Stepwise Oxidation of Aqueous Dicarboxylic Acids by Gas-Phase OH

Jan 21, 2015 - Shinichi Enami,*. ,†,‡,§. Michael R. Hoffmann,. ∥ and Agustín J. Colussi*. ,∥. †. The Hakubi Center for Advanced Research, ...
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Stepwise Oxidation of Aqueous Dicarboxylic Acids by Gas-Phase OH Radicals Shinichi Enami,*,†,‡,§ Michael R. Hoffmann,∥ and Agustín J. Colussi*,∥ †

The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302, Japan Research Institute for Sustainable Humanosphere, Kyoto University, Uji 611-0011, Japan § PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan ∥ Linde Center for Global Environmental Science, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States ‡

S Supporting Information *

ABSTRACT: A leading source of uncertainty in predicting the climate and health effects of secondary organic aerosol (SOA) is how its composition changes over their atmospheric lifetimes. Because dicarboxylic acid (DCA) homologues are widespread in SOA, their distribution provides an ideal probe of both aerosol age and the oxidative power of the atmosphere along its trajectory. Here we report, for the first time, on the oxidation of DCA(aq) by ·OH(g) at the air−water interface. We found that exposure of aqueous HOOC-Rn-COOH (Rn = C2H4, C3H6, C4H8, C5H10, and C6H12) microjets to ∼10 ns ·OH(g) pulses from the 266 nm laser photolysis of O3(g)/O2(g)/ H2O(g) mixtures yields the corresponding (n−1) species OC(H)-Rn−1COO−/HOOC-Rn−1-COO−, in addition to an array of closed-shell HOOCR n (-H)(OOH)-COO − , HOOC-R n (-2H)(O)-COO − , HOOC-R n (H)(OH)-COO−, and radical HOOC-Rn(-H)(OO·)-COO− species. Oxalic and malonic acids, which are shown to be significantly less hydrophobic and reactive than their higher homologues, will predictably accumulate in SOA, in accordance with field observations. aqueous microjets exposed to τ ≈ 10 ns ·OH(g) pulses.20 (See the Methods and Supporting Information (SI) Methods, Figure S1.) Note that our study most closely simulates the aging of atmospheric aerosol matter as a heterogeneous aging process driven by the ·OH radicals generated in the gas phase. Under such conditions, ·OH(g) first sticks to the surface of water microjets18,20,21 to subsequently react therein with DCAs via reaction 1 or recombine into H2O2 via reaction 2

D

icarboxylic acid (DCA, HOOC-Rn-COOH) homologues are ubiquitous components of atmospheric aerosols over rural, urban, polar, mountain, and open ocean environments.1−8 It has been consistently reported that the most abundant homologues are the lower members of the series, that is, oxalic (n = 0) and malonic (n = 1) acids. For example, a typical study found that oxalic acid is the most abundant (26 ng/m3), followed by malonic (13 ng/m3), succinic (n = 2) (10 ng/m3), glutaric (n = 3) (2 ng/m3), and adipic acid (n = 4) (0.6 ng/m3) in open ocean marine aerosols.4 Concentrations of oxalic acid range from tens of ng/m3 in remote locations to hundreds of ng/m3 in urban regions and up to more than 1 μg/m3 over forests.9−14 There has been speculation about the nature of the processes underlying these outcomes, and their detailed mechanisms. The low volatility DCAs, which are produced from the oxidation of biogenic and anthropogenic volatile organic compounds (VOC), largely partition to the aerosol phase where they continue to be slowly degraded by atmospheric oxidants.15,16 Because the hydrophobic character imparted to the higher DCA homologues by their longer alkyl chains drives them to the surface of aqueous aerosols, we deemed relevant to investigate the oxidation of DCA by ·OH(g), the key atmospheric oxidant.15−19 Here we present direct, mass-specific experimental evidence on the products of the degradation of DCA on the surface of © XXXX American Chemical Society

·OH + HOOC‐R n‐COO− (n ≥ 2) → products

(1)

·OH + ·OH → H 2O2

(2)

