Kinetics, Mechanism, and Secondary Organic Aerosol Yield of

Article pubs.acs.org/JPCA

Kinetics, Mechanism, and Secondary Organic Aerosol Yield of Aqueous Phase Photo-oxidation of α‑Pinene Oxidation Products Dana Aljawhary,*,† Ran Zhao,*,† Alex K.Y. Lee,† Chen Wang,‡ and Jonathan P.D. Abbatt† †

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M3S 3H6, Canada Department of Physical and Environmental Sciences and Department of Chemistry, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4, Canada

‡

S Supporting Information *

ABSTRACT: Formation of secondary organic aerosol (SOA) involves atmospheric oxidation of volatile organic compounds (VOCs), the majority of which are emitted from biogenic sources. Oxidation can occur not only in the gas-phase but also in atmospheric aqueous phases such as cloudwater and aerosol liquid water. This study explores for the first time the aqueousphase OH oxidation chemistry of oxidation products of αpinene, a major biogenic VOC species emitted to the atmosphere. The kinetics, reaction mechanisms, and formation of SOA compounds in the aqueous phase of two model compounds, cis-pinonic acid (PIN) and tricarballylic acid (TCA), were investigated in the laboratory; TCA was used as a surrogate for 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a known α-pinene oxidation product. Aerosol time-of-flight chemical ionization mass spectrometry (Aerosol-ToFCIMS) was used to follow the kinetics and reaction mechanisms at the molecular level. Room-temperature second-order rate constants of PIN and TCA were determined to be 3.3 (±0.5) × 109 and 3.1 (±0.2) × 108 M−1 s−1, respectively, from which were estimated their condensed-phase atmospheric lifetimes. Aerosol-ToF-CIMS detected a large number of products leading to detailed reaction mechanisms for PIN and MBTCA. By monitoring the particle size distribution after drying, the amount of SOA material remaining in the particle phase was determined. An aqueous SOA yield of 40 to 60% was determined for PIN OH oxidation. Although recent laboratory studies have focused primarily on aqueous-phase processing of isoprene-related compounds, we demonstrate that aqueous formation of SOA materials also occurs from monoterpene oxidation products, thus representing an additional source of biogenically driven aerosol formation.

1. INTRODUCTION In recent years, atmospheric aqueous phases (i.e., cloud, fog, and aerosol liquid water) have been recognized as important reaction media for the processing of water-soluble organic compounds.1−3 Aqueous-phase photochemistry can transform organic compounds into more functionalized and oxygenated forms, contributing to the formation and aging of secondary organic aerosol (SOA) with significant impact to air quality and global climate. Aqueous-phase chemistry is not considered in the traditional view of SOA formation involving only the gasparticle partitioning of semivolatile organic compounds,4 and thus, it has been proposed as an alternative SOA formation pathway. Formation of SOA is initiated by oxidation of precursor volatile organic compounds (VOCs) in the atmosphere. On the global scale, VOCs of biogenic origin dominate the total nonmethane hydrocarbon emission, with isoprene and monoterpenes (e.g., α-pinene) being the major compounds.5−7 A large number of studies have focused on the aqueous-phase photochemical processing of isoprene reaction products, given that isoprene is the VOC species emitted by far the most to the © XXXX American Chemical Society

atmosphere and that the contribution of isoprene to SOA formation has been recognized only a decade ago and has not been fully understood.8 Focus has been placed on the aqueousphase photochemistry of small carbonyl compounds arising from isoprene oxidation, including glycolaldehyde,9,10 glyoxal,11−15 methylglyoxal,15−18 methylvinylketone, and methacrolein.19−21 On the basis of the kinetics and mechanistic data, cloudwater models have also been developed to compute SOA yields from isoprene oxidation products.22−25 Despite monoterpenes also contributing to a major fraction of global VOC emissions, almost no attention has been given to aqueous-phase photochemistry of monoterpene reaction products. The gas-phase oxidation mechanisms of α-pinene, a major monoterpene species, have been studied extensively.7,26−29 Many proposed reaction products are highly Special Issue: James G. Anderson Festschrift Received: June 29, 2015 Revised: August 20, 2015

A

DOI: 10.1021/acs.jpca.5b06237 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Structures of compounds.

yield of MBTCA from PIN is low at less than 1%, MBTCA contributed to roughly 10% newly formed SOA in the chamber experiment, highlighting its importance in biogenic SOA formation. Recently, Lai et al.40 also detected MBTCA from heterogeneous OH oxidation of PIN. The triacid nature of MBTCA makes this species highly water-soluble, potentially partitioning into not only cloudwater, but also aerosol liquid water (Supporting Information). However, the aqueous-phase chemistry of MBTCA has never been investigated. Due to the lack of a commercial standard of MBTCA, we have employed a structurally similar compound, tricarballylic acid (TCA) (Figure 1), as a surrogate. The overall objective of this study is to provide fundamental information on the aqueous-phase processing of monoterpene oxidation products. As ambient biogenic SOA is highly complex in nature, investigating the behavior of individual compounds helps to detangle the chemical complexity. Specific goals of this study include obtaining kinetic and mechanistic data for the aqueous OH oxidation with PIN and TCA (as a representative for MBTCA) under cloudwater relevant conditions. As both PIN and MBTCA are employed as tracers for monoterpene oxidation, understanding their atmospheric stability is crucial toward accurate source apportionment. Detailed mechanistic analyses are also performed in this study, as a solid understanding of reaction mechanisms is important in the development of cloudwater chemistry models. As well, we have focused on the formation of MBTCA from PIN in the aqueous phase and on identification of other reaction products that are likely to contribute to aqueous-phase SOA formation.

