Reactions of Methanesulfonic Acid with Amines and Ammonia as a Source of New Particles in Air

: New particle formation (NPF) from gaseous precursors as a signi ﬁ cant source of aerosol needs to be better understood to accurately predict the impacts on visibility, climate change, and human health. While ternary nucleation of sulfuric acid, amines/NH 3 , and water is recognized as a signi ﬁ cant driver for NPF, increasing evidence suggests a contribution from methanesulfonic acid (MSA) and amines under certain conditions. Here we report the formation of particles 2.5 − 10 nm in diameter from the reactions of MSA with methylamine (MA), dimethylamine (DMA), and NH 3 at reaction times of 2.3 − 7.8 s in a ﬂ ow reactor and compare these particles with those previously reported to be formed from reaction with trimethylamine (TMA). The e ﬀ ects of water vapor and concentrations of gaseous precursors on the particle number concentration and particle size were studied. The presence of water signi ﬁ cantly enhances particle formation and growth. Under similar experimental conditions, particle number concentrations decrease in the order MA ≫ TMA ≈ DMA ≫ NH 3 , where NH 3 is 2 − 3 orders of magnitude less e ﬃ cient than DMA. Quantum chemical calculations of likely intermediate clusters were carried out to provide insights into the role of water and the di ﬀ erent capacities of amines/NH 3 in particle formation. Both gas-phase basicity and hydrogen-bonding capacity of amines/NH 3 contribute to the potential for particles to form and grow. Our results indicate that, although amines typically have concentrations 1 − 3 orders of magnitude lower than that of NH 3 in the atmosphere, they still play an important role in driving


■ INTRODUCTION
Atmospheric particles have well-known deleterious effects on human health 1,2 and visibility. 3−5 Those particles can globally affect radiative forcing and climate by absorbing and scattering solar radiation as well as by acting as cloud condensation nuclei (CCN) to change cloud coverage and properties. 6−8 However, great uncertainties remain in the mechanisms of formation of atmospheric particles, and these preclude accurate assessment of the impacts of atmospheric particles on human health, visibility, and climate. 9 New particle formation (NPF) from gasto-particle conversion represents a significant source of atmospheric particles 10,11 and contributes up to half of global CCN. 8,12 Understanding the potential species driving NPF and the underlying mechanisms is therefore critical for prediction and control of climate change. 9,10 Sulfuric acid (H 2 SO 4 ) formed from SO 2 oxidation in air has been well recognized as a key species driving NPF. 13−15 The reaction of H 2 SO 4 with ammonia (NH 3 ) and amines represents the initial step in gas-to-particle conversion to form small clusters, 10,11 which can further grow into detectable size particles in the atmosphere. Low-volatility, high-molecularweight organic compounds are likely also involved. 16−20 Although ternary nucleation of H 2 SO 4 −NH 3 −H 2 O has been shown to enhance particle formation by orders of magnitude compared to binary nucleation of H 2 SO 4 −H 2 O, 21−26 it is not sufficient to explain particle formation in the atmosphere. 25 Amines can play a more important role in NPF, 24,26−32 possibly due to the stronger bonds they form with H 2 SO 4 compared to NH 3 . 27 Laboratory experiments have demonstrated that amines are more effective in forming particles with H 2 SO 4 . 24,26,[28][29][30]32 Field studies have shown that aminium salts are widely present in ambient particles, 31,33−35 and NPF is correlated with the presence of amines. 36,37 In addition, amines have been found to effectively displace NH 3 in ammonium bisulfate clusters/ particles, 38−41 with a close-to-collision-limited rate for small clusters. 38,39 Thus, aminium salts are likely to be present even if ammonium salts are initially formed.
