Multiphase Ozonolysis of Aqueous Î±-Terpineol - ACS Publications

Article pubs.acs.org/est

Multiphase Ozonolysis of Aqueous α‑Terpineol Dani H. Leviss, Daryl A. Van Ry, and Ryan Z. Hinrichs* Department of Chemistry, Drew University, Madison, New Jersey 07940, United States S Supporting Information *

ABSTRACT: Multiphase ozonolysis of aqueous organics presents a potential pathway for the formation of aqueous secondary organic aerosol (aqSOA). We investigated the multiphase ozonolysis of α-terpineol, an oxygenated derivative of limonene, and found that the reaction products and kinetics differ from the gas-phase ozonolysis of α-terpineol. One- and twodimensional NMR spectroscopies along with GC-MS identified the aqueous ozonolysis reaction products as trans- and cis-lactols [4-(5-hydroxy-2,2-dimethyltetrahydrofuran-3-yl)butan-2-one] and a lactone [4-hydroxy-4-methyl-3-(3-oxobutyl)-valeric acid gamma-lactone], which accounted for 46%, 27%, and 20% of the observed products, respectively. Hydrogen peroxide was also formed in 10% yield consistent with a mechanism involving decomposition of hydroxyl hydroperoxide intermediates followed by hemiacetal ring closure. Multiphase reaction kinetics at gaseous ozone concentrations of 131, 480, and 965 parts-per-billion were analyzed using a resistance model of net ozone uptake and found the second-order rate coefficient for the aqueous reaction of α-terpineol + O3 to be 9.9(±3.3) × 106 M−1 s−1. Multiphase ozonolysis will therefore be competitive with multiphase oxidation by hydroxyl radicals (OH) and ozonolysis of gaseous α-terpineol. We also measured product yields for the heterogeneous ozonolysis of α-terpineol adsorbed on glass, NaCl, and kaolinite, and identified the same three major products but with an increasing lactone yield of 33, 49, and 55% on these substrates, respectively.

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1,2,3-trioxolane ring, or primary ozonide (POZ).8,9 Formation of the POZ is highly exothermic and leads to rapid homolytic cleavage of the C−C bond (and an O−O bond)10 yielding a carbonyl fragment and a carbonyl oxide biradical known as the Criegee intermediate (CI).8 The fate of these CI depends on their structure and the phase in which they are formed. In the gas phase, most CI, except CH2OO which has a long lifetime, undergo unimolecular decomposition releasing hydroxyl radicals. The major pathway for CH2OO and a minor pathway for all other gaseous CI involves collisional stabilization followed by bimolecular reactions, most often with water vapor to form organic acids and H2O2.9 CI formed via condensed phase ozonolysis reactions, especially in the aqueous phase, are quickly stabilized by collisions with the solvent and react with H2O to form hydroxyl hydroperoxide (HHP) intermediates, which may decompose to form H2O2(aq).11−13 Condensed phase CI can also rearrange to form a 1,2,4trioxolane, or secondary ozonide (SOZ),10 and SOZ have been observed for ozonolysis of several monoterpenes.14,15 Aqueous phase ozonolysis is competitive with aqueous oxidation by OH radicals for many alkene OVOCs.4

INTRODUCTION The ozonolysis of volatile organic compounds is a major source of secondary organic aerosol (SOA). The mechanism for SOA formation from biogenic hydrocarbons, including monoterpenes and isoprene, predominantly involves gas-phase oxidation that produces less volatile products which subsequently partition to the condensed phase.1,2 Recent studies have begun to evaluate alternative pathways for the production of SOA, including aqueous phase chemistry associated with cloud processing to form aqSOA.3−6 Atmospheric aqueous phase processes involving organics include reactions with radical oxidants, such as OH and NO3; reactions with nonradical oxidants, such as H2O2 and O3; organic accretion reactions including condensations, acetal, hemiacetal, and oligomerizations; hydrolysis and nucleophilic reactions; and photochemical processes.4 While aqueous phase SOA formation can involve biogenic hydrocarbons, oxygenated volatile organic compounds (OVOCs) are typically more relevant given their greater partitioning to the aqueous phase as characterized by larger Henry’s law coefficients.7 Unsaturated alcohols, such as α-terpineol and linalool, constitute one major class of atmospheric OVOCs that are highly reactive with O3, as well as OH and NO3 radicals.7 Ozonolysis mechanisms can differ significantly, whether they occur in the gas phase, aqueous phase, or adsorbed on aerosol surfaces. Ozonolysis of alkenes, regardless of phase, begins by 1,3-dipolar cycloaddition of ozone to CC bonds to form a © XXXX American Chemical Society

