an O atom in water that interacts with a C1 atom of the Criegee intermediate) is to be transferred to the surrounding water molecules in the Criegee−water reaction. Figure 4A,B compares the radial distribution function of Ow atoms in the water droplet surrounding OC in different Criegee intermediate−water droplet systems. The OC interacting with the smallest Criegee intermediate, CH2OO (black curve), forms a strong hydration layer with interfacial water, and the proton bonded to the OC atom can, thus, easily dissociate during the reaction with CH2OO. In contrast, only a weak hydration layer was formed around the OC atom in both syn- (blue curve) and antiCH3CHOO (red curve), which lowers the dissociation ability of the proton bonded to the OC atom. Figure 4C,D shows the probability of the Criegee intermediates and interfacial water molecules forming loop structures. Compared to CH2OO, both syn- and anti-CH3CHOO would rarely form the loop structure with the interfacial water molecules within tens of ps. These results imply that the strong hydrophobicity of methyl substituent hinders the formation of the necessary prereaction complex required for the Criegee−water reaction to occur at the water droplet surface. Additional simulations of C3 and C4 Criegee intermediates, (CH3)2COO and syn- and anti-CH2C(CH3)C(H)OO, on the water droplet surface further verified our viewpoint that the hydrophobicity of substituents in Criegee intermediates is a ubiquitous determinant of their reactivity at the water droplet surface. An important implication of these results is that at the aqueous surface, the Criegee intermediates with hydrophobic substituents (>C1) may survive longer than CH2OO and may, thus, participate in heterogeneous reactions with other trace species in the troposphere.
2.2. Large Criegee Intermediates
According to previous reports, the nature and location of the substituents in Criegee intermediates significantly impact their reactivity toward the water vapor.14,15 For example, antiCH3CHOO has the highest reactivity toward the water monomer in the gas phase, followed by CH2OO and synCH3CHOO (Figure 3). We recently used BOMD simulations
3. CRIEGEE INTERMEDIATE REACTIONS ON WATER SURFACES 3.1. Criegee−HNO3 Reaction
Organic acids (HCOOH and CH3COOH) and inorganic acids (HNO3 and HCl) are present in the troposphere at mixing ratios of parts per billion by volume (ppbv) to parts per trillion by volume (pptv),17 and their reactions with Criegee intermediates in the gas phase have been well studied. Welz et al. found that the rate constants for the reactions of CH2OO and CH3CHOO with HCOOH and CH3COOH are all greater than 1 × 10−10 cm3/s, suggesting that these reactions may significantly contribute to removal of these organic acids.18 On the other hand, Chao et al. suggested that the water vapor reaction would dominate even in dry conditions with relative humidity of ∼35%.10 Our air−water interface inclusive BOMD simulations suggest that in fog or cloud waters, inorganic or organic acids may enhance the probability of the Criegee− water reaction in urban or forested environments by forming hydrogen-bonded loop structures involving Criegee intermedi-
Figure 3. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated reaction profiles for the reactions of C1 and C2 Criegee intermediates with water. The zero-point-corrected electronic energies are given in kcal/mol. Adapted with permission from ref 16. Copyright 2017 John Wiley and Sons. 1231
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Figure 4. (A) An illustration of the interaction of OC (O atom in water interacting with a C1 atom) with other Ow atoms in the water droplet. (B) Computed radial distribution function of Ow atoms in the water droplet around the OC atom. (C) A cartoon of loop structures formed by Criegee intermediates with interfacial water molecules. (D) The probability of forming loop structures with different numbers of water molecules. Adapted with permission from ref 16. Copyright 2017 John Wiley and Sons.