oxygenated and expected to be sufficiently water-soluble to undergo aqueous-phase processing. Understanding the reaction kinetics and mechanisms of these reaction products is crucial to assessing the contribution of monoterpenes to SOA via the aqueous pathway. In this study, we direct our attention to two α-pinene oxidation products as model compounds: cis-pinonic acid (PIN) and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) (Figure 1). These two compounds are widely accepted as tracers for α-pinene oxidation,30,31 with PIN representing a first generation product, and MBTCA a later generation product. PIN is a well-established product of α-pinene, initiated by both OH and ozone oxidation.30 Although PIN is an intermediate-volatile organic compound (IVOC) and exists mostly in the gas phase under ambient conditions,32 the presence of carboxylic and ketone functional groups in its structure enhances its water-solubility. Using an equilibrium partitioning space,33−35 we predict that PIN largely resides in the aqueous phase under cloudy conditions (see Supporting Information), indicating that PIN can undergo cloudwater processing. Study of the aqueous-phase chemistry of PIN is exceedingly sparse. In previous studies from our group, Lee et al.14,36 investigated aqueous-phase OH oxidation of PIN by atomizing the bulk aqueous solution and monitoring the chemical composition of the particles formed using an aerosol mass spectrometer (AMS). Throughout the course of OH oxidation, the authors observed first an increase and a later decrease in the signal intensity of m/z 43 (f43, often observed from carbonyl functionalities), but a continuous increase in that of m/z 44 (f44, often observed from carboxylic acids). This observation was explained by formation of initial reaction intermediates (contributing to f43) and the conversion of these intermediates to more oxygenated products (contributing to f44). The authors concluded that water-soluble IVOCs, such as PIN, may contribute to SOA formation via aqueous-phase chemistry. Lignell et al.37 have investigated the direct photolysis of PIN in the aqueous phase, quantified its photolysis quantum yield, and proposed a detailed photolysis reaction mechanism. The authors have also claimed that aqueous-phase OH oxidation should be the dominant removal process of PIN under cloudy conditions. However, the OH rate constant of PIN photo-oxidation has not been reported. MBTCA is a low-volatility compound that has been detected in biogenic SOA from both laboratory experiments and field measurements (Hallquist et al.30 and references therein). While the presence of this triacid compound in biogenic SOA has long been known, Szmigielski et al. 38 for the first time unambiguously confirmed the identity of MBTCA and recommended MBTCA as a tracer for monoterpene oxidation. In a chamber experiment, Müller et al.39 have detected MBTCA as a gas-phase OH oxidation product of PIN. Although the

2. EXPERIMENTAL SECTION 2.1. Reaction Solutions and OH Oxidation. Batch solutions were prepared by dissolving commercial PIN (Sigma-Aldrich, 98%) and TCA (Sigma-Aldrich, 99%) into deionized water (18 mΩ-cm, total organic carbon ≤1 parts per billion, Veolia) without any further purification. The experimental solution (100 mL) was contained in a glass bottle (125 mL, Wheaton) for photo-oxidation experiments. Hydrogen peroxide (H2O2, Sigma-Aldrich, 30% in water, TraceSELECT) was added as a photolytic source of the OH radical. The concentration of H2O2 was 1 mM for all the experiments. Experiments for kinetic and mechanistic analyses were conducted under slightly different conditions, summarized in Table 1 and 2, respectively. The experimental solution was placed in the center of a photoreactor (Rayonet, RPR-200) with UVB lamps (RPR3000, peak emission at 310 nm) equipped in a circular manner. The glass bottle cut off light with wavelengths shorter than 300 nm. During experiments, the solution was constantly stirred by a magnetic stir bar, and a fan was equipped to minimize B


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enables detection of organic compounds at cloudwater-relevant concentrations (i.e., at the μM level;44 3) high mass resolution that separates compounds with identical nominal mass but different elemental compositions; and 4) flexibility in choosing a reagent ion most suited for the experimental objectives. In this study, the initial concentrations of PIN and MBTCA were chosen between 10 to 30 μM (Table 1 and 2) to represent cloudwater relevant conditions. Acetate (CH3C(O)O−) was chosen as the primary CIMS reagent ion because it was especially sensitive in detecting acidic organic compounds,45 including PIN and TCA. The protonated water cluster reagent ion ((H2O)nH+) was employed in the kinetic study of PIN to detect both PIN and the kinetic reference compound (see Section 2.3). The iodide (I(H2O)n−) reagent ion is sensitive to oxygenated organic compounds46 and was also employed in one PIN mechanistic study as a confirmation for the products detected using acetate. The ionization mechanisms of the three reagent ions are shown in RR1 to R3, respectively.

