Adsorption and heterogeneous reactions of ClONO₂ and N₂O₅ on/with NaCl aerosol

Abstract

The adsorption and heterogeneous reactions of ClONO₂ and N₂O₅ on the NaCl (100) surface have been investigated by performing density functional theory (DFT) calculations. Possible adsorption structures, energies and electronic properties of ClONO₂ and N₂O₅ were studied by considering multiple possible adsorption sites without symmetry restriction. The most stable adsorption configurations are a vertical adsorption mode for ClONO₂ and a horizontal adsorption mode for N₂O₅, with the adsorption energies of 14.81 and 13.67 kcal mol⁻¹, respectively. Two possible reactions were proposed on the NaCl (100) surface. Adsorbed ClONO₂ and N₂O₅ could react with HCl or hydrolyze on the NaCl surface to form HNO₃ and other gaseous species (Cl₂, HOCl and ClNO₂). More importantly, ClONO₂ and N₂O₅ can actively participate in the heterogeneous reaction with NaCl to generate Cl₂ and ClNO₂, which could be photolyzed to produce reactive chlorine atoms. The adsorbed H₂O molecule plays an important role in the key elementary step and promotes the formation of the products. The present results rationalize previous experimental studies well and enrich our understanding of the source of halogen atoms in the marine boundary layer.

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1. Introduction

Approximately 70% of the earth's surface is covered by oceans, making the exchange of marine aerosols and trace gases between the marine boundary layer (MBL) and the troposphere potentially important for atmospheric processes.¹ Sea salt aerosols, which are generated by wind-induced wave action and bubble bursting of seawater, contribute to most of the marine aerosol present in the atmosphere, with uncertain annual emission estimates ranging from 0.3 Pg to 30 Pg.^2,3 During the last few decades, growing interest has been given to understanding of the fundamental chemical and physical processes in the marine boundary layer, especially in the heterogeneous reactions of nitrogen oxides such as NO₂, N₂O₅ and ClONO₂ with sea salt aerosols, with the goal to reveal the role they play in atmospheric chemistry.^4–7 It has been proposed that the heterogeneous reactions taking place on or in sea salt aerosols may provide the main possible sources for gaseous chlorine-containing species (including HCl, Cl₂, ClNO₂, and HOCl etc.). Upon photolysis, the inert halogen reservoir species can convert into active halogen atoms, which may affect the ozone balance and oxidative potential of the local atmosphere, specifically in the marine boundary layer.

Of particular interest is the heterogeneous loss of ClONO₂ and N₂O₅ on the surface of solid NaCl (major component of sea salts aerosols). ClONO₂ is one of the most important temporary reservoir species for reactive chlorine. For example, in the winter, ClONO₂ accounts for approximately 50% of the total inorganic chlorine (Cl_y = HCl + ClO + HOCl + ClONO₂) at mid-northern latitudes stratosphere.^8,9 N₂O₅ represents a significant reactive intermediate in the atmospheric chemistry of nitrogen oxides and nitrate aerosol, especially during nighttime.¹⁰ Potential fates of ClONO₂ and N₂O₅ in the troposphere include deposition as well as reacting with HCl or their hydrolysis on surfaces. Furthermore, the gaseous ClONO₂ and N₂O₅ could react directly with NaCl according to:

ClONO₂ + NaCl → Cl₂ + NaNO₃ (1)

N₂O₅ + NaCl → ClNO₂ + NaNO₃ (2)

In the atmosphere, the gaseous products Cl₂ and ClNO₂ will photolyze rapidly (λ > 290 nm) and generate highly reactive chlorine atoms.¹¹ Chlorine atoms are known to have a significant impact on the formation and fate of tropospheric ozone. It can react with O₃ or react readily with organics in a manner similar to OH. In the presence of NO_x, the Cl-organic reaction may result in the formation of O₃, rather than its destruction.

