Glyoxal-Methylglyoxal Cross-Reactions in Secondary Organic Aerosol

Jul 22, 2010 - Glyoxal (G) and methylglyoxal (MG) are potentially important secondary organic aerosol (SOA) precursors. Previous studies of SOA format...
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Environ. Sci. Technol. 2010, 44, 6174–6182

Glyoxal-Methylglyoxal Cross-Reactions in Secondary Organic Aerosol Formation ALLISON N. SCHWIER, NEHA SAREEN, DHRUV MITROO, ERICA L. SHAPIRO, AND V. FAYE MCNEILL* Department of Chemical Engineering, Columbia University, New York, New York 10027

Received April 17, 2010. Revised manuscript received June 28, 2010. Accepted July 6, 2010.

Glyoxal (G) and methylglyoxal (MG) are potentially important secondary organic aerosol (SOA) precursors. Previous studies of SOA formation by G and MG have focused on either species separately; however, G and MG typically coexist in the atmosphere. We studied the formation of secondary organic material in aqueous aerosol mimic mixtures containing G and MG with ammonium sulfate. We characterized the formation of light-absorbing products using UV-vis spectrophotometry. We found that absorption at 280 nm can be described well using models for the formation of light-absorbing products by G and MG in parallel. Pendant drop tensiometry measurements showed that surface tension depression by G and MG in these solutions can be modeled as a linear combination of the effects of G and MG alone. Product species were identified using chemical ionization mass spectrometry with a volatilization flow tube inlet (Aerosol CIMS). Peaks consistent with G-MG cross-reaction products were observed, accounting for a significant fraction of detected product mass, but most peaks could be attributed to self-reaction. We conclude that crossreactions contribute to SOA mass from uptake of G and MG, but they are not required to accurately model the effects of this process on aerosol surface tension or light absorption.

Introduction Organic matter is a ubiquitous component of atmospheric aerosols. Inorganic aerosols may acquire an organic component via in situ interactions with volatile organic compounds (VOCs), a family of processes known as secondary organic aerosol (SOA) formation (1). SOA formation is one of the greatest sources of uncertainty in estimations of aerosol forcing on the climate (1, 2). Glyoxal (G) and methylglyoxal (MG) are gas-phase oxidation products of many anthropogenic and biogenic volatile organic compounds (VOCs) (3-5). In several cases, including isoprene and toluene oxidation, G and MG are both oxidation products of the same VOC (3-5). G and MG have recently attracted much attention as potential SOA precursors (6-26). SOA formation by these volatile R-dicarbonyl species occurs via their uptake into the aqueous phase of an aerosol particle or cloud droplet, followed by aqueous-phase chemistry leading to the formation of low-volatility oligomer products. While such heterogeneous SOA formation pathways have been identified for G and MG separately, these species are * Corresponding author phone: (212) 854-2869; fax: (212) 8543054; e-mail: [email protected]. 6174

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unlikely to be found in isolation in the atmosphere (3-5). These highly reactive species resemble each other structurally and participate in similar self-oligomerization chemistry in aqueous systems (24, 25). Therefore, significant cross-reaction may occur when both G and MG are present. Crossoligomerization between G and MG could increase the expected SOA yield from these precursors. Additionally, we have shown that the secondary organic products formed by G and MG in the presence of NH4+ and H3O+ may change the optical properties of the aerosol and MG may affect the aerosol surface tension (24, 25). In order to evaluate the atmospheric significance of these observations, we must study the SOA formation chemistry of G and MG as it would most likely occur in nature: simultaneously in a mixed system. In this work, we studied the chemistry of G and MG together in aqueous aerosol mimics containing ammonium sulfate (AS). We characterized the formation of lightabsorbing secondary organic products using UV-vis spectrophotometry. We found that a model based on the formation kinetics and optical properties of self-reaction products is sufficient to describe the observed kinetics of formation of products absorbing at 280 nm. We also showed that the effects of G and MG on the surface tension of the solution are additive. Aerosol chemical ionization mass spectrometry (Aerosol CIMS) was used to characterize reaction mixtures, and while most peaks observed corresponded to G or MG self-reaction products, some peaks consistent with products of G-MG cross-reaction by aldol condensation or hemiacetal formation mechanisms were observed. We conclude that G-MG cross-reactions contribute to SOA mass yield from these precursors but that they need not be explicitly taken into account when modeling the effects of G and MG uptake on aerosol surface tension or light absorption.

