Article pubs.acs.org/JPCA
Cite This: J. Phys. Chem. A 2018, 122, 6303−6310
Reactions of Criegee Intermediates with Benzoic Acid at the Gas/ Liquid Interface Junting Qiu,† Shinnosuke Ishizuka,‡ Kenichi Tonokura,† and Shinichi Enami*,‡ †
Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8563, Japan National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan
‡
J. Phys. Chem. A 2018.122:6303-6310. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/27/19. For personal use only.
S Supporting Information *
ABSTRACT: Secondary organic aerosol (SOA) found in polluted mega-cities contains benzoic acid (BA) as a major organic acid in addition to a variety of species including alkenes. In polluted air, ozone could be a major oxidizer for SOA and induces subsequent reactions involving Criegee intermediates (CIs, carbonyl oxide, RR′C•−O−O•/RR′CO+−O−) formed by the −CC− + O3 reaction at the gas/liquid interface. The possibility that abundant BA could be an effective scavenger of CIs at the interface remains to be investigated by direct experiments. Here, we showed that amphiphilic BA is able to compete with water molecules for the CIs produced in the prompt ozonolysis of β-caryophyllene on the surface of a water/acetonitrile solvent microjet by generating hitherto uncharacterized C22 ester hydroperoxide products. Competition between BA vs octanoic acid vs cis-pinonic acid toward CIs reveals that BA is a much less-efficient scavenger of CIs on aqueous organic surfaces. We attribute it to the surface-specific orientation of BA at the gas/liquid interface, where the reactive −C(O)OH group is fully hydrated and not available for CIs generated at the topmost layers.
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INTRODUCTION Criegee intermediates (CIs) are reactive carbonyl oxides (RR′C•−O−O• diradicals and/or RR′CO+−O− zwitterions)1 that contribute to the HOx cycle and particle formation in the atmosphere.2−9 Recent theoretical and field studies estimate that concentrations of CIs over the terrestrial boundary layer are ∼(1−5) × 104 molecules cm−3, just 1∼2 orders of magnitude smaller than the concentrations of OH radicals.10 Gaseous CIs can react with a variety of species, but the fate of gaseous CIs may be largely controlled by reactions with water vapors and unimolecular decomposition. 11 However, as result of the experimental difficulties, the information on larger, more complicated but more atmospherically relevant CIs generated from biogenic terpenes [isoprene, α-pinene, β-caryophyllene (β-C), etc.] is largely lacking for the moment. Furthermore, heterogeneous CI chemistry including reactions at the gas/liquid interface remain largely unexplored. Recent experiments have revealed that terpenes can be taken up via protonation on the surface of acidic aqueous aerosols.12−17 For example, laboratory experiments performed by Limbeck et al. showed the formation of higher-mass molecules with conjugated CC double bonds after the heterogeneous uptake of gaseous isoprene on sulfuric acid surfaces.18 Then, trapped terpenes will react with O3(g) to generate CIs at the gas/liquid interface. Derivatives of terpenes found in a condensed phase such as limononic acid are amphiphilic and contain CC bond(s), which should react with O3(g) to generate CIs under ambient conditions.19 Photochemistry at the air/seawater interface may convert © 2018 American Chemical Society
saturated fatty acid into a variety of unsaturated species containing CC bonds,20 which will further react with O3(g) to form CIs at the interface. Enami et al. recently found that hydroxylic species having large interfacial availability and gasphase acidity could compete with interfacial water molecules (H2O)n for sesquiterpene CIs in the outermost interfacial layers of water/acetonitrile (W/AN) mixtures.21−24 This finding is important, because it means that CIs on the surface of aqueous organic aerosol potentially bypass the reaction with water and form oligomer products of lower volatility that may contribute to the growth of atmospheric particles. In addition to this experimental evidence, theoretical studies by Francisco and co-workers showed that some large CIs are surprisingly inert toward water molecules at the air/water interface, while the simplest CI (i.e., CH2O2) reacts with water 102∼103 times faster at the interface than in the gas phase.25−27 Benzoic acid (BA) is a dominant anthropogenic acid found in the particulate matter (PM) over polluted urban areas (e.g., megacities). For example, a recent field measurement study showed that BA concentrations in PM2.5 sampled over Beijing are extremely high, i.e., 1496 ng m−3 (average) in Pekin University and 1278 ng m−3 in Yufa.28 These values exceed the concentrations of total diacids (∼1010 ng m−3) collected in the same campaign.28 The photochemical degradation of anthropogenic aromatic compounds and direct emission from traffic produce BA,29 which is taken up into the liquid phase Received: May 25, 2018 Revised: July 3, 2018 Published: July 10, 2018 6303
DOI: 10.1021/acs.jpca.8b04995 J. Phys. Chem. A 2018, 122, 6303−6310
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The Journal of Physical Chemistry A Scheme 1. Chemical Structures of the Reactants Used in the Present Studya
ΔGacidity values are taken from literatures38,39 (ΔGacidity of OA is taken from value for heptanoic acid).