Several experimental studies have been devoted to investigating the heterogeneous interactions of nitrogen oxides with NaCl. In 1994, Keyser and co-workers performed the quantitative study on the reaction of ClONO₂ with NaCl over a temperature range 220–300 K. They found Cl₂ was the sole product, and the determined uptake coefficient at 296 K was γ = (4.6 ± 3.0) × 10⁻³ on dry NaCl substrates. With added H₂O, a trace of HOCl was observed in addition to the Cl₂ product.¹² Subsequently, in the experiments of Gebel et al. and Zelenov et al., the reactions have been further confirmed. Two possible reaction paths were taken into consideration when water was present, ClONO₂ could react with NaCl to form Cl₂ and NaNO₃, or hydrolyze on the surface to generate HOCl and HNO₃.^5,13 The heterogeneous reaction of N₂O₅ with NaCl has been investigated by different techniques.^10,11,14,15 Results indicated that the uptake coefficients (γ) were in a wide range of 10⁻⁴ to 10⁻², and increased with increase in relative humidity (RH). The importance of water vapor in the real atmosphere has been proposed and demonstrated. Like ClONO₂, two different processes were also founded during the reaction of N₂O₅ with deliquesced NaCl or solid NaCl that holds surface-adsorbed water (SAW), both ClNO₂ and HNO₃ were observed as gaseous products from the N₂O₅–salt interactions.^4,10

In order to assess the source and role of halogen atoms, it is important to elucidate the kinetics, products, and mechanisms for reactions of nitrogen oxides with sea salt, especially for its major component, NaCl. However, while the overall kinetics and products are well-known by experiments,^4,5,10–15 there have been no studies conducted to the interaction mechanisms between ClONO₂ or N₂O₅ and NaCl. The experimental analysis helped to establish that the (100) surface of NaCl is dominant. Herein, we chose the (100) surface to study the adsorption and heterogeneous reactions of ClONO₂ and N₂O₅ using the density functional theory (DFT) slab calculations. This theoretical method has been successfully applied in the environmental investigations and can provide accurate predictions for the reaction mechanism through energy calculations.^16–18 Indices of crystal face is the inverse ratio of intercept coefficient for crystal face in three crystallographic axis (a, b and c). NaCl (100) surface is the crystal face, which is parallel to the two crystal axis (b, c), and the intercept in axis a is C. That is equivalent to the plane X = C when in the XYZ space. Stable adsorption structures as well as transition states for both clean NaCl (100) surface and water adsorbed NaCl have been determined. Particular attention was paid to explore the role of water in the formation of chlorine-containing species. We anticipate that results from the study of ClONO₂ and N₂O₅ adsorption and reaction on the ideal NaCl (100) surface will provide some insights into chemistry of the aerosol surfaces.

2. Computational methods

All calculations were performed in the framework of DFT by using the program package DMol³ in Materials Studio (version 6.0) of Accelrys Inc.¹⁹ The exchange–correlation energy was calculated within the GGA–PW91 approximation.^20,21 The double-numeric quality basis set with polarization functions (DNP) was used in the calculations. The fine quality mesh was used for numerical integration, in which the tolerances of energy, gradient, displacement, and self-consistent field (SCF) convergence were 1 × 10⁻⁵ hartree, 2 × 10⁻³ hartree Å⁻¹, 5 × 10⁻³ Å, and 1 × 10⁻⁶, respectively. The real space cutoff radius was maintained as 5.2 Å. On the basis of the convergence test, the surface Brillouin zone was sampled with a 2 × 2 × 1 Monkhorst–Pack k-point grid.²²

The (100) surface of NaCl was modeled by a p (3 × 3) supercell with a dimension of 12.10 Å × 12.10 Å × 23.55 Å. Thirty-six NaCl molecular units in the slab were distributed in four layers. The vacuum region separating the slabs along the z direction was set to 15 Å. In all calculations, the bottom two layers were kept fixed in their bulk lattice positions, while the top two layers together with the adsorbates were allowed to relax. Fig. 1 shows the super cell model used in this work. The flow of charges was estimated by Mulliken population analysis, which has been shown to be a useful tool.^23,24 Spin polarization was applied to all calculations.

Fig. 1 Structures of the clean NaCl (100) surface, ClONO₂ and N₂O₅. O atoms are red, N atoms are blue, Cl atoms are green and Na atoms are purple. Angles are in degree.

