Article pubs.acs.org/JPCA
Reactive Uptake of Gaseous Sesquiterpenes on Aqueous Surfaces Kohei Matsuoka,† Yosuke Sakamoto,†,‡ Tetsuya Hama,§ Yoshizumi Kajii,†,‡,∥ and Shinichi Enami*,∥ †
Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501, Japan Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8316, Japan § Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan ∥ National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan ‡
S Supporting Information *
ABSTRACT: Sesquiterpenes emitted from biogenic sources play important roles in atmospheric HOx cycles and new particle formation. Current atmospheric models, however, fail to account for their fates, possibly due to missing heterogeneous sinks. Here we apply interface-specific mass spectrometry to detect carbocation products of the reactive uptake of gaseous sesquiterpenes C15H24 (β-caryophyllene (β-C), αhumulene (α-H), and alloaromadendrene (a-d)) on the surface of aqueous microjets as functions of water acidity and gas concentration. We find that these gases are effectively protonated to C15H25+ upon colliding with the surface of pH < 5 water microjets. We determine inflection points from plots of product yields vs bulk pH: pH1/2 = 4.17 ± 0.05, 4.28 ± 0.06, and 4.36 ± 0.19, and kinetic isotope effects (KIEs) from H2O/D2O (1:1 = vol/vol) experiments: KIE = 2.31 ± 0.08, 1.95 ± 0.05, and 2.71 ± 0.11, for β-C, α-H, and a-d, respectively. These results are analyzed vis-a-vis previous reports on isoprene and monoterpenes experiments. We estimate 6.2 × 10−5 ≤ γ ≤ 3.1 × 10−4 for the reactive uptake of gaseous sesquiterpenes on acidic (1 < pH < 3) water surfaces. The atmospheric implications of present findings are discussed.
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INTRODUCTION Biogenic volatile organic compounds (BVOCs) contribute up to 90% of global VOC emissions to the troposphere, significantly influence the HOx cycle, and contribute to new particle formation.1−8 In spite of much work from field, modeling, and laboratory studies, it remains uncertain how BVOCs are removed from the atmosphere, and what fraction and how they are converted to atmospheric particles.9−16 Such uncertainties may originate from the existence of hitherto unknown missing heterogeneous sinks. Recent field measurements have shown that reuptake of some BVOCs occurs in forest canopies by an unidentified loss mechanism.13,14,17,18 The key issue appears to be a lack of experimental techniques for investigating in situ the processes taking place in collisions of atmospheric gases with aqueous nanofilms. Previously, we demonstrated in the laboratory that gaseous hemiterpene (isoprene) and monoterpenes (α-pinene, βpinene, and d-limonene) are protonated and oligomerized on pH < 4 aqueous microjets surfaces.15,19 Such acidities could be prevalent in the aqueous films lining the surfaces of leaves, soil, and aerosol particles in canopy.15,19−21 It was proposed that the reactive uptake is triggered by partly hydrated protons H+(H2O)n (n < 6) that emerge to the topmost layers of water surfaces at bulk pH < 4.22−25 These results are clearly consistent with the fact that the surfaces of aqueous droplets and films are positively charged only at bulk pH < 4 (cf. water surface at bulk pH ≈ 7 is negatively charged).25−29 Although there is experimental evidence that isoprene and α-pinene are © 2017 American Chemical Society
protonated and polymerize on highly acidic (pH < 0) aerosols or aqueous films,30,31 we deemed important to investigate whether such process could also take place on less acidic, more widespread environmental aqueous surfaces.15,19 Herein we extend our experimental studies on this issue to include hitherto unexplored sesquiterpenes (C15H24). Sesquiterpenes are larger, more nucleophilic and hence are expected to be more powerful particle-makers than hemi- or monoterpenes.3,32−37 A recent field observation study reports that gaseous β-caryophyllene, the most abundant sesquiterpene comprising ∼20% of total sesquiterpene mass concentrations in their study, in association with a variety of sesquiterpenes including α-humulene and alloaromadendrene, exists in the forest, especially at nighttime.33 Although a majority of sesquiterpenes is emitted from vegetation, they also originate from soil where microorganisms and plant root systems are using it as a communication tool between some plants, fungi, bacteria, and microbe in the air and the rhizosphere.38−41 To determine the actual fates of gaseous sesquiterpenes is, therefore, critical for atmospheric chemistry and other research fields. Here we report our new experimental results on sesquiterpene uptake on aqueous surfaces. We utilize continuously refreshed surfaces of free-flowing water microjets Received: November 23, 2016 Revised: January 5, 2017 Published: January 9, 2017 810
DOI: 10.1021/acs.jpca.6b11821 J. Phys. Chem. A 2017, 121, 810−818
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The Journal of Physical Chemistry A
Figure 1. Chemical structures of sesquiterpenes (C15H24, MW = 204) studied in the present study. β-C, α-H, and a-d stands for β-caryophyllene, αhumulene, and alloaromadendrene, respectively.