Received: July 19, 2016 Revised: September 26, 2016 Accepted: September 28, 2016

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DOI: 10.1021/acs.est.6b03612 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

ozonolysis was investigated by placing 5 mL of the aqueous α-terpineol solution in a 1-L round-bottom flask attached to a rotor spinning at 200 rpm to create a thin film on the inner surface of the glass with a surface area of 265 cm2. A photolytic ozone generator (Jelight 600) produced O3 in a flow of air (60 sccm) that continuously flushed the reaction flask. A Jelight 465L Ozone Monitor measured the gaseous O3 concentrations in the reaction flask, and all experiments were conducted between 131 and 965 parts-per-billion (ppb) O3. The reaction was monitored by recording 1H NMR spectra of aliquots removed from the reaction flask at 30- or 60 min intervals and then returning the samples to the flask for further reaction until completion. Quantofix peroxide 25 test strips monitored the formation of H2O2 in the reaction flask. Following reaction completion, the aqueous products were extracted into deuterated chloroform (Sigma-Aldrich, 98%) and concentrated with a gentle flow of nitrogen gas for further analysis using NMR (1H, 13C, COSY, and HSQC), GC-MS, and Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR). GC-MS analysis was performed on an Agilent 7890 Gas Chromatograph (SLB-5 ms column, Supelco) with Agilent 5975 Mass Selective Detector. GC-MS analysis was repeated after derivatizing the extracted products with BSTFA (N,O-bis(trimethylsilyl) trifluoroacetamide) containing 10% TMCS (trimethylchlorosilane) catalyst, which replaces alcohol and acid groups with trimethylsilyl moieties. A Thermo Nicolet iS-10 FTIR spectrometer recorded IR spectra (64 scans) of products by evaporating the extracted solution on a diamond ATR crystal. We also performed this complete analysis on a lactone standard, 4-hydroxy-4-methyl-3-(3oxobutyl)-valeric acid gamma-lactone (Sigma-Aldrich). Heterogeneous ozonolysis of α-terpineol adsorbed on glass, kaolinite, and sodium chloride were performed using the same experimental setup. α-Terpineol (0.005g) was dissolved in dichloromethane (5 mL), and the resulting solution was added to an empty reaction flask (glass), or the reaction flask containing 0.500 g kaolinite (Sigma-Aldrich) or sodium chloride (Sigma-Aldrich, 99.99%). The resulting slurries were coated on the inner surface of the reaction flask under a continuous flow of dry air, allowing the dichloromethane to evaporate. Samples were then equilibrated to a humidified air flow (∼50% relative humidity) for 25 min before introducing 900 ppb ozone. After reaction completion (5 h), products were extracted in 5 mL deuterated chloroform and analyzed using 1H NMR and GC-MS. Computational. Characteristic NMR assignments for each product were calculated using the Gauge-Independent Atomic Orbital (GIAO) method33 in Gaussian 09.34 Structures for αterpineol, secondary ozonide, lactone, and trans- and cis-lactol isomers were optimized at the B3LYP/6-311++G(d,p) level in an aqueous Polarizable Continuum Solvent Model (PCM). Optimized structures were confirmed as minima by the absence of any imaginary frequencies. Following optimization, the GIAO method with two-step spin−spin coupling calculations was used to calculate J coupling constants. Trans- and cis-lactol structures were also optimized with two water molecules hydrogen-bonded to the hemiacetal portion with PCM, and GIAO results were similar to calculations without the explicit inclusion of these two H2O molecules.