a
because of the hydrophilic C(O)OH moiety, followed by reactions with oxidants.28 It has recently been unveiled that aqueous-phase chemistry plays a central role in the formation of extensively oxidized secondary organic aerosol sampled in Beijing.30 Since BA is amphiphilic31 and relatively inert toward O3 (in bulk water, k = 1.2 M−1 s−1),32 not only the oxidation of BA(aq) by ·OH(g)33 but also CIs produced at the gas/liquid interface (from −CC− + O3) may control its fate.25 Previous molecular dynamics (MD) calculations, surfacetension measurements, and surface-specific mass spectrometry revealed a moderate affinity of BA for aqueous surfaces.34,35 Thus, the identification of products from the reaction of CIs with BA at the gas/liquid interface has emerged as an important issue in the atmospheric chemistry of polluted urban air.36,37 Since previous studies indicate that the reaction rate of CIs is correlated with gas-phase acidity, ΔGacidity (the more acidic, the more reactions),22,23,38 the process of how BA, which is more acidic (ΔGacidity = 1394 kJ/mol) than alkyl carboxylic acids of similar size, reacts with CIs at the gas/liquid interface is an important but unexplored issue (Scheme 1). Here we show the direct detection of products from reactions of BA with CIs generated on the surfaces of β-C, a representative terpene, in solutions in W/AN microjets exposed to O3(g) for ∼10 μs.35,40,41 The exceptionally high reactivity of β-C toward O3(g) is utilized as an in situ CI source. The microheterogeneous surface of a W/AN binary mixture is well studied42,43 and could be suitable as a surrogate for atmospheric aqueous organic media. Our results revealed that BA is a less-efficient scavenger of the CIs produced from ozonolysis of a terpene on aqueous organic surfaces, regardless of its higher gas-phase acidity. The unique orientation of BA at the gas/liquid interface will be responsible for the observed decreased reactivity.
Figure 1. Schematic diagram of the experimental method. β-C, BA, W, and AN stand for β-caryophyllene, benzoic acid, water, and acetonitrile, respectively.