Ammonia and amines are emitted from a wide range of sources, including biological processes in the ocean, animal husbandry, industrial operations, fuel and biomass burning, and agricultural activities. 42 The concentration of gas-phase amines in the atmosphere is typically at least one order of magnitude smaller than that of NH 3 , but this may increase with their use in CO 2 capture and storage as the technology becomes more widely adopted. 42−45 Methanesulfonic acid (MSA, MeSO 3 H) that is commonly detected in ambient particles contributes to NPF along with H 2 SO 4 . 33,34,46−52 MSA is formed along with SO 2 (H 2 SO 4 precursor) from the oxidation of organosulfur compounds that originate from biological processes, biomass burning, industrial operations, and agricultural activities. 53−57 Atmospheric gas-phase MSA concentrations are typically in the range of ∼10 5 −10 7 molecules cm −3 , about ∼10−100% of those of H 2 SO 4 . 58,59 The contribution of MSA to NPF is expected to become relatively more important with implementation of stricter environmental regulations on SO 2 emissions. 60 Binary nucleation of MSA−H 2 O was reported to be much less efficient than that of H 2 SO 4 −H 2 O. 50,52,61−64 However, previous studies from this laboratory showed that trimethylamine (TMA, Me 3 N) and dimethylamine (DMA, Me 2 NH) promote particle formation from MSA in the presence of water vapor. 50,52 These studies suggested that water not only is involved in particle growth but also participates in the formation of the initial clusters. 50,52 The objectives of this study are to investigate the effect of water on NPF and growth from MSA with ammonia and a series of amines including methylamine (MA, MeNH 2 ) and DMA, and to compare their capacities in NPF with previously reported TMA. 50 We conducted comprehensive experiments on the reaction of MSA with amines/NH 3 over a range of experimental conditions and at reaction times of 2.3−7.8 s. The structures and thermodynamic properties of likely intermediate clusters were predicted using a quantum chemical approach. Comparison to the previous studies of TMA 50,52 provides insight into the relative contributions of NH 3 and amines in NPF and into the molecular factors that determine the efficiency of these reactions. Figure S1 in the Supporting Information shows a schematic of the flow reactor, which is made of borosilicate glass and was designed to study the early stages, ∼2.3−32 s, of reaction (time includes 1.9 s in the sampling line). The flow reactor has been well characterized and tested previously. 50,51 Briefly, it is comprised of a major section that has a diameter of 7.6 cm and a length of 1.3 m, with end-caps at both ends to accommodate inlets and outlets. The major section is water-jacketed for temperature control. The upstream end-cap holds two hollow glass ring inlets ( Figure S1, ring A and ring B) that are perforated with 36 evenly spaced 0.05 cm holes to disperse gases. Two sets of perforated hollow spokes are fitted to two movable concentric glass tubes and guided into the reactor from the upstream end-cap to serve as the other two inlets ( Figure S1, spokes C and spokes D). Gas precursors and particles are sampled from a stainless steel sampling tube that is guided into the flow reactor by the downstream end-cap. Only part of the gas/particle flow is sampled through the sampling tube, and the majority of the flow is vented through a 0.5 in. glass port at the downstream end-cap.

■ EXPERIMENTAL METHODS
Prior to each experiment, the flow reactor was cleaned with Nanopure water (>18.0 MΩ cm; Model 7146; Thermo Scientific) and dried with purified air with the water jacket kept at ∼70°C. In all experiments, the mixture of purified air and MSA flow was introduced from the spokes C at a total flow of 2 slpm (standard liter per minute at 25°C and 101.32 kPa), and the mixture of purified air and the selected amines/NH 3 from the spokes D at a total flow of 1 slpm. Dry/humidified air at a total flow of 14 slpm was introduced from the upstream ring inlets (ring A and ring B) to make the total flow in the flow reactor 17 slpm. Thus, MSA was mixed with water vapor before reacting with the specific amines/NH 3 for experiments under humid conditions. All experiments were performed at ambient temperature. Because of wall losses of MSA, the gas flow of MSA was first introduced into the flow reactor to passivate the walls for 2−3 days. The gas flow of the selected amines/NH 3 was then introduced to initiate particle formation. Particle formation and growth as a function of time were monitored by adjusting the distance between the sampling tube and the spokes inlet D. A previously determined conversion factor (0.14 s cm −1 ) was used to convert the distance to reaction times. 51 All of the flows were controlled by high-precision mass flow controllers (Alicat or MKS), and were checked with a flow meter (Gilibrator 2; Sensidyne) prior to each experiment. Dry air was further purified through a purge gas generator (model 75-62; Parker Balston), carbon/alumina media (Perma Pure, LLC), and a 0.1 μm filter (DIF-N70; Headline Filters). Humid air was achieved by passing a portion of the dry air through a water bubbler filled with Nanopure water. The relative humidity (RH) was monitored with a RH probe (model HMT338; Vaisala) at the downstream end of the reactor. The gas-phase MSA flow was generated by passing dry purified air over liquid MSA (99.0%, Fluka) contained in a trap. The gasphase amines or ammonia were added into the system from gas tanks (1−10 ppm in N 2 , Airgas) without further purification.