Figure 5. (A) Population analysis of various pathways resulting from the reaction of anti-CH3CHOO with nitric acid on a water droplet. (B) Prereaction complexes for Criegee hydration mediated by nitric acid (left panel), sulfuric acid (middle panel), and formic acid (right panel) in different geographical regions.
reaction of anti-CH3CHOO with HNO3 only accounts for a minor fraction (30%) of the reactions at the air−water interface, whereas the newly observed mechanism, which involves HNO3-mediated Criegee hydration, is the dominant pathway, accounting for 70% of the total reaction. This new reaction between Criegee intermediates and acids at the air−water surfaces is expected to be relevant in the hazy environments of global polluted urban regions where nitrates and sulfates are abundant. During hazy periods, high relative humidity and the presence of fog droplets may favor HNO3- or H2SO4-mediated Criegee hydration (Figure 5B) over the
ate, water or water dimer, and an acid molecule. Kumar et al. employed BOMD simulations and two-layer ONIOM (QM/ MM) in an electronic embedding scheme to study the reaction and spectroscopic properties of anti-CH 3 CHOO-HNO 3 interaction at the air−water interface with HNO3.19 The Creigee−HNO3 reaction mechanism on the water surface is significantly different from that in the gas phase. In the gas phase, a direct reaction between anti-CH3CHOO and HNO3 is expected; on a water surface, the HNO3-mediated stepwise hydration of anti-CH3CHOO is dominantly observed. Figure 5A shows that out of the total 50 BOMD simulations, the direct 1232
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Figure 6. (A) (upper panel) Snapshot structures taken from BOMD simulation for the one water molecule mediated concerted reaction between CH2OO and H2S and (lower panel) time evolution of key bond distances (S−H1, C−S, O1−H1, O1−H2, and O2−H2). (B) (upper panel) Snapshot structures taken from BOMD simulation for the two water molecule mediated concerted reaction between CH2OO and H2S and (lower panel) time evolution of key bond distances (O1−H1, O2−H2, O3−H3, C−S, and S−H1). Adapted with permission from ref 24. Copyright 2017 Royal Society of Chemistry.
the CH2OO···H2S reaction occurs in the other eight. The most dominant mechanistic pathway in CH2OO···H2S reactions at the air−water interface involves a single water molecule mediated concerted reaction (50%, Figure 6A). A quarter of the reactions (Figure 6B) involve a two water molecule mediated concerted pathway. In addition, a small fraction of the CH2OO···H2S reactions proceed in a stepwise manner (12.5%) or a concerted manner (12.5%) without any involvement of H2O molecules. Further analysis shows that the ability of the air−water interface to mediate the CH2OO−H2S reaction is attributed to the specific hydration structures and the orientations of CH2OO and H2S on the water droplet. Specifically, one of the H atoms of H2S prefers to interact with water molecules via hydrogen-bonding interaction of H(H2S)···O(H2O). This orientation allows H2S to easily transfer one of its H atoms to H2O molecules. Simultaneously, the terminal O atom of CH2OO interacts strongly with a nearby H2O molecule in the water droplet via an O(CH2OO)···H(H2O) hydrogen bond. Thus, the H atoms of H2O in the droplet are easily transferred to CH2OO. As a result, the water-mediated CH2OO···H2S reaction on the water droplet, in which CH2OO and H2S, respectively, act as proton acceptor and donor, would be favored. The Criegee−H2S reaction on the water surface is expected to provide a novel non-photochemical pathway for the formation of C−S linkages in clouds; this pathway is especially important when the concentration of H2S is high, as in terrestrial, geothermal, and volcanic regions.
