Single-Molecule Catalysis Revealed: Elucidating the Mechanistic

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Single-Molecule Catalysis Revealed: Elucidating the Mechanistic Framework for the Formation and Growth of Atmospheric Iodine Oxide Aerosols in Gas-Phase and Aqueous Surface Environments Manoj Kumar, Alfonso Saiz-Lopez, and Joseph S. Francisco J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07441 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Journal of the American Chemical Society

Single-Molecule Catalysis Revealed: Elucidating the Mechanistic Framework for the Formation and Growth of Atmospheric Iodine Oxide Aerosols in Gas-Phase and Aqueous Surface Environments Manoj Kumar1,2, Alfonso Saiz-Lopez3 and Joseph S. Francisco1,2,* 1Department

of Chemistry, University of Nebraska-Lincoln, Lincoln, NE, USA 68588. of Earth and Environmental Sciences, University of Pennsylvania, Philadelphia, Pennsylvania, USA 19104. 3Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid, Spain, 28006.

2Department

Abstract. Iodine oxide aerosols are ubiquitous in many coastal atmospheric environments. However, the exact mechanism responsible for their homogeneous nucleation and subsequent cluster growth remains to be fully established. Using quantum chemical calculations, we propose a new mechanistic framework for the formation and subsequent growth of iodine oxide aerosols, which takes advantage of non-covalent interactions between iodine oxides (I2O5 and I2O4) and iodine acids (HIO3 and HIO2). Larger iodine oxide clusters are suggested to be formed in a facile manner and with enhanced exothermicity. The newly proposed mechanisms follow both concerted and stepwise pathways. In all these new chemistries, an O:I ratio of 2-2.5 is predicted, which satisfies an experimentally derived criterion recently proposed for identifying iodine oxides involved in atmospheric aerosol formation. Born Oppenheimer molecular dynamics simulations at the air-water interface suggest that I2O5 and I4O10, which are two of the most common nucleating iodine oxides, react with interfacial water on the picosecond time scale and result in novel nucleating species such as H2I2O6 and HI4O11- or I3O8. An important implication of these simulation results is that aqueous surfaces, which are ubiquitous in the atmosphere, may activate iodine oxides to result in a new class of nucleating compounds, which can form mixed aerosol particles with potent precursors, such as HIO3 or H2SO4, in marine air masses via typical acid-based interactions. Overall, these results give a better understanding of iodine-rich aerosols in diverse environments.

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INTRODUCTION Atmospheric aerosols affect air quality, human health, urban visibility, and the Earth’s radiation balance.1 By acting as cloud condensation nuclei (CCN) and ice nuclei, aerosols impact the frequency of occurrence and lifetime of clouds on local, regional, and global scales.2-4 Aerosols can be either directly emitted into the atmosphere from primary sources or formed in the atmosphere through the nucleation of gas-phase precursors.5-7 Aerosol nucleation events produce a large fraction of atmospheric aerosols. New particle formation (NPF) occurs in two distinct stages: nucleation of gaseous precursors to form a critical nucleus and subsequent growth of the critical nucleus to a larger size. Despite their impact on human health and global climate, the exact formation pathways for atmospheric particles remain largely unknown.5-7 While NPF events have often been linked with sulfuric acid (H2SO4),8-10 there is now evidence suggesting that other atmospheric species also play an important role in the aerosol formation.5,6,11-13 For example, a significant fraction of fine particulate matter (PM2.5) in the Eastern United States is composed of ammonium nitrate.14-17 Alkylaminium ions and carboxylate ions are detected in the mass spectra of nanoparticles collected during NPF events in Hyytiala, Finland.18 Measurements in Tecamac, Mexico also predict organic species to be relevant in the particle growth.13 Recently, Arquero et al.19 experimentally examined the role of the oxalic acid in new particle formation from vapor phase methanesulfonic acid, methylamine, and H2O. The addition of water to the mixture of oxalic acid and CH3NH2 was found to enhance the particle formation by an order of magnitude. In coastal iodine-rich environments that spread from mid-latitudes to the polar regions, the particle formation and initial growth processes are dominated by iodine oxoacids and iodine oxide vapors.20-28 These species are formed following the atmospheric photodecomposition of ocean-emitted iodine source gases which can be inorganic (I2, HOI), and organicbounded such as CH3I and polyhalogenated iodine species (e.g. CH2I2, CH2IBr, CH2ICl, C2H5I).29,30 Though most NPF events in the atmosphere occur at modest intensities, iodine aerosol particles in coastal air have unusually high formation and growth rates compared to other environments.20,24 Though considerable attention has been paid to the role of iodine oxides in the formation of ultra-fine aerosol and its potential to contribute to CCN formation,31-34 establishing the precise identity of iodine oxide aerosols continues to remain a significant challenge. Laboratory measurements have shown that gas phase IxOy (x=2-3; y=1-7) form in dry conditions35 and that iodine oxide particles have small hygroscopic growth factors.33,36,37 Generally, a small growth factor is indicative of a weakly soluble material or an insoluble material internally mixed with a small amount of soluble material.38,39 Considering high solubility of I2O5,40 Jimenez et al.33 concluded that I2O4 was the most likely composition of iodine oxide particles. However, elemental analysis of particles generated photochemically from I2 in the presence of O3 revealed a composition consistent with I2O5.41 It was in fact the first study to report the experimentally derived O:I ratio of 2-2.5 for aerosol-relevant iodine oxides in atmosphere. Ultra-fine particles (8 and 10 nm) detected during low tide particle “bursts” at Mace Head in Ireland are also reported to have a characteristic hygroscopic growth factor of less than 1.1 at an RH of 90%.39 This ruled out the possibility of any sulfate or sea salt aerosol formation which are known to have much greater growth factors. Glowa and Ball42 studied the formation of iodine oxide aerosols by irradiating I2 vapor in air. The O to I ratio, calculated from several energy dispersive X-ray spectroscopy, was in the range of 2.6 to 7. These findings were consistent with a number of potential products, such as I2O4, I2O5, I4O9 and HIO3. Väkevä et al.39 showed that the ultra-fine particles sampled at low-tide periods at the same coastal location contained iodine as well as sulphur. In addition, Vaattovaara et al.43 reported that appreciable quantities of organic compounds contribute to particles of 5–6 nm, whereas particles smaller than 3–4 nm are mainly composed of iodine oxides. A recent field study25 in Mace Head, Ireland, identified the key molecular steps involved in particle formation in an iodine-rich coastal environment; cluster formation primarily occurs via the sequential addition of iodic acid (HIO3), which is formed by the reaction of iodine

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with ozone in the presence of water vapor, followed by dehydration to I2O5, with average oxygen-toiodine ratios of 2.4 found in the clusters. Though this newly proposed mechanism for I2O5 formation is qualitatively consistent with their field data, previous quantum chemical calculations44 suggest that the concerted dehydration of two HIO3 molecules, which results in the formation of I2O5, involves a significant barrier and seems highly unlikely to occur under atmospheric conditions. Clearly, the exact mechanism for the formation and subsequent growth of I2O5 or I2O4 particles in coastal environments is far from being fully resolved. Though recent experimental20,25,33,41,45 studies suggest that the two iodine oxides I2O4 and I2O5 are the major nucleating iodine oxide precursors in these coastal environments, their formation and growth mechanisms are yet to be fully established. It is important to mention here that the atmospheric chemistry of iodine oxides have been examined using ab initial methods. For example, Misra and Marshall46 characterized isomers of various iodine oxides (IO2, I2O and I2O2) at the QCISD(T)/6-311+G(3df)//MP2/6-311+G(3df) level of theory. They also studied the IO + IO reaction, which was suggested to yield I + OIO as favorable products. Galvez et al.47 studied a range of potentially relevant aggregation reactions of different iodine oxides, as well as complexation with water molecules using high level ab initio correlated calculations, which employed a relativistic effective potential combined with a flexible valence-only REP-optimized basis set. Their results showed that the nucleation path most likely proceeds through dimerization of I2O4. It is also shown that water could somewhat hinder gas-to-particle conversion. Khanniche et al.44 studied the I2O5 hydration reaction using the CCSD(T)/CBS(D,T)//B3LYP/aug-cc-pVTZ and the CCSD(T)/CBS(D,T)//MP2/aug-cc-pVTZ levels of theory. The CCSD(T)/CBS(D,T)//MP2/aug-cc-pVTZ calculated barriers for the forward and reverse reactions were 22.4 and 27.7 kcal/mol, respectively. This suggests that the uncatalytic dehydration of two HIO3 molecules may not be responsible for the formation of I2O5 particles in air. This was also supported by their computational kinetic analysis within the framework of canonical transition state theory. It is quite surprising that though the gas-phase chemistry of iodine oxides has been extensively studied, the role of any metal or metal-free catalysis in impacting the formation of iodine oxides in the gas-phase has never been examined. Moreover, chemistry of iodine oxides at the aqueous surfaces, which is a unique reaction media in the troposphere,48-51 is yet to be explored. Herein, we have performed high-level quantum chemical calculations and Born Oppenheimer molecular dynamics (BOMD) simulations to investigate the formation and growth mechanisms of iodine oxide-based particles in the gas phase and at the air-water interface. The objective of performing gasphase calculations was to explore the possibility of any catalysis in the formation of iodine oxide aerosol particles whereas the BOMD simulations were aimed at providing fundamental insights into the dynamic behavior of iodine oxides. The BOMD simulations probe the bond forming and bond breaking events over a finite time scale and thus, may provide unique mechanistic insights into the chemistry of iodine oxides at the atmospherically-relevant aqueous surfaces. The results from these calculations and simulations may improve our understanding about the aerosol formation in atmospheric iodine-rich environments.