The noted preference of ·OH for the interfacial layers over the bulk liquid22,23 should enhance the probability of such events. (See the Methods and the SI Methods for more details.) Figure 1 shows a typical negative ion electrospray mass spectrum of 0.2 mM aqueous suberic acid (HOOC-R6-COOH) microjets exposed to O3(g)/O2(g)/H2O(g)/N2(g) mixtures with the 266 nm laser on and off. We found that the same products were observed on aqueous HOOC-R6-COOH microjets over the 0.01 to 1.0 mM range (Figure S2, SI). HOOC-R6-COOH largely exists as monoanion HOOC-R6Received: November 17, 2014 Accepted: January 21, 2015

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DOI: 10.1021/jz502432j J. Phys. Chem. Lett. 2015, 6, 527−534

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The Journal of Physical Chemistry Letters

reactor conditions, that is, E ≥ 2 × 1010 molecules cm−3 s.18,27,28 The molecular identity and progeny of the products are inferred from their mass-to-charge ratios. In Figure 1B, the m/z = 204 = 173 − 1 + 32 signal is assigned to isomeric peroxyl radicals HOOC-R6(-H)(OO·)-COO−, the m/z = 187 = 204 − 16 − 1 signal to HOOC-R6(-2H)(=O)-COO− carbonyls, and the m/z = 189 = 204 − 16 + 1 signal to HOOC-R6(-H)(OH)COO− alcohols. To our knowledge, this is the first report on the direct detection of DCAs peroxyl radicals at the air−water interfaces. Clearly, an H-atom abstraction from the alkyl chain by ·OH is rapidly followed by O2 addition, leading to peroxyl radical formation. Note that O2 is always in excess ([O2(g)] = 2.1 × 1018 molecules cm−3) over other reactants. The selfreactions of peroxyl radicals in bulk aqueous media are known to be extremely fast, with typical rate constants approaching the diffusional limit ∼109 M−1 s−1.29 The simultaneous occurrence of competitive peroxyl radical reactions such as the Russell disproportionation30 (self-reaction A, Scheme 1) versus the Bennett−Summers elimination of H2O231 (self-reaction B) is in line with the results on the reaction of aqueous monocarboxylic acids with ·OH(g) under similar conditions.20 The peroxide HOOC-R6(-H)(OOH)-COO−, at m/z = 205, is likely formed via the reaction with peroxyl radical with HO2 rather than from H-atom abstraction by peroxyl radicals.20,29 These products are alike those formed in the oxidation of monocarboxylic acids by ·OH(g).20 In contrast with monocarboxylic acid’s case, however, we observe shorter chain products, such as the m/z = 143 = 204 − 16 − 45 and m/z = 159 = 143 + 16 species, which can be assigned to OC(H)-R5-COO− carbonyls and HOOC-R5-COO−, respectively. We propose the reaction mechanism shown in Scheme 1 to explain our results. Notably, the β-scission of the alkoxy radical HOOC-R6(H)(O·)-COO− (m/z = 188, undetected, formed by selfreaction C) into the hydroxycarbonyl HOCO· radical plus a terminal carbonyl dominates over that into glyoxylic acid and an alkyl ·CH2Rn−3COO− radical, or its reaction with excess O2. In fact, this selectivity is the fundamental mechanistic reason that makes the degradation of DCA a stepwise process. Our findings, which closely approach atmospheric ·OH(g) exposures (rather than ·OH(g) concentrations) are in striking contrast with the broad product distributions reported in the oxidation of DCA(aq) by ·OH(aq) photogenerated in bulk water.32−34 (See below.) Importantly, the same (peroxyl radicals, peroxides, carbonyls, alcohols, and n−1 species) products, albeit in different yields, are observed in the case of n = 2−6 DCAs but are absent for n = 0 (oxalic acid) and 1 (malonic acid) in the HOOC-Rn-COOH (see SI, Figures S3− S8), thereby implying that the proposed mechanism should generally apply to the ·OH oxidation of n ≥ 2 DCAs at the air− water interfaces. Figure 2 shows the ratios of HOOC-Rn-COO− signal intensities measured in the presence versus those measured in the absence of identical ·OH(g) concentrations as a function of n. Note that these ratios approach unity for the least reactive DCAs. It is apparent that (1) oxalic (n = 0) and malonic (n = 1) do not react with ·OH under present conditions and (2) the higher n ≥ 3 DCA homologues are similarly reactive. This nonmonotonic dependence of reactivity with chain length is consistent with the fact that rate constants for the lower homologues: k(oxalate + ·OH) = 1.9 × 108 and k(malonate + · OH) = 3.6 × 108 M−1 s−1 in bulk water,35 are significantly smaller than the diffusionally controlled values (>1 × 109 M−1