with a β-caryophyllene (β-C), α-humulene (α-H), and alloaromadendrene (a-d) (Figure 1) beam and detect the formation of interfacial carbocations by online electrospray mass spectrometry (ES-MS).
dilution factor that can be calculated from the ratio of carrier (0.05−0.5 L min−1) and drying gas flow rates (12 L min−1). It is noted that reactant conversion is proportional to exposure, i.e., [β-C(g)] × τ, where [β-C(g)] is concentration and τ is contact time, rather than concentration. Therefore, the exposures (1.1 × 1013 molecules cm−3 × 10 μs) could be comparable with exposing atmospheric aqueous media (e.g., water films lining on leaf surfaces, fog, aerosol, etc.) to a relevant ∼0.1 pptv (2.46 × 10 6 molecules cm −3 ) 33 sesquiterpene (g) for ∼1 min. It can be assumed that the surface of water is not saturated with sesquiterpene at these concentrations. Conditions in the present experiments were: drying nitrogen gas flow rate, 12 L min−1; drying nitrogen gas temperature, 340 °C; inlet voltage, −3.5 kV relative to ground; fragmentor voltage value, 60. All solutions of various pH were prepared in purified water (resistivity ≥ 18.2 M cm at 298 K) from a Millipore Milli-Q water purification system. The carbocation products we observe are formed when the gaseous sesquiterpene collide with the intact electroneutral aqueous jets as they emerge from the nozzle, i.e., before jets are broken up by the nebulizer gas. The possibility that the species we monitor were produced in the reactions of gases with the highly concentrated, high surface-to-volume small droplets undergoing Coulomb explosions48,49 is excluded from experimental evidence.24,27,44,50,51 For example, no increase of product signal was observed by injecting the reactive gases 5 and 10 mm downstream relative to the standard position of the gas injector along the jet direction.52 We also note that the modest polarizations of the initial microjets do not affect the observed phenomena.24,25,27,51 The kinetics of the reaction of dissolved α-tocopherol with gaseous ozone determined by negative and positive ion detection were the same.53 The thickness of the sampled interfacial layers is controllable varying the nebulizer gas velocity vg, as evidenced by that both ion signal intensities and relative anion surface af f inities increase with higher gas velocities vg and extrapolate to zero as vg → 0.43,54,55 An important feature of our instrument is that the microjet issuing from the nozzle source is orthogonal to the polarized inlet to the mass spectrometer (Figure 2). Online mass-based sampling from the surface of continually refreshed water microjets under ambient temperature and pressure makes our instrument an original and promising surface-sensitive technique.56−58 Further experimental details could be found in our previous publications.23−25,59,60 Chemicals: β-caryophyllene (≥98.5%, Sigma-Aldrich or >90%, Tokyo Chemical Industry), α-humulene (≥96.0%, Sigma-Aldrich), alloaromadendrene (≥98.0%, Sigma-Aldrich),
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EXPERIMENTAL SECTION The experimental setup has been described in previous publications.15,19,42−44 Here we briefly summarize the specific features. A gaseous sesquiterpene is injected into a reaction chamber by flowing over the liquid sesquiterpene kept in a trap held at 298 ± 2 K carried by ultrapure (>99.999%) nitrogen gas at known flow rates (0.05−0.5 L min−1) regulated by calibrated digital mass flow controllers (Horiba, STEC, SEC-400 MARK 3). Figure 2 shows a schematic diagram of the present experimental setup.