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Figure 1. The CCSD(T)/aug-cc-pVTZ+LANL2DZ//M06-2X/aug-cc-pVDZ+LANL2DZ calculated gas-phase geometries of I2O5 and I2O4 with one and two molecules of iodous acid and iodic acid adsorbed, respectively. The zero-point-corrected electronic energies of these complexes are also given. These energies are relative to the separated constituents and are given in units of kcal/mol. For the sake of clarity, the structures of I2O5 and I2O4 are shown in wire representation, whereas those of iodous acid and iodic acid are given in ball-stick representation.

COMPUTATIONAL DETAILS Gas-phase Electronic Structure Calculations. All high-level quantum mechanical calculations reported

in this work were performed using the Gaussian0952 software at standard temperature (298.15 K) and pressure (1 atm). Two dehydration reactions, namely, HIO3 + HIO3  I2O5 + H2O and HIO3 + HIO2  I2O4 + H2O, in the absence and presence of a single I2O4 and I2O5 molecule were explored in the gas phase. The gas-phase geometries of all stationary points on the potential energy surfaces of both reactions were first fully optimized using the M06-2X53 density functional theory (DFT) method and aug-cc-pVDZ54 basis set for H and O, and effective core potential LANL2DZ basis set for Iodine atoms (M06-2X/aug-ccpVDZ+LANL2DZ), and subsequent normal-mode vibrational frequency analyses were performed to confirm that the stable minima had all positive vibrational frequencies. The energetics of the gas-phase reactions were further refined using the coupled-cluster single and double substitution method with a perturbative treatment of triple excitations (CCSD(T))55 and the aug-cc-pVTZ+LANL2DZ basis set. For all reactions, the M06-2X/aug-cc-pVDZ+LANL2DZ calculated unscaled vibrational frequencies were used to estimate the zero-point energy corrections.

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BOMD Simulations. BOMD simulations were performed based on DFT implemented in the CP2K56 code.

In the BOMD simulations, the droplet system contained 191 water molecules and one I2O5 or I4O10 molecule. The dimensions of the simulation box were x = 35 Å, y = 35 Å, and z = 35 Å, making the box large enough to avoid interactions between adjacent periodic images of the water droplet. Prior to the BOMD simulations, the system was fully relaxed using DFT, in which the exchange and correlation interaction was treated with the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional.57,58 Grimme’s dispersion correction (D3) was applied to account for the weak dispersion interaction.59,60 A double-ζ Gaussian basis set combined with an auxiliary basis set and Goedecker-Teter-Hutter (GTH) normconserved pseudopotentials were adopted to treat the valence electrons and core electrons, respectively.61,62 An energy cutoff of 280 Rydberg was set for the plane-wave basis set, and a cutoff of 40 Rydberg was set for the Gaussian basis set. The BOMD simulations were carried out in the constant volume and temperature (NVT) ensemble, using the Nose-Hoover chain method to control the temperature (300 K) of the system. The integration step was set as 1 fs, which has been proven to achieve sufficient energy conservation in water systems.50,63-68 Both systems were simulated for 35 picoseconds (ps).

RESULTS AND DISCUSSION a) Gas-Phase Calculation. The acidic iodine precursors HIO3 and HIO2 can interact favorably with I2O5 or I2O4 molecules via non-covalent interactions, which may play a role in lowering the barrier for the dehydration reaction. Indeed, our gas-phase results support that notion (Figure 1), i.e., both HIO2 and HIO3 form very strong complexes with I2O4 and I2O5, with binding energies of at least 15.4 kcal/mol at the CCSD(T)/aug-cc-pVTZ+LANL2DZ//M06-2X/aug-cc-pVDZ+LANL2DZ level of theory. It is important to mention here that besides I2O4 and I2O5, I2O2 and I2O3 are other potential iodine oxides that can interact favorably with HIO3 and HIO2.69 The role of I2O2 and I2O3 in the aerosol particle formation would be investigated in greater detail in near future. Considering the stereo-environment of a given I2O4 or I2O5 molecule, two HIO2 or HIO3 molecules may bind to the original molecule favorably. HIO2 forms stronger complexes than HIO3, i.e., the HIO2••I2O4 complex (ΔE = -22.5 kcal/mol) has 7.1 kcal/mol higher binding energy than that of the HIO3••I2O4 (ΔE = -15.4 kcal/mol). The HIO2••I2O5 complex (ΔE = -29.1 kcal/mol) has 9.1 kcal/mol higher binding energy than that of the HIO3••I2O4 (ΔE = -20.0 kcal/mol). Moreover, the HIO2••I2O5 interactions are ~3.0-6.6 kcal/mol stronger than the HIO2••I2O4 interactions. These favorable interactions between acidic iodine precursors and iodine oxides raise a mechanistically appealing question: Can these complexes play a catalytic role in the formation and subsequent growth of iodine oxide particles? We then performed quantum chemical calculations to investigate the role of the non-covalent interactions between HIO2 or HIO3 and I2O4 or I2O5 compounds in the formation of atmospheric iodine oxide particles. The results from our high-level quantum chemical calculations reveal that in the presence of one I2O4 or I2O5 molecule, the HIO3 + HIO3  I2O5 + H2O and HIO3 + HIO2  I2O4 + H2O reactions may proceed via both concerted and stepwise pathways (Scheme 1). Though the concerted mechanism for the formation of an iodine oxide particle has been previously reported,25,44 the details of the stepwise mechanism are reported here for the first time. The mechanistic beauty of this new proposal lies in the fact that the non-covalent interactions between HIO2 or HIO3 and I2O4 or I2O5 induce extremely favorable mechanistic paths en route to the formation of I2O4 or I2O5 particles. Considering that I2O4 and I2O5 particles are formed in these reactions, the subsequent growth of iodine oxide clusters is autocatalytic in nature. The O:I ratios in these new nucleation mechanisms are 2-2.5, which is consistent with the O:I ratios of 2-2.6 observed in coastal areas.25,41

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We first examined the dehydration reaction between two HIO3 molecules in the absence of any iodine oxide. The reaction follows both concerted and stepwise pathways and results in the formation of I2O5 (Figure 2). The results suggest that both reactions initiate with the formation of a prereaction complex, in which two HIO3 molecules are strongly held by non-covalent interactions. This complex has a zero-point-corrected binding energy of 20.9 kcal/mol, which is consistent with the previously CCSD(T)/CBS(D,T)//MP2/aug-cc-pVTZ calculated value of 20.1 kcal/mol.44 This complex then decomposes into I2O5 and water either by a concerted or stepwise mechanism. The concerted pathway is found to be favored over the stepwise one, i.e., the transition state for the concerted pathway lays below the TS2 for the stepwise addition reaction (Figures 2 and S1). The barrier height for the concerted reaction is 25.9 kcal/mol, which is 1.8 kcal/mol different from the previously CCSD(T)/CBS(D,T)//MP2/aug-cc-pVTZ calculated value of 27.7 kcal/mol.44 The stepwise reaction results in the formation of an adduct (Int2), which subsequently dehydrates into I2O5. The latter step has a barrier of 25.7 kcal/mol relative to Int2. The I2O5••H2O complex (Int3’) has a binding energy of 12.9 kcal/mol, which is again consistent with the previously CCSD(T)/CBS(D,T)//MP2/aug-cc-pVTZ calculated value of 12.8 kcal/mol.44 Galvez et al.47 have calculated the binding energy of the I2O5••H2O complex at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level of theory to be 11.9 kcal/mol, which is also in good agreement with our calculated value. Overall, the dehydration reaction has a reaction energy of 2.0 kcal/mol relative to the separated reactants.