To determine the concentration of gas-phase MSA, the gas flow out of the MSA trap was first passed through a 0.45 μm Durapore filter (Millex-HV) for 5−15 min for collection. The collected MSA was extracted with 10 mL of Nanopure water, and then the extract was analyzed using ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS, Waters). The concentrations of MA, DMA, and NH 3 were determined as previously described. 65 Briefly, the amines/NH 3 from the source were collected for 20−120 min using a high-concentration cartridge that contains a weak cation exchange resin, extracted with 10 mL of 0.1 M oxalic acid (Fluka), and analyzed with ion chromatography (Metrohm). The measured concentrations of ammonia and the amines were typically lower than values provided by the manufacturers. The analysis also confirmed that no significant contaminant of other amines or NH 3 was present in the selected amine/NH 3 (<0.1% of the specific base) flows. The initial concentrations of MSA and amines/NH 3 in the flow reactor were calculated on the basis of the measured concentrations at the sources and the total gas flow in the flow reactor. As a result, the initial concentrations may actually be lower than those reported here due to potential wall losses in the flow reactor inlets and the flow reactor itself.
Dry and humidified diluent air in the flow reactor was also collected from the sampling tube using a low-concentration cartridge 65 to examine possible amines and ammonia present as contaminants in the diluents air, and the concentrations of amines and ammonia were all lower than the 10 ppt detection limit.
Particle size distributions were measured using a scanning mobility particle sizer (SMPS) that consists of an electrostatic classifier (model 3080; TSI), a nano-differential mobility analyzer (model 3085; TSI), and a butanol-based condensation particle counter (model 3776; TSI). The 50% cutoff size of the SMPS for sucrose particles is reported by the manufacturer to The Journal of Physical Chemistry B Article be ∼2.5 nm. All particle diameters reported here are geometric mean mobility diameters obtained from the SMPS.

■ THEORETICAL METHODS
To examine the effect of water and differences in the nature of the amine on the stability of intermediate clusters, structures and thermodynamic parameters were determined for clusters consisting of one or two acid/amine molecules and up to two water molecules. For (MSA) 2 ·(amine) 2 ·(H 2 O) n [n = 0, 1, or 2], as well as their parent clusters and molecules, optimizations and frequency calculations at the B3LYP-D3/6-31+G(d,p) level of theory, including dispersion correction, 66 with electronic energies calculated at the latter level of theory, provided the structures and thermodynamic values. Test calculations indicate that use of the B3LYP-D3/6-31+G(d,p) geometries and harmonic frequencies leads to differences of ∼1 kcal mol −1 (and often less) for ΔH and ΔG values computed using RIMP2/aug-cc-pV(D+d)Z, which was used previously for MSA-amine systems. 52 Bork et al. 67 tested several DFT and ab initio methods against an experimentally determined ΔG value for the acetonitrile-HCl hydrogen-bonded complex and found that MP2 and B3LYP-D3 predictions were within the experimental range. For the smaller (MSA)·(amine)·(H 2 O) n [n = 0 or 1] clusters, optimizations and frequency calculations at the RIMP2/aug-cc-pV(D+d)Z level of theory with electronic energies calculated at the RIMP2/aug-cc-pV(D+d)Z level of theory were employed. All calculations were carried out with the Turbomole program package. 68,69 A detailed description of the theoretical methods is included in the Supporting Information.
■ RESULTS AND DISCUSSION Experimental Results. The effect of water on NPF and growth was first investigated for selected amines/NH 3 . The initial concentrations of the precursors and water vapor were kept at similar levels as the nature of the base was varied in order to make direct comparisons (Table 1). However, in the case of NH 3 , the concentrations of the base increased in order to form detectable particles (Table 1).  Figure 1c) either under dry conditions or with added water vapor. Clearly, particle formation and growth are much more favorable when water is present. An enhancement effect of water vapor on particle formation has also been reported in previous H 2 SO 4 -amines studies. 24,28,29,70,71 In some cases in this study, there are spikes in the size distributions of particles below the SMPS cutoff of 2.5 nm at short reaction times ( Figure 1). This suggests the presence of smaller particles or clusters <2.5 nm in size that can grow into the detectable size range at longer reaction times. Preliminary data obtained with a particle size magnifier (model A10; Airmodus) which has a cutoff size of ∼1.4 nm for ammonium sulfate particles confirms the presence of particles smaller than 2.5 nm in size. As indicated in the previous study, 50 we did not observe particle formation from MSA−H 2 O in the absence of amines/NH 3 , indicating that ammonia and amines present at ppt levels as contaminants from the water and air used in the experiment do not lead to particle formation.