Table 1. Summary of Conditions in Kinetic Experiments

cis-pinonic acid (PIN) tricarballylic acid (TCA)

concn (μM)

pH

10

2

20

4.5

CIMS reagent water cluster acetate

ref

ref concn (μM)

DMSO

10

propanoic acid

10

Table 2. Summary of Conditions in Mechanism Experiments concn (μM)

pH

CIMS reagent

20 30

4.6 4.3

acetate, iodide acetate

cis-pinonic acid (PIN) tricarballylic acid (TCA)

solution heating. The solution temperature reached roughly 28 °C during the experiments. For the kinetic study, the experiments continued until most of the PIN or TCA was consumed (i.e., roughly 35 min for PIN and 60 min for TCA), while the experiments were conducted for 4 h for the mechanistic study to ensure detection of products arising from later oxidation generations. 2.2. Analytical Techniques. An aerosol time-of-flight chemical ionization mass spectrometer (Aerosol-ToF-CIMS) was employed for online monitoring of the chemical composition in the experimental solution. The setup and operation principle of Aerosol-ToF-CIMS have been described in detail elsewhere.41,42 The experimental setup in the current study is shown in Figure 2. Briefly, the solution was constantly atomized using a TSI constant output atomizer (Model 3076). After dilution, the particles were introduced through a volatilization line heated to 150 C. The volatilization line was made of a Siltek-coated stainless steel tubing (Restek), which minimized the wall-loss of chemicals. Organic compounds volatilized in the heated line were introduced into an Aerodyne high-resolution time-of-flight chemical ionization mass spectrometer.43 As demonstrated in our previous studies,41,42 Aerosol-ToFCIMS is a highly attractive, new experimental technique with 1) high time resolution that enables online monitoring of a rapidly evolving chemical system; 2) superior detection sensitivity that

CH3C(O)O− + MH → CH3C(O)OH + M−

(R1)

(H 2O)n H+ + M → n(H 2O) + MH+

(R2)

I(H 2O)−n + M → n(H 2O) + IM−

(R3)

The operating conditions of the Aerosol-ToF-CIMS have been discussed in detail by Aljawhary et al.41 Briefly, the mass spectrometer was operated with the V-mode, with a mass accuracy at 5 μTh Th1− (ppm) and a resolving power at 3000 to 4000 Th Th1− in the relevant m/z range. Signals obtained by the CIMS have been normalized by the signal intensity of the respective reagent ion. The data were analyzed using the data analysis software, Tofware v.2.2.2. (Aerodyne Inc.) on the Igor platform. The instrument was operated with 1 Hz time resolution, but the time series presented in this study have been averaged every 5 min. In addition to Aerosol-ToF-CIMS, an aerosol particle flow was introduced to a scanning mobility particle sizer (SMPS) after diffusion drying to monitor the size distribution of the particles. This setup permits simulation of the evaporation of cloud droplets and measurement of the amount of SOA

Figure 2. Experimental apparatus. C


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The Journal of Physical Chemistry A material retained in the particle phase.9,47 The relative humidity (RH) after diffusion drying was estimated to be 10 to 20%. 2.3. Relative Rate Method. As in standard in the aqueous phase kinetics field, the relative rate method was used for rate constant determination. The advantage of this technique is that the OH radical concentration need not be monitored and the concentration of the reactants does not need to be known accurately.48 The relative rate method can be used to determine a compounds OH reactivity by using a reference compound with known OH reactivity. In such experiments, the reference compound (R) and the sample compound (S) are mixed in the same reaction cell with the oxidant, in a way that R and S follow the relationship illustrated in eq 1: ⎛ [S] ⎞ k II ⎛ [R ]0 ⎞ ln⎜ 0 ⎟ = SII ln⎜ ⎟ kR ⎝ [R ]t ⎠ ⎝ [S]t ⎠

We attempted to also perform kinetics experiments at pH 10 by adding NaOH to ensure complete dissociation of PIN to PINn. However, detection of PIN was not successful at this pH. This problem is possibly due to reduced vapor pressure of PINn compared to PIN (i.e., not volatilized in the heated line). Presence of the counterion (Na+) from the base added (NaOH) perhaps also assisted in further reducing the vapor pressure of PINn by forming salt. Ortiz-Montalvo et al.47 have also observed a similar pH-dependence in Aerosol CIMS measurements when ammonium hydroxide was added to the reaction solution. For this reason, the current study presents the kinetic data of PIN only at pH 2, representing the reactivity of the nondissociated form of PIN. 3.1.2. PIN Reaction Kinetics. Figure 3 illustrates the kinetics data obtained for PIN and DMSO solutions for three runs.

(1)

In eq 1, [X]0 and [X]t denote the concentration (or signal intensity) of compound X at time 0 and t, respectively. kIIS and kIIR are the second-order rate constants of the OH radical reacting with S and R, respectively. Plotting ln([S]0/[S]t) against ln([R]0/[R]t) should yield a straight line with slope of kIIS/kIIR. The measured slope and the kIIR obtained from the literature allow the determination of kIIS. Dimethyl suolfoxide (DMSO, Caledon Laboratories) was selected as the reference compound for PIN because the sulfur atom in the DMSO molecule made it unique in its exact mass, so that the Aerosol-ToF-CIMS could unambiguously distinguish it from other PIN reaction products. DMSO, being a nonacidic compound, cannot be detected by the acetate reagent ion; therefore, the protonated water cluster ion was chosen to conduct the kinetic study of PIN. DMSO could not be used as the reference compound for TCA, however, because TCA was not detected by the protonated water cluster reagent ion. Instead, propanoic acid (PRP, Sigma-Aldrich) was employed as the reference compound for TCA after confirming that PRP is neither produced from TCA oxidation nor that its own products overlapped mass spectrometrically with TCA. The concentrations of the reference compounds were 10 μM (Table 1).

Figure 3. Relative kinetic plot of cis-pinonic aicd (PIN) in accordance with eq 1 using dimethyl sulfoxide (DMSO) as the reference compound. The different colors represent triplicate runs. The DMSO rate constant (kIIDMSO) was adopted from Zhu et al.,51 shown along with the calculated rate constant of PIN (kIIPIN). The uncertainty represents standard deviation from the triplicate.