Scheme 1. Proposed mechanisms for the formation of iodine oxide particles (I2O5 and I2O4) in coastal environments that take advantage of the non-covalent interactions between potent precursors (HIO3 and HIO2) and I2O5 or I2O4 molecules. The concerted pathway is illustrated with green arrows.

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Figure 2. The CCSD(T)/aug-cc-pVTZ+LANL2DZ//M06-2X/aug-cc-pVDZ+LANL2DZ calculated reaction profiles for the HIO3-HIO3 (right panel) and HIO3-HIO2 (left panel) dehydration reactions. The zero-pointcorrected electronic energies are relative to the separated constituents and are given in units of kcal/mol. The concerted pathways for these two dehydrations are shown in wine color, and the key stationary points are given in Figure S1.

We next examined the same reaction but in the presence of an I2O5 molecule. Note that the reaction may occur via both concerted and stepwise pathways. However, we have only explored the stepwise reaction to illustrate the proof-of-concept. The reaction is triggered by the physisorption of two HIO3 molecules onto I2O5 in a sequential manner, resulting in the formation of Int1 and Int2, respectively (Figure 3). Int1 and Int2 have electronic energies of -20.0 and -31.8 kcal/mol, respectively, relative to the separated reactants. Int2 can then dehydrate in either a concerted or stepwise manner. The stepwise reaction involves the initial addition of I2O5-bound HIO3 across one of the I=O bonds of the other HIO3 molecule. This addition step has a barrier of 12.1 kcal/mol relative to Int2, with a transition state (TS1) lying 19.7 kcal/mol below the separated reactants. This results in the formation of an intermediate, Int3, which is 1.5 kcal/mol more stable than Int2 and has a reaction energy of -33.3 kcal/mol relative to the separated reactants. Int3 then decomposes into (I2O5)2 and water via TS2. This reaction step has a higher barrier of 23.7 kcal/mol relative to Int3, with TS2 lying 9.6 kcal/mol below the separated reactants. Overall, the formation of (I2O5)2 has a reaction energy of -19.2 kcal/mol relative to the separated reactants.

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TS2

Figure 3. The CCSD(T)/aug-cc-pVTZ+LANL2DZ//M06-2X/aug-cc-pVDZ+LANL2DZ calculated reaction profile for the I2O5-mediated (right panel) and I2O4-mediated (left panel) dehydration of two HIO3 molecules. The zero-point-corrected electronic energies are relative to the separated constituents and are given in units of kcal/mol. For the sake of clarity, the structures of I2O5 and I2O4 are shown in wire representation, whereas those of HIO3 are given in ball-stick representation.

It is interesting to compare the details of the stepwise HIO3-HIO3 reaction with and without an I2O5 molecule. In the absence of I2O5, the addition step involves a barrier of 21.5 kcal/mol. The presence of an I2O5 molecule causes a dramatic barrier lowering of 44% (by 9.4 kcal/mol). The barrier for the second dehydration step is lowered by 2.0 kcal/mol in the presence of I2O5. Moreover, Int2 is formed with an excess energy of 31.8 kcal/mol, which should easily compensate for the dehydration barriers. The uncatalyzed reaction has a reaction energy of 2.0 kcal/mol, whereas the I2O5-mediated reaction has a significantly favorable reaction energy of -19.2 kcal/mol. Considering that I2O5 is formed in the reaction, the reaction should be autocatalytic. We next examined the dehydration reaction between HIO3 and HIO2, which results in the formation of I2O4. Though the concerted dehydration of two HIO3 molecules has been previously reported,44 this is the first time the details of a mixed dehydration reaction are being reported. The mechanistic beauty of this dehydration reaction is that the end product, I2O4, has an O:I ratio of 2, which satisfies the criterion of an O:I ratio between 2 and 2.6 predicted by laboratory41 and recent field measurements25. As far as the mechanistic details of the reaction are concerned, the reaction follows both concerted and stepwise mechanisms (Figure 4), similar to the HIO3-HIO3 reaction shown in Figure 2. However, there are important distinctions between the two dehydration reactions. The prereaction complex in the HIO3-HIO2 reaction is 5.6 kcal/mol more stable than that in the HIO3-HIO3 reaction. Moreover, the former reaction has 2.7 kcal/mol more favorable reaction energy. The transition state for the concerted HIO3-HIO2 dehydration is 0.2 kcal/mol above the separated reactants whereas the transition state for the concerted HIO3-HIO3 dehydration is 5.0 kcal/mol above the separated reactants. In the stepwise reaction, the first barrier for the HIO3-HIO2 dehydration reaction is 6.5 kcal/mol larger than that for the HIO3-HIO3 dehydration reaction, while the second barrier is 5.9 kcal/mol smaller.

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Figure 4. The CCSD(T)/aug-cc-pVTZ+LANL2DZ//M06-2X/aug-cc-pVDZ+LANL2DZ calculated reaction profile for the I2O5-mediated (right panel) and I2O4-mediated (left panel) dehydration of HIO3 and HIO2 molecules. The zero-point-corrected electronic energies are relative to the separated constituents and are given in units of kcal/mol. For the sake of clarity, the structure of I2O5 is shown in wire representation, whereas that of iodic acid is given in ball-stick representation.

Under the catalytic presence of a single I2O5 molecule, the HIO3-HIO2 dehydration reaction occurs in a facile manner (Figure 4). The reaction can be initiated by the formation of either a non-covalent I2O5•HIO2 complex or a non-covalent I2O5•HIO3 complex. However, the results suggest that the former complex has a 9.1 kcal/mol larger binding energy and thus, is favored. Subsequently, HIO3 binds to this complex and results in the formation of Int2, which has a -43.3 kcal/mol binding energy relative to the separated reactants and is 11.5 kcal/mol more favorable than that for the analogous Int2 involved in the HIO3-HIO3 dehydration reaction. Though the addition and elimination barriers for the HIO3-HIO2 reaction are quite similar to those predicted for the HIO3-HIO3 reaction, its reaction energy is 11.0 kcal/mol more favorable. We also explored the effect of I2O4-induced non-covalent catalysis on the HIO3-HIO3 and HIO3HIO2 dehydration reactions (Figures 3 and 4). Though the I2O4-mediated reactions are less favorable than the I2O5-mediated ones, they are all downhill reactions with favorable reaction energies implying that both I2O4 and I2O5 may catalyze the growth of iodine oxide aerosol particles in the atmosphere.

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b) Air-Water Interface Inclusive BOMD Simulations. Previous computational studies44 suggest that the gas-phase reaction between I2O5 and a water molecule involves the breakage of an I-O bond and results in the formation of two HIO3 molecules (Pathway a in Scheme 2). Another interesting yet unexplored mechanistic pathway may involve the polar addition of water across one of the I=O bonds of an iodine oxide molecule, such as I2O5 or I4O10 (Pathway b in Scheme 2). We, thus, performed BOMD simulations to establish the precise mechanism of I2O5 and I4O10 hydration on an aqueous surface, i.e., on a water droplet of 191 water molecules. These BOMD simulations provide deeper insight into the dynamic behavior of I2O5 and I4O10 on an aqueous surface, which has not been explored previously. There is now an evolving body of evidence suggesting that aqueous surfaces provide unique reaction media for fundamental chemical events.48-51,63-68 For example, bimolecular reactions of Criegee intermediate at the air-water interface follow mechanisms that are distinctly different from those in the gas phase.50,63,66-68 Results from the BOMD simulations suggest that unlike the previously proposed gas-phase mechanism for the I2O5-H2O reaction,44 the interfacial water reaction of I2O5 follows a distinctly different mechanism that does not involve the breakage of any I-O bonds in I2O5 but rather the hydration of one of the terminal I=O bonds in I2O5 (Figure 5 and Movie S1). Though one of the I-O-I bonds is broken during interfacial I4O10 hydration, water addition across this bond does not occur (Figures 6-7 and Movie S2). Similar to interfacial I2O5 hydration, one of the terminal I=O bonds in I4O10 is hydrated. Another mechanistically appealing aspect of these interfacial I2O5 and I4O10 hydrations is that they follow stepwise mechanisms that involve multiple proton-transfer steps. These interfacial reactions occur on a ps time scale. The interfacial hydration mechanisms of I2O5 and I4O10 are also distinctly different; the product of I2O5 hydration, i.e., H2I2O6, remains dynamically stable over a simulated time scale of 35 ps, whereas that of I4O10 hydration, i.e., H2I4O11, remains stable only for a little over 5 ps and then deprotonates, forming a HI4O11- anion. This anion is found to be stable for up to 34 ps. After that, it decomposes into I3O8 and HIO3. Note that the products of I2O5 and I4O10 hydrations are retained on the water surface via interfacial hydrogen bonding, which implies that they may also escape out back to the gas-phase. The mechanistic details of these interfacial reactions are provided below.