Methods Solutions were prepared using Millipore water and a total mixed organic (G + MG) concentration ranging from 0-2.0 M. Three different molar ratios of G/MG were tested: 3:1, 1:1, and 1:3. Solutions were prepared from 40 wt % aqueous stock solutions of each organic (Fisher Scientific, Acros Organics). High, near-saturation salt concentrations (3.1 M ammonium sulfate (AS)) were used in most of the reaction mixtures studied in order to mimic the composition of aqueous atmospheric aerosol particles (24, 25). The AS concentration was varied from 0 to 3.1 M in order to probe the kinetics of the reactions of G and MG with the ammonium ion. The pH of each solution was tested using an Accumet model 15 pH/ conductivity meter (Fisher Scientific). The pH of these solutions with no buffering or addition of acid ranged from pH ) 2.7((0.1) to 4.9((0.1) depending on the amount of organics present. These pH values all fall within the range of pH relevant to tropospheric aerosols (27, 28). The acidity of these mixtures is due to trace impurities of pyruvic acid and glyoxylic acid in the stock solutions (24). In experiments where pH was varied, the desired pH was obtained by adding dilute HNO3 to the reaction mixtures. All solutions were prepared in 100 mL Pyrex volumetric flasks, were sampled at ambient temperature and pressure, and were not protected from ambient light. Exposure to light was not observed to affect the formation of light-absorbing products in the G-AS and MG-AS systems (24, 25). UV-Visible Spectrophotometry. The UV-vis absorption spectrum of samples was measured using an HP 8453 UV-visible spectrophotometer. A 10 mm open-top quartz cuvette (Agilent Technologies) was used. 10.1021/es101225q

 2010 American Chemical Society

Published on Web 07/22/2010

Surface Tension Measurements. Surface tension was measured using pendant drop tensiometry (PDT) as described in Shapiro et al. and Sareen et al. (24, 25). Droplets (40-50 µL) of the reaction mixtures were suspended from the tip of a glass capillary tube and allowed to equilibrate before an image was captured of the drop. The surface tension of the solution was calculated based on the drop shape according to σ)

∆Fgde2 H

(1)

where σ is the surface tension, ∆F is the difference of the density between the solution and the gas phase, g is acceleration due to gravity, de is the equatorial diameter of the drop, and H is a shape factor. The drop shape analysis procedure used to calculate de and H has been described previously (24, 25). Density measurements of the reaction mixtures were performed using an analytical balance (Denver Instruments). Aerosol CIMS. A custom built chemical ionization mass spectrometer with a volatilization flow tube inlet (Aerosol CIMS) was used to detect secondary organic material as described by Sareen et al. (24). Briefly, a reaction mixture of 2 M organics (G/MG 1:1) and 3.1 M AS was prepared as described above. After roughly 24 h reaction time, the mixture was diluted with Millipore water until the salt concentration was 0.2 M. The dilute solution was aerosolized using a constant-output atomizer (TSI), and the resulting aerosol stream was combined with dry N2 to achieve a relative humidity of 50-60%. On the basis of our previous observations of the MG-AS system, we expect oligomer formation to be irreversible upon dilution over the experimental time scale (24). Additionally, the time the solutions spent in aerosol form in our experiments was short (e3.5 s) compared to the time scale of the oligomerization reactions. Hence, the reaction products detected via Aerosol CIMS were most likely formed during the first 24 h after mixing. The equilibrated aerosol stream was sent through a poly(tetrafluoroethylene) (PTFE) tube heated to 135 °C for volatilization of the aerosol organics. The organic analyte molecules were detected following interaction with a reagent ion, either I- or H3O+ · (H2O)n, using a quadrupole mass spectrometer (Extrel CMS). In the I- detection scheme, the analyte molecule is detected as a cluster with I- or is ionized via proton abstraction. In the H3O+ · (H2O)n detection scheme, the reagent ion reacts through either proton transfer or ligand switching with the entering neutral species and is detected as either a protonated analyte molecule (RH+) or as a cluster with H3O+. These reagent ions are paired to utilize their complementary capabilities; I- forms clusters with organic acids, hemiacetals, and some aldol condensation products (24, 29), while H3O+ · (H2O)n is less selective and will ionize most organic molecules (30, 31).