Figure 1. (A) Negative ion electrospray mass spectra of 0.2 mM (pH 4.2) suberic acid (HOOC-R6-COOH) microjets exposed to 680 ppmv O3(g) in O2(g)/H2O(g)/N2(g) mixtures at 1 atm and 298 K. Blue: laser off. Red: under 40 mJ, ∼8 ns 266 nm pulses (at 10 Hz). ([· OH(g)] ≤ 63 ppmv). 1 ppmv = 2.46 × 1013 molecules cm−3. (B) Zooming in spectra of the ·OH oxidation products in the 140−210 Da range.

COO− (m/z = 173) and undissociated forms (neutral, undetected) at pH 4.2 (pKa1 = 4.5 and pKa2 = 5.5, respectively24). A minor signal at m/z = 195 is readily assigned to [Na+(−OOC-R6-COO−)]−. We confirmed that the addition of O3(g) to O2(g)/H2O(g)/N2(g) mixtures neither decreases the reactant signal (m/z = 173) nor generates new signals (see SI), in accordance with the inertness of these DCAs toward O3 (rate constants k ≤ 3.0 × 10−2 M−1 s−1 for n = 2−5).25,26 Upon irradiation with 266 nm laser pulses we observe, as expected, the depletion of the monoanion reactant and the simultaneous formation of new species, which we therefore ascribe to the products of reactions involving ·OH. We estimate [·OH(g)]0 ≈ 65 ppmv at the spot where ·OH(g) is generated in the experiments of Figure 1. (See [·OH(g)]0 estimates in the SI.) Because the concentrations of ·OH(g) at the surface of microjets are likely lower than [OH(g)]0, we consider that [· OH(g)] < 65 ppmv is an upper limit to [·OH(g)] on the surface of microjets. It should be emphasized that present exposures (E = [·OH(g)] × τ) to 1 ppmv = 2.5 × 1013 molecules cm−3 (at 1 atm, 298 K) for τ ≈ 10 μs: E = 2.5 × 108 molecules cm−3 s are equivalent to exposures to typical atmospheric ·OH(g) concentrations [·OH(g)] ≈ 2 × 106 molecules cm−3 for 100 s. Note in passing that present E values are much smaller than those prevailing under typical flow 528

DOI: 10.1021/jz502432j J. Phys. Chem. Lett. 2015, 6, 527−534

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The Journal of Physical Chemistry Letters Scheme 1. Mechanism of ·OH-Initiated Oxidation of Suberic Acid, HOOC-R6-COOH, at the Air−Water Interfacea

a

Note the interface-specific preference of HOCO· radicals + terminal carbonyls formation path. See the text for details.

Figure 2. HOOC-Rn-COO− ratios derived from signal intensities of 1 mM HOOC-Rn-COO− under [·OH(g)] ≤ 63 ppmv (1 ppmv = 2.46 × 1013 molecules cm−3) as a function of n.

Figure 3. Negative ion electrospray mass spectra of 0.6 mM suberic acid (HOOC-R6-COOH) in 99.9% D2O microjets exposed to 660 ppmv O3(g) in O2(g)/H2O(g)/N2(g) mixtures at 1 atm and 298 K. Blue: laser off. Red: under 40 mJ, ∼8 ns pulses (at 10 Hz) of 266 nm radiation ([·OH(g)] ≤ 63 ppmv). 1 ppmv = 2.46 × 1013 molecules cm−3.

