ARTICLE pubs.acs.org/est
Hydroxycarboxylic Acid-Derived Organosulfates: Synthesis, Stability, and Quantification in Ambient Aerosol Corey N. Olson,† Melissa M. Galloway,† Ge Yu,† Curtis J. Hedman,‡ Matthew R. Lockett,†,|| Tehshik Yoon,† Elizabeth A. Stone,§,^ Lloyd M. Smith,† and Frank N. Keutsch*,† †
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States Wisconsin State Laboratory of Hygiene, Environmental Health Division, Madison, Wisconsin 53718, United States § Environmental Chemistry and Technology, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States ‡
bS Supporting Information ABSTRACT: Organosulfates have been proposed as contributors to aerosol growth and have been detected in both chamber and atmospheric aerosol samples. We present a simple method for the synthesis of quantitative analytical standards of two small hydroxycarboxylic acidderived organosulfates, glycolic and lactic acid sulfate. Additionally, we discuss the stability of hydroxycarboxylic acid-derived organosulfates and their previously proposed sulfate hemiacetal isomers in commonly used solvents for filter extraction. The hydroxycarboxylic acid-derived organosulfates were found to be stable under acidic conditions comparable to those found in ambient aerosol. By using synthesized standards, quantitative organosulfate concentrations were measured from ambient particulate matter (PM2.5) collected in urban locations in the United States, Mexico City, and Pakistan. Lactic acid sulfate and glycolic acid sulfate concentrations ranged 0.4 3.8 ng/m3 and 1.9 11.3 ng/m3, respectively. We propose that glycolic acid sulfate represents an important tracer for atmospheric processes that form organosulfates in ambient particulate matter.
’ INTRODUCTION Secondary organic aerosol (SOA) can be formed by the partitioning of oxidized volatile organic compounds (VOCs) into aerosol, which can subsequently be processed within the condensed phase.1 3 Models constructed solely on vapor pressurebased partitioning theory4 6 underestimate the amount of ambient SOA,3 leading to the recent proposal of additional sources of SOA. One such pathway involves the uptake of small molecules (e.g., (methyl)glyoxal, glycolaldehyde, or isoprene-derived epoxides) followed by chemical processing within the aerosol,2,7 14 which is not accounted for in purely vapor pressure-based models. Such aerosol processing increases aerosol yields, as it can transform fairly volatile compounds into lower volatility oligomers or organosulfates. We must both identify and quantify the products arising from different SOA formation pathways to assess their relative importance to ambient SOA. Organosulfate formation can increase SOA yields, as they result from aerosol processing and have been observed in ambient and laboratory generated-SOA.15 27 Analysis of SOA by liquid chromatography (LC) coupled with electrosprayionization mass spectrometry (ESI-MS) has provided evidence for the presence of numerous organosulfates.16,19 21 Although high-resolution time-of-flight (TOF) MS provides exact chemical formulas, the unambiguous structural identification and quantification of these molecular species has been hampered r 2011 American Chemical Society
due to a lack of organosulfate standards. Quantitative standards greatly aid the reliability of LC-ESI-MS quantification and identification, as they provide the exact mass of the species as well as its fragmentation pattern and elution time, which in turn reflects the molecular structure. A very limited number of studies have used such quantitative standards to determine organosulfate concentrations in aerosol, leading to little quantitative data for their absolute contribution to SOA. Studies using a laboratorysynthesized β-pinene-derived organosulfate17,18 demonstrated the concentration of β-pinene-derived organosulfates, ranging up to 23 ng/m3, may be higher than the sum of the known Rpinene oxidation products in ambient SOA from a spruce forest. Chan et al.28 used a laboratory-synthesized analogue to show that the concentration of isoprene-derived 2-methyltetrol organosulfates was between 5 and 65 ng/m3 in both downtown Atlanta and a rural location in Georgia. Additional studies have used methods to measure the functional groups and elemental composition of organic aerosol to estimate the quantitative contribution of organosulfates.25,27 These studies provide insight into the Received: March 28, 2011 Accepted: June 17, 2011 Revised: June 10, 2011 Published: June 17, 2011 6468