ozonolysis of β-C on W/AN surfaces occurs ∼20 times faster than in the bulk liquid with dissolved O346 and even much faster than in the gas phase.47,48 W/AN solutions containing βC, NaCl, and BA were pumped (100 μL min−1) into the reaction chamber through a grounded stainless steel needle (100 μm bore) with a sheath flowing nebulizer N2(g) at a high gas velocity.49 Note that since an external application of voltage is not necessary to form detected ions,49−51 the present method resembles sonic spray ionization mass spectrometry51,52 rather than classical Taylor-cone electrospray ionization mass spectrometry.53,54 One of the most important features of the present method is that fragile hydroperoxide species (ROOH) can be detected as stable chloride adducts just by adding NaCl into the solution.21−24 Chloride was found to be unreactive toward O3(g) under the present conditions.55,56 It was shown that BA is inert toward O3(g) (in bulk water, k = 1.2 M−1 s−1) in the absence of β-C.33 Further experimental details could be found in previous publications.21,35,41,44,45,49,57 Ozone was produced by injecting O2(g) (>99.999%) through a commercial ozonizer (KSQ-050, Kotohira, Japan) and quantified via online UV−vis absorption spectrophotometry prior to entering the reaction chamber. The exposure values (E = [O3(g)] × time) in the text are derived by considering [O3(g)] measured by a UV−vis spectrometer (Agilent 8453) and a dilution factor by drying nitrogen gas. Conditions in the present experiments: 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 V. All solutions were prepared in purified water (resistivity ≥ 18.2 MΩ cm at 298 K) from a Millipore Milli-Q water purification system and used within a couple of
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EXPERIMENTAL SECTION The experimental method is the same as those we reported elsewhere.21,44 A mixture of [β-C + NaCl + BA] in W/AN (vol/vol = 1:4) microjets is introduced into the reaction chamber of a mass spectrometer (ES-MS, Agilent 6130 Quadrupole LC/MS Electrospray System at NIES, Tsukuba) with N2(g) at 1 atm, 298 K. Liquid microjets are then orthogonally exposed to the gas-phase O3/O2 beam (Figure 1). The observed mass spectrum represents species generated by heterogeneous processes in the interfacial layers (≤1 nm) of the liquid microjets, as validated by previous experiments.33,35,40,44,45 The small ozone exposures: E = [O3(g)] × τR ≤ 5.5 × 1011 molecules cm−3 s in our experiments enable us to measure the initial steps of alkene ozonolysis on liquid surfaces within ∼10 μs. Previous study reveals that the 6304
DOI: 10.1021/acs.jpca.8b04995 J. Phys. Chem. A 2018, 122, 6303−6310
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The Journal of Physical Chemistry A days. β-caryophyllene (≥98.5%, Sigma-Aldrich, or >90%, Tokyo Chemical Industry), benzoic acid (≥99.5%, SigmaAldrich), octanoic acid (>97%, Wako), cis-pinonic acid (≥98%, Sigma-Aldrich), acetonitrile (≥99.8%, Wako), D2O (>99.9 atom %D, Sigma-Aldrich), and NaCl (≥99.999%, SigmaAldrich) were used as received.
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Cl/37Cl ratio (Figure 2). The m/z 305,307 signals are assigned to the chloride adducts of hydroxy-hydroperoxides formed from the addition of O3 (+48) to a β-C (MW = 204) endo CC bond,46,47 followed by H2O addition (+18): 305,307 = 204 + 48 + 18 + 35,37, as already found in our previous study.21 The m/z 409,411 signals are assigned to the chloride adducts of α-acyloxy-hydroperoxides (C22 ester hydroperoxides) produced from the reaction of CIs with BA: 409,411 = 204 (β-C) + 48 (O3) + 122 (BA) + 35,37 (Cl−) (Scheme 2). Notably, neutral hydroperoxide products ROOH are detected as stable chloride adducts by mass spectrometry.21−24 The presence of signals for the C22 ester hydroperoxide products from the CIs + BA reaction is consistent with our previous findings that amphiphilic n-alkanoic acids Rn− C(O)OH (n ≥ 4) and cis-pinonic acid compete with interfacial water molecules for sesquiterpene CIs on the surface of W/ AN.21,24 Note that this observation means that BA is able to compete with (H2O)n from ∼11 M bulk water for the CIs at aqueous organic surfaces. To our knowledge, this is the first report on the products from the reaction of CIs with BA in any media (e.g., gas, liquid, and the interface). The present results imply that sesquiterpene CIs generated in situ could potentially react with BA on the surface of aqueous organic particles found in a polluted area. Not only CIs from β-C but also those from other precursors (e.g., monoterpenes) may also react with BA, while the reactivity will depend on the structures2,26 and the orientation of these reactants at the gas/liquid interface (see below). Importantly, since these products contain a reactive hydroperoxide group, −OOH, they will trigger polymerization processes or decompose to form radical species.5,58−61 Such a process may be linked to adverse health effects caused by inhalation of PM containing these species, inducing unexpected production of radical species in the lung epithelial lining fluid.62,63 Figure 3 shows a negative ion mass spectrum of 1 mM β-C + 0.2 mM NaCl + 10 mM BA in D2O/AN (1:4 = vol/vol) solution microjets in the presence of O3(g).
