Reactions of Criegee Intermediates with Benzoic Acid at the Gas

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Reactions of Criegee Intermediates with Benzoic Acid at the Gas/Liquid Interface Junting Qiu, Shinnosuke Ishizuka, Kenichi Tonokura, and Shinichi Enami J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04995 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

Reactions of Criegee Intermediates with Benzoic Acid at the Gas/Liquid Interface Junting Qiua, Shinnosuke Ishizukab, Kenichi Tonokuraa, Shinichi Enami*b a

Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8563, Japan. b

National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506, Japan *Author to whom correspondence should be addressed: S.E. [email protected], phone: +81-29-850-2770

ABSTRACT − Secondary organic aerosol (SOA) found in polluted megacities 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, that induces subsequent reactions involving Criegee intermediates (CIs, carbonyl oxide RR’C·-O-O·/RR’C=O+-O-) formed by the -C=C- + 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 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’C=O+-O- zwitterions)1 that contribute to HOx cycle and particle formation in the atmosphere.2-9 Recent theoretical and field studies estimate concentrations of CIs over terrestrial boundary layer are ~(1-5) x 104 molecules cm-3, that are just one ~ two orders of magnitude smaller than those of OH-radicals.10 Gaseous CIs can react with a variety of species but the fate of gaseous CIs may be largely controlled by the reaction with water vapors and the unimolecular decomposition.11 However, due to 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.] largely lacks for the moment. Furthermore, heterogeneous CI chemistry including reactions at the gas/liquid interface remains to be 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 C=C 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 condense phase such as limononic acid are amphiphilic and contain C=C bond(s), that should react with O3(g) to generate CIs under ambient conditions.19 Photochemistry at the air/seawater interface may convert saturated fatty acid into a variety of unsaturated species containing C=C bonds,20 that 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 gas-phase 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 2


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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 these experimental evidences, theoretical studies by Francisco and coworkers showed 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, surprisingly, exceed the concentrations of total diacids (∼1010 ng/m3) collected in the same campaign.28 The photochemical degradation of anthropogenic aromatic compounds and direct emission from traffics produce BA,29 that taken up to the liquid phase due to the hydrophilic C(O)OH moiety, followed by reactions with oxidants.28 It is recently 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 BzO(aq) by ⋅OH(g)33 but also CIs produced at the gas/liquid interface (from -C=C- + O3) may control its fate.25 Previous molecular dynamics (MD) calculations, surface-tension measurements and surface-specific mass spectrometry revealed a moderate affinity of BzO for aqueous surfaces.34-35 Thus, the identification of products from the reaction of CIs with BA at the gas/liquid has emerged as an important issue in the atmospheric chemistry of polluted urban air.36-37 Since previous studies indicate the reaction rate of CIs is correlated with gas-phase acidity ∆Gacidity (the more acidic, the more reactions),22-23, 38 how BA, that is more acidic (∆Gacidity = 1394 kJ/mol) than alkyl carboxylic acids of similar size, reacts with CIs at the gas/liquid interface is 3



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important but unexplored issue (Scheme 1).

O

OH

β-Caryophyllene (β β-C) MW: 204.35

Benzoic acid (BA) MW: 122.12 ΔGacidity= 1394 kJ/mol pKa = 4.2

Octanoic acid (OA) MW: 144.21 ΔGacidity= 1419 kJ/mol pKa = 4.8

SCHEME 1: Chemical structures of the reactants used in the present study. ∆Gacidity values are taken from Caldwell et al.39 (∆Gacidity of OA is taken from for heptanoic acid).

Here we show the direct detection of products from reactions of BA with CIs generated on surfaces of β-C, a representative terpene, solutions in W:AN microjets exposed to O3(g) for ~ 10 µs. The exceptionally high reactivity of β-C toward O3(g) is utilized as an in-situ CIs source.35, 40-41 The microheterogeneous surface of a W:AN binary mixture is well studied,42-43 and could be suitable as a surrogate of 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 will be responsible for the observed less reactivity.

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 gas-phase O3/O2 beam (Fig. 1).

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[β-C [β + NaCl + BA] in W:AN Stainless steel nebulizer

Nebulizer gas (N2)

O3(g)/O2(g)

- - Gas-phase ions

To quadrupole mass analyzer

To sink

Reaction chamber

Figure 1 – Schematic diagram of the experimental method. β-C, BA, W, and AN stand for β-caryophyllene, benzoic acid, water, and acetonitrile, respectively.