Figure 2. Schematic diagram of present experimental setup.
We assume that the carrier gas is saturated with sesquiterpene vapor in each case. We adopted the vapor pressure value ∼1.1 Pa (= 0.00825 Torr) for β-C at 298 K from Hoskovec et al.45 and assumed the same value for α-H and a-d. The gas molecule hit the surface of the pH-adjusted (by concentrated HCl, and the pH was already measured by a calibrated pH meter, Horiba LAQUA F-74, before the experiments) aqueous microjet (100 μm diameter) coaxial with a sheath issuing nebulizer gas N2(g) at high gas velocity (vg ≈ 160 m s−1)43 can stick to it by becoming protonated therein or rebound.46,47 The concentration of sesquiterpene in the reaction chamber is smaller than their vapor pressures by a 811
DOI: 10.1021/acs.jpca.6b11821 J. Phys. Chem. A 2017, 121, 810−818
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Figure 3. (A) Positive ion mass spectra of water microjets exposed to gaseous β-caryophyllene (β-C), (B) α-humulene (α-H), and (C) alloaromadendrene (a-d) at 1 atm, 298 K experimental conditions. The water pH is 2.32, 1.98, and 1.88, respectively.
Figure 5A, B shows the plots of the (β-C)H+ product (m/z = 205) as a function of bulk pH, in experiments performed on water microjets exposed to a low (2.2 × 1012 molecules cm−3) and a high (1.1 × 1013 molecules cm−3) concentration of βC(g), respectively. Under both concentrations, the (β-C)H+ product appears only below pH ≈ 5 and evolve along a sigmoidal titration curve possessing an inflection point at pH1/2 = 4.07 ± 0.05 and 4.17 ± 0.05, respectively. Note that product yields are negligible at pH ≈ 7, confirming that a reactive H+(H2O)n (n < 6) species22,23,67,68 is necessary for the formation. The experimental titration curves for α-H and a-d are found to be similar but not identical to β-C, having inflection points at pH1/2 = 4.28 ± 0.06 (Figure 6B) and pH1/2 = 4.36 ± 0.19 (Figure 6C), respectively. Interestingly, these inflection point values are considerably larger than those of other gaseous molecules determined by the similar setup, i.e., pH1/2(hexanoic acid) = 2.5 ± 0.1,23 pH1/2(isoprene) = 3.63 ± 0.05,15 pH1/2(αpinene) = 3.54 ± 0.26, pH1/2(β-pinene) = 3.51 ± 0.40, pH1/2(d-limonene) = 3.63 ± 0.05,19 and pH1/2(trimethylamine) = 3.8 ± 0.2.24,25 These different values indicate that they depend not only on the property of the surface of water, where strongly acidic hydronium species H+(H2O)n (n < 6) emerge to the surface of pH ≤ 4 water,23,25 but also on the proton affinity (PA) of the proton acceptors; PA(hexanoic acid) = 187 kcal mol−1, PA(isoprene) = 197 kcal mol −1 , PA(d-limonene) = 209 kcal mol −1 , and PA(trimethylamine) = 227 kcal mol−1.69,70 As far as we know, currently no PA data for sesquiterpenes are available. We note, however, that PAs increase as ionization energies (IEs) decrease.71,72 Hence, PAs for sesquiterpenes (IE ≈ 8.3 eV) are expected to be higher than those of monoterpenes (IE ≈ 8.5 eV).72 As a result, the protonation of sesquiterpenes takes place on less-acidic (i.e., larger pH1/2) water surfaces. In the present study, sesquiterpenes, whose gas-phase PAs are much larger than that of water (PA = 165 kcal mol−1),69 are readily protonated in interfacial layers of bulk pH < 5 giving rise to the corresponding carbocations. The implication is that gaseous sesquiterpene first attaches to water surface (R1) via a Langmuir−Hinshelwood (LH) mechanism, then partly rebounds to the air (R-1), or reacts H+(H2O)n (n < 6) on water forming detected carbocations of m/z = 205 (R2):
D2O (>99.9 atom % D, Sigma-Aldrich), and HCl (1 N solution, Wako) were used as received.