Scheme 2. Two possible pathways for the water reaction of I2O5 (left panel) and I4O10 (right panel). Pathway a (blue color) refers to the previously proposed mechanism for the gas-phase reaction, whereas Pathway b (magenta color) corresponds to a new yet unexplored mechanism either in the gas phase or at the air-water interface.

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I2O5 Hydration on a Water Droplet. I2O5 hydration on a water droplet occurs via a multi-step mechanism. In the first step, the formation of an I1-Ow1 bond involving I2O5 and an interfacial water molecule occurs at 1.26 ps (Figure 5 and Movie S1). Here, we use notations Ow1, Ow2, and Ow3 to represent oxygens in the interfacial water molecules that participate in the reaction. Hw1 and Hw2 are the hydrogens attached to Ow1 and Ow2 that are also involved in the reaction. I1 and O1 are two of the I2O5 atoms that are hydrated in the reaction. The I1-Ow1 bond distance at 1.26 ps is significantly reduced compared to that at the start of the simulations; the I1-Ow1 bond distance shrunk from 3.99 Å at 0 ps to 2.55 Å at 1.26 ps. Beyond 16 ps, the interfacial water bonded to the I1 atom of I2O5 begins to lose one of its protons to a nearby interfacial water molecule. The transition state-like complex for this proton-transfer step is observed at 16.29 ps, and the Ow1-Hw1 and Ow2-Hw1 bond distances are found to be 1.21 and 1.20 Å, respectively. At 16.32 ps, the formation of an interfacial hydronium ion is observed, and

Figure 5. (a) Snapshot structures taken from the BOMD simulations, illustrating the stepwise multi-protontransfer-based mechanism for the hydration of I2O5 on a water droplet of 191 water molecules. (b) Time evolution of key bond distances involved in the multi-proton-transfer-based interfacial hydration of I2O5. The magenta circles in panel (b) illustrate the proton-transfer steps involved in the interfacial hydration reaction.

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the Ow1  Ow2 proton transfer is deemed complete, i.e., Ow1-Hw1 = 1.54 Å and Ow2-Hw1 = 1.04 Å. The hydronium ion then transfers one of its protons to another nearby interfacial water molecule. The transition state-like complex for this proton transfer is formed at 16.35 ps, which is evidenced by the Ow2Hw2 (1.22 Å) and Ow3-Hw2 (1.23 Å) bond distances. This results in the formation of a hydronium ion involving the Ow3 atom at 16.36 ps, where Ow2-Hw2 = 1.48 Å and Ow3-Hw2 = 1.04 Å. This hydronium remains stable for only ~0.3 ps. Subsequently, the proton from Ow3 is transferred back to Ow2. The transition state-like complex for the reverse Ow3  Ow2 proton transfer is formed at 16.66 ps. At this point, the Ow3-Hw2 bond distance is 1.23 Å, whereas the Ow2-Hw2 bond distance is 1.21 Å. At 16.69 ps, the formation of a hydronium ion involving Ow2 is complete. The Ow2-Hw2 bond is now transformed into a proper covalent bond, i.e., Ow2-Hw2 = 1.04 Å, whereas the Ow3-Hw2 bond becomes a hydrogen bond, i.e., Ow3-Hw2 = 1.40 Å. In the final step, the hydronium ion involving the Ow2 atom transfers one of its protons to the O1 atom of I2O5, which completes the interfacial hydration of I2O5. Interestingly, the proton involved in the Ow2  O1 reaction is the one that was initially transferred from Ow1 to Ow2. At 16.75 ps, the transition state-like complex for the final proton transfer is formed; the O1-Hw1 bond distance is 1.27 Å, whereas the Ow2-Hw1 bond distance is 1.26 Å. The formation of the hydrated I2O5 product (H2I2O6) is complete at 16.81 ps, as indicated by the covalent and hydrogen bonding nature of the O1-Hw1 bond (1.00 Å) and Ow2-Hw1 bond (1.63 Å), respectively. We ran BOMD simulations over a 35 ps time scale and observed that the product of I2O5 hydration, H2I2O6, remains dynamically stable over the simulated time scale. I4O10 Hydration on a Water Droplet. Similar to the hydration of I2O5, I4O10 hydration on a water droplet also follows a multi-proton-transfer-based mechanism. However, there are important mechanistic distinctions between these two hydration reactions on the aqueous surface. For example, the I2O5 hydration reaction is directly mediated by two interfacial water molecules, whereas the I4O10 hydration reaction is mediated by three interfacial water molecules (Figures 6 and 7). Furthermore, the product of I2O5 hydration (H2I2O6) remains dynamically stable over the simulated time scale of 35 ps, whereas that of I4O10 hydration (H2I4O11) is quite acidic in nature and undergoes deprotonation within a few ps of its formation (Figures 6 and S2) and then decomposes into I3O8 and HIO3 (Figure 7). The interfacial I4O10 hydration reaction is also initiated with the formation of an I1-Ow1 bond, which is observed at 0.19 ps in the BOMD simulations (Figure 6 and Movie S2). The I1-Ow1 bond distance at that point is 2.26 Å, reflecting significant bonding between I1 and Ow1 compared to that at the start of the simulations (I1-Ow1 = 4.34 Å at 0 ps). The remaining steps of the hydration mechanism are composed of multiple proton transfers involving four interfacial water molecules and I4O10. These proton-transfer steps are accompanied by the simultaneous weakening of the I1-O2 bond. The first proton transfer occurs between Ow1 and Ow2. The transition state-like complex for this step is formed at 1.11 ps, in which the proton (Hw1) is situated at an equal distance from both Ow1 and Ow2, i.e., Ow1-Hw1 = Ow2-Hw1 = 1.20 Å. The I1-O2 bond is noticeably weakened at that point. At 0.19 ps, the I1-O2 bond distance was 1.97 Å, which was stretched to 2.18 Å at 1.11 ps. The Ow1  Ow2 proton transfer, which results in the formation of a hydronium ion on the Ow2 atom, is deemed complete at 1.14 ps. The time evolution profiles of the two key Ow1-Hw1 and Ow2-Hw1 bond distances support this hydronium ion formation, i.e., at 1.14 ps, the Ow1-Hw1 bond distance is 1.53 Å, whereas the Ow2-Hw1 bond distance is 1.02 Å. The I1-O2 bond distance at 1.14 ps is 2.16 Å. The second proton transfer occurs between Ow2 and Ow3, for which the transition state-like complex is formed at 1.28 ps, and the resulting hydronium ion formation on Ow3 is achieved at 1.29 ps. In the transition statelike complex, the Ow2-Hw2 bond distance is 1.25 Å, whereas the Ow3-Hw2 bond distance is 1.22 Å. At 1.29 ps, the Ow3-Hw2 bond is fully developed into a covalent bond, whereas the Ow2-Hw2 bond is transformed into a hydrogen bond with a length of 1.44 Å. The I1-O2 bond distance at 1.29 ps is 2.15 Å.

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Figure 6. (a) Snapshot structures taken from the BOMD simulations, illustrating the stepwise multi-protontransfer-based mechanism for the hydration of I4O10 on a water droplet of 191 water molecules. (b) Time evolution of key bond distances involved in the multi-proton-transfer-based interfacial hydration of I4O10. he black rectangles in panel (b) illustrate the proton-transfer steps involved in the interfacial hydration reaction. See Figure S2 for the time evolution of key bond distances over the 15-30 ps simulated time scale.