Following usual convention, the adsorption energy E_ad was defined as:

E_ad = E_total − (E_adsorbate + E_surf) (3)

where E_total, E_adsorbate, and E_surf represent the energies of the adsorbed system, free adsorbate, and the clean surface, respectively. With this definition, a negative E_ad value denotes an exothermic adsorption process, and more negative E_ad shows stronger adsorption between free adsorbate and the clean surface.

To determine the activation energy for a specific reaction path, a transition state (TS) connecting two structures through a minimum energy path was identified by complete synchronous transit (LST) and quadratic synchronous transit (QST) search methods. Each TS was confirmed by vibrational frequency analysis with one and only one imaginary frequency. The energy barrier (ΔE_b) and reaction heat (ΔE_r) of each elementary step were calculated by:

ΔE_b = E_TS − E_R (4)

ΔE_r = E_P − E_R (5)

where E_TS, E_R, and E_P are the energies of the transition state, reactant, and product, respectively.

3. Results and discussion

To test the reliability of the theoretical calculations, we compared several calculated results with available experimental values. (i) The optimized unit cell parameter of the bulk NaCl is 5.597 Å, while the experimental value is 5.640 Å.²⁵ (ii) The geometrical parameters of H₂O, ClONO₂ and N₂O₅ were calculated at the GGA–PW91 level. As shown in Fig. S1 of ESI,† the values match well with the available experimental data, and the maximum relative error is less than 2.0%.^26–28 (iii) The calculated adsorption energy of H₂O on the NaCl (100) surface is −13.99 kcal mol⁻¹, while previous experimental findings are in a range of −13.87 to −11.58 kcal mol⁻¹.^29,30 All the calculated data show good agreement with the corresponding reference values, indicating the acceptable accuracy and reliability of the method employed.

3.1 Adsorption of ClONO₂ and N₂O₅ on the NaCl (100) surface

ClONO₂ is a planar, near prolate molecule with a free electron pair residing on the N atom.³¹ In order to get an insight into the nature of ClONO₂–substrate interaction, we have carried out an extensive study for ClONO₂ molecule on NaCl (100) by considering different adsorption modes (flat and vertical) and various adsorption sites (top, bridge, etc.). The optimized stable adsorption configurations and key structural parameters are shown in Fig. S2 of ESI.† Table 1 lists the corresponding adsorption energies (E_ad) calculated by DMol³. As displayed in Fig. S2,† configurations I-1 to I-16 represent the vertical manner with single-atom adsorption, where the ClONO₂ molecule could either adsorb at the Cl⁻ top site (E_ad, −7.35 to −5.08 kcal mol⁻¹) or adsorb at the Na⁺ top site (E_ad, −9.45 to −7.32 kcal mol⁻¹). According to the definitions of E_ads, the calculated adsorption energies on Na⁺ site are more negative than that on Cl⁻ site, implying that the ClONO₂–Na⁺ interaction is much stronger than the ClONO₂–Cl⁻ interaction. Configurations II-1 to II-16 are the vertical manner with double-atom adsorption. Two atoms of ClONO₂ are bonded with the surface in a bridge configuration, including the Cl–Cl, Na–Na and Na–Cl sites. Table 1 reveals that the ClONO₂ adsorption at Na–Na bridge site (II-2, II-6, and II-10) is more energetically favorable than at the Cl–Cl bridge site (II-1, II-5 and II-9) and Na–Cl bridge site (II-3, II-4, II-7, II-8, II-11 and II-12). Configurations III-1 to III-7 stand for the flat manner with multi-atom adsorption. ClONO₂ can be horizontally adsorbed on NaCl (100) surface via three or more atoms as illustrated in Fig. S2.†

Table 1 Adsorption energy (E_ad) and Mulliken charges (e) for various adsorption configurations of ClONO₂ on the NaCl (100) surface