3. RESULTS AND DISCUSSION 3.1. cis-Pinonic Acid (PIN). 3.1.1. Detection of PIN. PIN is a monoacid with pKa = 4.82.49 Thus, it can exist in the protonated form (PIN) or as pinonate anions (PINn) in the aqueous phase. Previous studies have shown that the OH radical reacts with carboxylate acidic forms (e.g., PINn) more rapidly than nondissociated forms (e.g., PIN) due to an electron transfer reaction mechanism that prevails for the former.50 Thus, to determine the rate constant for the reaction of OH with PIN, it is important to monitor the pH. The fraction of each form (PIN and PINn) is dependent on the pH of the solution and can be calculated by eq 2: α=

[AH ] [H +] = [AH ] + [A−] [H +] + K a

Highly linear relationships were obtained for the three experiments with a y-intercept close to 0, in accordance with the relative kinetic relationship. Using eq 2 and a kIIDMSO value of 6.9 (±0.1) × 109 M−1 s−1 (at 301 K),51 kIIPIN was determined to be 3.3 (±0.5) × 109 M−1 s−1 at 301 K in a solution at pH = 2. The measured rate constant for PIN is within the diffusion limit of aqueous phase reactions of organic compounds with the OH radical (109 to 1010 M−1 s−1).2 3.1.3. MBTCA Formation from PIN Photo-oxidation. Formation of a large number of reaction products was observed by Aerosol-ToF-CIMS using both the acetate and the iodide reagent ions. An important observation from this experiment is the formation of MBTCA, as indicated by product signals consistent with its molecular weight. This is a complement to its formation observed previously from gas-phase39 and heterogeneous40 photo-oxidation of PIN. MBTCA is not a major product from PIN. From the signal intensity of MBTCA, assuming a detection sensitivity of MBTCA similar to that of citric acid,41 the molar yield of MBTCA from PIN is less than 1%. This yield is similar to the value reported from gas-phase oxidation of PIN.39 Despite its small molar yield, MBTCA

(2)

where α is ionization fraction, [AH] is the concentration of the carboxylic acid, [A−] is the concentration of the carboxylate, [H+] is the concentration of H+ and Ka is the acid dissociation constant. Using eq 2 to calculate for PIN at pH = 2, yields α = 99.8%, which indicates that the majority of the PIN is not dissociated at pH 2. D


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phase mechanism proposed by Müller et al.39 The reaction is initiated by a hydrogen abstraction on the 4-membered ring. This is followed by C−C cleavage leading to ring opening. Figure 4b illustrates the evolution of the C10 compounds (blue and green) and MBTCA (shown in pink) as a function of OH exposure which is obtained as the product of illumination time and steady-state concentration of the OH radical ([OH]ss). The [OH]ss value in the solution was calculated to be 1.7 × 10−13 M from the pseudo first-order decay of PIN, employing the rate constant obtained in Section 3.1.2. We note that a [OH]ss value at this level is relevant to that in cloudwater.52,53 As proposed by the mechanism in Figure 4a, the first onset observed is for the C10H14O5 aldehyde. C10H14O6 onset is close to that of C10H14O5 but still lags behind the onset of C10H14O5. The two C10 compounds can react with OH to form MBTCA as observed by the delayed onset of MBTCA. Figure 4b and time profiles thereafter are intended to effectively display the evolution profile of multiple products. The signal intensities of products have been scaled to be displayed on one y-axis and do not present absolute values. MBTCA is further photo-oxidized to form a myriad of reaction products. To provide insights into MBTCA reaction mechanism, we have conducted a mechanistic investigation of TCA as a surrogate of MBTCA. A detailed discussion about TCA reaction mechanism and its implication to the MBTCA reaction mechanism are provided later in this paper. 3.2. Tricarballylic Acid (TCA). 3.2.1. Acid-Dissociation Equilibria of TCA and PRP. TCA is a triacid with three acid dissociation constants in water, pKa1 = 3.47, pKa2 = 4.54 and pKa3 = 5.89.54 Thus, it can be found in the aqueous phase in four forms that are different in the extent of dissociation, which is dependent on the solution pH. Figure 5 shows the distribution of the different forms of TCA at different pH calculated from the given pKa values. It is shown that a minimum of two forms can exist simultaneously in the pH range of 1−8, which indicates that a single form of TCA (or MBTCA, assuming similar pKa) cannot be isolated alone. Under the extreme cases, at pH less than 1 and greater than 8, the fully protonated and dissociated forms of TCA can be isolated. However, those pH ranges are not relevant to atmospheric cloudwater. Several studies have measured the acidity of ambient cloudwater droplets and showed that cloud droplet water is acidic with an average pH of 4.5.55−57 As a result, attempts were not made to adjust the pH and the experiments were run at unadjusted pH (4.5) for rate constant determination for the reaction of TCA with OH.

represents a class of compound that can contribute to aqueous SOA formation (see Section 3.3). Hence, this study focuses on the reaction mechanisms associated with MBTCA. Key products associated with MBTCA formation, its oxidation mechanism, and the observed time profiles are illustrated in Figure 4. The time profiles of major products

Figure 4. Photo-oxidation mechanism of PIN adopted from Müller et al.39 (a) and the observed time profiles of PIN and its products as a function of OH exposure (b). The products were detected using the acetate reagent ion. All the product traces start from their minimum signal and normalized to their maximum signal.