Results and Discussion Similar to our previous observations of G and MG reacting separately in aqueous aerosol mimics containing ammonium salts, we observed that aqueous mixtures of G, MG, and AS changed color, becoming darker with time. The results of our characterization of this reactive system using UV-vis spectrophotometry, pendant drop tensiometry, and Aerosol CIMS are detailed in the following sections. UV-Visible Absorption and Kinetics. Figure 1 shows UV-vis absorption spectra of solutions containing 3.1 M AS and varying initial concentrations of organics (G/MG 1:1) 24 h after mixing. As we observed previously for the G-AS and MG-AS systems, a peak at ∼280 nm increases in magnitude with increasing total organic concentration

FIGURE 1. UV-vis spectra of aqueous solutions containing 3.1 M (NH4)2SO4 and varying total organic concentration (G/MG 1:1). Spectra were measured 24 h after mixing.

FIGURE 2. Time dependence of light absorption at (A) 280 nm and (B) 345 nm for solutions containing 3.1 M AS, 0.05 M total organics (G/MG 1:1), and pH ) 3.9. The curve shown in panel A is from the mathematical model described by eqs 2-6. See the text for details. (24, 25). A shoulder at 345 nm also appears at high organic concentrations (g0.03 M) after a delay of ∼10 h, consistent with our observations of the G-AS system (25). Figure 2 shows the time evolution of the peaks at 280 and 345 nm for an aqueous solution containing 0.05 M total organic concentration (G/MG 1:1) and 3.1 M AS. As we showed previously using density functional theory, the product species absorbing at 280 nm are likely to include a mixture of (hemi)acetal oligomers and aldol condensation products. Aldol condensation products and N-substituted species may absorb at 345 nm (24, 25). Additional experiments were performed to test the effects of varying the G/MG ratio, pH, AS concentration, and initial total organic concentration (Figure 3). The absorbance at VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of experimental variables on light absorption of aqueous aerosol mimics at 280 nm: (A) varying G/MG ratio, with 50 mM total organics, 3.1 M AS, and pH ) 3.9((0.2); (B) 1 mM total organics (G/MG 1:1), 3.1 M AS, and varying pH; (C) 32.4 mM total organics (G/MG 1:1) and varying initial concentrations of AS, pH ) 4.0((0.2); (D) 3.1 M AS, pH ) 4.1((0.1), and varying initial concentrations of total organics (G/MG 1:1). All measurements were made 3 h after mixing, except the data in panel A, which were obtained 1 h after mixing. The error bars reflect uncertainty in the absorbance based on variation in the baseline signal. The curves shown are from the mathematical model described by eqs 2-6. See the text for details.

FIGURE 4. Pendant drop tensiometry results. Surface tension is shown for (A) aqueous solutions with and without 3.1 M (NH4)2SO4, with G/MG 1:1 at various initial concentrations, and (B) for 3.1 M (NH4)2SO4 and a total organic concentration of 0-2 M, with varying ratios of G/MG, plotted as a function of MG concentration. The curves in panel A are weighted nonlinear least-squares fits to the data using the Szyszkowski-Langmuir equation (eq 7). MG-only data from Sareen et al. (24) along with the Szyszkowski-Langmuir curve and confidence intervals from fits to that data are shown for reference in panel B. Each point represents the weighted average of five to eight measurements, taken at 24 h after mixing, and the error bars indicate the standard deviation. 280 nm increases linearly with a decreasing G/MG ratio. This is consistent with our observations that the reaction products formed in the MG-AS system absorb more strongly at 280 nm than those in the G-AS system (24). It also suggests that the absorption of G-MG cross products at 280 nm can be represented by a linear combination of the absorptions of G-G and MG-MG self-reaction products. G-MG cross-reaction products are expected to have similar molecular structures to the self-reaction products. Therefore, they should absorb light at similar wavelengths (24, 25). Density functional theory 6176