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The hydronium ion involving the Ow3 atom remains stable for ~6.0 ps. After that point, one of its protons (Hw3) is transferred to a nearby interfacial water molecule. The activated complex for the Ow3  Ow4 proton transfer is observed at 7.39 ps, in which the Ow3-Hw3 and Ow4-Hw3 bond distances are found to be 1.22 Å and 1.24 Å, respectively. At 7.40 ps, this proton-transfer step, which results in the formation of a hydronium ion on Ow4, is deemed complete. The Ow3-Hw3 bond (1.41 Å) becomes a hydrogen bond, whereas the Ow4-Hw3 bond (1.05 Å) is fully formed at this point. The I1-O2 bond distance at 7.40 ps is 2.37 Å. The final step in the interfacial I4O10 hydration reaction involves proton transfer from Ow4 to O1 of I4O10. At 7.93 ps, Hw4 is partially transferred to O1, as indicated by the Ow4-Hw4 (1.24 Å) and O1-Hw4 (1.23 Å) bond distances. At 7.94 ps, the formation of the I4O10 hydrated product is achieved, with O1-Hw4 = 1.04 Å and Ow4-Hw4 = 1.41 Å. The I1-O2 bond is stretched to a length of 2.49 Å. Unlike the interfacial I2O5 hydration reaction, the product of the interfacial I4O10 hydration reaction (H2I4O11) remains stable for only ~5.0 ps. After that, H2I4O11 deprotonates and results in a HI4O11- anion. The activated complex for H2I4O11 deprotonation is formed at 13.33 ps, in which the bond distances of both Ow4-Hw4 (1.24 Å) and O1-Hw4 (1.23 Å) are nearly the same. At 13.35 ps, the formation of the HI4O11- anion is complete. The O1Hw4 bond distance is now 1.57 Å, whereas the Ow4-Hw4 bond distance is 1.03 Å. The I1-O2 bond distance at that point is 2.21 Å. The HI4O11- anion remains hydrogen bonded to Hw4 of the nearby interfacial water molecule for up to 34 ps. After that, it decomposes into I3O8 and HIO3. The time evolutions of all I-O bonds in I4O10 on the water droplet over the simulated time scale of 35 ps are shown in Figure 7. As I1 interacts with the interfacial water molecules, the I1-O2 bond begins to weaken, with maximum fluctuations of 2.00-3.00 Å among all I-O bonds. After 34 ps, the I1-O2 bond is permanently broken, resulting in I3O8 and HIO3.

ATMOSPHERIC IMPLICATIONS.

The results from our gas-phase calculations and simulations at the air-water interface have several important implications for aerosol particle formation in iodine-rich atmospheric environments where the occurrence of iodine-mediated particle formation has been established for the last two decades. Several studies have suggested that iodine particle formation could contribute to the local and regional CCN levels with a potential effect on radiative forcing. However, the lack of mechanistic information on iodine NPF prevents its inclusion in atmospheric models and hence a full assessment of its atmospheric impact. Our quantum chemical calculations suggest that I2O4- or I2O5-based single-molecule catalysis may primarily drive the formation of I2O4- and I2O5-based iodine oxide aerosol particles in the gas phase. The mechanistically appealing aspect of this single-molecule catalysis is that the formation of I2O4 or I2O5 occurs in a facile manner and with enhanced exothermicity. The catalytic mechanism can follow both concerted and stepwise pathways. Recent field measurements25 suggest that the iodine oxide particles participating in aerosol formation in coastal conditions should have an O:I ratio  2. According to our mechanistic proposal (Scheme 1), the single-molecule catalysis produces several iodine oxides, such as I2O4, I2O5, I4O8 and I4O10, which may subsequently grow into larger masses. The O:I ratio in I2O4 and I4O8 is 2.0, whereas that in I2O5 and I4O10 is 2.5. Both these O:I ratios are fully consistent with the recent suggested criterion,25 indicating that our mechanistic proposal is fully capable of explaining the formation and growth of atmospheric iodine oxide aerosols. The mechanisms of both I2O4- and I2O5-based catalysis involve the common iodine precursor HIO3, and thus, these catalytic processes strongly rely on the HIO3 concentration in the coastal air. Recent field data from Mace Head, Ireland, indicate that the HIO3 and HIO2 concentrations at the time of the nucleation event could rise to 108 and 106 molecules/cm3, respectively.25 Experiments further revealed the near linear dependence of the observed cluster formation on the HIO3 concentration in coastal environments. This experimental observation also supports our proposed catalytic mechanisms, which are initiated from a common precursor, HIO3. Considering that the gaseous iodine precursors that form HIO3 can be formed from gas-phase processes

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Figure 7. (a) Snapshot structures taken from the BOMD simulations, illustrating the interfacial watermediated breakage of the I1-O2 bond in I4O10 on a water droplet of 191 water molecules. The I1-O2 bond distance (in Å) is also given. (b) Time evolution of key I-O bond distances involved in the interfacial degradation of I4O10.

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not only in coastal areas but also in open ocean environments,70,71 as well as over Arctic and Antarctic sea ice,26,27,72-74 our proposed catalytic mechanism may have broad implications. In coastal Antarctica, where the highest iodine levels have been measured, iodine emissions are linked to seasonal sea ice and ocean biological activity.75 Therefore, there is potential for feedbacks between changing seasonal sea ice and iodine particle formation via aerosol indirect effects. Note also that recent ice core measurements76 have shown that atmospheric iodine levels have tripled from 1950 to 2010 in the Arctic region, driven by anthropogenic ozone pollution and the recent thinning of Arctic sea ice, which lead to enhanced iodine emissions from the ocean. This surprising finding also points to a potential recent increase in the formation of ultrafine iodine aerosol particles and suggests a linkage between human activities and climate change, and the enhanced occurrence of atmospheric iodine particle formation in the future atmosphere. The BOMD simulations at the air-water interface indicate that on a water droplet, both I2O5 and I4O10 react with interfacial water and are hydrated on a ps time scale. Interestingly, the product of I2O5 hydration (H2I2O6) remains dynamically stable over the simulated time scale of 35 ps, whereas that of I4O10 hydration (H2I4O11) undergoes deprotonation within a few ps of its formation (HI4O11-) and eventually decomposes into I3O8 and HIO3 on the aqueous surface. An important implication of these interfacial hydration reactions is that aqueous surfaces, which are ubiquitous in the atmosphere,48-51 may activate iodine oxide species for mixed aerosol formation with potent precursors in tropical conditions. The anticipated increase in ocean evapotranspiration in a future warmer climate will lead to higher

availability of atmospheric water surfaces and thus, points towards the broader implications of our simulation results. The products of the I2O5 and I4O10 hydration reactions on the aqueous surface

(H2I2O6 and HI4O11-) thus represent a new class of nucleating compounds that may interact with H2SO4, HIO3 or HIO2 via typical acid-base interactions (Scheme 3) to result in mixed aerosol particles in iodinerich environments. Though a variety of inorganic iodine species (e.g., I-, HOI, I2, ICl, IBr, and IO3-) have been observed in aerosols,29 this is the first time H2I2O6 and HI4O11- are suggested to be present in the atmospheric water surfaces. Note that HI4O11- remains stable on the aqueous surface for only ~23 ps, which implies that it can only participate in nucleation events that occur on a ps time scale. HI4O11-, which is produced in the interfacial I4O10 hydration reaction, eventually decomposes into I3O8 and HIO3. I3O8 could be a new nucleating iodine oxide that may be particularly relevant in aerosol formation on aqueous surfaces in coastal environments. As mentioned earlier, recent field measurements25 suggest that iodine oxides that have an O:I ratio of  2 may participate in aerosol-forming events in coastal environments. The O:I ratio of I3O8 is 2.6, which makes it an interesting yet unexplored nucleating compound. Though the role of I2O4 and I2O5 in aerosol-forming events has been previously suggested,20,25,33,41,45 this is the first time the involvement of I3O8 in coastal aerosol events has been proposed. Considering that the exact speciation of iodine in rainwater, snow, aerosols and seawater remains a significant challenge, our simulation results, which reveal an unaccounted fraction of soluble inorganic aerosol iodate, could play an important role in improving our understanding of iodine-enriched aerosols. For example, these interfacial insights will also provide useful leads for understanding the competition between I- and IO3- in European air77-80 and Danish rain77. It is also important to mention here that just like I2O4 and I2O5, I2O2 and I2O3 may also catalyze the gas-phase formation of various sized iodine oxides and play an important role in the atmospheric aerosol formation. I2O3 is very stable at atmospheric temperatures, and it forms from the species with highest observed concentration (IO).69 As a result, it has the largest concentration of all I2Ox (x = 2,3 and 4) in the lower troposphere. Even I2O2 in atmosphere has a typical lifetime of 1 second.69 Thus, it would be interesting to see whether these smaller iodine oxides can catalyze their own formation in the atmosphere and contribute towards the autocatalytic aerosol forming events. Finally, HIO2 and HIO3 also possess functionalities to catalyze the formation of iodine oxides and may constitute an interesting

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class of inorganic catalysts in the context of atmospheric aerosol formation. We are currently exploring these avenues using a combined approach of quantum chemical calculations and BOMD simulations.