indicate similarities in reaction mechanisms between the aqueous- and gas-phase chemistry. Figure 4a adopts the gas-

Figure 5. Calculated dissociation fractions (α) of tricarballylic acid (TCA) and propanoic acid (PRP) at pH range from 0 to 9. For TCA, α1 is the nondissociated form, α4 is the fully dissociated form, and α2 and α3 are the first and second dissociated forms. E


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The Journal of Physical Chemistry A The pKa of PRP is 4.87,57 and thus, PRP exists in the protonated (PRP) and dissociated (PRPn) forms, as shown in Figure 5. The reported rate constant for the reaction of OH with PRP is kIIPRP = 3.2 × 108 M−1 s−1 and with PRPn is kIIPRPn = 7.2 × 108 M−1 s−1.50 At pH = 4.5, α1 = 0.65 and α2 = 0.35 for PRP. Thus, an estimated rate constant for PRP was calculated using an α-weighted average rate constant of PRP and PRPn kIIPRP = 4.6 × 108 M−1 s−1). 3.2.2. TCA Reaction Kinetics. Figure 6 shows the data collected for the TCA rate constant using eq 1. As illustrated,

authentic MBTCA standard should be synthesized to determine its genuine OH reactivity. 3.2.3. Proposed Reaction Mechanism of TCA Photooxidation. The degradation mechanisms of MBTCA and TCA have not been reported in the literature. Given that TCA is a symmetric compound as compared to MBTCA, there are notably fewer plausible reaction pathways. We have performed a detailed mechanistic analysis for TCA based on widely accepted aqueous-phase radical chemistry. The OH radical initiates the radical chain reaction via H-abstraction from one of two possible locations, followed by addition of a dissolved O2 to form peroxy radicals (RO2). Due to the low solubility of NO,60 reaction between RO2 and NO is not expected to be significant in the aqueous phase. Instead, RO2 mainly reacts with the hydroperoxy radical (HO2) or another RO2 to form alkoxy radicals (RO) or carbonyl, alcohol, peroxide compounds.61 This occurs via the formation of the tetraoxide intermediate R4, followed by a variety of reaction pathways shown in R5 through R8.62 R 2CHO2 + R 2CHO2 → R 2CH − OOOO − CHR 2

(4)

R 2CH − OOOO − CHR 2 → R 2C(O) + R 2CHOH + O2 (5)

→2R 2C(O) + H 2O2

(6)

→2R 2CHO·+ O2

(7)

→R 2CHOOCHR 2 + O2

(8)

In addition, hydration can also proceed in an aqueous medium. For example, aldehydes can hydrate in water forming geminal diols R9:

Figure 6. Relative kinetic plot of tricarballylic acid (TCA) in accordance with eq 1 using propanoic acid (PRP) as the reference compound. Two replicates were performed and the rate constant for TCA (kIITCA) is derived using their average. The uncertainty represents the variation between the replicates.

RC(O)H + H 2O → RCH(OH)2

(9)

In the presence of OH, the geminal hydrogen is abstracted from the hydrated alkyl radical R10).50 The RO2 radical that forms subsequently (R11 decomposes to form the corresponding acid and HO2 R12. This aldehyde processing pathway is unique to the aqueous phase and does not occur via gas-phase oxidation.

the data for two runs yield a straight line with a y-intercept close to 0. The average slope of the two runs were used to calculate the rate constant for the reaction of OH with TCA, which was determined to be (kIITCA = 3.1 (±0.2) × 108 M−1 s−1 at 301 K in a solution at pH = 4.5. Although TCA and MBTCA are similar in structure, the two additional methyl groups on MBTCA may differentiate their OH reactivity. Given that the photo-oxidation rate constants of TCA and MBTCA have not been reported, an aqueous-phase structure−activity relationship58,59 is employed here for a rough estimation. To make a comparison based purely on the structures of TCA and MBTCA, we used parameters for a nondissociated carboxyl group. The calculated kIITCA is 1.5 × 108 M−1 s−1, in a reasonable agreement with the value determined in the current study (3.1 (±0.3) × 108 M−1 s−1), considering that the experimentally determined value is affected by complicated acid-dissociation equilibria. On the contrary, the calculated kIIMBTCA is 6.2 × 108 M−1 s−1. It turns out that the two additional methyl groups on MBTCA may account for up to a 4-fold difference in the OH reactivity. In addition, MBTCA is expected to have different pKa values to TCA, and hence, there may be a different pH dependence in their OH reactivity. On the basis of our prediction (Supporting Information), the three pKa values of MBTCA should be 3.76, 5.10 and 5.82, respectively. Overall, the TCA rate constant determined in the current work may be best considered as a lower limit for the MBTCA reactivity. In future studies, an

RCH(OH)2 + OH → RC(OH)2 ·+H 2O

(10)

RC(OH)2 ·+ O2 → RC(OH)2 OO·

(11)

RC(OH)2 OO· → RC(O)OH + HO2

(12)

On the basis of these general aqueous-phase reaction mechanisms, a reaction mechanism for TCA is proposed in Figure 7a. Products that are structural isomers are labeled with the same color as they appear at the same m/z, while products that are not detected in our experiment are colored in gray. TCA is shown in the fully protonated form in Figure 7a, but it is important to note that multiple dissociated forms of TCA would follow the same pathways. Figure 7b shows the evolution of major products (colored in accordance with Figure 7a) during photo-oxidation as a function of OH exposure. The [OH]ss value in the solution was determined to be 3.4 × 10−13 M by tracking the decay of TCA and employing the TCA reactivity determined in Section 3.2.2. The OH radical abstracts a hydrogen atom on the tertiary or secondary carbon (i.e., pathway 1 (P1) and pathway 2 (P2) shown in Figure 7a). This is supported by previous observations from dicarboxylic acid reactions with OH.63 The peroxy radical forming on the tertiary carbon of TCA (Pathway F