Further comparisons of particle number concentrations and diameters are shown in Figure 2. Similar plots for TMA reported earlier 50 are shown in Figure S2  When water is present, the particle number concentrations for the MA, DMA, and TMA reactions at 7.8 s are of the same order of magnitude (∼10 7 cm −3 ) (Figures 2 and S2). However, particle formation is slower initially in the cases of DMA and TMA, suggesting that there is more of a kinetic limitation compared to the MA reaction.
The effect of water was further investigated by monitoring particle formation and growth as a function of RH, while MSA and the amine concentrations were held constant. Figure 3 shows that particle number concentrations increase as the RH increases for all bases, MA (Figure 3a, upper panel), DMA (Figure 3b, upper panel), and NH 3 (Figure 3c, upper panel). Increasing RH also promotes particle growth in the cases of MA (Figure 3a, lower panel) and NH 3 (Figure 3c, lower panel),  The data in Figure 3 clearly show a strong dependence of particle formation on the water vapor concentration. In the context of classical nucleation theory, the slope of a log−log plot of the initial formation rate of detectable particles (J >2.5 nm ) against the gas-phase water concentration can provide information on the number of water molecules in the critical cluster, 73 although this is somewhat controversial. 74 Alternatively, the overall reaction order can reflect the ratedetermining steps in the formation and growth of the initial clusters; for example, if the MSA dihydrate was the key reactive species, a second order dependence on water could result.
The different shapes of the curves at short reaction times in the top panel of Figure 3 result in significant uncertainty in determining J >2.5 nm . Recognizing this caveat, as a first approach, values of J >2.5 nm were estimated using the 2.3−3.6 s regions in Figure 3. The resulting log−log plot of J >2.5 nm as a function of water vapor concentration for all of the amines/NH 3 investigated is shown in Figure 4. Experiments were repeated at least twice for each base. A similar dependence found for the MSA−TMA−H 2 O system 50 is included. The slopes obtained for different amines/NH 3 are in the range of 1.3−2.3, indicating that the particle formation rate is approximately overall second order with respect to water.
Experiments were also carried out in which the concentration of MA was varied in the presence of excess MSA ( Figure 5).
(Experiments under the reverse conditions of excess MA resulted in particle formation rates that were too rapid to be followed at short reaction times.) Both number concentrations and size increase with increasing concentrations of MA. The induction period ( Figure 5) that does not appear at higher concentrations of MSA and MA (Figure 2a) suggests that longer reaction times are required for small clusters to grow into detectable size particles at lower concentrations of gas precursors.  Experiments were also performed for particle formation from the reactions of MSA with MA, DMA, TMA or NH 3 at a constant concentration of MSA at 1.7 ppb at 55% RH. A comparison of J >2.5 nm for different amines/NH 3 is shown in Figure 6. Results from two separate experiments for each base are in good agreement. The different basic species show different abilities in forming particles with MSA. Consistent with previous studies in the H 2 SO 4 −amines/NH 3 system, 24,26−30 amines are much more effective than NH 3 in promoting particle formation from MSA. Of the three amines studied here, MA is by far the most effective, followed by TMA and DMA.
Theoretical Calculations. Both gas-phase basicity and hydrogen-bonding capacity can contribute to the potential for particles to form and grow. With increasing gas-phase basicity, the reaction with MSA is more exothermic. With increasing hydrogen-bonding capacity, the sites for addition of molecules from the gas phase increase in number. However, neither gasphase basicity, which increases with the number of methyl groups for the series MA < DMA < TMA, nor hydrogenbonding capacity, which decreases with the number of methyl   The lines between data points are drawn to as guides to the eye. Uncertainties represent two standard deviations from triplicate SMPS measurements and lie within the symbols in some cases. Measurements of the diameters have high uncertainties at short reaction times due to the experimental challenges of detecting low particle numbers, and those data have been disregarded in drawing lines.