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RESULTS AND DISCUSSION Figure 3 shows positive ion mass spectra of the surface of water microjets at pH ≈ 2, exposed to β-C(g)/N2(g), α-H(g)/N2(g), and a-d(g)/N2(g) mixtures for ∼10 μs. We observe the intense mass spectral signals at m/z = 205 (C15H24 + H+) in all cases. Note that carbocations C15H25+ may undergo rapid isomerization via intramolecular proton transfers and/or structural rearrangements.61,62 Smaller signal intensities at smaller mass are fragments of the collisionally induced decomposition of C15H25+.36,63−65 For example, the m/z = 135 and 149 signals (Figure 3A) are assigned to the reported fragment ions from protonated β-C, C15H25+ (m/z = 205), into C10H15+ (+C5H10) and C11H17+ (+ C4H8), respectively,36,63 in accord with the observed reciprocal evolution of m/z = 135 and 149 versus m/z = 205 signal intensities as a function of fragmentation voltage (Figure S1). No evidence for dimer product (m/z = 409) or mass >500 Da is found under present conditions, which is in contrast with previous results of isoprene and monoterpenes, where dimers and in some cases (isoprene and β-pinene) trimers were detected in a similar setup.15,19 We attribute it to a much smaller concentration of gaseous reactants under the present condition, i.e., [sesquiterpenes] < 1.2 × 1013 molecules cm−3 vs [isoprene], [monoterpenes] > 2.4 × 1014 molecules cm−3.15,19 Note that available concentration is limited by the much lower vapor pressure of sesquiterpene (∼1.1 Pa at 298 K), cf. α-pinene (∼590 Pa at 298 K).45,66 Figure 4A−C shows the plots of product signal intensities at m/z = 205 as a function of concentration of β-C(g), α-H(g), and a-d(g), respectively. Similar plots were obtained in all cases, except for relatively small signal intensities of (a-d)H+ and a plateau reached at a high [a-d(g)]. Alloaromadendrene (a-d) has constrained 3-, 5-, and 7-membered rings and an exo CC bond in the structure (see Figure 1). Previous study suggests that an exo CC double bond is a better nucleophile for oligomerization than the endo or vinyl CC double bonds.19 Thus, it could be speculated that a-d proceeds to dimerization at a high [a-d], while the dimer (m/z = 409, absent from mass spectra) decomposes immediately to MS-silent products. These results represent evidence that β-C(g), α-H(g), and a-d(g) undergo protonation upon hitting on mildly acidic water surfaces, consistent with previous results on hemi- and monoterpenes.19,25 812
β ‐C(g) → β ‐C(interface, int)
(R1)
β ‐C(int) → β ‐C(g)
(R-1) DOI: 10.1021/acs.jpca.6b11821 J. Phys. Chem. A 2017, 121, 810−818
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The Journal of Physical Chemistry A
Figure 5. Mass spectral signal intensities for (β-C)H+ (m/z = 205) as a function of bulk pH, in experiments performed on water microjets exposed to (A) 2.2 × 1012 molecules cm−3 and (B) 1.1 × 1013 molecules cm−3 β-C(g) for τ ≈ 10 μs contact times. All experiments are performed in 1 atm of N2(g) at 298 K. Lines are regression curves fitted with sigmoidal functions. See text for details.
