Gas-phase water-mediated equilibrium between SPECIAL FEATURE methylglyoxal and its geminal diol Jessica L. Axsona,b, Kaito Takahashic, David O. De Haand, and Veronica Vaidaa,b,1 aDepartment of Chemistry and Biochemistry, and bCooperative Institute for Research in Environmental Sciences, Campus Box 215, University of Colorado, Boulder, CO 80309; cThe Institute of Atomic and Molecular Sciences, P.O. Box 23-166, Taipei, Taiwan 10617, Republic of China; and dDepartment of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego, CA 92110 Edited by Barbara J. Finlayson-Pitts, University of California, Irvine, CA, and approved December 15, 2009 (received for review October 20, 2009) In aqueous solution, aldehydes, and to a lesser extent ketones, mined computationally that the aldehydic C ¼ O is more favor- −1 hydrate to form geminal diols. We investigate the hydration of ably hydrated in solution (ΔG ¼ −1.4 kcal mol ) than the −1 methylglyoxal (MG) in the gas phase, a process not previously ketonic C ¼ O(ΔG ¼þ2.5 kcal mol ) (21). In solution, MG considered to occur in water-restricted environments. In this study, is present primarily as MGD (60% diol to 40% tetrol) with we spectroscopically identified methylglyoxal diol (MGD) and the aldehydic group forming a geminal diol (21, 23). MGD obtained the gas-phase partial pressures of MG and MGD. These has a lower vapor pressure than MG, which allows the molecule results, in conjunction with the relative humidity, were used to ob- to partition more easily into the particle phase, lending to the tain the equilibrium constant, KP, for the water-mediated hydration formation of SOA. of MG in the gas phase. The Gibbs free energy for this process, ΔG°, obtained as a result, suggests a larger than expected gas-phase diol O OH concentration. This may have significant implications for under- CH [R1] standing the role of organics in atmospheric chemistry. + HOH 3 OH 3CH O O hydration ∣ equilibrium constant ∣ water clusters CHEMISTRY tmospheric aerosols are a major topic of current atmospheric Although the hydration of small aldehydes in aqueous solu- Astudies, given the important, but not fully understood, role tions is known to be extensive, gas-phase hydration of carbonyls that aerosols play in the Earth’s radiative balance (1). Aerosols has not yet been considered in atmospheric models because it is affect global radiative forcing directly by absorbing or scattering commonly believed that there is not enough water present to radiation and indirectly by enhancing cloud albedo (1). Because make such reactions favorable. Gas-phase studies of glyoxylic of their changing chemical composition, aerosol optical and acid performed in our laboratory observed hydration of glyoxylic physiochemical properties vary, which greatly complicates the acid and allowed for the identification of the gem diol through IR quantification of their global radiative forcing. Efforts at model- spectroscopy, suggesting this chemistry can occur in water re- ing aerosol effects on climate, as assessed by the Intergovernmen- stricted environments (28). In a CCl4 matrix with restricted tal Panel on Climate Change (1), are primarily based on sulfate water present, similar results were obtained for the hydration aerosol studies, though there is growing evidence that organic of pyruvic acid. aerosols play an important role in climate change (2–4). Organic In this IR spectroscopic study, we identify and assign the vibra- molecules formed by the oxidation of biogenic and anthropogenic tional features of gas-phase MGD and characterize the water- mediated gas-phase equilibrium between MG and MGD. The organic emissions have been identified as important components K of atmospheric aerosols (2, 5). The formation pathways of these equilibrium constant, P, is calculated using spectroscopically de- secondary organic aerosols (SOA) remain highly speculative, termined concentrations of MG, MGD, and water. This study leading to uncertainties in predictions of atmospheric models shows gas-phase hydration to be significant even under relatively (6–8). A large SOA source is missing from models as illustrated dry environmental conditions. This affects the gas-phase/aqueous by simultaneous field measurements of volatile organic com- particle partitioning of MG and could provide insight into the pounds (VOCs) and aerosol particles (6, 7, 9, 10). discrepancy between measured and modeled amounts of SOA Organic acids, especially oxalic and pyruvic acid, are found in (29, 30). SOA, though the origin of these acids is not predicted correctly by Results and Discussion the gas-phase chemistry considered in models (11–14). Recent Spectral Identification of MG and MGD. Fundamental gas-phase studies suggest that these acids are made in the aqueous phase, spectra MG and MGD. The fundamental