The different fates of CI in the gas- and aqueous-phases lead to different ozonolysis products and their yields. For example, ozonolysis of gaseous isoprene is a major source of OH radicals and forms formaldehyde, methacrolein (MACR), and methyl vinyl ketone (MVK) as major first-generation products; hydroxymethyl hydroperoxide forms in high yield from the secondary reaction of CH2OO with water vapor.16 Wang et al. recently studied the ozonolysis of aqueous isoprene and detected considerably higher yields for carbonyl products (i.e., formaldehyde, MVK, and MACR) and H2O2.13 Zhang and colleagues detected MACR in 40% yield from the ozonolysis of aqueous α-pinene, which is notable since MACR is not produced via ozonolysis of gaseous α-pinene.12 H2O2 was also formed in very high yields (60−100%) for these aqueous pinene + O3 reactions.12 Chen et al. also detected high H2O2 yields for the ozonolysis of aqueous MACR and MVK11 and found greater yields for organic peroxides from the heterogeneous reactions for MACR and MVK adsorbed on silica.17 While gas-phase chemistry is certainly the dominant pathway for these poorly soluble biogenic VOCs, these studies nonetheless demonstrate the potential impact of aqueous phase ozonolysis in fog and cloud droplets and on wet aerosol surfaces. α-Terpineol is a cyclic terpene alcohol that enters the atmosphere directly as an emission from plants18−20 and is a major indoor OVOC due to use in household cleaning products and air fresheners21−24 and its release by mold in sick buildings.25,26 Wells and colleagues studied the ozonolysis of gaseous α-terpineol, measuring a second-order rate constant of 3.0(±0.2) × 10−16 cm3 molecules−1 s−1 and identifying methylglyoxal as the major product.27,28 Yang and Waring measured SOA yields for this gas-phase process and concluded that this reaction could be a significant source of particulates in indoor environments.29 The heterogeneous ozonolysis of αterpineol adsorbed on typical indoor surfaces−glass, polyvinyl chloride (PVC), and latex paint−was also found to be competitive with gas-phase ozonolysis in indoor environments.30 Ham and Wells found that the products formed by the ozonolysis of α-terpineol adsorbed on glass and vinyl tiles were similar to the gas phase but with different yields, where unsaturated carbonyl products (e.g., 4-methyl-3-cyclohexen-1one) dominated.31 With a Henry’s law constant of 429 M atm−1,32 α-terpineol will also partition to the aqueous phase in addition to being present in the gas phase and adsorbed on surfaces. The present study seeks to understand the multiphase ozonolysis of aqueous α-terpineol, both its products and mechanism, using one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopies and gas chromatography− mass spectrometry (GC-MS). By analyzing multiphase ozonolysis kinetics, we assess whether such aqueous-phase chemistry may contribute to aqSOA formation and compete with other atmospheric oxidation pathways for this important OVOC.

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METHODS Experimental Section. Aqueous solutions (∼3 mM) of racemic α-terpineol (Sigma-Aldrich, 98%) were prepared in deuterated water (Sigma-Aldrich, 98%) containing tetramethylammonium chloride, N(CH3)4Cl (91 mM), which served as a nonvolatile and nonreactive internal standard. A Bruker 400 MHz NMR spectrometer analyzed the resulting solution: 1H spectra were taken with 64 scans, 13C with 16384 scans, COSY with 64 scans, and HSQC with 256 scans. Multiphase

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RESULTS AND DISCUSSION Product Identification. Figure 1 shows the 1H NMR spectrum of α-terpineol (i.e., t = 0 min) along with spectra B


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have previously been reported in the organic synthesis literature following ozonolysis of α-terpineol in organic solvents.35,36 Given the identification of lactol diastereomers as the major products, it is curious that hemiacetal proton peaks at δ = 5.37 and 5.42 exhibit different splitting patterns even though each structure has two neighboring protons. Electronic structure calculations for each lactol were optimized [B3LYP/6-311+ +G(d,p)] in a polarizable continuum solvent model while explicitly including two water molecules hydrogen bonded to the hemiacetal (see Figure S7). Figure 2 shows the optimized trans- and cis-lactol structures and also presents the dihedral angle between the hemiacetal proton (Figure 2 blue atom) and its neighboring protons. The Karplus equation describes the impact of dihedral angle on spin−spin coupling constants (J)

Figure 1. 1H NMR spectra of α-terpineol in D2O exposed to 965 ppb gaseous O3 recorded in 30 min intervals; peak at δ = 1.18 cutoff to show other spectral features, and intensity in region between δ = 5.3 and 5.6 doubled. Red arrows indicate α-terpineol features lost during reaction, while blue arrows highlight formation of product peaks.