values (i.e., the kinetic isotope effect, KIE) for products are found to be independent of bulk pH. Average KIE of β-C, α-H, and a-d at these pHs is determined to be 2.31 ± 0.08, 1.95 ± 0.05, and 2.71 ± 0.11, respectively. These relatively small KIEs and their pH-independence are consistent with those found for monoterpenes under similar conditions,15,19 but in contrast with those observed in the protonation of hexanoic acid and trimethylamine, whose KIE values strongly depend on pH.23−25 The acid-induced H−D exchange of isobutene with D2SO4 has been reported to involve a fast, reversible protonation/ deprotonation reaction between isobutene and the trivalent tert-butyl carbocation, followed by H−D exchange with the solvent.22,73,74 We infer that a similar rapid protonation/ deprotonation process applies in the case of terpenes (that has (a) CC bond(s)) and the corresponding trivalent carbocations prior to their detection, leading to partial loss of isotope labeling via H−D exchange with the solvent.75,76 A reactive uptake coefficient for β-C(g) in the present experiments, γ, is derived from the expression for the number of reactive collisions per unit area during contact times, N, given by the kinetic theory of gases (eq E1)15,47
Figure 4. Mass spectral signal intensities from aqueous microjets exposed to gaseous (A) β-caryophyllene (β-C), (B) α-humulene (αH), and (C) alloaromadendrene (a-d) as a function of gaseous sesquiterpene concentration. The water pH is 2.32, 1.98, and 1.88, respectively. Exposure time is ∼10 μs at 1 atm and 298 K experimental conditions.
β ‐C(int) + H+(H 2O)n (n < 6) → (β ‐C)H+ + (H 2O)n (R2)
or the protonation occurs directly via an Eley−Rideal (ER) mechanism: β ‐C(g) + H+(H 2O)n (n < 6)(int) → (β ‐C)H+ + (H 2O)n (R3) +
+
Figure 7 displays the isotopic ratios RH /RD measured in experiments on β-C(g), α-H(g), and a-d(g) uptake on H2O/ D2O (1:1 = vol/vol) microjets as a function of bulk pH. The contributions of the 13C satellites of RH+ signals to RD+ signal intensities were carefully subtracted in all cases. The RH+/RD+
γβ ‐ C = 4N /(vnβ ‐ Cτ )
(E1) −1
where v = 1.76 × 10 cm s is the mean thermal speed of gaseous β-C at 298 K, nβ‑C is number density, and τ = 10−50 μs 4
813
DOI: 10.1021/acs.jpca.6b11821 J. Phys. Chem. A 2017, 121, 810−818
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Figure 6. Normalized (A) (β-C)H+, (B) (α-H)H+, and (C) (a-d)H+ (m/z = 205) mass spectral signal intensities as a function of bulk pH, in experiments performed on water microjets exposed to 1.1 × 1013 molecules cm−3 sesquiterpene(g) for τ ≈ 10 μs contact times. Signal intensities are normalized to the maximum values observed in the pH range. All experiments are performed in 1 atm of N2(g) at 298 K. Lines are regression curves fitted with sigmoidal functions. See text for details.
Figure 7. Ratios of 205/206 mass spectral signal intensities as a function of bulk pH, in experiments performed on H2O/D2O (1:1 = vol/vol) microjets exposed to 1.1 × 1013 molecules cm−3 β-C(g) (A), α-H (B), and a-d (C) for τ ≈ 10 μs contact times. All experiments are performed in 1 atm N2(g) at 298 K. The contributions of the 13C satellites of RH+ signals to RD+ signal intensities were subtracted in all cases. Lines are linear-regression fittings.
