Investigation of Aqueous-Phase Photooxidation of Glyoxal and

Feb 1, 2012 - Nitrogen-Containing, Light-Absorbing Oligomers Produced in Aerosol Particles Exposed to Methylglyoxal, Photolysis, and Cloud Cycling...
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Investigation of Aqueous-Phase Photooxidation of Glyoxal and Methylglyoxal by Aerosol Chemical Ionization Mass Spectrometry: Observation of Hydroxyhydroperoxide Formation R. Zhao,* A. K. Y. Lee, and J. P. D. Abbatt Department of Chemistry, University of Toronto, Toronto, ON, Canada ABSTRACT: Aqueous-phase processing of glyoxal (GLY) and methylglyoxal (MG) produces highly oxygenated, less volatile organic acids that can contribute to SOA formation and aging. In this study, aerosol chemical ionization mass spectrometry (aerosol CIMS) is employed to monitor aqueous-phase photooxidation of GLY and MG. Using iodide (I−) as the reagent ion, aerosol CIMS can simultaneously detect important species involved in the reactions: organic acids, peroxides, and aldehydes, so that the reconstructed total organic carbon (TOC) concentrations from aerosol CIMS data agree well with offline TOC analysis. This study also reports the first direct detection of hydroxyhydroperoxide (HHP) formation from the reaction of H2O2 with GLY or MG. The formation of HHPs is observed to be reversible and an estimate of their equilibrium constants is made to be between 40 and 200 M−1. Results of this study suggest that HHPs can form additional formic acid and acetic acid via photooxidation and regenerate GLY or MG during photooxidation, compensating their loss. HHP formation needs to be further studied for inclusion in aqueous-phase chemical models given that it may affect the aqueous partitioning of carbonyls in the atmosphere.

1. INTRODUCTION It is now evident that aqueous-phase processing can be a significant formation and aging pathway of secondary organic aerosol (SOA),1−7 a major fraction of atmospheric submicrometer particles8 that causes adverse health effects, visibility degradation and affects global climate. Aqueous-phase oxidation can convert water-soluble volatile organic compounds (VOCs) into highly oxygenated, less volatile compounds that can contribute to SOA mass upon water evaporation. Inclusion of this SOA formation mechanism may improve agreement between field observations and traditional models9,10 in which SOA formation is governed by thermodynamic partitioning of gas-phase oxidation products alone.11 Aqueous-phase processing is also an additional chemical aging process causing continuous modification of physicochemical properties, such as hygroscopicity,12,13 light absorption,14 and oxidative stress to the human body. However, due to its complicated nature, many aspects of aqueous-phase processing and its contribution to the SOA budget are still not fully understood. Glyoxal (GLY) and methylglyoxal (MG) are ubiquitous dicarbonyl VOCs in the atmosphere. They originate from both biogenic and anthropogenic precursors, with photooxidation of isoprene being their dominant source.15 They have been frequently employed as model compounds in numerous laboratory and modeling studies of atmospheric aqueousphase processing.3−5,16−24 Specifically, GLY and MG can efficiently partition into pure water via formation of their © 2012 American Chemical Society

hydrated structures (i.e., geminal diols), resulting in high Henry’s law constants (HLCs) of 4.19 × 105 M atm−1 for GLY25 and 3.71 × 103 M atm−1 for MG.26 There is growing evidence that the presence of inorganic and organic species in aqueous solution can further enhance their Henry’s law solubilities,25,27,28 in part due to various types of aqueousphase reactions including enhanced hydration effects,25,28 oligomerization,5,29 and formation of nitrogen-containing (C− N)30,31 and sulfur containing (C−S) compounds.20 Under solar radiation, aqueous-phase GLY and MG can be rapidly photooxidized,16,18,22 via irreversible reactions that can form SOA materials which have larger molecular weight and/or lower volatility.5 Depending on the conditions of gas−aqueous phase transitions, aqueous-phase photooxidation of GLY and MG can be a loss mechanism comparable to their gas-phase photooxidation and has been studied in detail.5,16,18,22,24,32 The proposed reaction scheme is shown in Figure 1.5,17,32 The major products include highly oxygenated organic acids such as oxalic acid, glyoxylic acid, and pyruvic acid. The products from multiple oxidation generations are expected to participate in oligomer, C−N, and C−S formation reactions. Furthermore, Special Issue: A. R. Ravishankara Festschrift Received: November 30, 2011 Revised: January 31, 2012 Published: February 1, 2012 6253

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Figure 1. Simplified reaction mechanisms of aqueous-phase photooxidation of GLY5 and MG.17,32 The two-way arrows represent reversible processes, whereas the one-way arrows represent irreversible OH oxidation.