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calculations confirm that an example aldol condensation cross-product absorbs at ∼280 nm with an oscillator strength f > 0.2 (see the Supporting Information for details). Absorbance at 280 nm also varied linearly with total organic concentration, pH, and AS content. This suggests that the protonation of the organic reactants by NH4+ or H3O+ is the rate-limiting step in the formation of the products responsible for the absorbance at 280 nm (24). Both aldol condensation and acetal formation are initiated by protonation of the carbonyl.

TABLE 1. Szyszkowski-Langmuir Fit Parameters mixture

σ0 (dyn cm-1)

a (dyn cm-1 K-1)

b (kg water (mol C)-1)

G-MG (H2O, no salt) G-MG (3.1 M AS) MG (3.1 M AS) (24) G (3.1 M AS) (25)

72.0 78.5 78.5 78.5

0.023 ( 0.003 0.0189 ( 0.0006 0.0185 ( 0.0008 0.00

2(1 83 ( 13 140 ( 34 0.00

TABLE 2. Proposed Peak Assignments for Aerosol CIMS Mass Spectra Obtained Using I- as the Reagent Iona

a

Reported mass uncertainty reflects the peak width at half maximum at 126.9 amu. *, reported by Sareen et al. (24).

We modeled the kinetics of the formation of lightabsorbing products. The modeling effort was focused on absorption at 280 nm since this was the only significant peak observed at atmospherically relevant organic concentrations and time scales. The rate of formation of absorbing products was modeled based on the rate-limiting initiation step: the protonation of G or MG. The linear dependence of absorption at 280 nm on the G/MG ratio as shown in Figure 3a was used to create the overall rate equation as follows: dAbs d[G] d[MG] l ) - εG + εMG dt dt dt

(

)

(2)

where Abs is the absorbance at 280 nm, εG and εΜG are the average molar absorptivities of products formed by the self-reactions of G and MG at 280 nm, respectively, and

l is the path length. The time evolution of the reactants was described by the following equations: d[MG] ) -kAII[MG][NH4+] - kBII[MG][H3O+] dt

(3)

d[G] II [G][H3O+] ) -kIIC[G][NH4+] - kD dt

(4)

d[NH4+] ) -kAII[MG][NH4+] - kIIC[G][NH4+] dt

(5)

d[H3O+] II [G][H3O+] ) -kBII[MG][H3O+] - kD dt

(6)

Here, kAII and kCII are the second-order rate constants for the protonation of G and MG, respectively, by NH4+. VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Proposed Peak Assignments for Aerosol CIMS Mass Spectra with H3O+ · (H2O)n as the Reagent Iona

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TABLE 3. Continued

a Reported mass uncertainty reflects the peak width at half maximum at 55.1 amu. *, reported by Sareen et al. (24); **, reported by Galloway et al. (22); ***, reported by Liggio et al. (9) and Loeffler et al. (40).