Species	Ead (kcal mol^−1)	Charges	Species	Ead (kcal mol^−1)	Charges
I-1	−6.76	−0.012	II-5	−7.15	−0.015
I-2	−7.35	−0.014	II-6	−9.25	−0.002
I-3	−9.45	−0.001	II-7	−8.16	−0.002
I-4	−8.22	−0.002	II-8	−8.60	−0.008
I-5	−5.08	−0.004	II-9	−7.00	−0.017
I-6	−5.11	−0.005	II-10	−9.11	−0.002
I-7	−7.32	0.011	II-11	−7.54	−0.011
I-8	−7.33	0.009	II-12	−8.36	−0.011
I-9	−5.70	−0.005	II-13	−12.67	−0.172
I-10	−6.26	−0.001	II-14	−8.00	−0.001
I-11	−8.75	0.010	II-15	−14.81	−0.227
I-12	−8.46	0.008	II-16	−8.57	−0.005
I-13	−6.93	−0.174	III-1	−9.95	−0.004
I-14	−6.66	−0.173	III-2	−10.11	−0.001
I-15	−8.46	−0.015	III-3	−9.42	0.001
I-16	−8.21	−0.014	III-4	−10.19	−0.002
II-1	−5.52	−0.009	III-5	−8.95	−0.002
II-2	−9.67	0.004	III-6	−10.87	0.001
II-3	−8.49	0.009	III-7	−9.03	0.000
II-4	−8.05	0.010	III-8	−10.30	−0.008

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In all the optimized ClONO₂-adsorbed structures, the bridgelike configuration II-15 is found to be most stable with the adsorption energy of −14.81 kcal mol⁻¹. In II-15, ClONO₂ adsorbs on NaCl (100) in an upright fashion, where the O atom of ClONO₂ binds to the surface Na⁺ ion, and the Cl atom of ClONO₂ attaches to an adjacent Cl⁻ ion. The distances of the newly formed O–Na and Cl–Cl bonds are 2.695 and 2.548 Å, respectively. The Mulliken population analysis³² was performed to understand the charge redistribution upon ClONO₂ adsorption on NaCl. The results are summarized in Table 1. It's clearly show that, in configurations II-15, the adsorbed ClONO₂ molecule is partially negatively charged (−0.227|e|), indicating that ClONO₂ acts as a Lewis acid and accepts the electrons from the NaCl surface.

N₂O₅ is a flat-shaped molecule, with C₂ symmetry.²⁸ Studies show that N₂O₅ can either lie flat or stand vertically on the surface. Twenty-one different adsorption configurations of N₂O₅ molecule on the NaCl (100) surface were explored. The optimized structures and calculated adsorption energies (E_ad) are summarized in Fig. S3† and Table 2. Configurations I-1′ to I-14′ represent the vertical adsorption manners with single or multiple atom adsorptions. In these configurations, N₂O₅ molecule is initially placed perpendicular to the NaCl surface. According to the calculated adsorption energies in Table 2, N₂O₅ preferentially adsorbs on the Na⁺ site over the Cl⁻ site, which is similar to the property of ClONO₂ adsorption on the NaCl (110) surface. Configurations II-1′ to II-7′ are the parallel adsorption manners with multi-atom adsorption. The plane of the N₂O₅ molecule is nearly parallel to the surface, with adsorption energies ranging from −13.67 to −11.21 kcal mol⁻¹. The parallel adsorption has stronger interaction with NaCl surface than the vertical adsorption.

Table 2 Adsorption energy (E_ad) and Mulliken populations (e) for various adsorption configurations of N₂O₅ on the NaCl (100) surface

Species	Ead (kcal mol^−1)	Charges	Species	Ead (kcal mol^−1)	Charges
I-1′	−9.00	−0.011	I-12′	−9.21	−0.015
I-2′	−9.71	−0.008	I-13′	−12.08	0.004
I-3′	−11.86	−0.008	I-14′	−12.24	−0.004
I-4′	−9.64	−0.013	II-1′	−12.96	−0.003
I-5′	−9.07	−0.013	II-2′	−11.77	−0.015
I-6′	−9.30	−0.014	II-3′	−11.21	−0.021
I-7′	−10.30	−0.011	II-4′	−13.42	−0.013
I-8′	−9.76	−0.008	II-5′	−12.98	−0.003
I-9′	−8.29	−0.013	II-6′	−11.24	−0.019
I-10′	−12.56	0.003	II-7′	−13.67	−0.002
I-11′	−11.50	0.007

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Comparing above different adsorption models of N₂O₅ on the NaCl (100) surface, structures (II-4′) and (II-7′) are identified as the favorable adsorption configurations. The calculated adsorption energies for the two structures are −13.42 and −13.67 kcal mol⁻¹, respectively. Mulliken population analysis shows that the net electronic charges transferred from the surface to N₂O₅ in (II-4′) and (II-7′) are −0.013|e| and −0.002|e|, respectively.