Figure 2. Schematic description of Aerosol CIMS and photooxidation cell.

chemistry models may be incomplete. Our previous work monitored aqueous-phase photooxidation of GLY with online aerosol mass spectrometry (AMS), offline ion chromatography (IC), and total organic carbon (TOC) measurement.24 It was observed that the reconstructed TOC calculated from the AMS and IC speciated data was significantly lower than the measured TOC in the early stages of photooxidation, suggesting incomplete characterization of the oxidizing solution. On the basis of the AMS measurement, Lee et al.23,24 first proposed that this unrecognized compound was a hydroxyhydroperoxide (HHP) resulting from the rapid reaction between GLY and H2O2. HHP formation from other atmospherically relevant water-soluble aldehydes (e.g., formaldehyde and acetaldehyde) and H2O2 is also favored.36 Because AMS is not a highly species-specific technique when one observes a mixture of compounds, and HHPs are likely to be unstable for offline analysis, it is necessary to develop an online analytical technique that allows direct detection of HHPs. For the

volatile products such as formic acid and acetic acid can repartition into the gas phase. Their formation via aqueousphase processing may partially explain the current underestimation of the global budget of gas-phase formic acid and acetic acid.33 The major advances described above in our understanding of the mechanism of the aqueous-phase photooxidation of these dicarbonyls have arisen from both offline analysis16,17,34 or a combination of online and offline analytical methods, many conducted by Turpin and co-workers.18,22,24 These studies focused on the identification of products and the determination of their formation kinetics, based on which aqueous-phase chemistry models have been developed. However, all offline analysis is subject to potential secondary reactions occurring in the solution prior to analysis.18,35 In addition, detection and quantification of unstable (e.g., hydroxyhydroperoxides) or highly volatile species (e.g., acetic acid and formic acid) are especially challenging, so that the current aqueous-phase 6254

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effects, whereas MG photooxidation was only conducted at 3 mM. H2O2 (Sigma Aldrich, 30 wt % in water) was used as the source of OH radicals upon photolysis. The H2O2 concentrations used in the experiment were 13 and 1.3 mM for the 3 and 0.3 mM concentration regimes, respectively (Table 1). These H2O2 concentrations were expected to give a OH radical concentration of approximately 10−14 to 10−13 M in the solution during photooxidation, which corresponds to ambient cloudwater concentrations.39 After H2O2 addition, 30−40 min was given for HHPs to form and reach their equilibria without exposure to light. Some 3 mM GLY photooxidation experiments were also conducted without any waiting period to elucidate the effect of HHP to the photooxidation. In these experiments, the photooxidation was initiated immediately after the addition of H2O2. In all experiments, photooxidation was initiated by a 254 nm mercury lamp (UVP, constructed to remove the 185 nm line) immersed in the reaction solution, and the length of each experiment was approximately 4 h. During the entire experiment, the solution was atomized by a constant output atomizer (TSI, Model 3076) using ultrapure compressed air (BOC, grade 0.1), and the generated aerosol particles were analyzed by Aerosol CIMS. A series of control experiments was also conducted (Table 1). A dark control was conducted to investigate dark reactions of GLY or MG with H2O2, whereas a H2O2 control was performed to investigate direct photoreactions of GLY and MG without addition of H2O2. The purity of the water used was also examined by H2O2 addition and by exposure to direct and indirect photolysis to elucidate any potential organic formation from water impurities. 2.2. Aerosol CIMS. The aerosol CIMS system employs a heated inlet line to volatilize particle-phase organic compounds, thus detecting gas- and particle-phase organics simultaneously. Aerosol CIMS was first developed by Hearn and Smith37 and has been used very successfully for studies of heterogeneous and aqueous-phase chemistry.20,40 Here, we apply aerosol CIMS to investigate bulk aqueous-phase photooxidation. In the current study, the particle flow generated from the atomizer was diluted with 3.0 standard liter per minute (slpm) of ultrapure N2 gas (BOC, grade 4.8). The diluted flow was subsequently sent into a volatilization line made of 40 cm long silicon-coated metal tubing (SilcoNert 2000), which was heated to 105 ± 5 °C using a heating tape and monitored by a thermocouple. This temperature was chosen because it enables the detection of HHPs and both monohydrated and dihydrated GLY. The sample flow entering the CIMS was measured to be ∼4.5 slpm, which is controlled by a critical orifice at the entrance to the ion molecular region (IMR), with excess flow going to the exhaust line. The CIMS is a home-built unit using I− as the reagent ion and a quadrupole as the mass analyzer; see Escorcia et al.41 The reagent ion is generated by flowing 6 splm of N2 gas over a homemade CH3I permeation tube held at 70 °C. The flow containing CH3I vapor is then sent through a Po210 radioactive cell (NRD, Model 2031) from which I− is generated. The reagent ion meets the sample flow in the IMR of the CIMS, where organic acids, peroxides, and aldehydes are ionized by reactions R1, R2, and R4:20,40

current study, we have applied a real-time mass spectrometry measurement system with a soft ionization source, aerosol chemical ionization mass spectrometry (aerosol CIMS),37 to provide simultaneous detection of GLY, MG, HHPs and both volatile and less-volatile photooxidation products. This is a complement to earlier online analyses conducted using electrospray ionization mass spectrometry (ESI-MS),16−18,22 making use of a different sample preparation method and mass spectrometric ionization technique. The goal of the study was to demonstrate the oxidation mechanisms of GLY and MG using aerosol CIMS, and to confirm the formation of HHPs in the reaction systems. In particular, total organic carbon (TOC) concentration in the solution was reconstructed using aerosol CIMS to examine its agreement with the measured TOC concentration, and to assess the degree of oligomer formation.