Scheme 3. Plausible association complexes of hydrated I2O5 with sulfuric acid and periodic acid.

CONCLUSION

In summary, high-level quantum chemical calculations and BOMD simulations are used to propose new gas-phase and aqueous surface mechanisms for the formation and growth of new particles in iodine-rich coastal atmospheric environments. This mechanism involves I2O4- or I2O5-based single-molecule catalysis, which takes advantage of the non-covalent interactions between potent precursors, such as HIO3 and HIO2, and an I2O4 or I2O5 molecule. Particle formation via single-molecule catalysis occurs in a facile manner and has enhanced exothermicity. This makes the single-molecule catalysis mechanism better than the previously proposed concerted mechanism, which involves a larger barrier and has very low exothermicity. This new mechanism fully explains previously observed experimental trends and thus, provides a unique mechanistic framework for understanding the formation and growth of iodine oxide aerosol particles in coastal conditions. BOMD simulations at the air-water interface suggest that I2O5 and I4O10, which are two of the most common nucleating precursors in coastal environments, react with interfacial water on a ps time scale. An important implication of these results is that interfacially hydrated I2O5 and I4O10 molecules (H2I2O6 and HI4O11- or I3O8) represent a new class of nucleating precursors, which may react with potent acids such as HIO3, HIO2 and H2SO4 to form mixed aerosol particles in iodine-rich environments. We are currently investigating the role of other potent iodine oxides such as I2O2 and I2O3 in the aerosol particle formation in atmosphere.

ASSOCIATED CONTENT Supporting Information

Reaction profiles for the concerted gas-phase HIO3 + HIO3  I2O5 + H2O and HIO3 + HIO2  I2O4 + H2O dehydration reactions, time evolution of key bond distances involved in the interfacial hydration of I4O10 over the 15-30 ps simulated time scale, table containing optimized geometries of key species involved in all gas-phase reactions, and videos of trajectories of the BOMD simulations for the I2O5 and I4O10 reaction pathways at the air-water interface. The Supporting Information is available free of charge on the ACS Publications website. Trajectories of the BOMD simulation for the I2O5 hydration reaction at the air-water interface (MPG)

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Trajectories of the BOMD simulation for the I4O10 hydration reaction at the air-water interface (MPG)

ACKNOWLEDGEMENTS

This work was supported by the University of Nebraska Holland Computing Center.

AUTHOR INFORMATION Corresponding Authors

*Email: [email protected]; [email protected]

NOTES

The authors declare no competing financial interest.

REFERENCES 1. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, 2013: Climate Change 2013: The Physical Science Basis. Stocker, T. F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V., Midgley, P. M., Eds.; Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2013. 2. Rosenfeld, D.; Lohmann, U.; Raga, G. B.; O’Dowd, C. D.; Kulmala, M.; Fuzzi, S.; Reissell, A.; Andreae, M. O., Flood or drought: how do aerosols affect precipitation? Science 2008, 321, 1309. 3. Li, G.; Wang, Y.; Lee, K.-H.; Diao, Y.; Zhang, R., Impacts of aerosols on the development and precipitation of a mesoscale squall line. J. Geophys. Res. 2009, 114, D17205. 4. Li, Z.; Li, C.; Chen, H.; Tsay, S.-C.; Holben, B.; Huang, J.; Li, B.; Maring, H.; Qian, Y.; Shi, G.; Xia, X.; Yin, Y.; Zheng, Y.; Zhuang, G., East Asian studies of Tropospheric Aerosols and their Impact on Regional Climate (EAST-AIRC): An overview. J. Geophys. Res. 2011, 116, D00K34. 5. Zhang, R.; Khalizov, A.; Wang, L.; Hu, M.; Xu, W., Nucleation and growth of nanoparticles in the atmosphere. Chem. Rev. 2012, 112, 1957. 6. Zhang, R.; Wang, G.; Guo, S.; Zamora, M. L.; Ying, Q.; Lin, Y.; Wang, W.; Hu, M.; Wang, Y., Formation of urban fine particulate matter. Chem. Rev. 2015, 115, 3803. 7. Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics From Air Pollution to Climate Change. Wiley Interscience, New York, 2006. 8. Weber, R. J.; Chen, G.; Davis, D. D.; Mauldin III, R. L.; Tanner, D. J.; Eisele, F. L.; Clarke, A. D.; Thornton, D. C.; Bandy, A. R., Measurements of enhanced H2SO4 and 3–4 nm particles near a frontal cloud during the First Aerosol Characterization Experiment (ACE 1). J. Geophys. Res. 2001, 106, 24107. 9. Weber, R. J.; Marti, J. J.; McMurry, P. H.; Eisele, F. L.; Tanner, D. J.; Jefferson, A., Measured atmospheric new particle formation rates: implications for nucleation mechanisms. Chem. Eng. Commun. 1996, 151, 53.

18


Page 18 of 24

Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10. Weber, R. J.; Marti, J. J.; McMurry, P. H.; Eisele, F. L.; Tanner, D. J.; Jefferson, A., Measurements of new particle formation and ultrafine particle growth rates at a clean continental site. J. Geophys. Res. 1997, 102, 4375. 11. Zhang, R. Y.; Suh, I.; Zhao, J.; Zhang, D.; Fortner, E. C.; Tie, X. X.; Molina, L. T.; Molina, M. J., Atmospheric new particle formation enhanced by organic acids. Science 2004, 304, 1487. 12. Zhang, R.; Wang, L.; Khalizov, A. F.; Zhao, J.; Zheng, J.; McGraw, R. L.; Molina, L. T., Formation of nanoparticles of blue haze enhanced by anthropogenic pollution. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 17650. 13. Smith, J. N.; Dunn, M. J.; VanReken, T. M.; Iida, K.; Stolzenburg, M. R.; McMurry, P. H.; Huey. L. G., Chemical composition of atmospheric nanoparticles formed from nucleation in Tecamac, Mexico: Evidence for an important role for organic species in nanoparticle growth. Geophys. Res. Lett. 2008, 35, L04808. 14. Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics. Wiley, New York, 1998. 15. Ansari, A. S.; Pandis, S. N., Response of Inorganic PM to Precursor Concentrations. Environ. Sci. Techno. 1998, 32, 2706. 16. Blanchard, C. L.; Roth, P. M.; Tanenbaum, S. J.; Ziman, S. D.; Seinfeld, J. H., The use of ambient measurements to identify which precursor species limit aerosol nitrate formation. J. Air. Waste Manag. Assoc. 2000, 50, 2073. 17. West, J. J.; Ansari, A. S.; Pandis, S. N., Marginal PM25: nonlinear aerosol mass response to sulfate reductions in the eastern United States. J. Air. Waste Manag. Assoc. 1999, 49, 1415. 18. Smith, J. N.; Barsanti, K. C.; Friedli, H. R.; Ehn, M.; Kulmala, M.; Collins, D. R.; Scheckman, J. H.; Williams, B. J.; McMurry, P. H., Observations of aminium salts in atmospheric nanoparticles and possible climatic implications. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6634. 19. Arquero, K. D.; Gerber, R. B.; Finlayson-Pitts, B. J., The role of oxalic acid in new particle formation from methanesulfonic acid, methylamine, and water. Environ. Sci. Technol. 2017, 51, 2124. 20. O'Dowd, C. D.; Jimenez, J. L.; Bahreini, R.; Flagan, R. C.; Seinfeld, J. H.; Hämeri, K.; Pirjola, L.; Kulmala, M.; Jennings, S. G.; Hoffmann, T., Marine aerosol formation from biogenic iodine emissions. Nature 2002, 417, 632. 21. Yoon, Y. J.; O’Dowd, C. D.; Jennings, S. G.; Lee, S. H., Statistical characteristics and predictability of particle formation events at Mace Head. J. Geophys. Res. 2006, 111, D13204. 22. O’Dowd, C. D.; de Leeuw, G., Marine aerosol production: a review of the current knowledge. Phil. Trans. R. Soc. A 2007, 365, 1753. 23. McFiggans, G.; Bale, C. S. E.; Ball, S. M.; Beames, J. M.; Bloss, W. J.; Carpenter, L. J.; Dorsey, J.; Dunk, R.; Flynn, M. J.; Furneaux, K. L.; Gallagher, M. W.; Heard, D. E.; Hollingsworth, A. M.; Hornsby, K.; Ingham, T.; Jones, C. E.; Jones, R. L.; Kramer, L. J.; Langridge, J. M.; Leblanc, C.; LeCrane, J.-P.; Lee, J. D.; Leigh, R. J.; Longley, I.; Mahajan, A. S.; Monks, P. S.; Oetjen, H.; Orr-Ewing, A. J.; Plane, J. M. C.; Potin, P.; Shillings, A. J. L.; Thomas, F.; von Glasow, R.; Wada, R.; Whalley, L. K.; Whitehead, J. D., Iodine-mediated coastal particle 19