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Figure 7. Proposed reaction mechanism for TCA oxidation by the OH radical in the aqueous phase (a). Time profiles for the decay of TCA oxidation and the evolution of selected products plotted over OH exposure (b). The coloring of the compounds is consistent between (a) and (b). Gray compounds were proposed but either were not detected or showed no change in signal. All the compounds in (b) were detected by the acetate reagent ion. All the product traces in (b) start from their minimum signal and are normalized to their maximum signal.

shows the evolution of malonic acid (purple), glyoxylic acid (red) and C3.1 (pink). The traces for malonic acid and glyoxylic acid show a very similar time profile, appearing to be products of later generations. C3.1 arises earlier, in accordance to the proposed mechanism where malonic acid can be produced from C3.1 (P2.1.2). If glyoxylic acid (C2.1) arises from P2.1.1 and P2.1.3, it should appear as a first generation product of TCA, which does not agree with its observed time profile. The formation profile of glyoxylic acid is likely complicatd by multiple formation pathways. P2.2 leads to the formation of C5.2a, a structural isomer with C5.1. However, we believe P2.2 is a minor pathway, as the subsequent product of C5.2a, C5.2b, is not observed. This indicates that the signal detected was most likely not that of C5.2a but for C5.1. The C6 products formed from P2.3 were also observed and appeared to form very early in the reaction, as shown in Figure 7b. These two C6 products appear to be very reactive with the OH

1, or P1) likely results in an alkoxy radical which induces decomposition of the TCA backbone leading to a carbon− carbon bond breakage. Due to the symmetry of TCA around the tertiary carbon, two possible C−C bonds can be cleaved (P1.1/P1.2). The resultant products from the cleavage are C5.1 through P1.1 and C4.1/C2.1 through P1.2. C4.1 is not detected by the acetate reagent ion, as indicated by the gray color in Figure 7a. However, as will be mentioned in the Section 3.2.4, the same compound has been detected instead by the iodide reagent ion. Compound C4.1 is likely present but not detected by the acetate reagent ion. P2 in Figure 7a has three branches resulting from hydrogen abstraction by OH on the secondary carbon. Two pathways lead to C−C bond cleavage (P2.1 and P2.2), and the third (P2.3) is due to O2 abstraction of a hydrogen. P2.1 gives rise to products with small carbon numbers: malonic acid (C3.2), glyoxylic acid (C2.1) and C3.1. Figure 7 G


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Figure 8. Proposed reaction mechanism of MBTCA (a) and the time profiles of MBTCA and proposed products observed during the PIN photooxidation experiment (b). The coloring of the compounds is consistent between (a) and (b). Compounds bracketed in (a) are the analogous products observed from TCA photo-oxidation. Products in (b) are detected using the acetate reagent ion and displayed in their deprotonated forms, except for C4H4O5, which is detected using the iodide reagent ion from a separate experiment. The products shown in (b) start from their minimum signal and are scaled arbitrarily for clarity of display.

Compound C4H4O5, shown in brown in Figure 8, which is the same compound as C4.1 in Figure 7, was not detected by the acetate reagent ion. However, the presence of this compound was confirmed by using the iodide reagent ion in a separate experiment under the same conditions. The formation profile of this compound (Figure 8b) shows agreement with the proposed mechanism, but we cannot rule out the possibility that this compound is a nonacid isomer of C4H4O5 (i.e., can be detected by iodide but not acetate). 3.3. Aqueous-Phase SOA Formation during Photooxidation of PIN. As shown in previous sections, photooxidation of PIN is highly complex, with products forming via both functionalization (i.e., less volatile) and fragmentation (i.e., more volatile). To investigate how the overall reaction contributes to aqueous SOA formation, a SMPS was used to monitor the evolution of particle size distribution throughout the photo-oxidation. Figure 9 shows a particle volume profile as the reaction of PIN with the OH radical proceeds. In the presence of PIN, the particle volume is the same as the background level, indicating that PIN alone does not contribute to particle volume. However, when the reaction is initiated, the

radical. It is proposed that C6.2a reacts mostly with OH by a hydrogen abstraction on the carbon bearing the electron donating hydroxyl group to produce C6.2b. This is supported by the earlier onset and peak of C6.2a observed in Figure 7b compared to C6.2b. Compounds C6.2a and b undergo subsequent reactions to give rise to products with smaller carbon numbers. 3.2.4. MBTCA Reaction Mechanism. Although the two additional methyl groups on MBTCA may affect its reaction kinetics (Section TCA Reaction Kinetics), many of the TCA products observed are equivalent to those of MBTCA, indicating that the TCA mechanism is useful in deriving the reaction mechanism of MBTCA. The proposed mechanism for MBTCA is shown in Figure 8a, where the analogous products from the TCA experiment are shown with brackets. This proposed mechanism is supported by the time profiles of the proposed products observed from the PIN oxidation experiment (Figure 8b). The proposed products all arise later than MBTCA; however, we cannot rule out the possibility of reaction pathways other than MBTCA also give rise to the same products or their structural isomers. H


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Figure 9. Atomized particle total volume concentration and size distribution (top) as the oxidation of PIN proceeds. The circle trace illustrates the volume concentration, color-coded according to different stages of the experiment. The diamond trace shows the MBTCA signal from the same experiment.