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Scheme 1. Reaction Scheme and Conditions Used for Synthesis of Hydroxy Carboxylic Acid Sulfates
quantitative contribution of organosulfates to SOA and highlight the need for additional standards. In addition to the quantification of organosulfates in ambient aerosol, the use of standards in conjunction with LC separation can distinguish the chemical structures of ambient organosulfates observed with ESI-MS. The work by Galloway et al.23 highlights the importance of using authentic standards in conjunction with LC-MS analysis to unambiguously assign chemical structures to ambient organosulfates. The study of glyoxal uptake under irradiated conditions showed that the organosulfate C2H3SO6 , which had been assigned as glyoxal sulfate in previous chamber and ambient studies,19 21 in fact corresponded to glycolic acid sulfate (GAS). The assignment of GAS was unambiguous due to the combination of a qualitative, laboratory-synthesized standard and UPLC-ESI-TOF-MS, which showed the exact mass and the elution time for both the standard and the aerosol C2H3SO6 species. Organosulfate identification and quantification is useful for providing information on the sources of both organosulfates and SOA. The specific conditions required for organosulfate formation could provide insight into both aerosol processing and SOA sources. Minerath et al.29 have shown that the acid-catalyzed reaction of inorganic sulfate with alcohol groups is not an important pathway for normal tropospheric aerosol conditions, whereas organosulfate formation via reaction of isoprene-derived epoxides with inorganic sulfate has been shown to be feasible.30,31 The photochemical formation of organosulfates via OH radicals, sulfate, and either alkenes14 or glycolaldehyde32 has been demonstrated. In this work, we aim to extend the range of the available quantitative organosulfate standards, characterize their stability, and present their observed concentrations in ambient aerosol.
’ EXPERIMENTAL SECTION L(+)-Lactic acid (95% Sigma), glycolic acid (99% Sigma Aldrich), N,N-diisopropylethylamine (DIEA, 99.5% Anaspec, Inc.), chlorosulfonic acid (99% Sigma-Aldrich), acetonitrile (ACN, anhydrous, 99.8% Sigma Aldrich), and dichloroacetic acid (ReagentPlus g99% Sigma Aldrich) were used as purchased without further purification. The glycolic acid sulfate standard was prepared using the same methods as Galloway et al.,23 with the addition of DIEA, a base with low nucleophilicity, to achieve less acidic conditions (see Scheme 1) and cooling for increased purity and stability based on the general method described by Ramdahl et al.33 Glycolic acid (0.761 g) or lactic acid (0.901 g) and 2.00 mL of DIEA were added to 50 mL of ACN. The solution was cooled to 0 C, and 0.731 mL of chlorosulfonic acid was added dropwise over 15 min. See Table S1 of the Supporting Information for the reaction components. The solution reacted on ice for three hours, and then, the ACN was removed by rotary evaporation.
Figure 1. 1H NMR (A) and 13C NMR (B) spectra of synthesized GAS. The organosulfate product has a 1H chemical shift of 4.4 ppm and 13C chemical shifts of 63 ppm and 171 ppm (carboxylic acid carbon). Impurities resulting from synthesis include acetonitrile (2.006 ppm), water (4.8 ppm), and N,N-diisopropylethylamine (1.29, 3.13, and 3.64 ppm). No measurable starting material, glycolic acid (4.0 ppm), is observed.