The observed mass spectrum represents species generated by heterogeneous processes in the interfacial layers (≤ 1nm) of the liquid microjets, as validated by previous experiments.33, 35, 40, 44-45

The small ozone exposures: E = [O3(g)] × τR ≤ 5.5 x 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 ozonolysis of β-C on W:AN surfaces occurs ~20 times faster than in the bulk liquid dissolved with O3,46 and even much faster than in the gas-phase.47-48 W:AN solutions containing β-C, 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 present method is that fragile hydroperoxide species (ROOH) can be detected as stable 5



chloride-adducts just by adding NaCl into the solution.21-24 Chloride was found to be unreactive toward O3(g) under 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)] x 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 were: drying nitrogen gas flow rate: 12 L min-1; drying nitrogen gas temperature: 340 oC; 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 days. Chemicals: β-caryophyllene (> 98.5 %, Sigma-Aldrich, or > 90 %, Tokyo Chemical Industry), benzoic acid (> 99.5%, Sigma-Aldrich), 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 %, Sigma-Aldrich) were used as received.

RESULTS AND DISCUSSION Fig. 2 shows 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).

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305

no O3(g) with O3(g)

0.6

5

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Signal intensity / 10 ion counts

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0.4

307 0.2

409 411 0.0 300

320

340

360

380

400

420

m/z

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 x 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 = 35Cl/37Cl ratio (Fig. 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 C=C 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).

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Hydroxy-hydroperoxides (305;307) O

O3

H

H

OA, Chloride

OOH Cl

O

H H

β-Caryophyllene (MW 204)

O

C23 Acyloxy-hydroperoxides (431;433)

Criegee Intermediate (MW 252)

Other products

C22 Acyloxy-hydroperoxides (409;411)

SCHEME 2: Reaction schemes of β-caryophyllene’s Criegee intermediate + (H2O)n, octanoic acid (OA), benzoic acid (BA) at the gas/liquid interface. Here we show most likely structures among possible isomers. See the text for details.

Notably, neutral hydroperoxide products ROOH are detected as stable chloride-adducts by the 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 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 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 reactive hydroperoxide 8


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group -OOH, they will trigger polymerization processes or decompose to form radical species.5, 58-61 Such 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 Fig. 3 shows 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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0.30

307

D2O:AN

0.25

0.20

0.15

0.10

309 410

0.05

412 0.00 300

320

340

360

380

400

420

m/z

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 x 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 (Fig. 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 (Fig. 3), consistent with the formation of α-acyloxy-hydroperoxides having one exchangeable peroxide -OOH group (Scheme 2). These shifts also imply both structures do not have an aldehydic H-atom -C(=O)H, otherwise they will shift + 3 and +2 mass units, respectively, in the D2O:AN experiment.24,

64

Thus, Scheme 2 represents most likely structures among 9



possible isomers. Next, we evaluate the relative reactivity of BA toward CIs at the gas/liquid interface by comparing other competitive reactants. Fig. 4 shows 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 surface-active carboxylic acid commonly observed in ambient particles (Scheme 1).65 The mass spectrum represents the competition between (H2O)n vs. BA vs. OA towards CIs at the gas/liquid interface.

0.30

with 10mM BA + 10mM OA

305

no O3(g) with O3(g)

0.25

5


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0.20 0.15

431

307

0.10

409

0.05

433

411 0.00 300

320

340

360

380

400

420

440

m/z

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 x 1011 molecules cm-3 s) at 1 atm and 298 K.