II Similarly, kBII and kD are the rate constants for the protonation of G and MG by H3O+. Sareen et al. reported the following parameters for the MG-AS system: εΜG g 4938 L mol-1 cm-1, kAII ) 5 × 10-6 M-1 min-1, kBII e 10-3 M-1 min-1 (24). Shapiro et al. reported εG g 15 L mol-1 cm-1 and kCII e 2.5 × 10-4 M-1 min-1 for G-AS (25). An additional II G-AS experiment was performed in order to constrain kD (see the Supporting Information for details). It was found II that εG g 681 L mol-1 cm-1 and kD e 0.60 M-1 min-1. We used POLYMATH 6.10 (Polymath Software) to numerically integrate eqs 2-6 and found that the kinetic model describes the data in Figures 2 and 3 well when values of kAII ) 5 × 10-6 M-1 min-1, kBII ) 10-3 M-1 min-1, kCII ) 3.25 II × 10-5 M-1 min-1, kD ) 0.15 M-1 min-1, εG ) 750 L mol-1 cm-1 and εΜG ) 7500 L mol-1 cm-1 are used. Fit statistics are provided in the Supporting Information. This observation that the formation of light-absorbing products in the mixed G-MG system can be modeled without explicitly accounting for cross-reactions simplifies the future representation of this process in atmospheric chemistry and climate models. This does not imply that cross-products do not contribute to the observed light absorption but rather that their formation mechanisms share a common rate-limiting step with the self-reaction products (protonation of the carbonyl), and their absorptivity at 280 nm can be modeled as a linear combination of those of the self-reaction products. Surface Tension. Results of surface tension measurements of solutions containing a 1:1 mixture of G and MG is shown in Figure 4A. Surface tension decreases with increasing total organic content. The effect is enhanced when AS is also present in the solution, probably due to

“salting-out” (24, 32). The observed surface tension depression can be described by the Szyszkowski-Langmuir equation (33, 34): σ ) σ0 - aT ln(1 + bC)

(7)

where σ and σ0 are the surface tension of the solution with and without the addition of organics, respectively, T is temperature (in K), C is the carbon content of the mixture (in mol carbon/kg water), and a and b are fit parameters for the organic compounds. Values of σ0 were taken from the International Critical Tables (2003). Fit parameters obtained using eq 7 are listed in Table 1. Experiments shown in Figure 4B were performed at 2 and 0.05 M total organics in 3.1 M AS, with ratios of G/MG varying from 3:1 to 1:3. The data are plotted as a function of MG content for comparison with our previous study of the MGAS system (24). It is apparent that the surface tension depression in the mixed G-MG system is determined only by MG content and is not affected by the presence of G (25). G and its self-oligomerization products were not observed to depress the surface tension in aqueous AS (25). Therefore, surface tension depression in the G-MG system can be described by an additive model such as that of Henning et al. for nonreacting organic mixtures (34). Aerosol CIMS. Aerosol CIMS proposed peak assignments and mass spectra for the G-MG/AS system are shown in Tables 2 and 3 and Figure 5. Both detection schemes (H3O+ · (H2O)n and I-) show products of G-G and MG-MG self-reaction. Peaks consistent with products of G-MG crossreaction via aldol condensation and hemiacetal pathways were also observed. In control experiments, aqueous soluVOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Aerosol CIMS spectra of aerosolized solutions containing G/MG 1:1 in 3.1 M AS. See the text for details of sample preparation and analysis. (A) Negative-ion mass spectrum obtained using I- as the reagent ion. (B) Positive-ion mass spectrum. H3O+ · (H2O)n was the reagent ion. tions containing G and MG were tested and show observable signal at many of the peaks observed in the G-MG/AS spectra. This is consistent with the notion that a major role of NH4+ in this system is to promote the formation of oligomers that may also be present (in lower quantities) in the absence of salt (24, 35-38). Data from the control experiments are available in the Supporting Information. (a). Negative Ion Detection with I-. The mass spectrum shown in Figure 5A was obtained using I- as the reagent ion. The use of Aerosol CIMS with I- as a reagent ion for the detection of organic acids, acetal oligomers, and aldol condensation products has been described previously (24, 29). Most of the peaks in the mass spectrum, including those at 217.1, 225.0, 275.1, and 289.5 amu, correspond to masses of MG self-reaction products (24). Peak assignments are listed in Table 2. Those compounds include hemiacetal oligomers and aldol condensation products formed by MG self-reaction. For detailed discussion of the MG self-oligomerization compounds, the reader is referred to Sareen et al. (24). The peak at 203.1 amu is consistent with monohydrated G, I- · C2H4O3. The remaining major peaks are attributed to products of G self-reactions and G-MG cross-reactions. Each of the proposed products features multiple hydroxyl groups and therefore is expected to form clusters with I- (24). The peak at 260.8 amu corresponds to either I- · C4H6O5, a G selfreaction hemiacetal dimer (9, 20, 22, 26, 39), or I- · C5H10O4. The molecular formula C5H10O4 is consistent with a G-MG dimer, but its formation mechanism is unknown. The peaks 6180