2. EXPERIMENTAL METHODS The schematic diagram of the experimental apparatus is shown in Figure 2, with each component of the system explained in this section. 2.1. Photooxidation of Aqueous Solution. The conditions of the photooxidation experiments conducted are listed in Table 1. GLY and MG solutions were prepared in 1 L Table 1. Experimental Conditions photooxidation experiment 3 mM GLY 0.3 mM GLY 3 mM MG control experiments dark control GLY dark control MG H2O2 control GLY H2O2 control MG (NH4)2SO4 control photolysis of water H2O2 to water

GLY (mM) 3 0.3 3 GLY (mM) 3 0 3 0 3 0 0

MG (mM)

H2O2 (mM)

(NH4)2SO4 (mM)

light

0 0 0 MG (mM)

13 1.3 13 H2OH2O2 (mM)

0.02 0.02 0.02 (NH4)2SO4 (mM)

yes yes yes light

0 3 0 3 0 0 0

13 13 0 0 13 0 13

0.02 0.02 0.02 0.02 0 0.02 0.02

no no yes yes yes yes no

pyrex reaction bottles using 18 mΩ Milli-Q water as described by Lee et al.24 Because GLY and MG may exist in dimer or trimer forms in the concentrated GLY (Sigma-Aldrich, 40 wt % in water) and MG (Sigma-Aldrich, ∼60% in water) stock solutions, sufficient time (∼12 h) was given for them to reach their hydration equilibria in the reaction bottle under dark conditions prior to the experiments. For consistency, ammonium sulfate ((NH4)2SO4) (0.2 mM) was added to the solution because it was used as an internal standard for AMS measurement in our previous study.24 Because concentrated solutions of (NH4)2SO4 only react slowly with GLY and MG to form C−N and C−S compounds,19,20,30,38 it is assumed that a low concentration of (NH 4 ) 2 SO 4, 0.2 mM, does not significantly react with organics in the solution compared to OH radical reaction during our experimental time scale. The (NH4)2SO4 control experiment (Table 1), where GLY photooxidation was performed without addition of (NH4)2SO4, did not show major differences, confirming that (NH4)2SO4 does not significantly affect the photooxidation kinetics. Photooxidation of GLY was conducted in two concentration regimes, 3 and 0.3 mM, to investigate possible concentration 6255

RC(O)OH + I−·H2O → I−·RC(O)OH + H2O

(R1)

ROOH + I−·H2O → I−·ROOH + H2O

(R2)

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Figure 3. Pathways showing formation of hydroxyhydroperoxides.

RCHO + H2O ↔ RC(OH)2

(R3)

RC(OH)2 + I−·H2O → I−·RC(OH)2 + H2O

(R4)

background. The background values were subtracted from analyte signals. The linearity of the calibrations was generally excellent (R2 > 0.98), and the measurement uncertainty for most of the organic acids was determined to be ∼5% for a single run, except for pyruvic acid whose ionization efficiency drifted over time. We estimate its measurement uncertainty to be ∼15%. There is more uncertainty associated with GLY and MG quantification due to the sensitivity of their hydration equilibria to temperature and the chemical composition of the solution, making it more difficult to estimate the overall uncertainties. These uncertainties will be discussed in the TOC results section. The detection sensitivity of the analytes, in general, is constant during an experiment but varies to some degree between days. For this reason, a calibration was performed prior to each photooxidation experiment. The following species were quantified with this calibration method: GLY, MG, formic acid (FA) (Fluka, 50% in water), glyoxylic acid (GA) (Sigma Aldrich, 50 wt % in water), oxalic acid (OA) (Fisher Scientific, in dihydrate form), pyruvic acid (PA) (Sigma Aldrich, 98%), glycolic acid (GCA) (Sigma Aldrich, 99%), acetic acid (AA) (Fisher Scientific, 99.7%), malonic acid (MA) (Fisher Scientific, 99%), and succinic acid (SA) (Fisher Scientific). The detection limits of the analytes are generally below 0.02 mM, except for GLY and AA. Their detection limits are estimated to be 0.05 mM and 0.07 mM, respectively. 2.3. Offline TOC and Complementary IC Analysis. The total organic carbon (TOC) concentration was analyzed by an offline TOC analyzer (Shimadu TOC-VCPN) in a similar way as our previous studies.24,42 The TOC analyzer quantifies the total carbon concentration in the solution by converting all the carbon containing species into CO2 at a high temperature using an oxidation catalyst. The CO2 generated is detected by a nondispersive infrared detector. Meanwhile, the inorganic carbon concentration in the sample solution is quantified by converting inorganic carbon species into CO2 using HCl, followed by the same CO2 detection. Potassium hydrogen phthalate was used to calibrate the total carbon measurement,