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RESULTS AND DISCUSSION Figure 2 shows a negative ion mass spectrum of 1 mM β-C + 0.2 mM NaCl + 10 mM BA in W/AN (1:4 = vol/vol) solution microjets in the absence and presence of O3(g).
Figure 2. Negative ion mass spectrum of 1 mM β-caryophyllene + 0.2 mM NaCl + 10 mM benzoic acid in W/AN (1:4 = vol/vol) solution microjets (gray) or those exposed to O3(g) (red, E = 9.5 × 1010 molecules cm−3 s) at 1 atm and 298 K. The m/z 305,307 and 409,411 signals correspond to chloride adducts of α-hydroxy-hydroperoxides and α-acyloxy-hydroperoxides (C22 ester hydroperoxides), respectively. Here, we show representative structures among possible isomers.
In the presence of O3(g), distinguished peaks appear at m/z 305,307 and 409,411 in the characteristic M/M + 2 = 3/1 =
Scheme 2. Reaction Schemes of β-Caryophyllene’s Criegee Intermediate + (H2O)n, Octanoic Acid (OA), and Benzoic Acid (BA) at the Gas/Liquid Interfacea
a
Here, we show the most likely structures among possible isomers. See the text for details. 6305
DOI: 10.1021/acs.jpca.8b04995 J. Phys. Chem. A 2018, 122, 6303−6310
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between (H2O)n vs BA vs OA toward CIs at the gas/liquid interface. The peaks at m/z 431,433 correspond to the products of OA addition to CIs: 431,433 = 204 (β-C) + 48 (O3) + 144 (OA) + 35,37 (Cl −) (Scheme 2), as observed before. 21 The observation that signal intensities of m/z 431,433 are ∼2 times larger than those of m/z 409,411 at [BA] = [OA] = 10 mM indicates that BA is less reactive toward CIs or less surface available.57 Previously, Enami et al. showed that the relative reactivity toward CIs at the gas/liquid interface is negatively correlated with ΔGacidity values (note that the smaller the values, the greater the acidity) and the interfacial availability of the OH species.21−24 The reported ΔGacidity,BA is 1394 kJ/mol, which is smaller (more acidic) than that of heptanoic acid (1418 kJ/mol) (Scheme 1), suggesting that the BA + CIs reaction should have been more favorable. We infer that this unexpectedly small reactivity of BA toward CIs at the gas/ liquid interface is correlated with the surface orientation of BA. A previous study revealed that the interfacial availability of benzoate (not benzoic acid, though) is smaller than heptanoate (by ∼3.5 times at 1 mM).35 Since π-electrons of the benzene ring of BA can interact with H atoms of water molecules,66 the reactive C(O)OH group of BA will be located at deeper layers of the air/water interface than OA where CIs are generated. We found that the ratio of the signal intensity of m/z 431 (from CIs + OA) to the signal intensity of m/z 409 (from CIs + BA) does not depend on the pH of the solution acidified by HCl in the range of 2.1 ≤ ws pH ≤ 3.4 (Figure S1).67 This result implies that the extent of acid dissociation into carboxylate anions (pKa = 4.2 for BA vs 4.8 for OA in bulk water) at the gas/liquid interface of W/AN mixtures does not contribute to the preferable formation of the m/z 431 product over the m/z 409 product. Note that because of the lack of active −(O)OH, benzoate (BA−) does not react with CIs, as suggested by the absence of signals at m/z 373, which would have arisen from 373 = 204 (β-C) + 48 (O3) + 121 (BA−), consistent with the cases of alkyl carboxylate and cis-pinonate.21,24 Figure 5 shows mass spectral signals acquired from 1 mM βC + 0.2 mM NaCl + 20 mM BA + 20 mM OA in W/AN (1:4 = vol/vol) microjets under the exposure of gaseous O3/O2 mixtures as functions of E(O3).
Figure 3. Negative ion mass spectra of 1 mM β-caryophyllene + 0.2 mM NaCl + 10 mM benzoic acid in D2O/AN (1:4 = vol/vol) solution microjets exposed to O3(g) (E = 3.2 × 1011 molecules cm−3 s) at 1 atm and 298 K.