3.1.1 Reactions of ClONO₂ and N₂O₅ with HCl or H₂O on the NaCl (100) surface. The chemistry of ClONO₂ and N₂O₅ with HCl or their hydrolysis has been recognized as a major contributor to ozone depletion, especially in the Antarctic stratosphere. Both experimental and theoretical investigations suggested that these reactions cannot occur in the gas phase easily and thus must be heterogeneous. Water clusters, ice, sulfate aerosols, as well as the sea salt aerosols, have been demonstrated to promote these processes.^4,33–35 In order to understand the catalytic effect of sea salt aerosols, we explored the relevant reaction mechanisms of ClONO₂ and N₂O₅ on the NaCl (100) surface.

Previous studies indicate that the interaction between ClONO₂ and HCl or H₂O molecule are prone to present planar bimolecular reactions, thus we chose the flat configuration III-6 in the following researches.^35,36 For N₂O₅, considering the effect of steric hindrance on the reactions, the stable configuration II-4′ is selected for the following discussions. The optimized geometries of intermediates and transition states involved in these reactions on the NaCl (100) surface are shown in Fig. 2. Fig. 3 is the corresponding profile of the potential energy surface (PES), where the sum of the energies of the isolated molecule (ClONO₂ or N₂O₅) and the clean NaCl surface is taken as zero energy. In order to show the zero-point vibrational energy contributions to the barriers and reaction energies, the reaction schemes labeled with energy barriers (ΔE_b) and reaction heats (ΔE_r) are supplied in Fig. S5 of ESI.†

Fig. 2 Optimized geometries for the intermediates and the transition states involved in the reactions on the NaCl (100) surface. Distances are in angstroms.

Fig. 3 Calculated potential energy surface profiles for the reactions on the NaCl (100) surface.

As show in Fig. 2, HCl molecule could adsorb on the NaCl surface firstly, combine with ClONO₂ and form a ClONO₂–HCl complex (IM1). The distance of H–Cl⁻ bond is 2.078 Å. Then via a six-membered cyclic transition state TS1, the products coadsorbed HNO₃ and Cl₂ can be obtained. In TS1, the O–Cl bond in ClONO₂ and H–Cl bond in HCl stretch synchronously, and the adjacent O–H and Cl–Cl bond lengths decreased to 1.462 Å and 2.683 Å, respectively. The energy barrier is calculated to be 5.42 kcal mol⁻¹, which is notably lower than that occurs in pure gas phase (40.90–64.00 kcal mol⁻¹).³⁵ If give the energy of 9.26 kcal mol⁻¹ to P1, desorption reaction can be occur to get “free products” P1-p. The optimized geometries of the “free products” are displayed in Fig. S4 of ESI.†

H₂O molecule could also adsorb on the NaCl surface, combine with ClONO₂ and form a complex IM2. This process is strongly exothermic and releases energy of −18.22 kcal mol⁻¹. The O–Cl⋯O in IM2 has an almost linear configuration. Upon complexation, the hydrolysis reaction on the surface may happen subsequently. TS2 is the transition state corresponding to this process, in which the H atom of H₂O transfers to the O atom of ClONO₂, meanwhile the Cl atom of ClONO₂ migrates to the O atom of H₂O. The optimized structure and its key parameters are depicted in Fig. 2. The products HNO₃ and HOCl are still absorbed on the NaCl surface. As can be seen from the PES profile (Fig. 3), the reaction could readily occur on NaCl surface. This result is consistent with the experimental studies of Timonen et al. and Zelenov et al.^5,12