The molecular masses present in the product solutions were determined with ESI-MS by directly infusing the diluted product solution (1/10000 in 80:20 ACN/H2O) into a MicrOTOF timeof-flight mass spectrometer (Bruker Daltonics, Billerica, MA). Mass scans were performed in negative ionization mode, with a potential difference of 4500 V between the spray tip and the inlet, using 0.4 bar N2 nebulizer gas and 3.5 L/min N2 drying gas at 200 C. The flow rate of the product solutions was 4 μL/min. The product components were also identified with 1H NMR (see Figure 1), which showed the presence of the desired organosulfate (Table 1 and Table S2 of the Supporting Information) as well as base precursor DIEA and less than 10% of the initial ACN, and 13C NMR using a Varian MercuryPlus 300 MHz spectrometer with D2O as solvent. Sixteen scans were acquired for 1H spectra and 256 for 13C spectra; relaxation delays of 25 and 5 s were used for 1H and 13C spectra, respectively. To investigate the stability of the hydroxycarboxylic acid sulfates with respect to pH, solutions of GAS and dichloroacetic acid were prepared in D2O. Dichloracetic acid has a single, pHdependent 1H NMR peak that does not interfere with the peaks of interest for quantification34,35 and is ideal for use as a pH indicator with the small sample sizes in this work. Dichloroacetic acid concentrations were varied to control the pH between 0.9 and 3.0 in order to mimic ambient aerosol pH levels. 1H NMR spectra were taken both within 24 h after the solutions were prepared and 7 days later to determine product stability. In addition, solutions were prepared in water and methanol/water mixtures (0.18 M in 1:1 methanol/D2O), reflecting common filter extraction conditions, to further analyze the stability of these compounds over longer time scales by 1H NMR. Fine particulate matter (PM2.5) samples from Mexico City (MILAGRO, March 2006 T0, T1 sites),36 Riverside, CA (SOAR, 6469
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Table 1. Organosulfate Standards Developed in This Studya
a
Molecular weights were determined using high-resolution mass spectrometry and structures were confirmed by 1H NMR and 13C NMR spectroscopy.
July 2005),37 Cleveland, OH (LADCO, July 2007),38 and Lahore, Pakistan (ABC, November 2007)39 were collected as described in the references. At the Bakersfield, CA site (CalNexSJV, June 2010), PM2.5 was collected using three URG-2000B sampling devices. URG 10 μm 16.7 LPM cyclones were used to achieve a 2.5 μm cutoff size at a sampling rate of 92 L/min1. PM2.5 was collected on 90 mm quartz fiber filters that were prepared in the laboratory by baking at 550 C for a minimum of 18 h. After sampling, filters were wrapped in prebaked aluminum foil and stored frozen in dark. Filters were extracted with milli-Q water then analyzed with a liquid chromatograph interfaced to a triple quadrupole mass spectrometer (API 4000, Applied Biosystems/MDS Sciex) as described by Stone et al.25 A negative TurboIon Spray source was applied, following a Multiple Reaction Monitoring (MRM) analysis, where the desired ions (m/z 155 for GAS and 169 for LAS, switched at 8 Hz) were isolated at the first quadrupole, fragmented in the second quadrupole, and the resulting product ions were scanned in the last quadrupole. Two major product ion channels for each compound were monitored: bisulfate ion (m/z 97) and the hydroxy acid ion (at MW 80, i.e., 75 for GAS and 89 for LAS). Plotting counts per second (cps) of these channels as a function of elution time gives the chromatogram, a sample of which is shown in Figure 2. MRM gives independent chromatograms for GAS and LAS and no interference was observed, so coelution likely does not affect quantification. Coeluting components in sample extracts can have the potential to cause suppression or enhancement in the ionization source of the mass spectrometer. Normalization of target compound response can be best addressed by one of the following methods: standard addition, mass labeled analogues of the compounds of interest as internal standards, or performing experiments at different dilutions. GAS and LAS are not commercially available in a mass labeled form, and the cost of synthesis of such mass labeled forms was too high and beyond the scope of this project. However, ion suppression is also not expected to be significant in aerosol samples as it increases with the concentration of ionizing species and aerosol samples have very low analyte concentrations relative to applied charge. Inorganic ions (e.g., bisulfate) that are most likely to cause ion suppression due to high concentrations and ionization efficiency are chromatographically separated from analytes of interest, eliminating such interferences.