is the estimated gas−liquid contact time.25 If we could assume that (1) the thickness of the interfacial liquid layer in which these processes take place is Δ ≈ 5 × 10−10 m,25 (2) the minimum detectable ion concentration (∼0.5 μM) in this setup is produced under n β‑C ≈ 1.1 × 10 12 molecules cm −3 concentration; we derive N = (0.5 × 10−6 × 6.02 × 1020 molecules cm−3) × (5 × 10−8 cm) = 1.5 × 107 molecules cm−2, and 6.2 × 10−5 ≤ γβ‑C ≤ 3.1 × 10−4 for the reactive uptake of β-
C on fresh, i.e., nonaged, water surfaces at 1 < pH < 3. We find γα‑H and γa‑d are within the same range of γβ‑C. These values are comparable with 4.6 × 10−5 < γ < 1.1 × 10−3 for monoterpenes, but >120 times larger than 5 × 10−7 < γ < 2 × 10−6 for isoprene measured in the similar setup.15,19 These values are also in reasonable agreement with the reported uptake coefficient 1.2 × 10−5 < γ < 1.3 × 10−3 for pinonaldehyde on aerosols of pH < 0.77 As another reference, 1 × 10−6 < γ < 6 × 10−6 for gaseous 814
DOI: 10.1021/acs.jpca.6b11821 J. Phys. Chem. A 2017, 121, 810−818
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predominant during new particle formation.88 It could be speculated that oligomerization and/or oxidations of carbocations formed on the acidic surfaces of seed particles may play critical roles in the growth. Importantly, a recent aircraft measurement study revealed aerosols collected in altitudes up to 5000 m of the northeastern US wintertime are more acidic than previously assumed and reach the pH as low as 0.77 ± 0.96.89 The pH range is similar to those reported in other areas.90−92 Thus, our results suggest that reactive uptake of sesquiterpenes and other VOCs whose PA is higher than water’s (i.e., 165 kcal mol−1) onto such acidic (not necessarily pH < 0, but 1 < pH < 3) particles could trigger the formation and augment new particles. Thus, our results may narrow the current gap between field observations and atmospheric model calculations where models always underestimate actual secondary organic aerosol (SOA) yields.9 A modeling study incorporating the present findings under actual conditions is necessary to fully elucidate their impact on atmospheric chemistry.
limonene on kaolinite under 25% relative humidity is recently reported.78 It is noted that since the contribution of diffusion from gas to interface on γ is negligible (γ ≪ 1), the observed uptake could be largely driven by the protonation reaction at the air−water interface.79 The atmospheric implications are discussed below. The lower limit value γβ‑C > 6.2 × 10−5 determined by our experiments indicates surface resistances: RC < 4/(vγβ‑C) = 367 s m−1 for the deposition of β-C(g) on acidic water surfaces. The product of 1/RC times the surface-to-volume ratio (S/V) = 5 m−1 of typical canopies80 yields a first-order rate constant for β1
C dry deposition: k = 4 γv
( VS ) = 0.0136 s
−1
, and half-lives of
τ1/2 = 0.69/k ≈ 1 min for β-C toward this process. This value is comparable or even shorter than the lifetimes of these sesquiterpenes toward their gas-phase oxidations, which range from a few minutes to an hour.81−85 Present estimates are consistent with the previous conclusion that significant reuptake of gaseous terpenes is expected to take place on the surfaces of leaves or soils that are only mildly acidic, i.e., pH < 4−5, and the rates of dry deposition are largely controlled by their aerodynamic approach to foliar surfaces rather than by uptake itself.15,19 It is recently reported that the pH of infusion from ambient leaves taken from fields and arboretums in 20 mL of pure water was measured to be around 5.5, supporting our claim that leaf surfaces are indeed sufficiently acidic (note that the actual pH before-dilution must be much lower than the value).20 Although the fates of carbocations remaining on the surface of leaves or soil are unknown, it is noted Connelly and Tolbert reported that gaseous isoprene forms yellow/red-colored mono- and sesquiterpene-like products via dry deposition on concentrated (60−80 wt %) sulfuric acid, which are then released to the gas phase upon addition of water.31 They claimed an equilibrium exists between the product and its carbocation, and the addition of water drives the reaction