J(φ) = A + B cos(φ) + C cos(2φ)

taken at 30 min intervals upon exposure to a continuous flow of air containing 965 ppb ozone. NMR characterization of aqueous α-terpineol (1H and 13C with spectral assignments) is presented in Figures S1 and S2. The loss of α-terpineol features, indicated by red arrows in Figure 1, correlated with the formation of product features, noted by blue arrows. Two major products in the t = 120 min spectrum are identified as trans- and cis-lactol [4-(5-hydroxy-2,2-dimethyltetrahydrofuran3-yl)butan-2-one], and the complete NMR characterization of this product mixture, including 1H, 13C, COSY, and HSQC, is presented in Figures S3−S6. The integrated areas of reaction products were slightly smaller than that predicted by complete conversion of α-terpineol, and control experiments without O3 confirmed a steady loss of α-terpineol via evaporation at a rate of ∼8.5 × 10−3 mM min−1. After accounting for this α-terpineol evaporative loss, the observed products accounted for >90% of the consumed α-terpineol based on 1H NMR integrations. Upon reaction completion, the lactol product signals remained constant under continued air flow indicating that they did not evaporate like α-terpineol. These trans- and cis-lactol products

(E1)

where A, B, and C are empirical parameters. In this equation, J coupling constants are largest near 0° and 180° and smallest at 90°. J coupling constants calculated using the gaugeindependent atomic orbital (GIAO) method for the optimized trans-lactol were 5.7 and 0.0 Hz, corresponding to the doublet observed at δ = 5.37, and for the cis-lactol were 7.4 and 6.3 Hz, which produces a doublet of doublets consistent with the triplet observed at δ = 5.42 since the coupling constants are similar. After complete reaction of α-terpineol, the organic products were extracted in deuterated chloroform for further analysis. An 1 H NMR spectrum of the aqueous layer following extraction showed no product signal, while the spectrum of the chloroform layer confirmed the complete extraction of all products without changes in their spectral features. GC-MS analysis of the extracted products revealed the formation of two major and two minor products. Figure 3 shows the total ion current (TIC) chromatograms for extracted products (black) and after derivatization by BSTFA (red), which silates carboxylic acids and alcohols [i.e., −OH → −OSi(CH3)3]. Mass spectra for all labeled chromatogram peaks are included in

Figure 2. Optimized structures for trans- and cis-lactol, and dihedral angles affecting proton−proton coupling to δ = 5.37 doublet and 5.42 triplet, respectively. C


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conclusively identified but did undergo derivitization with BSTFA. This peak also appears to have the same molecular weight as the lactols (MW = 186) and may represent the aldehyde precursor to lactol formation (ALD in Scheme 1 below), although no aldehyde signal was observed in the 1H NMR. The derivatized peak at 25.4 min was present in methodological control experiments adding BSTFA to the solvent and does not correspond to a product. The heterogeneous ozonolysis of α-terpineol adsorbed on glass, kaolinite, and sodium chloride produced the same reaction products but in different yields. For α-terpineol thinfilms coated on glass, trans- and cis-lactols and lactone accounted for 35%, 32%, and 33% of the identified product signal, respectively. Lactone yields increased with a corresponding decrease in lactols for NaCl and kaolinite. For NaCl translactol, cis-lactol, and lactone accounted for 34%, 21%, and 47% of observed products, respectively, while for kaolinite, the percents were 21%, 25%, and 55%. As is discussed below, the products identified in the current study are different than the products previously detected for α-terpineol adsorbed on glass and vinyl tiles.31 The major products detected by Ham and Wells included the unsaturated carbonyl products 6-hydroxyhept-5-en-2-one, 5-(1-hydroxy-1-methylethyl)-2-methylcyclohex-2-en-1-one, and 3-(hydroxy-1-methylethyl)-6-methylcyclohex-2-en-1-one.31 As discussed in the Supporting Information, these products are not consistent with the mass spectrum of the unidentified product noted above. Reaction Mechanism. Scheme 1 shows our proposed mechanism for the formation of the two diastereomeric lactols, each formed as a pair of enantiomers, and the lactone, which together account for 93% of the observed products. No experimental evidence for the secondary ozonide was observed, indicating that if formed, this intermediate quickly proceeded on to form HHP intermediates. The mechanism for lactol formation requires the release of hydrogen peroxide, H2O2, to form the aldehyde intermediate [3-(2-hydroxypropan-2-yl)-6oxoheptanal, ALD]. The concentration of H2O2, detected using semiquantitative test strips, steadily increased during the course of the reaction achieving a final yield of ∼10% (Figure S9), which is consistent with the mechanism included in Scheme 1. Although the H2O2 test strips may also be sensitive to organic peroxides (e.g., HHP), the absence of any significant HHP