Figure 2. Sample HILIC MS/MS chromatogram from Bakersfield, CA (CalNex-SJV) field data on 6/17/10. Each chromatogram shows the most sensitive precursor-to-product ion transition: 155/97 for GAS and 169/97 for LAS. Elution times were determined by lab standards.
’ RESULTS Organosulfate Product Identification and Quantification. The 1H NMR chemical shifts of the organosulfate products are distinct from those of the precursor compounds (see Table S2 of the Supporting Information), making NMR a useful tool for product identification. No polymers or reaction byproducts were observed in the NMR or mass spectra. Glycolic acid and GAS each exhibit a single peak in the 1H spectrum. The addition of the sulfate group shifts the alkyl proton (position 2, Tables 1 and S2 of the Supporting Information) from 4.0 ppm for glycolic acid to 4.4 ppm for GAS, as shown in Figure 1 and Tables 1 and S2 of the Supporting Information. In the 13C NMR spectrum, the addition of the sulfate group shifts the carboxylic acid carbon signal from 177 to 171 ppm and the alkyl carbon signal from 60 to 63 ppm. Lactic acid sulfates exhibited similar shift changes in 1H NMR spectra (Figure S1 of the Supporting Information and Tables 1 and S2 of the Supporting Information). To determine the concentration of the organosulfates in solution, known amounts of the reaction products were dissolved in D2O with a known amount of dichloroacetic acid added as an internal standard and were quantified as a mass fraction, based upon the relative areas of the dichloroacetic acid and product 1H NMR peaks and the number of protons in each species. To ensure quantitative sampling, all measurements were taken with 45 pulse width and a 75 s relaxation delay. Stability of Organosulfates. GAS (∼90 mM), dissolved in D2O at pH values as low as 0.9, showed neither detectable hydrolysis nor any other side reactions over a period of 21 days via NMR spectroscopy. This indicates that GAS, a primary alcohol sulfate ester, can tolerate acidities much greater than those commonly found in tropospheric aerosol, which are usually between pH 1.5 and 4.0.40 Similarly, the secondary alcohol sulfate ester (LAS) showed no hydrolysis or other reaction over 21 days at a pH of 0.9. These findings are consistent with those of Darer et al.41 who determined that isoprene-derived primary and secondary organosulfates are quite stable in strongly acidic solution. Whereas the organosulfate moiety of the species in this study was found to be stable, the carboxylic acid moiety quickly reacted to form methyl carboxylates in methanol solutions (>50% of GAS formed methyl ester in 0.18 M, 1:1 methanol/ D2O solution after two days), such as those often used in filter extraction of aerosol samples. 6470
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Table 2. Ambient Concentrations of GAS and LAS, PM2.5, and Organic Carbon (OC) location Riverside, CA (SOAR) b
Mexico City, (MILAGRO T0)
d
Mexico City, (MILAGRO T1) Cleveland, OH (LADCO) Lahore, Pakistan (ABC) Bakersfield, CA (CalNex-SJV) Bakersfield, CA (CalNex-SJV) Bakersfield, CA (CalNex-SJV)
date
GAS ng/m3
LAS ng/m3
PM2.5 μg/m3
07/27/05
3.3 ( 1.1
0.8 ( 0.3
16.5a
7.6h
03/26/06
4.1 ( 1.1
1.2 ( 0.4
c
8.5g
03/26/06
7.0 ( 01.4
1.8 ( 0.6
07/15/07
1.9 ( 0.6
0.4 ( 0.2
11/02/07
11.3 ( 3.5
3.8 ( 0.9
06/16/10
5.4 ( 1.5
06/17/10 06/18/10
40
e
OC μg/m3
5.2g
33
f
3.9h
12.7
327.5
i
174.7i
0.6 ( 0.2
j
11.3
4.0k
4.9 ( 0.6
0.6 ( 0.2
j
12.0
4.8k
4.5 ( 0.5
0.7 ( 0.2
j
a
4.7k
11.1
b
PM2.5 data at Riverside, CA were obtained from the Riverside-Rubidoux (A) California Air Resources Board (CARB) site. The MILAGRO T0 site was located in downtown Mexico City. c Obtained from Mugica et al.48 d The MILAGRO T1 site was a peripheral site located 35 km downwind of the T0 site. e Daily average of 3/1/06 3/29/06 from Querol et al.49 f Daily average of 7/14/05 7/22/05 from Stone et al.38 g OC and PM2.5 data were collected by Stone et al.36 h OC and PM2.5 data were collected by Stone et al.38 i OC and PM2.5 data were collected by Stone et al.39 j PM2.5 data at Bakersfield, CA were obtained from the Bakersfield-55558 California Avenue (A) CARB site. k OC data in Bakersfield, CA (CalNex-SJV 2010) were collected by CARB.