Chemical ionization using I− is a soft ionization technique, causing minimal fragmentation of the parent molecules. Organic acids are ionized mostly via ligand transfer reaction from water clusters of I− (I−·H2O) (R1) and are detected at the mass to charge ratio (m/z) of the molecule plus that of I− (m/z = 127). Hydrogen peroxide and organic peroxides are also detected by the same ligand transfer reaction (R2). Aldehydes are not ionized by I− directly, but they form geminal diols via hydration reactions in the aqueous phase (R3). These geminal diols are ionized via the same ligand transfer reaction (R4); therefore, aldehydes can be detected at the m/z of the molecule plus 18 (from H2O) and 127 (from I−). For dialdehydes such as GLY, geminal diols at the two hydration steps were both detected. Herein, the terms GLY·1H2O and GLY·2H2O will be used to represent GLY monohydrate and dihydrate, respectively. GLY·1H2O was the dominant GLY peak detected by Aerosol CIMS, so that GLY·1H2O was used to estimate the GLY concentration in the solutions. Only one geminal diol (MG·1H2O) was detected for MG, and it is used to estimate MG concentration in the solution. The calibration of the aerosol CIMS was conducted by atomizing standard solutions with known concentrations of GLY, MG, and their major products using the same experimental settings mentioned above. This method enables the direct quantification of the target compounds, at the same time accounts for the effects of temperature, ionization efficiency, and water vapor that control the detection sensitivity of Aerosol CIMS. The raw signal of each species was normalized to the reagent ion (I− at m/z 127) to account for the effect of any changes in the number of reagent ions during the experiment. At the beginning of each experiment, Milli-Q water with 0.2 mM (NH4)2SO4 was atomized for 30−40 min to stabilize the volatilization line temperature and to obtain a 6256

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whereas sodium carbonate/sodium bicarbonate were used for the calibration of inorganic carbon. The difference between the measured total carbon and inorganic carbon is the TOC concentration. TOC samples were collected from the experimental solution every 30 min during the photooxidation. The samples were sealed, covered with aluminum foil, and refrigerated at 5 °C until analysis. The measurement uncertainty of the TOC analysis is 10%. Because the m/z of MG·1H2O overlaps with the m/z of OA, our CIMS with unit mass resolution could not distinguish these two compounds. Offline ion chromatography (IC) was employed to quantify OA in the MG photooxidation experiments to assist the aerosol CIMS data analysis. The analysis was conducted using a Dionex ICS 2000 system with AS19 anion exchange column under suppressed conductivity detection using an ASRS 300 conductivity suppressor. The eluent was potassium hydroxide (KOH) with a flow rate of 1 mL min−1. The eluent concentration gradient was programmed as by VandenBoer et al.43 The measurement uncertainty of the IC analysis was estimated to be within several percent. IC samples were collected from the photooxidation solution every hour and were analyzed on the same day of the experiment, keeping them sealed, covered with aluminum foil and refrigerated. A polynomial curve was fitted to the OA concentration time profile obtained from IC analysis, and this expression was used to back-calculate the aerosol CIMS signal responsible for OA. From there, the OA and MG·1H2O signals were separated, and the concentration profile of MG was obtained.

3. RESULTS AND DISCUSSIONS 3.1. Formation of Hydroxyhydroperoxides (HHPs) in Dark Control Experiments. Formation of a class of compounds was witnessed from the dark control experiment of GLY and MG, but not from addition of H2O2 into pure water. These compounds have m/z of the geminal diols of GLY or MG plus 127 (from I−) and 34, the m/z of a H2O2 molecule. We propose that these compounds are hydroxyhydroperoxides (HHPs) formed via reactions between H2O2 and GLY or MG (Figure 3). The mechanism of HHP formation is the nucleophilic attack of an O atom in H2O2 to the carbonyl group, followed by migration of one of the H atoms on H2O2 to the oxygen on the carbonyl group.44,45 HHP formation from reactions between H2O2 and carbonyls has been long recognized.35,36 In the case of the current study, H2O2 reacts with GLY and GLY·1H2O (R5 and R6, Figure 3) to form 2hydroxy-2-hydroperoxyethanal (HHPE) and its hydrated counterpart (HHPE·1H2O). For MG, H2O2 can react with either the aldehyde (R7) or the ketone (R8) functional group to form a pair of structural isomers: hydroxyhydroperoxyacetone (HHPA) and 2-hydroxy-2-hydroperoxypropanal (HHPP). Note that Aerosol CIMS was unable to separate these two structural isomers. MG·1H2O can also react with H2O2 (R9) to form hydrated HHPP (HHPP·1H2O). Formation of the four HHPs shown in Figure 3 has been confirmed using aerosol CIMS by observations of the signal increase at their corresponding m/z (Figure 3) in the dark control experiments. HHPE and HHPA/HHPP were the major HHP peaks observed. Figure 4a shows an example dark control experiment of 3 mM of GLY. Immediately after H2O2 was added to GLY solution at time (I), a significant increase of HHPE signal was detected until equilibrium was established in 40 min. At this

Figure 4. Formation of HHPs in dark control experiments. H2O2 (13 mM) was added to 3 mM of GLY (a) or 3 mM of MG (b) solutions at time (I), and quenched at time (II) by addition of catalase from bovine liver. HHPs formed sharply after the addition of H2O2 and reached equilibrium values approximately 40 min after the addition. After quenching of H2O2, HHPs decayed to zero and reversibly formed GLY or MG.