Figure 10. Atomized mass of cis-pinonic acid (PINm(t)), particles (Cm(t)), as well as the calculated time-dependent yield of aqueous SOA (YaqSOA).

particle volume measured by the SMPS increases significantly higher than the background. This shows that the photooxidation of PIN in the aqueous phase results in low volatility products that form particles even in the absence of seed inorganic particles. In addition, the total particle volume starts decaying past 2 h of oxidation. This provides evidence of fragmentation reactions leading to the degradation of the low volatility organics and resulting in small oxygenated compounds that are relatively volatile. Although this observation is consistent with the conclusion from our previous study36 using AMS, the Aerosol-ToF-CIMS measurements provide molecular-level information to support this conclusion. Figure 9 also shows the signal of MBTCA

detected during the same experiment, where the rise of MBTCA corresponds well with that of SOA volume. It is likely that MBTCA and products of similar generations (i.e., second to third generation products from PIN) are contributing to the initial rise of SOA. As photo-oxidation proceeds, the products arising from MBTCA and similar compounds can also contribute to SOA mass, but fragmentation likely starts dominating, as represented by a gradual decay of the SOA volume. Indeed, most of the proposed products of MBTCA contain less carbon and are presumably more volatile than MBTCA (Figure 8). We define a novel aqueous-phase SOA yield (YaqSOA), as the ratio of the change in particle mass detected by the SMPS and I


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tricarballylic acid (TCA), a surrogate for 3-methyl-1,2,3butanetricarboxylic acid (MBTCA). The second-order rate constants were determined at 301 K to be 3.3 (±0.5) × 109 and 3.1 (±0.2) × 108 M−1 s−1 for PIN and TCA, respectively. To translate these rate constants to atmospheric lifetimes, we have considered two scenarios. In these scenarios, the atmospheric lifetime of MBTCA is assessed based on the kinetic data obtained for TCA. We note, however, that MBTCA may react more rapidly with the OH radical than TCA (see Section 3.2.2) so the actual photochemical lifetime of MBTCA can be shorter. The first scenario is to consider decay of PIN and MBTCA in remote cloudwater, where the [OH]ss value modeled by Herrmann et al.53 (2.2 × 10−14 M) is employed. The corresponding e-fold lifetimes with respect to aqueous-phase photo-oxidation (τaq,OH) are calculated to be 3.8 and 41 h for dissolved PIN and MBTCA, respectively. The overall atmospheric lifetimes with respect to cloud processing are much longer than these τaq,OH values because an air mass is not always in the cloud, and a compound does not partition to the aqueous phase entirely. The aqueous fraction of a compound exhibits complicated dependencies on the liquid water content of an air mass, temperature, cloud pH, as well as the Henry’s law constant of the compound. Our equilibrium partitioning space calculations (Supporting Information) show that both PIN and MBTCA reside almost entirely in the aqueous phase under cloudy conditions (liquid water content ≥0.3 g m−3). Assuming a typical in-cloud time of 15%,67 the atmospheric lifetime with respect to cloudwater processing should be approximately 7 times as long as the τaq,OH values. This calculation leads to the conclusion that cloud processing can be a significant atmospheric sink for PIN, but not for MBTCA, because its cloud processing lifetime becomes longer than the typical residence time of aerosol in the troposphere. The second scenario involves processing of PIN and MBTCA in aerosol liquid water. Although smaller in volume than cloudwater, aerosol liquid water is constantly present, depending on the surrounding relative humidity and the hygroscopicity of the aerosol. Based on our predictions, a significant fraction of MBTCA can partition into aerosol liquid water with a typical liquid water content of 10 μg m−3, but PIN is predominantly in the gas phase under this condition (Supporting Information). Therefore, PIN is not considered in the aerosol liquid water scenario. The [OH]ss value in aerosol liquid water is highly uncertain and remains controversial. It has been estimated to be on the order of 10−12 M by models,53 but measurements have reported much lower concentrations on the order of 10−15 M.68 This large gap in the proposed [OH]ss in aerosol liquid water results in a wide range of possible τaq,OH values for MBTCA, from 0.9 to 900 h. We propose that processing in aerosol liquid phase is likely a significant sink for MBTCA, but its specific lifetime is highly dependent to the [OH]ss. In summary, we conclude that PIN can be removed efficiently via photo-oxidation in cloudwater, while MBTCA is more likely removed in aerosol liquid water. Recently, the stability of tracer compounds used in mass balance receptor models has been questioned. The tracer compounds are commonly considered to be chemically inert,69 but have been shown to react with the OH.70−72 The current study adds to this argument by showing that PIN and MBTCA, considered as tracers for SOA from biogenic sources, can degrade through aqueous-phase photo-oxidation. For successful use of receptor models for source apportionment, such degradation should be taken into consideration when

the change in the mass of atomized PIN. The time dependent yield (YaqSOA(t)) is expressed by eq 3: YaqSOA(t ) =

(Cm(t ) − Cm(0)) ΔCm = (PINm(t ) − PINm(0)) ΔPINm

(3)

where Cm(t) is the total mass (per minute of atomization) of atomized particles at time t, and PINm(t) is the total mass of PIN atomized at time (t) (per minute of atomization). Cm(t) is determined by conversion of the volume concentration (Cv(t)) assuming an average SOA density (d) of 1.4 g mL−1, as well as the flow rate of the atomizer (fa, 3 L min−1) as expressed by eq 4: Cm(t ) = Cv(t ) × d × fa