s−1) for the higher homologues. In our experiments, this effect is likely compounded by the fact that the latter, by being more hydrophobic, have larger propensities for the air−water interface than oxalic and malonic acids, thereby exposing readily abstractable >CH−H H-atoms to ·OH(g) attack.36−39 (See below.) Experiments in D2O solvent provided additional, compelling evidence of our molecular assignments (Figure 3). All products have exchangeable RCOO−D(H) atoms and hence would generate (M + 1) species and ES mass signals. The

corresponding alcohols and hydroperoxides further possess extra exchangeable (O−)H atoms and would generate (M + 2) species. The finding that m/z = 143 shifted to 144 in D2O reveals that aldehydic (OC)−H atoms also exchange with the solvent. Additional experiments in 97% H218O solvent confirmed that the O atoms present in the products exclusively arise from gas-phase O species and do not exchange O with the 529

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The Journal of Physical Chemistry Letters solvent, as expected from the proposed mechanism (Figure S9 in the SI). Because the rate coefficients of R1 and R2 are extremely large, k1 ≈ (0.5−5.0) × 109 M−1 s−1 and k2 = 5.5 × 109 M−1 s−1,35,40 respectively, and these n ≥ 2 DCAs are surface-active, as confirmed by vibrational sum frequency spectroscopy (VSFS) by Richmond and coworkers,36−38 the ensuing reactions are deemed to take place in interfacial layers. VSFS studies revealed that unlike monocarboxylic acids the hydrophobic DCA backbones lie on the interface, which makes them prone to ·OH(g) radical attack in surface-specific reactions.36,38 A recent study showed that longer chain DCAs form monolayers in which hydrophobic alkyl groups or even − COOH groups protrude toward the air side.41 In other words, most ·OH cannot survive residence times long enough to diffuse into deeper bulk layers under the present conditions. Theoretical calculations also predict the preference of ·OH for the interfacial layers over the bulk water.22,23 Previously, we demonstrated that the heterogeneous reaction of ·OH(g) on water surface predominantly proceeds via a Langmuir− Hinshelwood rather than an Eley−Rideal mechanism,20 in accordance with related studies.21,42 Because the reported thermal accommodation coefficient of ·OH at the air−water interface approaches unity, S ≈ 0.95,22 most ·OH(g) will initially stick to the surface of water and then react with the sparse alkyl chains presented by sub-millimolar DCAs solutions. It is remarkable that the products from the reactions HOOCRn-COO− (n = 2−6) + ·OH include OC(H)-Rn−1-COO− and HOOC-Rn−1-COO− (Scheme 1). This observation implies that α-CH2 groups of undissociated terminal HOOC moieties residing at the topmost layers of the water surfaces react selectively with interfacial ·OH, followed by conversion into alkoxy −C−O· radicals that split HOCO·. The latter, similarly to its behavior in the gas phase, should form HO2· and CO2 in the presence of O2.43 Note that if ·OH reacted with other CH2 groups we should have detected Rn−2 products, at variance with present observations. The above observations strongly support our claim that we have observed early generation, interfacespecific processes, whose behaviors contrast with those of bulk water phase reactions, which generate broad product distributions.32−34 For example, succinic acid irradiated by a xenon lamp in the presence of H2O2 yields malonate, glyoxalate, and oxalate.34 The absence of the latter two products, −2C smaller products that should have appeared as m/z = 73 and 89, under ·OH exposure in the present study (see SI figures) suggests that the reaction mechanisms depend on whether ·OH is generated in the gas phase or in bulk water. Our study most resembles the oxidation of atmospheric aerosol matter as a genuinely heterogeneous aging process in which · OH initially sticks to the surface of water to subsequently react therein with sparse DCAs alkyl chains. Thus, product identities and yields derived from the attack of amphiphilic organic species by ·OH radicals coming from the gas phase could be, in general, different from those arising from ·OH generated in bulk water, such as those produced from H2O2(aq) via photolysis or Fenton chemistry.44−51 Figure 4 shows electrospray mass spectral signals acquired from aqueous suberic acid (HOOC-R6-COOH) microjets exposed to irradiated gaseous O3/O2/H2O/N2 mixtures as a function of laser energy per pulse. We estimate that 1, 5, 10, 20, 30, and 40 mJ pulse−1 laser pulse energies in Figure 4 generate [·OH(g)]0 ≈ 0.6, 3.0, 5.7, 10.5, 14.6, and 18.1 ppmv, which, from the mean ·OH speed c = 6.09 × 104 cm s−1 at 298 K, may