The two different reactions with regard to N₂O₅ on the NaCl (100) surface are exhibited in Fig. 3. Unlike the reaction of ClONO₂, HCl molecule doesn't need to adsorb on the NaCl surface, the reaction of N₂O₅ with HCl proceeds via the formation of a complex (named IM3). The bond lengths of O–H and Cl–N in IM3 are 2.040 and 3.480 Å, respectively. Once formed, IM3 can perform H–Cl and O–N bonds cleavage through transition state TS3 lead to the formation of ClONO₂ and adsorbed HNO₃. As shown in the structure of TS3, the distances of O–H and Cl–N bond have been significantly shortened to 1.462 and 2.699 Å, respectively. Energy barrier for this transformation is found to be only 4.80 kcal mol⁻¹, indicating facile occurrence of such a process. The overall reaction is calculated to be exothermic by 15.67 kcal mol⁻¹.

The hydrolysis of N₂O₅ on particle surfaces including NaCl is considered as a major source of nitric acid aerosols in the atmosphere. As shown in Fig. 2, IM4 is the reactant complex of N₂O₅ with H₂O, which is calculated to be more stable by 9.42 kcal mol⁻¹ than the separated reactants. Along this channel, IM4 is converted to two HNO₃ via transition state TS4, with a lower barrier of 5.75 kcal mol⁻¹. In TS4, the breaking N–O bond is elongated to 2.344 Å and the forming H–O and O–N bonds of HNO₃ decreased to 1.428 Å and 1.990 Å, respectively. The adsorbed HNO₃ has been observed experimentally from the heterogeneous interactions of N₂O₅ with NaCl aerosol.^4,10 The homogeneous gas-phase reaction of N₂O₅ with H₂O is slow and considered to be relatively unimportant, while the heterogeneous reaction on NaCl surface is more easier and can occur rapidly. In the subsequent reaction, the product HNO₃ can react further with NaCl to form gaseous HCl.³⁷

3.1.2 H₂O adsorption and reconstruction on the NaCl (100) surface. Water is a ubiquitous atmospheric constituent and plays a key role in many atmospheric heterogeneous chemical reactions. In particular, its adsorption on salt aerosols is an important process that can affect their surface structure and chemical reactivity.³⁸ Recent studies have demonstrated the catalytic effect of water during the interaction between NaCl and gaseous pollutants, such as ClONO₂, N₂O₅, and NO₂.^10,13 However, the role of adsorbed water in the reaction mechanisms at the molecular level is not well understood. Thus, we proposed a possible H₂O adsorption and reconstruction mechanism on the NaCl surface. As shown in Fig. 4, water is preferably adsorbed on the NaCl (100) surface, producing a partially hydrated NaCl surface (named IM5), in which the water molecule locates near the top site with its O atom adjacent to Na⁺ and its H atoms attracted by the nearest Cl⁻. The distance of the newly formed O–Na bond is 2.399 Å. The calculated adsorption energy of H₂O on the NaCl (100) surface is −13.99 kcal mol⁻¹ (∼0.61 eV), which is in reasonable agreement with the previous experimental results of 0.50–0.60 eV.^29,30 Subsequently, the adsorbed H₂O molecule can enter into NaCl lattice via transition state TS5 with a barrier of 14.03 kcal mol⁻¹ and form intermediate IM6. In TS5, the O–Na bond has been slightly stretched to 2.470 Å and the adjacent Cl⁻ ion emerges from the plane. IM6 is the reconstructed NaCl configuration, where the water molecule embedded into the bulk NaCl, and the Cl⁻ ion move upward about 1.610 Å from the surface. Apparently, the raised chloride ion could act as the reactive component of the NaCl and positively participate in further reactions.

Fig. 4 Potential energy surface (PES) profile with the optimized geometries of intermediate, transition state, and product for H₂O adsorption and reconstruction on the NaCl (100) surface. Distances are in angstroms.