Figure 3. Total ion current (TIC) chromatograms for α-terpineol reaction products extracted in chloroform without (black) and with derivatization with BSTFA (red).

Figure S8. The underivatized TIC chromatogram shows three main peaks with retention times of 12.7, 20.0, and 22.2 min, which account for 5%, 73%, and 20% of the total peak area, respectively. The mass spectrum of the 22.2 min peak was identified as 4-hydroxy-4-methyl-3-(3-oxobutyl)-valeric acid gamma-lactone, hereafter referred to as the “lactone,” by a retention time and mass spectrum match to a standard. Derivatization with BSFTA did not alter this compound’s GCMS features consistent with the absence of acid or alcohol functional groups in the lactone. Subsequent comparison of the 1 H NMR spectrum of a lactone standard confirmed its presence in the aqueous reaction spectra with similar yield. The largest TIC peak in Figure 3 at 20.0 min was broad, and upon derivatization split into two peaks (Rt = 21.4 and 21.6) that exhibited nearly identical mass spectra showing the addition of a single trimethylsilyl group indicating a single acid or alcohol site. This behavior is consistent with the formation of two unresolvable cis/trans stereoisomers that were not initially separated by gas chromatography but were resolved after silation. Fragmentation patterns for these peaks (see Figure S8) are consistent with trans- and cis-lactol. Furthermore, the relative area of these peaks (60%:40%) is in agreement with the integrated areas of the NMR peaks at δ = 5.37 and 5.42 (55%:45%). Finally, the minor peak at 12.7 min could not be Scheme 1. Mechanism for Ozonolysis of Aqueous α-Terpineol

D


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Environmental Science & Technology signal in the NMR suggests that H2O2 is the main peroxide detected here. No H2O2 was detected in control experiments exposing aqueous solutions without α-terpineol to ozone. Hemiacetal formation via alcohol attack of the aldehyde likely produces both enantiomers of the trans- and cis-lactol diastereoisomers in proportion to their equilibrium constant, with the trans-lactol thermodynamically favored. This mechanism is consistent with several previous aqueous ozonolysis studies. Ozonolysis of aqueous α-pinene and βpinene produced H2O2 in 60% and 100% yields, respectively, through decomposition of analogous HHP intermediates.12 Isoprene and its first generation oxidation products MVK and MACR also produce H2O2 in high yields during aqueous ozonolysis.11,13 Acid-catalyzed hemiacetal formation is a major mechanism for the formation of dimers contributing to SOA formation, although the reversible nature of this reaction makes it challenging to detect.37 The stability of the lactol hemiacetals observed in the present study is likely due to the more favorable entropy for unimolecular hemiacetal formation compared to bimolecular dimer formation. Similar hemiacetal ring closure products have been detected for the ozonolysis of gaseous linalool in a mechanism analogous to Scheme 1.38 The products and mechanism for the multiphase ozonolysis of aqueous α-terpineol differ from those detected for the same reaction in the gas phase27,28 and for the heterogeneous ozonolysis of α-terpineol surface adsorbed on glass and vinyl.31 For gas-phase ozonolysis, significant fragmentation of αterpineol was observed with methylglyoxal as the major product along with ethanedial, 4-methyl-3-cyclohexen-1-one, 6-hydroxy-hept-5-en-2-one, 1,4-butanedial, and 4-oxopentanal.28 The proposed gas-phase mechanism differs from Scheme 1 in that CI abstract hydrogen atoms to form peroxy intermediates that fragment to smaller products via reactions with oxygen gas. In an aqueous environment, solvent molecules can collisionally stabilize the CIs and add to the biradical to form HHP, which explains the different products detected herein. The present observation of lactone and lactol products for heterogeneous ozonolysis of α-terpineol adsorbed on glass, kaolinite, and sodium chloride, however, also differs from the unsaturated carbonyl products (e.g., 4-methyl-3-cyclohexen-1one) previously detected for ozonolysis of α-terpineol adsorbed on glass and vinyl.31 Although differences in the abundance of surface-adsorbed water could explain this discrepancy, both sets of experiments were conducted under similar humidity conditions (50% RH). Other experimental factors, such as αterpineol surface concentrations, might explain these differences but require further investigation. The increasing lactone yields for surface-adsorbed reactions compared to aqueous reactions in the current study implies that elimination of H2O, rather than H2O2, is preferred on wet surfaces as compared to the bulk aqueous environment. Multiphase Reaction Kinetics. Figure 4 shows the increase in total product concentration as a function of time, as determined by 1H NMR integrated areas, for gaseous ozone concentrations ranging from 131 to 965 ppb. The initial rate of reaction varied linearly with the gas-phase ozone concentration. Product formation is nearly linear with respect to time, suggesting that uptake of ozone into the aqueous solution is a dominant factor controlling the rate of reaction. To analyze these multiphase ozonolysis kinetics, we used a resistance model to calculate the net uptake of gaseous ozone into the liquid, γnet39