Figure 4. Reactant (A) and products (B,C) electrospray mass spectral signal intensities from aqueous 0.1 mM suberic acid (HOOC-R6COOH) microjets (pH 4.5) exposed to O3(g)/O2(g)/H2O(g)/N2(g) mixtures, [O3(g)] ≈ 190 ppmv, irradiated with 266 nm laser beams as a function of laser energy (in mJ pulse−1). 1 ppmv = 2.46 × 1013 molecules cm−3.

correspond to 0.2 × 1018, 1.1 × 1018, 2.1 × 1018, 3.9 × 1018, 5.5 × 1018, and 6.8 × 1018 molecules cm−2 s−1 fluxes on the surface of the microjets, respectively.27 Thus, the ·OH flux = 6.8 × 1013 molecules cm−2 at 40 mJ pulse−1 laser pulse energy during 10 μs exposure times is, for example, comparable to the number density of n-octanoic acid on the surface of its ∼0.1 mM solutions.20,52 Note that the mass spectral signals of the peroxyl radical derived from suberic acid are the most intense among the detected products over the entire [·OH] range, which is in striking contrast with the results for n-octanoic acid under 530

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The Journal of Physical Chemistry Letters similar experimental conditions.20 We believe that this is due to kinetic reasons, that is, the rate constants for the self-reactions of the suberic peroxyl radical are smaller than those for their octanoic acid correlates. This is consistent with the reported yields of H2O2 production (0.7 vs 1.7) during the photolysis of NO3− as a source of ·OH(aq) in the presence of suberic versus octanoic acid,31 respectively. The implication is that the suberic peroxyl radical self-reaction via the Bennett−Summers pathway producing H2O2 is slower than that of its octanoic peroxyl radical counterpart. It is apparent that reactant losses and concomitant product formation display double-exponential behaviors (Figure 4). Significant initial effects are followed by attenuated responses. We interpret these findings as evidence that ·OH rapidly reacts with and therefore depletes alkyl chains in the outermost interfacial layers. Note that the hydrophobic DCAs backbones lie parallel to the interface, a phenomenon made possible by the limited solvation of both carboxylic acid head groups.36−38 Thus, above a certain dose, the ·OH excess would recombine into H2O2 in the outermost layers via the second-order reaction 2 (k = 5.5 × 109 M−1 s−1), leaving a small fraction to reach the underlying layers, a phenomenon that may account for the observed attenuated DCAs depletion. Similar observations were made in the case of ·OH reactions with monocarboxylic acids.20 Total mass balances (TMBs) evaluated before and after laser pulses provide further information on the reactions course. We define TMB as the sum of the products mass signal intensities divided by suberic acid monoanion losses, [SA]withoutOH − [SA]withOH. We found that TMB ≈ 0.5 at the highest laser energy, implying that ∼50% the products are undetectable via negative ion electrospray mass spectrometry. Notably, the value is consistent with the results on the reaction of octanoic acid with ·OH at the air−water interface.20 VOCs yields (∼34%) have been reported for the oxidation of stearic acid (C17H35COOH) films by ·OH in the presence of O2.42 Thus, our results imply the partial fragmentation of reaction intermediates into volatile products. Alternatively, we note that neutral products could be formed by direct electron transfer from the carboxylate anions to ·OH.19,53 Because DCAs homologues make a significant contribution to the water-soluble organic carbon (WSOC) fraction of the aerosol,5,54−57 our results indicate that their oxidative degradation in the aerosol phase by gas-phase ·OH radicals is an important source of oxalic acid in the troposphere.1,5−7,14,54−69 Thus, the weak increase in 13C depletion in the lower DCA homologues of aged organic aerosols,70 corresponds to a genuine secondary kinetic isotope effect (KIE) on activated H-atom abstraction by ·OH radicals rather than to the mass dependence of collision frequencies in the gas phase.71,72 An important finding is that the oxidation of DCAs initiated by gas-phase ·OH radicals leads not only to their stepwise degradation but also to the formation of more reactive species, such as hydroperoxides, which can propagate radical reactions in the condensed aerosol phase. We demonstrate that the reactivities of DCAs homologues toward ·OH(g) drop dramatically below n = 3, in line with the higher hydrophilicity of the shorter chain monoanions that inhibits their partitioning to the air−water interface. Our results suggest that interfacespecific ·OH oxidation of DCAs may play a far more significant role in photochemical aging process of atmospheric aerosols than previously assumed. Thus, the present results, in addition to photo-oxidation of DCAs in bulk phase,32−34 would explain why and how shorter-chain DCAs naturally accumulate in aged