3.1.3 Heterogeneous reactions of ClONO₂ and N₂O₅ with the reconstructed NaCl (100) surface. There are four different Na⁺ ions around the adsorbed H₂O molecule and the raised Cl⁻ ion, which are labeled as Na₁⁺, Na₂⁺, Na₃⁺ and Na₄⁺. Calculations show that, ClONO₂ and N₂O₅ molecule preferentially adsorb on the unrestricted Na₁⁺ or Na₂⁺ ions. Starting from these stable adsorption structures, we have searched reasonable mechanisms for ClONO₂ and N₂O₅ with the reconstructed NaCl (100) surface. Fig. 5 shows the optimized geometries of intermediates and transition states involved in the heterogeneous reactions, and Fig. 6 is the calculated potential energy surface (PES) profile. The reaction schemes labeled with energy barriers (ΔE_b) and reaction heats (ΔE_r) are supplied in Fig. S7 of ESI.† For convenience, the oxygen atoms in ClONO₂ and N₂O₅ have been numbered. As seen in Fig. 5, ClONO₂ molecule adsorbs on the NaCl surface in a slant fashion (IM7), where the O₁ atom binds to the Na₁⁺ site. The newly generated O₁–Na₁ bond is 2.442 Å, and the calculated adsorption energy is −20.37 kcal mol⁻¹. After adsorption, the Na₁⁺ ion in IM7 shifts 0.10 Å upward with respect to the original position on the surface. Then, the ClONO₂ moiety tilts toward the surface through its O₃ atom binding to adjacent Na₂⁺ ion, forming a bridgelike configuration IM8. In IM8, ClONO₂ was adsorbed at the Na₁–Na₂ site through O₁–Na₁ and O₃–Na₃ bonds with the distances of 2.368 Å and 3.787 Å, respectively. From the calculated PES profile, IM8 is calculated to 0.24 kcal mol⁻¹ higher than IM7. Next, the bidentate configuration IM8 can be further converted into products NaNO₃ and Cl₂ through transition state TS6, where the O–Cl bond is elongated to 2.616 Å and Cl–Cl bond is shortened to 2.122 Å. This process is exothermic by 5.35 kcal mol⁻¹ and needs to overcome a barrier of 5.36 kcal mol⁻¹. The generated product Cl₂ adsorbs on the NaCl (100) surface in a tilted mode with two adjacent Na⁺ ions. ClONO₂ molecule can also adsorb on the Na₂⁺ site, resulting in a unidentate configuration IM9, in which the O₂–Na₂ distance is 2.448 Å. After absorbing the energy of 0.45 kcal mol⁻¹, ClONO₂ is found to bind the surface with two O–Na bonds. The further conversion of the adsorbed ClONO₂ in IM10 is shown in Fig. 5. In this process, the Cl atom is transferred from the adsorbed ClONO₂ to the raised Cl⁻ ion through a transition state TS7, producing NaNO₃ and adsorbed Cl₂ (P6). This process is exothermic by 5.35 kcal mol⁻¹ with a low energy barrier of 4.38 kcal mol⁻¹. As shown in Fig. 6, adsorbed Cl₂ could deviate from NaCl easily after absorb little energy. The product Cl₂ is a photochemical labile species, given its absorption cross section in the actinic region, it will photolyze rapidly and generate highly active chlorine atom, which may affect the fate of tropospheric ozone and the oxidative balance of the atmosphere.¹³

Fig. 5 Optimized geometries for the intermediates and the transition states involved in the heterogeneous reactions on the reconstructed NaCl (100) surface. Distances are in angstroms.

Fig. 6 Calculated potential energy surface profiles for the heterogeneous reactions on the reconstructed NaCl (100) surface.