point, one-third of GLY has been consumed, indicating that approximately 1 mM of total HHP was present in the solution. At time (II), 2 drops of catalase from bovine liver (Sigma Aldrich, 28 mg protein/mL; 21600 units/mg protein) were added to the solution to quickly quench H2O2. It was observed that HHPE decayed away, with the GLY signal recovering. Similar behavior was observed for HHPA/HHPP formed from MG and H2O2, as shown in Figure 4b. These observations indicate that HHPs are equilibrium products that can regenerate GLY and MG upon quenching of H2O2. To further test the ubiquity of the formation of HHPs and their behavior, we performed the same experiment by adding 13 mM of H2O2 to a 3 mM aqueous solution of propionaldehyde. Formation of a peak at m/z 219 was confirmed, corresponding to 1hydroxypropylhydroperoxide, and this compound also reversibly produced propionaldehyde upon quenching of H2O2. This observation further confirms the HHP formation in our system and the generality of this reaction with aldehydes. From Figure 4, the formation and decomposition rate constants of HHPs can be roughly evaluated. The concentration of HHPs are estimated from the fraction of GLY or MG that decay upon addition of H2O2. The second-order rate constant of HHP formation from GLY/MG plus H2O2 is estimated from the initial slope of HHPE and HHPP/HHPA 6257

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formation (Figure 4a,b) to be 0.06 ± 0.03 M−1 s−1 for GLY plus H2O2 and 0.04 ± 0.02 M−1 s−1 for MG plus H2O2. The decomposition rate constants of HHPs forming H2O2 and GLY/MG are determined from the decay of HHPE and HHPP/HHPA signals after the quenching of H2O2 (Figure 4a,b). Both the HHPE and HHPP/HHPA signals showed firstorder decays, and the decomposition rate constants were determined to be 3 × 10−4 s−1 for HHPE and 1 × 10−3 s−1 for HHPP/HHPA. From the ratio of the formation and decomposition rate constants, the equilibrium constants were calculated to be 200 M−1 for HHPE and 40 M−1 for HHPP/HHPA, which are comparable to those that can be estimated from the equilibrium concentrations of HHP, H2O2, and GLY, i.e., roughly 40 M−1 for each. The disagreement in the equilibrium constants of HHPE (200 and 40 M−1) using the two calculation methods demonstrates uncertainties in the approaches used to determine these quantities. For example, a fraction of the HHPs is likely to decompose to re-form GLY and MG in the heated volatilization line. Also, the decay of the dicarbonyls leads to an estimate of the total concentration of the HHPs forming in each experiment, and not to speciated values. Further experiments need to be conducted to better determine these rate and equilibrium constants. Our previous work24 has inferred the formation of HHPs during photooxidation of GLY from the fragmentation pattern of AMS spectra. However, the current study presents the first direct detection of HHPs produced from GLY and MG in the aqueous phase. The fast response time of aerosol CIMS and its aqueous-phase detection mechanism using the volatilization line enabled the detection of HHPs, a class of compounds that are otherwise considered to be unstable and difficult to be detected with offline analytical methods. We are currently uncertain why others have not identified HHPs in their studies. 3.2. Photooxidation of GLY. Parts a and b of Figure 5 show the decay profiles of GLY and formation of its major products (FA, OA, GA, and HHPE) from the 3 and 0.3 mM concentration solutions. The organic acid data in the figure are the average of two to three experimental replicates, and the error bars account for the fluctuations of the concentration profiles across the experimental replicates. We are unable to easily quantify the HHP species at the moment, so that the HHPE signal from one of the replicates is shown in Figure 5. The H2O2 control experiments of GLY did not result in detection of any compounds nor in any observable decay of GLY, indicating that the results shown in Figure 5 are indeed due to OH driven oxidation. The concentration profiles of FA, GA, and OA agree with the mechanism proposed by Lim et al.,5 where FA and GA being the first generation products and OA a second generation product. The FA formation before the initiation of photooxidation (Figure 5a,b) is due to reaction of H2O2 with impurities in water, which is confirmed from the control experiment, where H2O2 was added to pure water. Although the formation of GA and OA in the two concentration regimes is proportional to the initial GLY concentrations, the formation of FA appears to be highly concentration dependent as shown by the significantly lower production in the 0.3 mM regime (Figure 5a,b), implying the existence of different reaction mechanisms between the two concentration regimes. We propose that the different behavior in the two concentration regimes is associated with the HHP formation. A recent computational study46 proposed that an acyl radical of HHPE

Figure 5. Results of photooxidation experiments with 3 mM (a) and 0.3 mM (b) initial GLY concentration. Photooxidation was initiated at time 0 (dashed line). Data shown here are the average of 2−3 replicates, and the error bars represent fluctuations between replicates (1 σ). The signal of 2-hydroxy-2-hydroperoxyethanal (HHPE) overlaps with that of hydrated GA (GA·1H2O). This normalized signal from one typical experiment is shown (right axis).

can decompose to form FA (Figure 6, R10). These mechanisms are proposed to occur in the gas phase in the original paper, but formation of acyl radicals due to H-abstraction reactions is common, and the subsequent decomposition may also occur in the aqueous phase. HHP formation in the 0.3 mM concentration regime is expected to be less important as implied from the expression for HHP equilibria (eq 1): KHHP = [HHP]/([aldehyde] × [H2O2 ])