secondary organic aerosol (SOA). Our results support the recent model simulation that the only significant sink of SOA oxalic acid in the atmosphere would be its removal via dry and wet aerosol deposition.14 In summary, we present direct, mass-specific evidence of the prompt formation of peroxyl radicals/peroxides and shorterchain products on the surface of aqueous DCAs microjets exposed to gaseous ·OH pulse beams. To the best of our knowledge, this is the first report on the direct detection of DCAs peroxyl radicals at the air−water interface. We propose an interface-specific oxidation mechanism for the heterogeneous reaction of HOOC-Rn-COOH (n ≥ 2)(aq) with · OH(g). We confirmed that the shortest DCAs oxalic acid and malonic acid, in contrast with its higher homologues, are unreactive toward ·OH under present conditions.

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METHODS The prompt (within the ∼10−50 μs lifetime of the intact microjets) formation of anionic products at the air−water interfaces of microjets from the reaction of aqueous reactants with gaseous species at 1 atm at 298 K is monitored in situ by an electrospray mass spectrometry (ES-MS, Agilent 6130 Quadrupole LC−MS Electrospray System, Kyoto University). Aqueous solutions are pumped (100 μL min−1) into the spraying chamber of the mass spectrometer through a grounded stainless-steel needle (100 μm bore) coaxial with a sheath issuing nebulizer N2(g) at high gas velocity (vg ≈ 160 m/s).73 The surface specificity of our experiments had been previously demonstrated by showing that (i) anion signal intensities I in the mass spectra of equimolar salt solutions adhere to a normal Hofmeister series rather than being identical, for example, I(ClO4−) > I(I−) > I(Br−),74−77 (ii) the depth of the sampled interfacial layers can be controlled by varying the nebulizer gas velocity vg, as evidenced by the fact that ion signal intensities and relative anion surface affinities increase with higher gas velocities vg and extrapolate to zero as vg → 0,73 and (iii) they allow the detection of products of gas− liquid reactions that could only be formed at the air−water interface.78−80 The 266 nm beam emitted by our Nd3+:YAG laser setup is used to generate ·OH(g) at or near the gas−liquid interface. [·OH(g)]0 can be varied from a few tens of ppbv to 100 ppmv. (See [·OH(g)]0 estimates in the SI.) Note that reported [·OH(g)]0 values are considered as the upper limits to [·OH]. (See above.) We confirmed that reactant depletion and product formation require both the participation of O3(g) and actinic 266 nm photons. (See SI Figures S10 and S11.) Because the microjets break up within 10−50 μs after being ejected from the nozzle whereas the laser pulses every 100 ms, we assume the phenomena we observe take place in fresh solutions. Note that the products we observe are formed when gaseous reactants collide with the intact aqueous jets as they emerge from the nozzle, that is, before jets are broken up into submicron droplets by the nebulizer gas. (See the SI experimental details.)

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ASSOCIATED CONTENT

S Supporting Information *

Additional data and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*S.E.: E-mail: [email protected]. 531

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The Journal of Physical Chemistry Letters *A.J.C.: E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.E. is grateful to Kurita Water and Environment Foundation and the Japan Science and Technology Agency (JST) PRESTO program. S.E. also thanks Prof. Hiroshi Masuhara for stimulating discussions. M.R.H. and A.J.C. acknowledge support from the National Science Foundation (U.S.A.) Grant AC-1238977.

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