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 at m/z 431;433 are ~2 times larger than m/z 409;411 at [BA] = [OA] = 10 mM indicates that BA is less reactive toward CIs or less surface available.57 Previously, 10


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Enami et al. showed the relative reactivity toward CIs at the gas/liquid interface is negatively correlated with ∆Gacidity values (note that the smaller values, the more acidic) and the interfacial availability of OH-species.21-24 Intriguingly, the reported ∆Gacidity, BA is 1394 kJ/mol, that is smaller (more acidic) than that of heptanoic acid (1419 kJ/mol) (Scheme 1), suggesting the BA + CIs reaction should have been more favorable. We infer that this unexpectedly small reactivity of BA toward CIs at the gas/liquid is correlated with the surface orientation of BA. A previous study revealed 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 benzene ring of BA can interact H-atoms of water molecules,66 the reactive C(O)OH group of BA will locate at deeper layers of the air-water interface than OA where CIs are generated. The observation that the ratio of signal intensity of m/z 431 (from CIs + OA) to m/z 409 (from CIs + BA) does not depend on the pH of the solution acidified by HCl in the range of 2.1
10 mM) decreases absolute signal intensities of the m/z 409 and 431 products (Fig. 6), implying the surface-active (and inert toward O3) BA and OA 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 of 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] (Fig. 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 carboxylate-moiety -C(O)O- projecting into the liquid-phase and the 14


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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 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 a favorable π-electrons---H-atoms bonding between the aromatic ring and water molecules.66 The implication is that BA similarly stands up nearly perpendicularly to the surface, by pointing 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 as molecular forms R-C(O)OH at the air-water interface, more than expected from bulk pKa values.72 The reported small ∆Gacidity,

BA

(Scheme 1) predicts that the reaction rate constant for CIs + BA in the gas-phase or in homogeneous liquid phase are sufficiently large, although there are no data available for the moment. Thus, we infer that the less reactivity of BA toward CIs observed in this study stem from the orientation of BA at the air-water interface, that 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, 53, 76 that is widely found in PM over forest regions.24, 57, 77 Figure 8 shows 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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with 10mM BA + 10mM CPA 0.20

0.15

0.10

471

no O3(g) with O3(g)

389

305 473

0.05

307 409

0.00 300

350

400

450

500

m/z

Figure 8 – Negative ion mass spectra of 1 mM β-caryophyllene + 0.2 mM NaCl + 10 mM benzoic acid + 10 mM cis-pinonic acid in W:AN (1:4 = vol:vol) solution microjets (gray), or those exposed to O3(g) (red, E = 9.0 x 1010 molecules cm-3 s) at 1 atm and 298 K.

The signal at m/z 389 appearing without O3(g) exposure is assigned to Na(CPA)2-, as observed before.24 The observation that signal intensities at m/z 471;473 (from CIs + CPA) are ~14 times larger than m/z 409;411 at [BA] = [CPA] = 10 mM indicates that BA is much less efficient scavenger toward CIs than CPA at the gas/liquid interface. Previous experiments suggested that CPA is an exceptionally efficient scavenger of CIs on aqueous organic surfaces, due the large interfacial availability of -C(O)OH group,24 stemming from the quasi-planar backbone structure of CPA. CPA creeps parallelly to the surface, being anchored by the −C(O)OH and carbonyl −C(=O) groups as shown by MD calculations.76 On the other hand, BA would nearly perpendicularly float on the surface and the -C(O)OH group is fully hydrated in deeper layers, that make it difficult for CIs to access the moiety before being trapped by other species, i.e., (H2O)n, OA, and CPA. It is noted, however, given the condition where actual concentration of BA in atmospheric particles are exceedingly higher than other acids 16


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(e.g., PM2.5 in Beijing),28 BA may still act as the dominant scavenger of CIs on aqueous organic surfaces. The experimental results in this study confirmed the two important facts; competitive reactions at the gas/liquid interface largely depend on interfacial rather than bulk reactant concentrations, and the depth of Å-level where reactions occur is critically important. The latter conclusion is relevant to 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 Å-level.78-80 In another example, surface-active C≥4 dicarboxylic acids on water react with gas-phase OH-radical via a gas/liquid interface specific mechanism in which α-CH2 groups attached to undissociated terminal -C(O)OH locating at the topmost layers of water surface exclusively react with OH-radical.64 This observation originates from the features that OH-radical prefers to remain at the topmost layers of interface rather than in bulk,81 and only the α-CH2 groups locate at a suitable depth that are accessible for interfacial OH-radical.64

CONCLUSION Our experiments show that Criegee intermediates produced by ozonolysis of β-caryophyllene react with amphiphilic benzoic acid in the interfacial layers of model aqueous organic particles for the first time. Mass-specific identification of the products including labile hydroperoxides within a very short (

Reactions of Criegee Intermediates with Benzoic Acid at the Gas

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