The Journal of Physical Chemistry B
Article groups for the series MA > DMA > TMA, alone can explain the observed lack of particle formation for DMA relative to both TMA and MA under dry conditions. Calculations for alternating addition of acid and base molecules to the initial acid−base ion pair were carried out in an attempt to identify differences in the growth mechanisms that would explain the differences in particle formation rates observed experimentally for the series of amines.
The stability of the cluster formed from addition of MSA to the initial acid−amine ion pair is similar for MA and DMA  (Figure 7a). This leads to a somewhat weaker interaction compared to the MA and DMA cases where an additional hydrogen bond to an available −NH group is formed (Figures  8a and 9a), but MSA is still strongly bound to the TMA ion pair (ΔH = −25 kcal mol −1 , ΔG = −12 kcal mol −1 ). As MSA addition is favorable in all cases, and not less favorable for DMA where there is limited particle formation under dry conditions, we move to the next step: amine addition to the (MSA) 2 · (amine) cluster.
The addition of MA to a (MSA) 2 ·(MA) cluster to form a stable (MeSO 3 − ) 2 ·(MeNH 3 + ) 2 ( Figure 8c) may proceed through a weakly bound complex (Figure 8b) similar to that of TMA (Figure 7b). The binding enthalpy of this complex is similar to the TMA case, ΔH = −5 kcal mol −1 (ΔG = +5 kcal mol −1 ). However, in the case of MA an alternate route for cluster growth may facilitate particle formation under dry conditions. The (MSA) 2 ·(MA) cluster consists of a (MeSO 3 − )· (MeNH 3 + ) ion pair similar to the TMA cluster, but with the additional −NH groups of MA, the MSA molecule is hydrogenbonded to both the anion and cation (Figure 8a). The MeNH 3 + Figure 6. Comparison of particle formation rates (J >2.5 nm ) for MSA with the basic species, i.e., MA, DMA, TMA, and NH 3 , at ∼55% RH. The initial concentration of MSA is 1.7 ppb. Two separate experiments were performed for each base, and the data are represented as symbols with different shapes. , is slightly greater than the corresponding MA system. The two routes for cluster growth, addition either at the cation or at the neutral acid molecule, both seem to be feasible for the MSA−DMA system, and potentially more likely based on enthalpies of formation of the complexes. The conclusion from these calculations is that understanding the anomalous dry behavior of DMA will require consideration of intermediates at the level of four or more molecule clusters.
Dawson et al. 72 described structures and binding for (MSA) 2 · (MA) 2 , (MSA) 2 ·(DMA) 2 , and (MSA) 2 ·(TMA) 2 clusters, including speculation on the nature of interactions in extended systems. The endothermicity for dissociation of (MSA) 2 · (TMA) 2 to (MeSO 3 − )(Me 3 NH + ) ion pairs, ΔH = 24 kcal mol −1 , is similar to the exothermicity of TMA addition to (MSA) 2 ·(TMA) presented above. Due to the lack of hydrogen bonds between (MeSO 3 − )(Me 3 NH + ) ion pairs, the single ion pair formed on fragmentation may then play a significant role in growing the stabilized clusters to detectable size particles. 50 For comparison, the binding energies of ion pairs in forming (MSA) 2 ·(MA) 2 and (MSA) 2 ·(DMA) 2 clusters (ΔH = −36 and −37 kcal mol −1 , respectively) are close and are significantly larger than the binding energy informing (MSA) 2 ·(TMA) 2 due to a cyclic hydrogen-bonding structure between alternating cations and anions. 72 However, the availability of an NH group that is not hydrogen-bonded in the MA cluster may lead to an extended hydrogen-bonding network, an arrangement seen in the crystal structure of ammonium methanesulfonate. 75 While growth of a perfect crystal may not occur in this case, the additional hydrogen-bonding site may play a role in more effectively growing the MSA-MA clusters to detectable sizes even under dry conditions. For DMA, the large binding energy between MSA-DMA ion pairs enhances the formation and stability of (MSA) 2 ·(DMA) 2 clusters, so that these double ion pairs may play an important role in growth to detectable particle sizes. Coagulation of two of these larger clusters may exhibit steric effects, whereas growth through single ion pairs for TMA would be less sterically hindered, leading to more rapid formation of detectable particles in the TMA system.