The observation that the m/z = 305,307 product signals shift by +2 mass units into 307,309 signals in D2O/AN (Figure 3) is in accordance with the formation of hydroxy-hydroperoxides.21 The m/z = 409,411 product signals shift by +1 mass units into 410,412 signals in D2O/AN (Figure 3), consistent with the formation of α-acyloxy-hydroperoxides having one exchangeable peroxide −OOH group (Scheme 2). These shifts also imply that both structures do not have an aldehydic H atom −C(O)H; otherwise, they would shift +3 and +2 mass units, respectively, in the D2O/AN experiment.24,64 Thus, Scheme 2 represents the most likely structures among possible isomers. Next, we evaluated the relative reactivity of BA toward CIs at the gas/liquid interface by comparison with other competitive reactants. Figure 4 shows the negative ion mass spectra of 1 mM β-C + 0.2 mM NaCl + 10 mM BA + 10 mM octanoic acid (OA, see Scheme 1) in W/AN (1:4 = vol/vol) solution microjets in the absence or presence of O3(g). OA is a surfaceactive carboxylic acid commonly observed in ambient particles.65 The mass spectrum represents the competition
Figure 5. Products’ mass spectral signal intensities from 1 mM βcaryophyllene + 0.2 mM NaCl + 20 mM benzoic acid + 20 mM octanoic acid in W/AN (1:4 = vol/vol) solution microjets exposed to O3(g) as functions of O3(g) exposure (in 1012 molecules cm−3 s). The lines are regression curves fitted with y = a[1 − exp(−bx)].
Figure 4. Negative ion mass spectra of 1 mM β-caryophyllene + 0.2 mM NaCl + 10 mM benzoic acid +10 mM octanoic acid in W/AN (1:4 = vol/vol) solution microjets (gray) or those exposed to O3(g) (red, E = 3.3 × 1011 molecules cm−3 s) at 1 atm and 298 K. 6306
DOI: 10.1021/acs.jpca.8b04995 J. Phys. Chem. A 2018, 122, 6303−6310
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The Journal of Physical Chemistry A All signals show nonzero initial slopes, implying that these species are indeed primary rather than secondary products generated within a few μs. Again, we note that CIs react comparably quickly with BA, OA, and interfacial (H2O)n, instead of diverse bulk concentration ratios: [BA]/[H2O] = [OA]/[H2O] = 20 mM/11 M ≈ 2 × 10−3 M at W/AN = 1:4 (= vol/vol). This observation will stem from (1) the low water density of aqueous organic surfaces, (2) the enrichment of amphiphilic BA and OA therein, and (3) the lower reactivity of sesquiterpene CIs toward water molecules. A recent quantum mechanics/molecular mechanics (QM/MM) study showed that α-humulene CIs preferentially react with n-alkanoic acids as opposed to water at the air−W/AN interface because of the smaller energetic barriers (i.e., ≤5.6 kcal mol−1 for n-alkanoic acids (n = 1−7) vs ≥6.4 kcal mol−1 for water molecules).68 These results demonstrate that sesquiterpene CIs may bypass the reaction with water molecules and react with amphiphilic acids to form the larger-mass and lower-volatility products that potentially contribute to the growth of atmospheric particles. The competition between (H2O)n vs BA vs OA toward CIs at the gas/liquid interface at constant E(O3) is quantified by the signal intensity (SI) and yields (defined by eq 1) as a function of equimolar (BA + OA) concentration (Figures 6 and 7, respectively). Y (P) = SI(P)/[SI(305) + SI(409) + SI(431)]
(1)
Decay of the signals at m/z 305 in both figures means that CIs at the gas/liquid interface are scavenged by BA and OA rather than (H2O)n as a function of concentration of the solutes, as expected. The addition of a larger concentration of solutes (>10 mM) decreases the absolute signal intensities of the m/z 409 and 431 products (Figure 6), implying the surface-active BA and OA, which are also inert toward O3, start to occupy the surface, replace β-C, and suppress the formation of CIs at the gas/liquid interface. This observation is consistent with previous results regarding the addition of surface-active cis-pinonic acid.24 The observation that the yields of m/z 409 and 431 increase while that of m/z 305 decreases as a function of [solutes] (Figure 7) also implies that BA and OA replaces (H2O)n at the gas/liquid interface. Thus, the present findings suggest that the observed CI reactions