(1)

where KHHP is the equilibrium constant for HHPs in general, and [HHP], [aldehyde], and [H2O2] are the equilibrium concentrations of HHP, aldehydes, and H2O2. Because the concentrations of GLY and H2O2 used in the 3 mM regime were both 1 order of magnitude higher than those used in the 0.3 mM experiment, eq 1 indicates that the HHPE concentration in the 3 mM case is expected to be 2 orders of magnitude higher. Indeed, the normalized signal of HHPE in the 3 mM concentration regime (Figure 5a) is significantly higher than that in the 0.3 mM regime (Figure 5b). If a significant fraction of FA has been produced from the HHP pathway, the much higher yield of FA in the 3 mM concentration regime can be explained. To further examine 6258

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Figure 6. Possible formation mechanisms of formic and acetic acids from HHPs (from da Silva46).

this possibility, we performed an experiment in which HHPE equilibrium was not allowed to establish prior to the initiation of photooxidation. With HHPE absent at the beginning of the photooxidation, FA should appear to be a second generation product. The result of this experiment (Figure 7) shows that formation of FA indeed exhibits characteristics of a second generation product, with slower formation immediately after the initiation of the photooxidation. Lee et al.24 also did not allow HHPE to equilibrate prior to the initiation of photooxidation and observed slow formation of FA at the early stage of photooxidation. This evidence supports the HHPE pathway of FA formation. By contrast, Lim et al.5 suggests that FA is directly produced from photooxidation of GLY. As described in the Experimental Methods, the concentration of GLY was estimated using the signal of GLY·1H2O. The decay of GLY in the 3 mM concentration regime was observed to be non-first-order and significantly slower than Tan et al.18 and Lee et al.24 We propose that the slow decay of GLY arises via the regeneration of GLY·1H2O from HHPE, as discussed in detail in the TOC section.

Figure 8. Concentration profiles of MG and its products. Photooxidation was initiated at time 0 (dashed line). The oxalic acid profile obtained from IC and its fitted line are shown on the graph. Using the fitted line, the MG concentration profile was calculated. The data represent the average of two independent replicates, with the error bars showing fluctuation between the replicates (1 σ). The normalized signal of 2-hydroxy-2-hydroperoxypropanal (HHPP) and hydroxyhydroperoxyacetone (HHPA) from one experiment is shown (right axis).

second-, and third-generation (or later) products, respectively, and that the yield of GA is low. Tan et al. found the quantification of AA challenging because of its small m/z (outside of the ESI-MS mass range) and the overlap of AA and GCA peaks in the IC. Here, we successfully quantified AA and GCA using aerosol CIMS. It is observed that GCA is only a minor product, whereas AA turns out to be the dominant product of MG photooxidation. Note that the yields of FA and AA are observed to be significantly higher than the model prediction of Tan et al.22 Direct photolysis of MG can, at least in part, give rise to the high yields of FA and AA. A significant amount of FA and AA formation was observed from a H2O2 control experiment of MG, as shown in Figure 9. Approximately 0.4 mM of FA and AA were produced by 2 h of direct MG photolysis. Tan et al.22 did not include direct photolysis of MG into the model due to the assumption that the effects of direct photolysis are small compared to photooxidation. This is likely to be true in their

Figure 7. Three mM GLY photooxidation without HHP equilibrium. The experiment was conducted as with the 3 mM GLY photooxidation, except that photooxidation was initiated immediately after H2O2 was added to the GLY solution at time 0. The error bars represent fluctuations between replicates (1 σ).

3.3. Photooxidation of MG. The time profiles of 3 mM MG photooxidation are shown in Figure 8. The data are the average of two experiments, and the error bars represent fluctuations between the replicates. The signal of HHPP plus HHPA from one experiment is also shown in the figure. The overall trend of the photooxidation agrees with the observation of Tan et al.22 in that PA, AA, and OA appear to be the first-, 6259

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Figure 9. H2O2 control experiment for MG. MG solution (3 mM) was exposed to irradiation without addition of H2O2. The irradiation was initiated at time 0 (dashed line). A significant amount of FA and AA was produced. The initial increase of the MG signal was due to equilibration of MG in the inlet line, and the initial signal of FA and AA are due to impurities in solution or due to decomposition of MG prior to the experiment.

Figure 10. Normalized signal of OA (blue) was obtained from the fitted line of IC data. The normalized signal of MG (yellow) was calculated by subtracting OA normalized signal from total signal of MG + OA (black) obtained from the Aerosol CIMS.