nitrooxyethyl hydroperoxide-forming reaction that was previously assumed to dominate in the gas phase. Similar reactions of Criegee intermediates with organic acids, which possesses HNO3-like functionalities (Figure 5B), could be expected to occur on the water surfaces in forested areas. Understanding these mechanisms may improve our knowledge of the organic acid budget in terrestrial equatorial regions and high northern latitudes. To provide the benchmarks for future experiments, the infrared spectra of nitrooxyethyl hydroperoxide (product from traditionally believed mechanism) and hydrooxyethyl hydroperoxide (product from our proposed mechanism) on the water surface were calculated using the ONIOM calculations. The results suggest that the N−O stretching bands (at approximately 1600−1200 cm−1) and the NO2 bending band (at approximately 750 cm−1) in nitrooxyethyl hydroperoxide could be used as spectroscopic markers to distinguish it from hydrooxyethyl hydroperoxide on the water surfaces. 3.2. Criegee−Hydrogen Sulfide Reaction
Thioaldehydes are an important class of carbon−sulfur compounds that play crucial roles in atmospheric chemistries.20,21 Thioaldehydes were previously considered to be formed by the reaction of SH− ions with carbonyl groups in the atmosphere. It was recently reported that the water- or acidcatalyzed gas-phase reactions between Criegee intermediates and hydrogen sulfide (H2S) also contribute to atmospheric formation of thioaldehydes.22,23 Approximately half of the H2S in the troposphere is emitted from oceans, implying that the aqueous chemistry of H2S exerts a significant role in its oxidation in the troposphere. The BOMD simulations of Kumar et al. showed that the reaction between CH2OO and H2S at the air−water interface occurs within a few picoseconds and leads to the formation of α-thioxymethyl hydroperoxide.24 This atmospheric reaction occurs in two steps: the Criegee intermediate first interacts with H2S to form a Criegee−H2S complex in the gas phase, and then this complex collides with the water surface and results in the adduct formation. Due to the high reactivity of CH2OO with the interfacial water molecules, the CH2OO−H2O reaction accounts for 50% of the 16 independent BOMD simulations on the droplet, whereas
4. OTHER REACTIONS ON WATER SURFACES 4.1. SO3 + NH3 Reaction
Atmospheric nucleation generally occurs with gaseous compounds that have very low volatility,25−27 such as sulfuric acid and highly oxidized organic compounds. Compounds that can promote the formation of low-volatility complexes also play crucial rules in atmospheric nucleation; these compounds include gaseous ammonia or amines that form acid−base complexes with inorganic or organic acids. 28,29 Many experimental studies have shown that ammonium sulfate is 1233
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Figure 7. (A) (upper panel) Snapshots of structures taken from BOMD simulation of the water trimer system with NH3 and SO3, in which gray spheres mark the protons transferred during the reaction, and (lower panel) time evolution of the N−H1, S−O2, O1−H2, O1−H1, O2−H2, and O3− H3 bond lengths during BOMD simulation. (B) (upper panel) Snapshot structures taken from BOMD simulation of the NH3 and SO3 molecules reacting on the surface of a water nanodroplet, in which gray spheres mark the protons transferred during the reaction and (lower panel) time evolution of the O2−H2, O2−H4, O3−H3, O5−H5, O1−H6, N−H1, and S−O4 bond lengths during the BOMD simulation. Reproduced with permission from ref 33. Copyright 2016 American Chemical Society.
Figure 8. (A) Relative energies as a function of torsion angle, τ, obtained from electronic structure calculations in vacuum. (B) Free energy profile (blue curve) of trans to cis isomerization at liquid water interface, obtained by combining AIMD and thermodynamics. The black and yellow curves represent the relative isomerization energies in the gas and four-water cluster, respectively. Reproduced with permission from ref 38. Copyright 2017 American Chemical Society.
