SOA from aqueous reactions of phenols with two oxidants - CiteSeerX

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Aug 19, 2014 - In this study, we investigate the reactions of phenol and two methoxy- phenols (syringol and guaiacol) with two major aqueous phase oxidants ...

This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available.

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Atmos. Chem. Phys. Discuss., 14, 21149–21187, 2014 www.atmos-chem-phys-discuss.net/14/21149/2014/ doi:10.5194/acpd-14-21149-2014 © Author(s) 2014. CC Attribution 3.0 License.

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Correspondence to: Q. Zhang ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union.

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L. Yu et al.

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Received: 3 August 2014 – Accepted: 4 August 2014 – Published: 19 August 2014

SOA from aqueous reactions of phenols with two oxidants

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Department of Environmental Toxicology, University of California, 1 Shields Ave., Davis, CA 95616, USA 2 Department of Land, Air and Water Resources, University of California, 1 Shields Ave., Davis, CA 95616, USA 3 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA 4 Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA

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L. Yu , J. Smith , A. Laskin , C. Anastasio , J. Laskin , and Q. Zhang

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Chemical characterization of SOA formed from aqueous-phase reactions of phenols with the triplet excited state of carbonyl and hydroxyl radical

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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Secondary organic aerosol (SOA) is ubiquitous in the atmosphere (Murphy et al., 2006; Zhang et al., 2007; Jimenez et al., 2009) and plays an important role in climate, human

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Phenolic compounds, which are emitted in significant amounts from biomass burning, can undergo fast reactions in atmospheric aqueous phases to form secondary organic aerosol (aqSOA). In this study, we investigate the reactions of phenol and two methoxyphenols (syringol and guaiacol) with two major aqueous phase oxidants – the triplet 3 ∗ excited states of an aromatic carbonyl ( C ) and hydroxyl radical (·OH). We thoroughly characterize the low-volatility species produced from these reactions and interpret their formation mechanisms using aerosol mass spectrometry (AMS), nanospray desorption electrospray ionization mass spectrometry (nano-DESI MS), and ion chromatography (IC). A large number of oxygenated molecules are identified, including oligomers containing up to six monomer units, functionalized monomer and oligomers with carbonyl, carboxyl, and hydroxyl groups, and small organic acid anions (e.g., formate, acetate, oxalate, and malate). The average atomic oxygen-to-carbon (O / C) ratios of phenolic aqSOA are in the range of 0.85–1.23, similar to those of low-volatility oxygenated organic aerosol (LV-OOA) observed in ambient air. The aqSOA compositions are overall 3 ∗ similar for the same precursor, but the reactions mediated by C are faster than ·OHmediated reactions and produce more oligomers and hydroxylated species at the point when 50 % of the phenol had reacted. Profiles determined using a thermodenuder indicate that the volatility of phenolic aqSOA is influenced by both oligomer content and O / C ratio. In addition, the aqSOA shows enhanced light absorption in the UVvis region, suggesting that aqueous-phase reactions of phenols are likely an important source of brown carbon in the atmosphere, especially in regions influenced by biomass burning.

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health, and air quality. Thus understanding the impacts of SOA requires a thorough knowledge of the formation, evolution, and composition of SOA. This knowledge, however, is still limited because atmospheric organic chemistry is extremely complex. Numerous sources emit organic compounds and organic aerosol is formed and transformed via complicated chemical and physical processes in the atmosphere (Kanakidou et al., 2005). SOA formation can take place in both gas and condensed phases. Much of the previous research on SOA has mainly focused on gas-phase reactions of volatile organic compounds (Hallquist et al., 2009). Recent work, however, has shown that SOA can also be produced efficiently in cloud and fog drops and water-containing aerosol (Blando and Turpin, 2000; Lim et al., 2005; Altieri et al., 2006; Ervens et al., 2011). Understanding the characteristics of SOA formed from aqueous-phase reactions (aqSOA) is important for properly representing its formation pathways in models and for elucidating its climatic and health effects. Phenols are important precursors of aqSOA because (1) they are emitted in large quantities from biomass burning (Hawthorne et al., 1989; Schauer et al., 2001); (2) they have high Henry’s Law constants and can partition significantly into atmospheric aqueous phases (Sagebiel and Seiber, 1993; Sander, 1999); and (3) they can undergo fast reactions with hydroxyl radical (·OH) and triplet excited states of organic 3 ∗ compounds ( C ) formed via light absorption by dissolved chromophores (Anastasio et al., 1997; Canonica et al., 2000; Smith et al., 2014). In the aqueous phase, ·OH is typically considered a dominant oxidant for organics. However, a recent study by 3 ∗ Smith et al. (2014) showed that the destruction rates of phenols by C are comparable to or faster than those by ·OH under typical ambient conditions in areas influenced 3 ∗ by biomass burning. An important source of C in the atmosphere is non-phenolic aromatic carbonyls – a group of compounds that are emitted from wood combustion in significant amounts (Hawthorne et al., 1992; Simoneit et al., 1999) and have been detected in fog and cloud droplets (Leuenberger et al., 1985; Sagebiel and Seiber, 1993). These compounds, once dissolved in water, can catalyze the photooxidation of