Figure 4. Total product formation as a function of time for aqueous terpineol exposed to 131 (red), 480 (blue) and 965 ppb O3 (black). Solid lines show kinetic fits generated using multiphase model discussed in text.

1 1 1 1 = + + γnet Γg α Γrxn + Γsol

(E2)

where Γg is the conductance term for diffusion of gaseous ozone to the air−water interface, α is the mass accommodation for defining the efficiency of ozone uptake into the aqueous solution (2 × 10−2),40 Γsol is the conductance term for ozone solubility, and Γrxn is the conductance associated with reactivity of ozone with α-terpineol in the bulk. Given the relatively long reaction times for these experiments, we can assume that the solubility term becomes negligible compared to the bulk reactivity term (i.e., Γsol ≪ Γrxn) as the aqueous ozone concentration approaches equilibrium. In this scenario, eq E2 simplifies to39 uav 1 1 1 = + + γnet Γg α 4HRT D1k O3

(E3)

The final term describes the bulk reactivity where uav is the mean thermal velocity of ozone, H is Henry’s law coefficient for ozone [(0.82−1.3) × 10−2 M atm−1],39 R is the ideal gas constant, T is temperature, D1 is the diffusion constants for ozone in water (1.90 × 10−9 m2 s−1),41 and kO3 represents the pseudo-first order rate coefficient, which equals the secondorder rate constant times the α-terpineol concentration (kO3 = k [terpineol]). The experimental data was analyzed by combining eq E3 with the frequency of gaseous ozone surface collisions, calculated using the kinetic theory of gases, to calculate the rate of ozone uptake (d[O3(aq)]/dt), which was assumed to equal the rate of product formation. To calculate the net uptake of ozone, γnet, we used the experimentally determined αterpineol concentration at each time point and treated the second-order bulk rate constant, k, as a variable. Optimized rate constants ranged from 6.15 × 106 M−1 s−1 for 480 ppb O3 to 1.21 × 107 M−1 s−1 for 131 ppb O3, with an average value of 9.9(±3.3) × 106 M−1 s−1. This second-order rate constant is an order of magnitude faster than that for the aqueous ozonolysis of isoprene (4.1 × 105 M−1 s−1)42 and styrene (3.00 × 105 M−1 s−1),43 which could be explained by a lower energy barrier for the cycloaddition of electrophilic ozone to an alkene with more electron donating substituents. It should be noted that this analysis ignores interfacial reactivity, which may account for a substantial fraction of O3 + α-terpineol reactivity and could possibly explain the more linear shape of the experimental data. E