Concentration in Ambient Samples. Figure 2 shows a typical HILIC-MS/MS chromatogram for an ambient sample from Bakersfield, CA. The most sensitive product ion for each organosulfate is shown: m/z 97 product ion of 155 precursor ion (m/z 155/97) for GAS and m/z 169/97 for LAS. GAS eluted at 6.4 min, and LAS eluted at 6.6 min (see Figure S2 of the Supporting Information for chromatograms of standards). Ambient concentrations of LAS were a factor 3 10 lower than those of GAS. Table 2 summarizes GAS and LAS in ambient PM2.5 during the CalNex-SJV 2010 sampling period and other sampling sites from Stone et al.25,36 39 Field and lab blanks for Bakersfield, CA and a field blank for Mexico City showed no signal of GAS, whereas the lab blank of Mexico City showed a small contribution to GAS that was less than 5% of the lowest ambient sample, which is significantly smaller than experimental uncertainty and may be due to lab contamination.
’ DISCUSSION The target organosulfates (GAS and LAS) in this study were chosen because the corresponding hydroxycarboxylic acids were found in rainwater dissolved organic matter by Altieri et al.,42 in a cloud processing study of glyoxal by Tan et al.,11 and based on the work by Galloway et al.23 GAS had been identified in chamber and ambient aerosol.19 21 Other ambient hydroxycarboxylic acids, such as lactic, malic, and tartaric acid have been observed.11,42 Synthesis and characterization of additional hydroxycarboxylic acid derived organosulfates was beyond the scope of this study and is the focus of future work. The simple synthesis method described in this work could be used for many other quantitative hydroxycarboxylic acid organosulfate standards and also for hydroxyketones and hydroxyaldehydes. Combined with analysis by LC-MS, these standards can provide a means to unequivocally identify and quantify numerous organosulfates that have been observed in ambient aerosol via ESI-MS methods and thus will allow a more reliable quantification of the contribution of organosulfates to SOA. The use of the GAS and LAS standards demonstrated that the previous assignments of both C2H3SO6 as glyoxal sulfate and C3H5SO6 as methylglyoxal sulfate, respectively, are likely incorrect. This conclusion is supported by chemical considerations (e.g., nucleophilicity and acetal stability), which make it highly unlikely that aldehyde-derived hemiacetal sulfates could be stable in either aqueous or methanol solutions (see Scheme 2),
Scheme 2. Equilibrium between a Hemiacetal Organosulfate and Its Hydrated Acetal (Geminal-Diol), which Is Shifted to the Hydrated Form in Aqueous Filter Extractsa
a
An analogous equilibrium exists in methanolic solutions to form methanol derived hemiacetals and acetals.