one of the main constituents of aerosols (e.g., fine particles (PM2.5)).30,31 Neutralizing sulfuric acid with NH3 is a wellknown and conventional pathway for the formation of ammonium sulfate.32 Li et al. found a new pathway for the formation of ammonium bisulfate (NH4HSO4) from hydrated NH3 and SO3 molecules in a water trimer as well as on the surface of a water droplet using the BOMD simulations.33 This new mechanism for the formation of ammonium bisulfate is especially important when the atmospheric concentration of NH3 is high (e.g., ∼10 μg/m3).34 Although the SO3−NH3 reactions with the water monomer and dimer involve relatively large barriers, the water trimer enables nearly barrier-less proton transfer via the formation of a unique loop structure. When NH3 and SO3 molecules react with a water trimer or on the surface of a water droplet, a direct reaction between NH3 and SO3 is not observed; in contrast, a
loop-structure-meditated proton-transfer mechanism was found to be dominant, leading to the formation of the NH4+/HSO4− complex (Figure 7). Two water molecules directly interact with NH3 and SO3 in this mechanism, while the other water molecules serve as a “bridge” that interacts with SO3 to form a loop structure. The loop structures formed in the water trimer and on the water surface greatly enhance the ability to transfer protons, which leads to a nearly barrier-less reaction. The loopstructure-promoted proton-transfer mechanism is expected to be ubiquitous in NH3−SO3 reactions on the cloud droplets and, thus, play a crucial role in aerosol nucleation. 4.2. Isomerization of Glyoxal
Glyoxal is an oxygenated organic compound that is mostly produced by anthropogenic and biogenic emissions.35 Glyoxal is easily hydrated, even in the presence of a small number of water molecules. This feature suggests that glyoxal plays crucial 1234
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Accounts of Chemical Research roles in promoting water uptake and the growth of aerosols.36 Hydrated glyoxal can also strongly bind with sulfates, which are an important component of aerosol particles.37 However, all these processes are speculative because conclusive information about the solvation behavior of glyoxal on the aerosol surfaces is still lacking. Zhu et al. compared glyoxal solvation in the gas phase and at the air−water interface by performing high-level electronic structure calculations as well as both classical MD and BOMD simulations.38 The minimum of the energy profile with respect to the torsion angle (τ) of glyoxal in the gas phase is located at 0°, which corresponds to the trans isomer. In addition, the rotational barrier from the trans to the cis isomer peaks at 100° and involves a barrier of approximately 6 kcal/mol (Figure 8A). The addition of two water molecules lowers the gas-phase barrier to 4.5 kcal/mol. Figure 8B shows the free energy profiles of glyoxal isomerization along the τ-coordinate at the air−water interface (blue), in the gas phase (black), and in a four-water cluster (yellow). The rotational barrier in the water cluster (∼4.5 kcal/mol) and at the water surface (∼2 kcal/mol) is lower than that in the gas phase. Figure 8B also shows that the free energy of the cis isomer is ∼3 kcal/mol lower at the water interface than in the gas phase, corresponding to a higher cis/ trans population ratio at the interface. To explore the catalysis mechanism by the water surface, the dipole moment of glyoxal is calculated along with torsion angle (τ) at the air−water interface. The results show that there is a significant change in the dipole moment during the isomerization of glyoxal. The dipole moment of trans-glyoxal is almost 0, whereas cis-glyoxal exhibits a large dipole value of 3.03 D. Such large polarity of cis-glyoxal favors its interaction with polar water solvent. Furthermore, the classical MD simulations suggest that the cis conformation of glyoxal prefers to be located at the interface, while the trans isomer is more easily solvated in the bulk. The population of the cis isomer at the interface can, thus, be enhanced, even if the trans isomer has lower energy. Because glyoxal is generally assumed to be present in its trans form in the gas phase and interfacial environments, these results have implications in the interpretation of experimental data and modeling of glyoxal chemistry at the interfaces of the water aerosols.