experimental setup where the OH radical concentration is 1 order of magnitude higher than in the current study, where we have observed that FA and AA formation via direct photolysis of MG may be important. In particular, the FA concentration profile shows rapid formation immediately after the initiation of photooxidation (Figure 9), further implying that this arises from direct photolysis of MG. The higher yield of AA may also arise by two additional formation pathways associated with HHP formation. The first pathway is PA plus H2O2, as suggested in Stefan et al.:35 H2O2 reacts with the ketone group of PA, forming a HHP intermediate which decomposes to form AA. The second pathway is the oxidation of HHPP as illustrated by R11 in Figure 6.46 H-abstraction of the aldehydic hydrogen of HHPP results in an acyl radical which may subsequently decompose to form AA. The formation of AA is thus complicated, and deconvolution of the relative contributions is difficult at present. As explained previously, OA was quantified by offline IC analysis in the MG photooxidation experiments. It is known that H2O2 reacts with organic species in the dark to cause secondary reactions prior to offline analysis, such as FA formation from GA plus H2O216,18 and AA formation from PA plus H2O2.22,35 However, OA is expected to be relatively stable against H2O2 reaction and volatilization loss due to its low volatility. For these reasons, the offline quantification of OA is expected to be more reliable compared to other organic acids. Indeed, the fitted line from the IC data matched within 10% error the MG·1H2O and OA signal from aerosol CIMS in the last hour of photooxidation (Figure 10). The excellent agreement between IC and Aerosol CIMS suggests that most of MG was depleted at the end of photooxidation, and the aerosol CIMS signal is solely attributed to OA at this point. The decay of MG is not first-order, and it appears that a fraction of MG is constantly regenerated during the photooxidation. This feature is similar to the GLY decay in the 3 mM regime (Figure 5a). We propose this observation is also associated with HHP formation. 3.4. TOC Concentration and Carbon Balance. The TOC concentrations from 3 mM GLY, 0.3 mM GLY, and 3 mM MG photooxidation experiments are shown in Figure 11a−c. The reconstructed TOC concentrations are calculated

Figure 11. Measured and reconstructed TOC concentration in 3 mM glyoxal (GLY) (a), 0.3 mM GLY (b), and 3 mM methylglyoxal (MG) photooxidation experiments. Photooxidation was initiated at time 0 (dashed line). The measured TOC shows the results from the offline TOC analyzer whereas the reconstructed TOC is calculated from the total of the quantified organic species (i.e., excluding HHPs and oligomers); see text. CIMS data represent the average of 2−3 independent experimental replicates, and the error bars represent fluctuations between the replicates (1 σ).

as the total of GLY or MG and their major products. Both the measured and reconstructed TOC profiles are the average of the experimental replicates. Note that the measured TOC data were not necessarily obtained from the same experimental replicates of the reconstructed TOC. As shown in the figures, the measured TOC concentrations constantly decrease, most likely due to formation of CO2 and volatile species during the photooxidation. The measured TOC profiles in the current study show excellent agreement with that reported in our previous TOC-AMS study,24 confirming the reproducibility of 6260

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perturbation in the hydration equilibria of GLY, making the estimation of total GLY concentration not straightforward. A question arises why Lee et al.24 underestimated the TOC at the early stage of photooxidation using coupled AMS and IC measurements, if HHPs are indeed converted to GLY or FA which are detectable by AMS and offline IC analysis. A possible explanation is that the AMS in Lee et al. mainly detected the GLY·2H2O form of GLY in solution. Indeed, in the current study, we found that if the total GLY concentration is estimated by using the GLY·2H2O signal instead of GLY·1H2O, the decay of GLY is found to be faster and resembles the GLY decay obtained in Lee et al. Also, Lee et al. employed a diffusion drier to dry the generated particle flow prior to online AMS analysis. Because nonhydrated GLY and GLY·1H2O are more volatile than GLY·2H2O, they may have partitioned into the gas phase during the drying process and may not have been detected by the AMS, leading to an underestimation of the TOC concentration. The arguments above lead to the conclusions that the missing carbon in Lee et al is likely to be the fraction of GLY·1H2O regenerated from HHPs in the photooxidation. 3.5. Evidence of Oligomer Formation. It is now evident that oligomers, including C−N and C−S species, are formed via photochemistry of GLY and MG.16−18,22,38,48,49 Lim et al.5 proposed that oligomerization becomes dominant when initial GLY concentrations exceed the mM level due to more active radical−radical reactions. From the 3 mM GLY photooxidation experiment, formation of malonic acid (MA) and succinic acids (SA) was observed using aerosol CIMS. MA, a C3 diacid, and SA, a C4 diacid, are considered to be oligomers formed during photooxidation. The observed formation of these two species was significantly lower than the other organic acids, and their signals are close to the detection limits: 0.002 mM for MA and 0.003 mM for SA. The time profiles of these two (Figure 13)

our experimental setup. In all the three experiments conducted, the reconstructed TOC profiles match the measured profiles fairly well (to within 20%), indicating that aerosol CIMS detects all the major compounds involved in the photooxidation. However, note that the reconstructed TOC of 3 mM GLY and MG (Figure 11a,c) underestimates the carbon concentration immediately after the initiation of the photooxidation but better matches with the measured TOC later on. This initial underestimation of carbon concentration was observed to be less obvious in the case of 0.3 mM GLY (Figure 11b). We propose that the initial underestimation of carbon concentration is due to HHPs, which are excluded from TOC reconstruction due to the inability to quantify these species. These species convert to compounds that are considered in the reconstructed TOC in the later period of photooxidation. It was observed that GLY·1H2O and GLY·2H2O show different decay profiles during photooxidation (Figure 12), with

Figure 12. Decay time profiles of GLY·1H2O and GLY·2H2O during one typical photooxidation experiment. The HHP equilibrium was fully established before the photooxidation was initiated at time 0 (dashed line).