since hemiacetal sulfates exist in equilibrium with the hydrated aldehyde (geminal diol) form. Due to the large excess of water in aqueous filter extracts, the hemiacetal sulfates will hydrolyze to the hydrated (geminal diol) form. Similarly, the hemiacetal sulfates will react to form methanol-derived hemiacetals or acetals in methanolic solutions. Thus, the hemiacetal sulfates will not contribute substantially in filter extract solutions. However, the high sulfate activity may result in a substantial concentration of the hemiacetal sulfate within sulfate aerosol. Based on the results by Galloway et al.23 and the chemical considerations detailed above, we propose the aldehyde-derived hemiacetal-like organosulfates are misidentified and actually correspond to isomeric structures. When filters were extracted with methanol and standards were prepared in methanol solutions, GAS reacted with methanol to form the methyl carboxylates. In addition, the organosulfates investigated in this study are very hydrophilic, so using methanol as a cosolvent provides no benefit. Therefore, all ambient and standard solutions used in this study were aqueous. This finding highlights a potential for the formation of artifacts when filter samples containing carboxylic acids are extracted with methanol. We propose that aqueous extraction for very hydrophilic organosulfates (e.g., the ones studied here) or extraction with acetonitrile for less hydrophilic organosulfates (e.g., monoterpene-derived organosulfates) should be considered to address this problem when studying carboxylic acids and acid-derived organosulfates. Structural identification will also shed insight into the sources and formation mechanisms of organosulfates. Originally, C2H3SO6 was believed to be glyoxal sulfate, which had a simple proposed formation mechanism corresponding to nucleophilic attack of sulfate on the aldehyde group forming a hemiacetal, which would be facilitated by acid catalysis. As C2H3SO6 has 6471
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’ ASSOCIATED CONTENT
bS
Supporting Information. Summary of compounds used during synthesis, LAS 1H and 13C NMR spectra and detailed chemical shifts, and chromatograms for GAS and LAS standards. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses
)
been shown to instead correspond to GAS, this raises the question of the formation pathway. Minerath et al.29 have shown that the formation of organosulfates from alcohols, such as the case of glycolic acid, is not important under most atmospheric conditions. In addition, Galloway et al.23 showed that glyoxal uptake onto acidic aerosol seed (AS/H2SO4) did not produce C2H3SO6 . Investigations into potential ammonium catalysis43 of sulfate ester formation did not produce C2H3SO6 in experiments that used saturated ammonium sulfate solutions with glycolic acid. Therefore, C2H3SO6 is likely formed via photochemical reactions, although the mechanism is not clear. A photochemical formation mechanism of organosulfates is supported by the results of Noziere et al.14 and Perri et al.32 and is in agreement with observations of GAS only in irradiated experiments by Galloway et al.23 As glycolic acid has an alcohol group, it likely forms GAS under highly acidic conditions such as sulfuric acid aerosol. Our filter sample analysis in conjunction with previous work indicates that GAS and LAS are common in ambient aerosol.19 21 Ambient GAS and LAS concentrations ranged 1.9 11.3 ng/m3 and 0.4 3.8 ng/m3, respectively. GAS concentrations and the fraction of OC from GAS (Table 2) were higher at the downwind Mexico City site (T1), which experiences more processed air masses than the T0 Mexico City site. This is consistent with work by DeCarlo et al.44 which reported increased O/C ratios downwind of Mexico City, although further studies are clearly needed to investigate the hypothesis that GAS scales with aerosol processing time. In Lahore, Pakistan, the highest concentrations of GAS and LAS were observed, but their relative contribution to both PM2.5 and OC were smaller due to extremely high PM2.5 levels. The observation of GAS at appreciable quantities at all of the study sites across different seasons indicates the global relevance of GAS to ambient aerosol. The relevance of GAS does not lie in its quantitative contribution to SOA but that it is an organosulfate observed throughout the troposphere in a variety of conditions. Compounds with the formula C2H3SO6 have also been observed in numerous other studies, such as aircraft measurements using a single-particle MS-based instrument in both the boundary layer and the free troposphere45 and as a gas-phase ion.46 From structural arguments, it is likely that C2H3SO6 also corresponds to GAS in the single-particle MS studies, as preliminary results show that GAS is usually detected unfragmented with these methods.47 GAS is particularly interesting as it corresponds to a commonly observed SOA component that only contains two carbon atoms and thus is an example of a small organic molecule contributing to SOA. The availability of quantitative organosulfate standards will provide for an increase in data on the absolute contributions of organosulfates to ambient SOA mass and thus will help to determine the importance of these compounds. In this work, we have synthesized standards for GAS and LAS and used these standards to quantify ambient concentrations from aerosol filter samples from five diversely located field campaigns. Due to the widespread observation of GAS and the likelihood of its photochemical production, we propose that GAS is an important tracer for the contribution of small organic molecules to ambient SOA, most likely via aqueous-phase photochemical production pathways. More work must be done to advance our understanding of the sources of organosulfates, their contribution to SOA formation, and their usefulness as tracers of aerosol processes.