Similar to ClONO₂, there are two different Na⁺ ions (Na₁⁺ and Na₂⁺) participate in the following reactions of N₂O₅ with the reconstructed NaCl surface. As displayed in Fig. 5, N₂O₅ molecule initially binds to the Na₁⁺ ion through its O₃ atom, forming configuration IM11. PES profile shows that this adsorption process is exothermic by 14.35 kcal mol⁻¹. The distance of formed O₃–Na₁ bond is 2.972 Å. The adsorption of N₂O₅ causes the Na₁⁺ ion to relax downward by 0.07 Å along the z direction. Next, O₁ atom of N₂O₅ could weakly bind to adjacent Na₂⁺ ion and transform into configuration IM12, where O₁ and O₃ coadsorbed onto the NaCl surface. It is a barrierless process and slightly endothermic by 0.99 kcal mol⁻¹. The structure of the adsorbed N₂O₅ in IM12 is distorted significantly with respect to that of the free N₂O₅ molecule. Subsequently, IM12 can react with raised Cl⁻ ion and perform the rupture of N–O₃ bond and the formation of N–Cl bond, finally resulting in NaNO₃ and absorbed ClNO₂. As shown in Fig. 5, the distance between O₃ and N increased to 2.147 Å from 1.704 Å and the N–Cl bond length significantly decreased to 2.511 Å from 3.250 Å. This process has a potential barrier of 4.14 kcal mol⁻¹ and is exothermic by 10.63 kcal mol⁻¹. The low energy barrier and the strong exothermicity of the reaction indicate that the formation of ClNO₂ on the NaCl (100) surface is both kinetically and thermodynamically favorable. IM13 represents the stable configuration for N₂O₅ adsorption on the Na₂⁺ site with an adsorption energy of 14.29 kcal mol⁻¹, and it could further rearrange to a bidentate configuration IM14. Once formed, IM14 would convert into NaNO₃ and ClNO₂ through breaking the N–O₃ bond and forming the new N–Cl bond. The barrier associated with the reaction is found to be 3.78 kcal mol⁻¹. And the overall reaction is calculated to be exothermic by 9.91 kcal mol⁻¹. The nitrate species NaNO₃ is a common product of sea salt aerosols with nitrogen oxides. Gaseous molecule ClNO₂ can subsequently be photo-dissociated by sunlight to give chlorine atoms and NO₂.

As discussed above, the energy barriers for the formation of Cl₂ and ClNO₂ are low (3.78–5.36 kcal mol⁻¹), and the exothermicity are large (5.35–10.63 kcal mol⁻¹), which demonstrates that with the aid of water the heterogeneous reactions of ClONO₂ and N₂O₅ with the reconstructed NaCl (100) surface are facile. According to previous experiments, the heterogeneous reactions could occur in a few minutes.^4,5,13 The results are in accordance with the experimental discoveries, and further complement the experiment researches.^10,13 We conjecture that the suggested mechanism involved with water could be applied to other similar heterogeneous reaction of sea salt aerosols with other nitrogen oxides.

4. Conclusion

In this paper, the detailed adsorption modes and heterogeneous reaction mechanism of ClONO₂ and N₂O₅ on the NaCl (100) surface have been systematically analyzed by the density functional theory (DFT) method (periodic DMol³). Several valuable findings are obtained:

(1) Various adsorption configurations for ClONO₂ and N₂O₅ on the clean NaCl (100) surface have been explored. For ClONO₂, the vertical adsorption mode II-15 is the most stable with the adsorption energy of −14.81 kcal mol⁻¹, and for N₂O₅, the horizontal adsorption mode II-7′ is determinately the most favorable with the adsorption energy of −13.67 kcal mol⁻¹.

(2) On the NaCl (100) surface, adsorbed ClONO₂ and N₂O₅ could react with HCl or via hydrolyze to form chlorine-containing species (Cl₂, HOCl and ClNO₂) and nitric acid. Acting as a catalyst, NaCl can promote these reactions.

(3) Possible heterogeneous reaction mechanism of ClONO₂ and N₂O₅ with the reconstructed NaCl (100) surface in the presence of adsorbed water was proposed. Water molecule plays an important role for the formation of active chloride ion and insures the consequent reactions. Cl₂, ClNO₂ as well as NaNO₃ are the relevant products. The present results may help to improve our understanding of the interaction between nitrogen oxides and sea salt aerosols.

Acknowledgements

This work was supported by NSFC (National Natural Science Foundation of China, Project No. 21337001, 21177076).

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Footnote

† Electronic supplementary information (ESI) available: Supplementary material contains Fig. S1–S7. See DOI: 10.1039/c6ra03961h

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