formation: an overview of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) Roscoff coastal study. Atmos. Chem. Phys. 2010, 10, 29752999. 24. Mahajan, A. S.; Sorribas, M.; Gómez Martín, J. C.; MacDonald, S. M.; Gil, M.; Plane, J. M. C.; Saiz-Lopez, A., Concurrent observations of atomic iodine, molecular iodine and ultrafine particles in a coastal environment. Atmos. Chem. Phys. 2011, 11, 2545. 25. Sipilä, M.; Sarnela, N.; Jokinen, T.; Henschel, H.; Junninen, H.; Kontkanen, J.; Richters, S.; Kangasluoma, J.; Franchin, A.; Peräkylä, O.; Rissanen, M. P.; Ehn, M.; Vehkamäki, H.; Kurten, T.; Berndt, T.; Petäjä, T.; Worsnop, D.; Ceburnis, D.; Kerminen, V.-M.; Kulmala, M.; O’Dowd, C. D., Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3. Nature 2016, 537,532. 26. Allan, J. D.; Williams, P. I.; Najera, J.; Whitehead, J. D.; Flynn, M. J.; Taylor, J. W.; Liu, D.; Darbyshire, E.; Carpenter, L. J.; Chance, R.; Andrews, S. J.; Hackenberg, S. C.; McFiggans, G., Iodine observed in new particle formation events in the Arctic atmosphere during ACCACIA. Atmos. Chem. Phys. 2015, 15, 5599. 27. Roscoe, H. K.; Jones, A. E.; Brough, N.; Weller, R.; Saiz-Lopez, A.; Mahajan, A. S.; Schoenhardt, A.; Burrows, J. P.; Fleming, Z. L., Particles and iodine compounds in coastal Antarctica. J. Geophys. Res. 2015, 120, 7144. 28. Saiz-Lopez, A.; Plane, J. M. C.; McFiggans, G.; Williams, P. I.; Ball, S. M.; Bitter, M.; Jones, R. L.; Hongwei, C.; Hoffmann, T., Modelling molecular iodine emissions in a coastal marine environments: the link to new particle formation. Atmos. Chem. Phys. 2006, 6, 883. 29. Saiz-Lopez, A.; Plane, J. M. C.; Baker, A. R.; Carpenter, L. J.; von Glasow, R.; Martín, J. C. G.; McFiggans, G.; Saunders, R. W., Atmospheric chemistry of iodine. Chem. Rev. 2012, 112, 1773. 30. Carpenter, L. J.; MacDonald, S. M.; Shaw, M. D.; Kumar, R.; Saunders, R. W.; Parthipan, R.; Wilson, J.; Plane, J. M. C., Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine. Nat. Geosci. 2013, 6, 108. 31. O’Dowd, C. D.; Geever, M.; Hill, M. K.; Smith, M. H.; Jennings, S. G., New particle formation: nucleation rates and spatial scales in the clean marine coastal environment. Geophys. Res. Lett. 1998, 25, 1661. 32. Hoffmann, T.; O’Dowd, C. D.; Seinfeld, J. H., Iodine oxide homogeneous nucleation: an explanation for coastal new particle production. Geophys. Res. Lett. 2001, 28, 1949. 33. Jimenez, J. L.; Bahreini, R.; Cocker III, D. R.; Zhuang, H.; Varutbangkul, V.; Flagan, R. C.; Seinfeld, J. H.; O’Dowd, C. D.; Hoffmann, T., New particle formation from photooxidation of diiodomethane (CH2I2) J. Geophys. Res. 2003, 108, 4318. 34. Saiz-Lopez, A.; Plane, J. M. C., Novel iodine chemistry in the marine boundary layer. Geophys. Res. Lett. 2004, 31, L04112. 35. Martín, J. C. G.; Gálvez, O.; Baeza-Romero, M. T.; Ingham, T.; Plane, J. M. C.; Blitz, M. A., On the mechanism of iodine oxide particle formation. Phys. Chem. Chem. Phys. 2013, 15, 15612. 20


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36. McFiggans, G.; Coe, H.; Burgess, R.; Allan, J.; Cubison, M.; Alfarra, M. R.; Saunders, R.; SaizLopez, A.; Plane, J. M. C.; Wevill, D.; Carpenter, L.; Rickard, A. R.; Monks, P. S., Direct evidence for coastal iodine particles from Laminaria macroalgae-linkage to emissions of molecular iodine. Atmos. Chem. Phys. 2004, 4, 701. 37. Saunders, R. W.; Kumar, R.; Gomez Martin, J. C.; Mahajan, A. S.; Murray, B. J.; Plane, J. M. C., Studies of the formation and growth of aerosol from molecular iodine precursor. Int. J. Res. Phys. Chem. Chem. Phys. 2010, 224, 1095. 38. Swietlicki, E.; Hansson, H. C.; Hameri, K.; Svenningsson, B.; Massling, A.; McFiggans, G.; McMurry, P. H.; Petaja, T.; Tunved, P.; Gysel, M.; Topping, D.; Weingartner, E.; Baltensperger, U.; Rissler, J.; Wiedensohler, A.; Kulmala, M., Hygroscopic properties of submicrometer atmospheric aerosol particles measured with HTDMA instruments in various environments-a review. Tellus B Chem. Phys. Meteorol. 2008, 60, 432. 39. Väkevä, M.; Hämeri, K.; Aalto, P. P., Hygroscopic properties of nucleation mode and Aitken mode particles during nucleation bursts and in background air on the west coast of Ireland. J. Geophys. Res.-Atmos. 2002, 107, 8104. 40. Kumar, R.; Saunders, R. W.; Mahajan, A. S.; Plane, J. M. C.; Murray, B. J., Physical properties of iodate solutions and the deliquescence of crystalline I2O5 and HIO3. Atmos. Chem. Phys. 2010, 10, 12251. 41. Saunders, R. W.; Plane, J. M. C., Formation pathways and composition of iodine oxide ultra-fine particles. Environ. Chem. 2005, 2, 299. 42. Glowa, G.A.; Ball, J.M. Gas Phase Radiolysis of Iodine, Atomic Energy of Canada Limited, Report, 2012, 153-126530-440-017. 43. Vaattovaara, P.; Huttunen, P. E.; Yoon, Y. J.; Joutsensaari, J.; Lehtinen, K. E. J.; O’Dowd, C. D.; Laaksonen, A., The composition of nucleation and Aitken modes particles during coastal nucleation events: evidence for marine secondary organic contribution. Atmos. Chem. Phys. 2006, 6, 4601. 44. Khanniche, S.; Louis, F.; Cantrel, L.; Cernušák, I., Computational study of the I2O5 + H2O = 2 HOIO2 gas-phase reaction. Chem. Phys. Lett. 2016, 662, 114. 45. Saunders, R. W.; Kumar, R.; Gómez Martín, J. C.; Mahajan, A. S.; Murray, B. J.; Plane, J. M. C., Studies of the formation and growth of aerosol from molecular iodine precursor. Z. Phys. Chem. 2010, 224, 1095. 46. Misra, A.; Marshall, P., Computational investigations of iodine oxides. J. Phys. Chem. A 1998, 102, 9056. 47. Gálvez, O.; Martín, J. C. G.; Gómez, P. C.; Saiz-Lopez, A.; Pacios, L. F., A theoretical study on the formation of iodine oxide aggregates and monohydrates. Phys. Chem. Chem. Phys. 2013, 15, 15572. 48. Gerber, R. B.; Varner, M. E.; Hammerich, A. D.; Riikonen, S.; Murdachaew, G.; Shemesh, D.; Finlayson-Pitts, B. J., Computational studies of atmospherically-relevant chemical