Structures of (MSA) 2 ·(DMA)·(H 2 O) 2 and (MSA) 2 ·(DMA) 2 · (H 2 O) 2 illustrating how water facilitates amine addition are included in Figure 10  The binding of water to the initial ion pair is slightly greater for MA and DMA (ΔH = −15 kcal mol −1 , ΔG ≈ −5 kcal mol −1 ) than for TMA (ΔH = −11 kcal mol −1 , ΔG ≈ −2 kcal mol −1 ). While there is not a significant difference in the binding energy of water to the MA vs DMA ion pair, the MA ion pair has an additional hydrogen-bonding site that may increase the chances of picking up water from the gas phase. If hydrated clusters are formed more quickly or are more stable, and hydration aids cluster growth, this could explain the faster formation of detectable particles from MA (Figure 2a) compared to DMA (Figure 2b) and TMA ( Figure S2). The growth of DMA and TMA particles is also significantly faster than the MA particles (Figure 2 and S2). This can be due to (1) a larger contribution of growth from water because of their higher hygroscopicities, 72 (2) a larger molecular volume, and/ or (3) smaller particle formation rates so that higher concentrations of gas precursors and small clusters that can contribute to growth are left in the DMA and TMA systems.
If gas-phase basicity and hydrogen-bonding capacity are the two competing factors in particle formation and growth for the series of amines studied here, the success of MA in forming particles indicates that the hydrogen-bonding capacity of the amine plays a larger role, though the ineffectiveness of ammonia indicates that threshold basicity is required. This differs from the case of sulfuric acid, 29,32 which provides greater hydrogen-bonding capacity, so it is not surprising that the relative effects of amines are different than for MSA.
The conceptual approach taken in this paper involves calculations for small clusters, with extrapolation to likely growth possibilities. Calculations for larger particles are obviously very desirable. One appealing direction is to carry out calculations for crystalline particles, using periodic boundary conditions. As the crystalline salt is likely not the most relevant model for growth of the early-stage clusters and at the disordered surface, and high-level calculations with periodic boundary conditions remain a challenge for systems of this size, this approach was not pursued here, but it is a very interesting direction for the future.

■ ATMOSPHERIC IMPLICATIONS
Particle formation and growth are observed for reactions of MSA with ammonia and three small alkyl amines, MA, DMA, and, as reported earlier, 50 TMA. Thus, the reaction of MSA with ammonia and amines must contribute to NPF in the atmosphere. Consistent with the previous studies, 50,52 water shows a significant enhancement effect on particle formation and growth from MSA. Relative humidity is therefore expected to be a critical factor affecting NPF in the atmosphere.
Similar to previous studies on the H 2 SO 4 −amines/NH 3 system, 24,26−30 amines are much more effective in forming particles with MSA than ammonia. Different bases show capacities in NPF with MSA in the order of MA ≫ TMA ≈ DMA ≫ NH 3 .
While particle formation rates of amines with MSA are about 2−3 orders of magnitude higher than that of ammonia, field measurements in agricultural, ocean, and landfill areas usually show concentrations of amines approximately 1−3 orders of magnitude lower than those of ammonia. 42 Together these indicate that amines, despite having relatively smaller concentrations in the atmosphere, are expected to play a significant role in the initial stages of NPF.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b07433. Figure S1, schematic of the flow reactor; Figure S2, particle formation and growth in the MSA−TMA system; detailed description of the theoretical methods; and Cartesian coordinates and B3LYP-D3/6-31++G** and RIMP2/aug-cc-pV(T+d)Z electronic energies of the structures in

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
The authors are grateful to the National Science Foundation (grant nos. 0909227 and 1443140) for funding. We thank Metrohm USA for their support and help in ion chromatography analysis. Computational resources include the Greenplanet cluster at University of California, Irvine, the CSC cluster Taito in Helsinki, Finland, and those of the Fritz-Haber Center in Jerusalem, Israel. It is a pleasure to dedicate this paper to Bruce Garrett, whose contributions from molecular interactions with liquid surfaces to nucleation theory and particle growth greatly impacted the study of atmospheric chemistry and have been a major influence on work done at AirUCI. His support of a strong interaction between theory and experiment is one important example. The Journal of Physical Chemistry B Article