(4)

PINm(t) is determined using eq 5: PINm(t ) = R sol × PINc(t ) × MWPIN

(5)

where Rsol is the rate at which the experimental solution is consumed (i.e., the rate at which the solution is converted into aerosol particles), and was determined by monitoring the mass of the experimental solution over time. PINc(t) is the molarity of PIN in the experimental solution at time t. This is tracked by the Aerosol-ToF-CIMS signal of PIN. Finally, MWPIN is the molecular weight of PIN, 184 g mol−1. A similar YaqSOA was defined by Ortiz-Montalvo et al.9 based on the mass of organic matter in single droplets. The definition of YaqSOA in the current study is based on the total output of the atomizer, under the assumption that the output is constant throughout the course of each experiment. Figure 10 shows the calculated Cm(t), PINm(t), and YaqSOA up to an experimental time of 2 h, the time at which PIN is completely consumed and SOA mass reaches its maximum. YaqSOA was high at around 60% at the beginning of the photooxidation but later reached a relatively stable level of 40%. This trend is somewhat similar to the observations by OrtizMontalvo et al.,9 where the authors found that the YaqSOA from glycolaldehyde OH oxidation gradually decreased from 120% to 50% during OH oxidation. The authors proposed that oligomerization of glycolaldehyde and glyoxal in the evaporation droplets may have contributed to the high yield at the beginning of the experiments. Given that PIN also contains a carbonyl functional group, PIN and its early products may undergo similar type of reactions in the evaporated droplets. We recommend that the YaqSOA reported here be used as a rough benchmark value for a number of reasons. A certain amount of water is likely associated with particles after drying and has been considered as SOA in our calculation. Particularly, the particles can be semisolid after drying,64 where the enhanced viscosity can impede mass transfer of water from the particles to the gas phase65,66 The volatility of the aqueous SOA can be highly dependent on the particle acidity. As demonstrated by Ortiz-Montalvo et al.,9,47 organic acids are converted into salts under less acidic conditions, reducing the volatility of organic acids by orders of magnitude. Inorganic salts are absent in our experiment, and so it is possible that YaqSOA will change as the acidity and composition of particles vary in the ambient.

4. ATMOSPHERIC IMPLICATIONS AND CONCLUSIONS 4.1. Atmospheric Lifetimes of PIN and MBTCA. This study is the first kinetic and mechanistic investigation of the aqueous-phase photo-oxidation of cis-pinonic acid (PIN) and J


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The Journal of Physical Chemistry A considering atmospheric residence times comparable to or longer than the chemical lifetime. 4.2. Aqueous-Phase SOA Formation from PIN. A large number of reaction products was detected using Aerosol-ToFCIMS, and possible reaction mechanisms of PIN and MBTCA were proposed. Many of these compounds are highly oxygenated and will contribute to SOA formation upon cloud droplet evaporation. The employment of a SMPS after drying the particles was valuable in addressing the potential of SOA formation. As shown in Section 3.3, the yield of aqueous SOA (YaqSOA) was determined to be 40 to 60%. Aerosol-ToF-CIMS provided further molecular-level information to the chemistry. In particular, the formation profile of MBTCA exhibited excellent correlation with the formation of aqueous SOA. We conclude that aqueous SOA from monoterpenes, such as αpinene, is likely attributed to MBTCA and low-volatility products of similar oxidation generations. Using a YaqSOA of 40% as a lower limit, we have made a preliminary measurement of aqueous-phase SOA formation from an oxidation product of α-pinene. The molar yield of PIN from α-pinene ozonolysis has been determined by a number of chamber studies (Ma et al.73 and references therein) and exhibits a large variation from less than 1% to approximately 8%, reflecting the highly variable conditions employed in these chamber experiments. Although the yield of PIN alone is small, a number of structurally similar compounds have been identified, with the representative species being pinic acid, norpinic acid, pinonaldehyde, and so forth.7 The total molar yield of these identified products ranges between 10% to 40%.73 Assuming these products can all contribute to aqueous SOA formation to a similar extent as PIN (i.e., YaqSOA of 40%), the aqueous SOA yield of α-pinene can be roughly estimated to be between 4 to 16%, with the upper bound similar to the generally accepted SOA yield from gasphase oxidation of α-pinene.30 In an environment characterized by α-pinene emissions and persistent sunlit clouds, aqueousphase chemistry can likely replace its gas-phase counterpart and contribute to a similar amount of SOA formation. Under ambient conditions, the relative humidity and liquid water content affect the gas−aqueous partitioning of -pinene oxidation products and likely determine the relative importance of the aqueous-phase SOA formation pathway. The current study represents a novel effort to determine SOA yields via aqueous-phase OH oxidation of α-pinene oxidation products. The measurement setup involving particle drying and SMPS is useful for determining such yields, which have not been systematically quantified by the community previously. Although the majority of work on aqueous-phase processing has focused on isoprene related compounds, our results demonstrate the importance of aqueous SOA formation from monoterpene reaction products. Furthermore, aqueousphase processing can also lead to degradation of reactive SOA components (e.g., photolysis of organic peroxides).74 Future study should investigate the potential of aqueous SOA formation and aging from the reaction products arising from α-pinene and other major monoterpenes.

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Detailed information about the equilibrium partitioning space (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: (1)416 9467359. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The funding of this study was provided by NSERC and Environment Canada.

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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.5b06237. K


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