surfaces provide more solvation environment than that in gasphase water. This may significantly lower the energy barriers for the proton transfer-based reactions in the atmosphere. Such catalytic effects of the aerosol water surface are beneficial in discovering new and efficient pathways for the formation of important species (e.g., forming NH4HSO4 on water surfaces from SO3 and NH3 instead of from H2SO4 collisions with NH3). (4) The aerosol water surface provides a solvent medium for acids residing in neutral form, especially for the weak acids, such as formic acid and acetic acid. The strong acids, for example, HNO3 and HCl, can also survive as neutral molecules for a short period at the air−water interface, although the formation of a critical cluster with two additional interfacial waters can cause their dissociation.7 The neutral acids at the air−water interface can participate in atmospheric reactions via mechanisms that are different from those in the gas and bulk aqueous phases. Although theoretical studies have improved our understanding about the effects of aerosol surfaces on atmospheric chemical processes at the molecular level, there are several issues that need to be addressed in future studies. First, the surface concentrations of the prereaction complexes need to be estimated in order to evaluate the importance of specific reactions on the aerosol water surfaces. Second, the energy barriers for the reaction pathways on the aerosol surface need to be estimated to determine their actual impact on chemistry in the troposphere. Third, more complex aerosol water surfaces need to be considered in future work, such as water droplets with different shapes, sizes, pH values, and ion concentrations and ice with different exposed surfaces. Such models may provide new insights into chemical processes in the atmosphere. Fourth, more advanced simulation techniques need to be developed to investigate the atmospheric reactions on the water surface. In the present scenario, the BOMD simulation can only probe the low-barrier reactions within the picosecond time scale. This hampers the full discovery of reaction pathways, especially for those with relatively high barrier. Moreover, the BOMD simulation does not consider quantum-tunneling and zero-point effects, which might alter the outcome quantitatively if they were correctly taken into consideration.
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5. SUMMARY AND OUTLOOK Aerosol water surfaces play crucial and multiple roles in atmospheric reaction processes. Complementary to reactions in the gas phase, new atmospheric processes as well as new reaction pathways are revealed on water surfaces by recent theoretical investigations. According to these studies, aerosol surfaces can impact atmospheric processes in the following manner: (1) Aerosol water surfaces can confine the atmospheric species into a specific orientation, which is generally determined by the hydrophilicity of the substitution of the atmospheric species or the hydrogen-bonding interactions between atmosphere species and interfacial water. Such specific orientation can align atmospheric species into a reaction-favorable position, thereby catalyzing or changing its reaction pathways on the aerosol water surface. (2) An aerosol water droplet is a polar solvent, which would favorably interact with high polarity substrates. Thus, it can accelerate the transformation between different conformers (e.g., anti and syn) of atmospheric species depending on their polarity (conformer with less polarity tends to transform to conformer with high polarity on aerosol water surface). (3) The aerosol water
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.8b00051.
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Computational methods (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Joseph S. Francisco: 0000-0002-5461-1486 Xiao Cheng Zeng: 0000-0003-4672-8585 Funding
The authors acknowledge computational support by UNL Holland Computing Center. 1235
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Accounts of Chemical Research Notes
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The authors declare no competing financial interest. Biographies Jie Zhong obtained his Master degree with Prof. Jun Zhang at China University of Petroleum (East China) in 2014. He is currently a Ph.D. candidate at University of NebraskaLincoln working with Professors Xiao Cheng Zeng and Joseph S. Francisco. Manoj Kumar obtained his Ph.D. with Prof. P. M. Kozlowski at the University of Louisville in 2012. He is currently working as a postdoctoral researcher with Prof. Joseph S. Francisco. Joseph S. Francisco is currently Elmer H. and Ruby M. Cordes Chair in Chemistry and Dean of the College of Arts and Sciences at the University of NebraskaLincoln. He received his B.S. degree at the University of Texas at Austin and his Ph.D. degrees from the Massachusetts Institute of Technology and did his postdoctoral training at Cambridge University. His research interests are primarily in the areas of spectroscopy, chemical kinetics, computational chemistry, and atmospheric chemistry. Xiao Cheng Zeng is the Chancellor’s University Professor of Chemistry at the University of NebraskaLincoln. He received his B.S. degree from Peking University in 1984 and Ph.D. from The Ohio State University in 1989. He did his postdoctoral work at the University of Chicago and UCLA. Zeng is a fellow of the American Association for the Advancement of Science, the American Physical Society, and the Royal Society of Chemistry. He has mentored 25 graduate students and 32 postdoctoral fellows. His research interests include phase behavior of confined water and ice, superhydrophobicity and wetting, structural evolution of gold clusters, electronic properties of nanostructures, nanocatalysis, and atmospheric chemistry.
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