to favor the product, which then partitions into the gas phase.31 We also note that the recent analysis of infusion from ambient leaves reveals the existence of hitherto unidentified larger-mass organic species on the leaf surfaces (m/z− = 221, 305, 356, and 445),20 possibly originating from subsequent reactions of deposited carbocations such as observed here. Thus, it is envisioned that under ambient conditions carbocations as observed here should go (1) back to neutral sesquiterpenes (of not necessarily the identical structures), (2) oligomerization, (3) fragmentation, and/or (4) further reaction with atmospheric oxidants (i.e., O3, OH, and NO3). Another important implication would be the contribution toward the growth of atmospheric particles.86 It is noted that atmospheric acidic particles may not be a significant sink for gaseous sesquiterpenes due to the limited surface areas (cf. leaf surfaces), but they may grow by absorbing gaseous sesquiterpenes and other VOCs.30,77,87 There was the experimental evidence that isoprene and α-pinene are protonated and polymerized on highly acidic (pH < 0) aerosols in chamber experiments.30 Although whether oligomerization or oxidation of carbocations (C15H25+) occurs would largely depend on actual conditions such as concentrations of sesquiterpene(g) and oxidants(g), our results underscore that such process could also take place on less acidic, more representative environmental aqueous surfaces. A recent field measurement performed at a rural mountaintop station in central Germany reveals the contribution of sesquiterpenes is
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CONCLUSION We report that gaseous sesquiterpenes β-caryophyllene (β-C), α-humulene (α-H), and alloaromadendrene (a-d) are protonated on the surface of pH < 5 water microjets for the first time. We determined inflection points from titration curves of the products versus bulk pH for β-C, α-H, and a-d: pH1/2 = 4.17 ± 0.05, 4.28 ± 0.06, and 4.36 ± 0.19, respectively. The observed titration curves indicate that partly hydrated proton, H+(H2O)n (n < 6), must be present on the topmost layers of mildly acidic water and can transfer a proton to colliding sesquiterpene molecules, consistent with previous results on other VOCs.15,19,23−25 It is inferred that proton affinities (PAs) of acceptors also contribute to the pH1/2 values. We determined kinetic isotope effects (KIEs) from H2O/D2O (1:1 = vol/vol) experiments: 2.31 ± 0.08, 1.95 ± 0.05, and 2.71 ± 0.11, for β-C, α-H, and a-d, respectively. We also derived 6.2 × 10−5 ≤ γ ≤ 3.1 × 10−4 for the reactive uptake of gaseous sesquiterpenes on acidic (1 < pH < 3) water surfaces. It is shown that the heterogeneous loss process on acidic water films of leaves93 and soil would be comparably important as homogeneous oxidation for determination of the fate of gaseous sesquiterpenes under relevant conditions. Another implication is that reactive uptake of sesquiterpenes onto relevant particles, recently revealed to be sufficiently acidic (0 < pH < 3),89,90,92 could trigger the formation and augment new particles. Although a detailed modeling study incorporating the present results under actual conditions is necessary to fully elucidate their impact on atmospheric chemistry, it is anticipated that our results narrow the gap between field observations and atmospheric model calculations.
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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.jpca.6b11821. Additional data on fragmentor voltage effects on product mass signal intensities (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +81-29-850-2770. 815
DOI: 10.1021/acs.jpca.6b11821 J. Phys. Chem. A 2017, 121, 810−818
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The Journal of Physical Chemistry A ORCID
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Shinichi Enami: 0000-0002-2790-7361 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Drs. A. J. Colussi and M. R. Hoffmann of Caltech and Drs. Satoshi Inomata, Akihiro Fushimi and Kei Sato of NIES for stimulating discussion. This work is partly supported by JSPS KAKENHI grant numbers 15H05328 and 15K12188.
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