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Atmospheric Implications. Multiphase ozonolysis of aqueous organics may provide an alternate pathway contributing to secondary organic aerosol formation, especially for unsaturated oxygenated organic compounds.4 We identified the formation of lactol diastereomers and a lactone accounting for 93% of observed aqueous products following multiphase ozonolysis of α-terpineol. To contribute to aqSOA formation, the Henry’s law constant of the oxidized products must be significantly larger than the reactants. Experimental values have not been measured for the products identified in the current study, however the empirical methods of Brockbank and colleagues can be used to estimate Henry’s law constants based on primary and secondary functional group analysis.44 For example, the geometric average of the two empirical methods predict H = 684 M atm−1 for α-terpineol, in reasonable agreement with the experimental value (429 M atm−1).32 The same methodology predicts lactol and lactone Henry’s law constants of 2.73 × 107 M atm−1 and 8.70 × 106 M atm−1, respectively, which corresponds to an increase by 4 orders of magnitude. Henry’s law estimates calculated by EPI Suite software also predict a 4−5 order of magnitude difference for the lactol compared to α-terpineol and a 2−4 order of magnitude change for the lactone,45 further suggesting that these products can contribute to aqSOA formation. Hydrogen peroxide is a secondary product detected experimentally and predicted mechanistically, demonstrating the potential for this reaction, like many aqueous ozonolysis reactions,11−13 to initiate further oxidative processes following photolysis of H2O2. The second-order rate constant for the reaction of αterpineol + O3, k = 9.9(±3.3) × 106 M−1 s−1, means multiphase ozonolysis will be competitive with oxidation by hydroxyl radicals (OH) and ozonolysis of gaseous α-terpineol. Assuming an in-cloud aqueous O3 concentration of 2 × 10−9 M, which is typical of urban environments,4,46 the lifetime for aqueous αterpineol is 51 s. To the best of our knowledge, the rate constant for α-terpineol + OH in aqueous solutions has not been measured. Using the typical range of organic + OH rate constants (108 − 1010 M−1 s−1) along with a typical urban incloud OH concentration of 1 × 10−14 M,4,46,47 the lifetime for aqueous α-terpineol from OH reactivity is estimated between 2.7 and 277 h. In these conditions, aqueous ozonolysis of αterpineol is almost 200 times faster than aqueous α-terpineol + OH reactivity. Gas-phase ozonolysis of α-terpineol has a rate constant of 3.0(±0.2) × 10−16 cm3 molecules−1 s−1.28 Urban O3 concentrations of 40 ppb (1 × 1012 molecules cm−3) are common and would correspond to a lifetime of just under an hour (3384 s). However, the in-cloud aqueous O3 concentration of 2 × 10−9 M used above would correspond to a gaseous concentration of 150−250 ppb using the Henry’s law coefficient,39 and these concentrations would result in lifetimes on the order of 555−880 s, which is still an order of magnitude slower than aqueous ozonolysis of α-terpineol. Aqueous phase ozonolysis of α-terpineol will also be competitive with gaseous α-terpineol + OH, which has an estimated lifetime of 526− 2632 s using a rate constant of 1.9(±0.5) × 10−10 cm3 molecules−1 s−1 and an OH concentration of 2−10 × 106 radicals cm−3.28,39 Combined, these results indicate that multiphase ozonolysis will be a significant pathway for αterpineol oxidation and the products can contribute to the formation of aqueous secondary organic aerosol.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b03612. 1 H- and 13C-NMR of aqueous α-terpineol, 1H- and 13CNMR of aqueous reaction products extracted in chloroform, COSY and HSQC NMR of aqueous reaction products extracted in chloroform, optimized structures for trans-lactol and cis-lactol, mass spectra for underivatized and derivatized products extracted in chloroform, and H2O2 product concentration as a function of time (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 973-408-3853. Fax: 973408-3572. Author Contributions

The manuscript was written by D.H.L. and R.Z.H., and revised by all authors. D.H.L. conducted all NMR and kinetic experiments. D.V.R. completed all GC-MS analysis. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Alan Rosan for helpful discussions of the mechanism and Dr. Andrew Evans for assistance with NMR analysis. This material is based on work supported by the National Science Foundation under grant AGU-1400556.

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ABBREVIATIONS aqSOA aqueous secondary organic aerosol BSTFA N,O-bis(trimethylsilyl) trifluoroacetamide CI Creigee intermediate MACR methacrolein MVK methyl vinyl ketone OVOC oxygenated volatile organic compound POZ primary ozonide RH relative humidity SOA secondary organic aerosol SOZ secondary ozonide

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REFERENCES

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