21



reactions in water clusters and on liquid water and ice surfaces. Acc. Chem. Res. 2015, 48, 399. 49. Ravishankara, A. R., Heterogeneous and multiphase chemistry in the troposphere. Science 1997, 276, 1058. 50. Zhong, J.; Kumar, M.; Francisco, J. S.; Zeng, X. C., Insight into chemistry on cloud/aerosol water surfaces. Acc. Chem. Res. 2018, 51, 1229. 51. Donaldson, D. J.; Vaida, V., The influence of organic films at the air-aqueous boundary on atmospheric processes. Chem. Rev. 2006, 106, 1445. 52. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian 09, revision D.01; Gaussian Inc.: Wallingford, CT, 2009. 53. Zhao, Y.; Truhlar, D. G., The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215. 54. Kendall, R. A.; Dunning Jr., T. H.; Harrison, R. J., Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions. J. Chem. Phys. 1992, 96, 6796. 55. Noga, J.; Bartlett, R. J., The full CCSDT model for molecular electronic structure. J. Chem. Phys. 1987, 86, 7041. 56. VandeVondele, J.; Krack, M.; Mohamed, F.; Parrinello, M.; Chassaing, T.; Hutter, J., Quickstep: fast and accurate density functional calculations using a mixed Gaussian and plane waves approach. Comput. Phys. Commun. 2005, 167, 103. 57. Perdew, J. P.; Wang, Y., Pair-distribution function and its coupling-constant average for the spin-polarized electron gas. Phys. Rev. B 1992, 45, 13244. 58. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. 59. Grimme, S., Accurate description of van der Waals complexes by density functional theory including empirical corrections. J. Comput. Chem. 2004, 25, 1463. 60. Grimme, S., Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 2006, 27, 1787. 61. Goedecker, S.; Teter, M.; Hutter, J., Separable dual-space Gaussian pseudopotentials. Phys. Rev. B 1996, 54, 1703. 62. Hartwigsen, C.; Goedecker, S.; Hutter, J., Relativistic separable dual-space Gaussian pseudopotentials from H to Rn. Phys. Rev. B 1998, 58, 3641. 22


Page 22 of 24

Page 23 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


63. Zhu, C. Q.; Kumar, M.; Zhong, J.; Lei, L.; Francisco, J. S.; Zeng, X. C., New mechanistic pathways for Criegee-water chemistry at the air/water interface. J. Am. Chem. Soc. 2016, 138, 11164. 64. Li, L.; Kumar, M.; Zhu, C. Q.; Zhong, J.; Francisco, J. S.; Zeng, X. C., Near-barrierless ammonium bisulfate formation via a loop-structure promoted proton-transfer mechanism on the surface of water. J. Am. Chem. Soc. 2016, 138, 1816. 65. Zhong, J.; Zhao, Y.; Li, L.; Li, H.; Francisco, J. S.; Zeng, X. C., Interaction of the NH2 radical with the surface of a water droplet. J. Am. Chem. Soc. 2015, 137, 12070. 66. Zhong, J.; Kumar, M.; Zhu, C. Q.; Francisco, J. S.; Zeng, X. C., Surprising stability of larger Criegee intermediates on aqueous interfaces. Angew. Chem. Int. Ed. 2017, 56, 7740. 67. Kumar, M.; Zhong, J.; Zeng, X. C.; Francisco, J. S., Criegee intermediate-hydrogen sulfide chemistry at the air/water interface. Chem. Sci. 2017, 8, 5385. 68. Kumar, M.; Zhong, J.; Zeng, X. C.; Francisco, J. S., Reaction of Criegee intermediate with nitric acid at the air-water interface. J. Am. Chem. Soc. 2018, 140, 4913. 69. Saiz-Lopez, A.; Fernandez, R. P.; Ordóñez, C.; Kinnison, D. E.; Gómez Martín, J. C.; Lamarque, J.-F.; Tilmes, S., Iodine chemistry in the troposphere and its effect on ozone. Atmos. Chem. Phys. 2014, 14, 13119. 70. Mahajan, A. S.; Plane, J. M. C.; Oetjen, H.; Mendes, L.; Saunders, R. W.; Saiz-Lopez, A.; Jones, C. E.; Carpenter, L. J.; McFiggans, G. B., Measurement and modelling of tropospheric reactive halogen species over the tropical Atlantic Ocean. Atmos. Chem. Phys. 2010, 10, 4611. 71. Lawler, M. J.; Mahajan, A. S.; Saiz-Lopez, A.; Saltzman, E. S., Observations of I2 at a remote marine site. Atmos. Chem. Phys. 2014, 14, 2669. 72. Saiz-Lopez, A.; Plane, J. M. C.; Mahajan, A. S.; Anderson, P. S.; Bauguitte, S. J. -B.; Jones, A. E.; Roscoe, H. K.; Salmon, R. A.; Bloss, W. J.; Lee, J. D.; Heard, D. E., On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O3, HOx, NOx and the Hg lifetime. Atmos. Chem. Phys. 2008, 8, 887. 73. Mahajan, A. S.; Shaw, M.; Oetjen, H.; Hornsby, K. E.; Carpenter, L. J.; Kaleschke, L.; TianKunze, X.; Lee, J. D.; Moller, S. J.; Edwards, P.; Commane, R.; Ingham, T.; Heard, D. E.; Plane, J. M. C., Evidence of reactive iodine chemistry in the Arctic boundary layer. J. Geophys. Res. 2010, 115, D20303. 74. Atkinson, H. M.; Huang, R.-J.; Chance, R.; Roscoe, H. K.; Hughes, C.; Davison, B.; Schönhardt, A.; Mahajan, A. S.; Saiz-Lopez, A.; Hoffmann, T.; Liss, P. S., Iodine emissions from the sea ice of the Weddell Sea. Atmos. Chem. Phys. 2012, 12, 11229. 75. Cuevas, C. A.; Maffezzoli, N.; Corella, J. P.; Spolaor, A.; Vallelonga, P.; Kjær, H. A.; Simonsen, M.; Winstrup, M.; Vinther, B.; Horvat, C.; Fernandez, R. P.; Kinnison, D.; Lamarque, J.-F.; Barbante, C.; Saiz-Lopez, A., Rapid increase in atmospheric iodine levels in the North Atlantic since the mid-20th century. Nat. Commun. 2018, 9, 1452. 76. Saiz-Lopez, A.; Blaszczak-Boxe, C. S.; Carpenter, L., A mechanism for biologically induced iodine emissions from sea ice. J. Atmos. Chem. Phys. 2015, 15, 9731.

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77. Hou, X. L.; Aldahan, A.; Nielsen, S. P.; Possnert, G., Time series of 129I and 127I speciation in precipitation from Denmark. Environ. Sci. Technol. 2009, 43, 6522. 78. Truesdale, V. W.; Jones, S. D., The variation of iodate and total iodine in some UK rainwaters during 1980-1981. J. Hydrol. 1996, 179, 67. 79. Campos, M. L. A. M.; Nightingale, P. D.; Jickells, T. D., A comparison of methyl iodide emissions from seawater and wet depositional fluxes of iodine over the southern North Sea. Tellus B 1996, 48, 106. 80. Baker, A. R.; Tunnicliffe, C.; Jickells, T. D., Iodine speciation and deposition fluxes from the marine atmosphere. J. Geophys. Res. 2001, 106, 28743.

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