should take place in interfacial layers where reactant concentrations change as a function of the depth.57,69−71 Previous MD simulations showed a preferential orientation of benzoate at the air/water interface with the carboxylatemoiety −C(O)O− projecting into the liquid phase and the aromatic ring being partly exposed toward the gas phase.34 Minofar et al. found that the decrease of surface tension by adding benzoate is moderate, indicating that this carboxylate has a considerably smaller surface affinity than typical surfactants.34 Another study revealed that the relative interfacial affinity follows the order: n-heptanoate > cyclohexanecarboxylate > benzoate at the air/water interface.35 The smallest availability of benzoate at the air/water interface among the C7 carboxylates is responsible for favorable πelectron···H atom bonding between the aromatic ring and water molecules.66 The implication is that BA similarly stands up nearly perpendicularly to the surface, by pointing the hydrophilic −C(O)OH group into the liquid phase with full hydration, where it is hard for CIs to reach the moiety. It is generally assumed that carboxylic acids exist in the molecular form R−C(O)OH at the air/water interface, more than expected from bulk pKa values.72 The reported small
Figure 6. Signal intensities at m/z 305 (A), 409, and 431 (B) as a function of concentration of added solutes (equimolar benzoic acid and octanoic acid) to 1 mM β-caryophyllene + 0.2 mM NaCl in W/ AN (1:4 = vol/vol) solution microjets in the presence of O3 (E ≈ 3.3 × 1011 molecules cm−3 s). The lines are regression curves fitted with y = aexp(−bx) + cexp(−dx) for the m/z 305 product and y = a[1 − exp(−bx)] + c[1 − exp(−dx)] for the m/z 409 and 431 products, respectively.
Figure 7. Product yields Y(P) (from eq 1) as a function of concentration of added solutes (equimolar BA + OA) from experiments on [1 mM β-caryophyllene + 0.2 mM NaCl] in W/AN (1:4 = vol/vol) liquid microjets exposed to E(O3) ≈ 3.3 × 1011 molecules cm−3 s. The lines are regression curves fitted with y = aexp(−bx) + cexp(−dx) for Y(305) and y = a[1 − exp(−bx)] + c[1 − exp(−dx)] for Y(409) and Y(431), respectively.
ΔGacidity,BA (Scheme 1) predicts that the reaction rate constant for CIs + BA in the gas phase or in homogeneous liquid phase 6307
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many interface-specific reaction mechanisms. For example, the extent of dissociation of nitric acid (HNO3) at the air/water interface is precisely depth-dependent with the Å-level.78−80 In another example, surface-active C≥4 dicarboxylic acids on water react with the gas-phase OH radical via a gas/liquid interfacespecific mechanism, in which α-CH2 groups attached to the undissociated terminal −C(O)OH located at the topmost layers of water surface exclusively react with the OH radical.64 This observation originates from the features that the OH radical prefers to remain at the topmost layers of interface rather than in bulk,81 and only the α-CH2 groups are located at a suitable depth that is accessible for the interfacial OH radical.64
are sufficiently large, although there are no data available for the moment. Thus, we infer that the decreased reactivity of BA toward CIs observed in this study stem from the orientation of BA at the air/water interface, which will be a prominent example of interface-specific reaction mechanisms as previously observed.33,44,63,64,73−75 Finally, we evaluate the relative reactivity toward CIs between BA and cis-pinonic acid (CPA). CPA is an exceptionally surface-active acid,24,76 which is widely found in PM over forest regions.77 Figure 8 shows the negative ion mass spectrum of 1 mM β-C + 0.2 mM NaCl + 10 mM BA + 10 mM CPA in W/AN (1:4 = vol/vol) solution microjets in the absence or presence of O3(g).
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CONCLUSION Our experiments show, for the first time, that Criegee intermediates produced by ozonolysis of β-caryophyllene react with amphiphilic benzoic acid in the interfacial layers of model aqueous organic particles. Mass-specific identification of the products including labile hydroperoxides within a very short (