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Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, United States. ^ Department of Chemistry, University of Iowa, Iowa City, IA 52242, United States.
’ ACKNOWLEDGMENT Samples and instrumentation assistance was appreciated from James Schauer. We acknowledge support from the NMR facility in the Department of Chemistry at UW-Madison, which was supported by the National Science Foundation under grant no. NSF CHE-0342998. Funding was provided courtesy of the National Science Foundation, NSF-ATM 0904134, and the California Air Resources Board (CARB). We also thank CARB for providing OC data as well as the CalNex-SJV 2010 team. We also thank Lindsay Hatch and Kim Prather for making preliminary data from experiments using the standards in ATOF-MS studies available. ’ REFERENCES (1) Kroll, J. H.; Seinfeld, J. H. Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere. Atmos. Environ. 2008, 42 (16), 3593–3624. (2) De Haan, D. O.; Corrigan, A. L.; Smith, K. W.; Stroik, D. R.; Turley, J. J.; Lee, F. E.; Tolbert, M. A.; Jimenez, J. L.; Cordova, K. E.; Ferrell, G. R. Secondary organic aerosol-forming reactions of glyoxal with amino acids. Environ. Sci. Technol. 2009, 43 (8), 2818–2824. (3) Jimenez, J. L.; Canagaratna, M. R.; Donahue, N. M.; Prevot, A. S. H.; Zhang, Q.; Kroll, J. H.; DeCarlo, P. F.; Allan, J. D.; Coe, H.; Ng, N. L.; Aiken, A. C.; Docherty, K. S.; Ulbrich, I. M.; Grieshop, A. P.; Robinson, A. L.; Duplissy, J.; Smith, J. D.; Wilson, K. R.; Lanz, V. A.; Hueglin, C.; Sun, Y. L.; Tian, J.; Laaksonen, A.; Raatikainen, T.; Rautiainen, J.; Vaattovaara, P.; Ehn, M.; Kulmala, M.; Tomlinson, J. M.; Collins, D. R.; Cubison, M. J.; Dunlea, E. J.; Huffman, J. A.; Onasch, T. B.; Alfarra, M. R.; Williams, P. I.; Bower, K.; Kondo, Y.; Schneider, J.; Drewnick, F.; Borrmann, S.; Weimer, S.; Demerjian, K.; Salcedo, D.; Cottrell, L.; Griffin, R.; Takami, A.; Miyoshi, T.; Hatakeyama, S.; Shimono, A.; Sun, J. Y.; Zhang, Y. M.; Dzepina, K.; Kimmel, J. R.; Sueper, D.; Jayne, J. T.; Herndon, S. C.; Trimborn, A. M.; Williams, L. R.; Wood, E. C.; Middlebrook, A. M.; Kolb, C. E.; Baltensperger, U.; Worsnop, D. R. Evolution of organic aerosols in the atmosphere. Science 2009, 326 (5959), 1525–1529. (4) Pankow, J. F. An absorption-model of the gas aerosol partitioning involved in the formation of secondary organic aerosol. Atmos. Environ. 1994, 28 (2), 185–188. (5) Pankow, J. F. An absorption-model of the gas/aerosol partitioning involved in the formation of secondary organic aerosol. Atmos. Environ. 1994, 28 (2), 189–193. (6) Odum, J. R.; Hoffmann, T.; Bowman, F.; Collins, D.; Flagan, R. C.; Seinfeld, J. H. Gas/particle partitioning and secondary organic aerosol yields. Environ. Sci. Technol. 1996, 30 (8), 2580–2585. 6472
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