that of GLY·1H2O slower than GLY·2H2O. This observation is rather surprising considering the rapid hydration equilibria of GLY in the aqueous solution.4 One possible explanation for this observation is that GLY·2H2O is more reactive to OH radicals because the bond dissociation energy of a C−H bond on a diol carbon is lower.47 Alternatively, a fraction of HHPs may regenerate GLY and MG in the solution during photooxidation. Under a condition in which aldehydes, H2O2 and HHPs are all being consumed, such as during photooxidation, eq 1 indicates that the equilibrium is more likely to shift toward the decomposition of HHP. In the case of GLY photooxidation, GLY·1H2O is directly regenerated from HHPE·1H2O via the reverse reaction of R6 (Figure 3), compensating a fraction of the GLY·1H2O decay. However, a significant amount of GLY·2H2O is assumed not to be directly regenerated because GLY·2H2O is not expected to effectively form HHP species (see Figure 3). We cannot exclude the possibility that this regeneration of GLY·1H2O also occurs in the heated volatilization line. The observed slow decay of MG is very likely due to the same regeneration of MG·1H2O from HHPs (R9). Other deviations between measured and reconstructed TOC may have been caused by the uncertainties associated with the quantification of GLY and MG. In particular, the different decay kinetics of GLY·1H2O and GLY·2H2O have caused

Figure 13. Formation of MA and SA in 3 mM GLY photooxidation.

acids imply that SA is formed first, and a fraction of it may have converted into MA during the photooxidation. This observation agrees with the aqueous-phase photooxidation mechanism of SA that is described in Gao and Abbatt.42 We believe that small concentrations of MA and SA at early times (Figure 13, from 0 to 25 min) are due to contamination carried over from the calibration. Oligomer formation was also observed in 3 mM MG photooxidation, but MA and SA peaks overlap with those of hydroxypyruvic acid17 and mesoxalic acid,22 respectively. Oligomer formation was not observed from the 0.3 mM GLY concentration regime. According to the simulation conducted by Lim et al.,5 the mass based yield of oligomers may be around 10% when 3 mM 6261

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systematically investigated to determine its effects upon partitioning of gases into aerosols in the environment. We are currently engaged in laboratory studies of this type.

initial GLY concentration is used. With the uncertainties in our observed and reconstructed TOC, we cannot rule out that 10% of the C resides in an oligomeric form that we do not observe. Also, it is possible that weakly bound oligomers decompose to smaller species in the heated transfer line leading to the Aerosol CIMS. However, succinic and malonic acids are present at low concentrations, representing roughly 1% of the mass. Because Lee et al.24 also did not observe significant oligomer formation, it is possible that the relatively low H2O2 photolysis efficiency in our system may lead to little oligomer formation; i.e., it is likely that we do not have sufficient OH radical concentration to bring the reaction into the regime in which radical−radical reactions, and therefore oligomer formation, are activated.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: 416-946-7359. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of NSERC and QEIISST. We thank Jennifer Murphy, Milos Marcovic, Greg Wentworth, and Philip Gregoire for IC support and Edgar Acosta and Shawna Gao for TOC support.

4. CONCLUSION AND ENVIRONMENTAL IMPLICATIONS Aerosol chemical ionization mass spectrometry (aerosol CIMS) with the I− reagent ion was used to monitor aqueous-phase photooxidation of glyoxal (GLY) and methylglyoxal (MG). The major organic acids produced from photooxidation matched those reported from previous studies. Oligomer formation was confirmed but was a minor contribution to the total organic carbon (TOC) concentration. The reconstructed TOC concentrations are in good agreement with directly measured values, indicating that the current method can simultaneously detect all the major species involved in the photooxidation of GLY and MG. The current study reports the first direct detection of hydroxyhydroperoxides (HHPs) from reactions between GLY and MG with H2O2 in the aqueous phase. Significant, rapid HHP formation occurred once H2O2 was added to the GLY or MG solutions in the dark. HHPs were determined to be equilibrium products and can reversibly regenerate GLY and MG upon H2O2 quenching. The equilibrium constants of the HHP equilibria with H2O2 and GLY/MG were calculated to be between 40 and 200 M−1. HHP formation has implications for laboratory studies, and perhaps for some situations in the ambient environment also. In cloud or fogwater, typical concentrations of GLY and MG are at the range of micromolar levels50 and the H 2O 2 concentration does not usually exceed 100 μM.51 Under these cloud and fogwater conditions, HHP formation is unlikely to be very active. In aerosol, however, Paulson and coworkers52−54 have reported unexpectedly high concentrations of H2O2. From filter sample analyses and model simulations, they estimated an average aqueous-phase H2O2 concentration of 70 mM in aerosol liquid water, nearly 3 orders of magnitude higher than the value predicted from the Henry’s Law partitioning of H2O2.53 It is possible that the unexpectedly high H2O2 concentrations resulted from decomposition of existing HHPs in the aerosol liquid water. Being equilibrium products, HHPs are expected to decompose and regenerate H2O2 upon dilution from the volume of aerosol liquid water to that of the filter extracts. The current study has shown that HHP formation occurs with aldehydes in general and has impacts on aqueous-phase chemistry, which are expected to be especially important in aqueous aerosols where concentrations of H2O2 and aldehydes are high. In particular, photooxidation of HHPs provides an additional formation pathway of FA and AA in addition to previously proposed mechanisms. HHPs may also lead to slower effective loss rates of GLY and MG by regeneration during the reactions. Finally, HHP formation needs to be more

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