gas-phase IR spectrum particularly in cloud water, by the oxidation of aldehydes (e.g., of MG has been previously observed (31) and provides the basis glyoxal and methylglyoxal) with hydroxyl radicals and other aque- for determining the presence of MGD in this study (Fig. 1). In the ous radical species (14–17). Methylglyoxal (MG) is one of the lower energy region of the MG spectrum (Fig. 1A), from 1,050 to most abundant α-dicarbonyls present in the atmosphere and is −1 1; 500 cm , there are three distinguishable bands which are as- produced from VOCs of both biogenic and anthropogenic origin −1 signed to the CCC asymmetric stretch at 1; 240 cm (ν4), the CH (14, 18–20). −1 bend (ν6)at1; 373 cm , and the CH3 bends (ν7) in the 1,415 to Water-mediated aldehyde chemistry is expected to have impor- −1 1; 440 cm region (Table1). MG contains a ketonic and aldehydic tant consequences to the formation of SOA. For example, labora- −1 C ¼ O, which are present in the MG spectrum at 1; 723 cm (ν8) tory studies of MG hydration performed using theoretical (21) −1 and 1; 741 cm (ν9), respectively (Fig. 1A). The CH stretching and spectroscopic techniques (14, 22–24) suggest that in aqueous environments MG can become hydrated to form methylglyoxal diol (MGD) via reaction 1 and undergo further reactions to form Author contributions: J.L.A., D.O.D.H., and V.V. designed research; J.L.A. and K.T. performed oligomers (14, 21, 24). Like other aldehydes and ketones in research; J.L.A. and V.V. analyzed data; J.L.A. wrote the paper. aqueous solution, MG hydrates through proton addition to the The authors declare no conflict of interest. aldehyde carbonyl and reaction to form MGD (21–23, 25–27). This article is a PNAS Direct Submission. MG has both an aldehydic and ketonic C ¼ O and it was deter- 1To whom correspondence should be addressed: E-mail: [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.0912121107 PNAS Early Edition ∣ 1of6 Downloaded by guest on October 2, 2021 MGD features under very low relative humidities. In the absence of literature assignments, theoretical frequency and intensity cal- culations aided in assigning MGD vibrational modes (Table 1). The formation of MGD can be observed by the coincidence of the distinct MGD vibrational modes that appear in the lower energy region from 1,050 to 1; 500 cm−1 (Fig. 1B, Inset), the de- crease in the ν9 intensity relative to ν8 intensity, and the appear- −1 δ ance of OH stretching vibrations at 3; 505 cm (ν13) and −1 δ 3; 585 cm (ν14) (Fig. 1B) with the addition of water vapor. With water present, the CH stretching mode of MGD is observed at −1 δ 2; 895 cm (ν12). Because the lower energy region of the spectra from 1,050 to 1; 500 cm−1 becomes increasingly complex due to overlapping bands from the addition of water, the intensities of δ δ the ν8, ν9, ν13, and ν14 stretches were used to quantify and follow the formation of MGD. C ¼ O fundamental and overtone stretching region. With the addi- tion of trace amounts of water, the C ¼ O stretching region is altered by the decrease in intensity of ν9 relative to the ν8, pre- sumably due to the formation of MGD. To better understand these changes in the fundamental C ¼ O stretching region, the intensity and peak width (FWHM) of the fundamental and over- tone C ¼ O stretches for MG and MGD were modeled by two Lorenzian curves. The fundamental MG C ¼ O stretching region shows ν8 and ν9 with roughly the same intensity (Fig. 2A). As MG is hydrated to form MGD, the aldehydic C ¼ O is hydrated to form the gem diol, which can be observed spectroscopically by the decrease in the relative intensity of ν9 in Fig. 2B, intensity ν8∶ν9 ¼ 1.0∶0.75. In addition to the MG and MGD fundamental C ¼ O stretches near 1; 730 cm−1, there is a third peak around −1 δ 1; 780 cm (ν10), which is attributed to the diol because of its intensity increase mirroring the increase in partial pressure of 1; 050 −1 3; 600 −1 water.TheMGandMGDC¼ O first overtone region near Fig. 1. Fundamental gas-phase spectra from cm to cm of −1 −1 (A) MG and (B) MG and MGD. MGD frequencies are labeled with δ. 3; 450 cm has the ketonic C ¼ Oovertoneat3; 443 cm −1 (2ν8) and the aldehydic C ¼ O overtone at 3; 458 cm (2ν9). vibration of MG has been previously assigned (31) and occurs at In both MG and MGD spectra, the intensity of the first overtone −1 C ¼ O stretches drops by at least a factor of 10 from the intensity 2; 835 cm (ν11). The MG CH stretch is very prominent and was ¼ ¼ used to compare the MG and MGD spectra and to identify MGD. of the fundamental C O stretches. In the MG C O first over- −1 C 2ν ∶2ν ¼ The peak at 3; 450 cm , which normally corresponds to the OH tone spectrum in Figure 2 , the relative intensity of 8 9 1 00∶0 23 D 1 00∶0 17 stretching region, we assign to the first overtone, 2ν8 and 2ν9, of the .