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observed at 275.1 and 293.1 amu are consistent with ion formulas of I- · C5H8O5, I- · C6H12O4, and I- · C5H10O6. Multiple species with these molecular formulas may be formed via MG-MG self-reaction or G-MG cross-reaction via aldol condensation or hemiacetal pathways; possible structures can be seen in Table 2. (b). Positive Ion Detection with H3O+ · (H2O)n. The mass spectrum in Figure 5B was obtained using H3O+ · (H2O)n as the reagent ion. Many peaks in this spectrum including 125.9, 143.8, 163.1, 165.1, 167.1, 181.2, and 235.3 amu were observed previously in the MG-AS system (24). Peak assignments and possible structures are listed in Table 3. A detailed discussion of MG self-oligomerization chemistry is given in Sareen et al. (24). Peaks analogous to those observed using I- as the reagent ion were detected, along with several new peaks; this is consistent with the notion that I- is a more selective reagent than H3O+ · (H2O)n. The peaks at 77.0 and 95.0 amu correspond to the G monomer and hydrates or water clusters thereof. A nitrogen-substituted MG-G cross-reaction product with the molecular formula C5H5O2N could yield the peaks observed at 112.0 and 130.0 amu. Unresolved signal was observed in the mass spectra of both the MG-G/AS solution and the control solution between 114 and 116 amu. 1Himidazole-2-carboxaldehyde, a product of G self-reaction in the presence of NH4+ reported by Galloway et al. (22) would appear at 115.1 amu; significant signal is observed at that mass. The peak at 115.1 amu is also consistent with an ion formula of C5H7O3+, corresponding to a MG-G reaction

product of unknown formation mechanism. A peak is observed at 116.1 amu, which could correspond to an N-substituted G-G dimer with ion formula C4H6O3N+; the cluster of this species with H2O would appear at 134.1 amu. Because of the unresolved signal at these masses in the control spectrum, for which no NH4+ was present, it is not possible to make a conclusive identification of these N-substituted species. Peaks at 149.1, 167.1, and 185.3 amu correspond to either cross-reaction products of G and MG via aldol condensation and hemiacetal pathways or MG-MG selfreaction products of unknown formation mechanism and structure. The peaks at 153.0 and 229.1 amu correspond to G-G hemiacetal oligomers with the ion formulas C4H9O6+ and C6H11O8+ · H2O, respectively. The peak at 193.1 amu is consistent with an ion formula of C6H7O6+ · H2O or C6H9O7+; this formula could correspond to a MG dimer or a G trimer (25). The peak at 221.3 amu is consistent with a mixed MG-G trimer (C8H13O7+) or a MG trimer (C9H17O6+). (c). Cross-Reaction Products. Analysis of five Aerosol CIMS mass spectra obtained using the H3O+ · (H2O)n detection scheme shows that cross-reaction products make up 20((6)% to 55((5)% of detected product mass. The lower bound was calculated assuming that masses consistent with either selfreaction or cross-reaction products are self-reaction products; for the upper bound, these peaks were counted as crossreaction products. Details of the calculation are provided in the Supporting Information. On the basis of this result, we conclude that cross-reaction products of G and MG contribute to SOA mass formed when both species are present in the aqueous phase.

Acknowledgments This work was funded by the NASA Tropospheric Chemistry program (Grant NNX09AF26G), the NSF BRIGE program (Grant EEC-0823847), and the ACS Petroleum Research Fund (Grant 48788-DN14). The authors gratefully acknowledge the Koberstein group for use of the pendant drop tensiometer.

Supporting Information Available UV-vis data for G/AS as a function of pH and the associated kinetic analysis, description and results of density functional theory calculations, model fit statistics, Aerosol CIMS control spectra, and details of the calculation of the detected product mass attributable to cross-reactions. This material is available free of charge via the Internet at http://pubs.acs.org.

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