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Phenolic aqSOA samples

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Experimental Methods

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phenols and generate aqSOA with little or no loss of the aromatic carbonyl (Anastasio et al., 1997; Smith et al., 2014). Recent studies have shown that phenols react with ·OH and 3 C∗ to form aqSOA with mass yields close to 100 % (Smith et al., 2014) and that the reaction products include small organic acids, hydroxylated phenols, and oligomers (Sun et al., 2010). However, since Sun et al. (2010) mainly used an Aerodyne high-resolution time-of-flight aerosol mass spectrometer with an electron impact (EI) ionization source, in which analyte molecules are generally extensively fragmented (Canagaratna et al., 2007), the molecular composition of the phenolic aqSOA was not sufficiently characterized. 3 ∗ In addition, almost nothing is known about the chemistry of aqSOA formed from C reactions. In this study, we thoroughly characterize the aqueous reaction products of phenols 3 ∗ with C produced from a non-phenolic aromatic carbonyl and ·OH from hydrogen peroxide (HOOH) under simulated sunlight illumination. We studied three basic structures of biomass-burning phenols – phenol (C6 H6 O), guaiacol (C7 H8 O2 ; 2-methoxyphenol), and syringol (C8 H10 O3 ; 2,6-dimethoxyphenol). We examine the molecular and bulk compositions of low-volatility species produced from these reactions and use this information to interpret the formation pathways of phenolic aqSOA.

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The aqSOA samples of phenol, guaiacol, and syringol were prepared during simulated 3 ∗ sunlight illumination under two oxidant conditions: (1) via reaction with C formed from −1 5 µmol L 3,4-dimethoxybenzaldehyde (3,4-DMB) and (2) via reaction with ·OH generated from 100 µmol L−1 hydrogen peroxide (HOOH; Table 1). Details of the experiments are given in Smith et al. (2014) and a brief summary is given here. Initial solutions were composed of air-saturated Milli-Q water (resistance > 18 MΩ cm; Millipore) containing

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In this study, a High Resolution Time-of-Flight Aerosol Mass Spectrometer (Aerodyne Res. Inc., Billerica, MA; thereinafter referred to as AMS) was used to characterize the bulk chemical composition and elemental ratios of the low-volatility substances in both blown-down and flash-frozen samples. The working principles of the AMS have been discussed previously (DeCarlo et al., 2006; Canagaratna et al., 2007). Briefly, the AMS analyzes nonrefractory aerosols that can be evaporated at ∼ 600 ◦ C via 70 eV EI mass spectrometry. In this study, the AMS was operated alternatively between “V” and “W” ion optical modes (mass resolutions of ∼ 3000 and ∼ 5000, respectively) to acquire mass spectra up to m/z 500 and m/z 300, respectively. Prior to AMS analysis, each blown-down sample was dissolved in 6.0 mL Milli-Q water and the flash-frozen samples were thawed overnight inside a refrigerator (∼ 4 ◦ C). The liquid samples were atomized in argon (Industrial Grade, 99.997 %) using a constant output atomizer coupled with a diffusion dryer and the resulting particles were analyzed by the AMS downstream of a digitally controlled thermodenuder (TD) (Fierz et al., 2007). The TD consists of 21153

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Aerosol Mass Spectrometry (AMS) measurement and data analysis

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100 µmol L of a single phenol and adjusted to pH = 5 using sulfuric acid. Each solution was illuminated in an RPR-200 Photoreactor System (George et al., 2014) until approximately half of the initial phenol was degraded (as monitored by a high performance liquid chromatograph (HPLC) with a UV-vis detector). At that point, 12.0 mL of the illuminated solution was placed in an aluminum cup and blown gently to dryness with pure N2 at room temperature. Another aliquot of the illuminated solution was ◦ flash frozen with liquid nitrogen and stored at −20 C in the dark until analysis. HPLC analysis of the blown-down samples detected negligible amounts of the initial phenols, indicating that they were completely removed. Dark control experiments, carried out under the same condition except in the dark, showed negligible loss of phenol and no formation of aqSOA.

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a bypass line and a heated line terminating in a section with activated carbon cloth. The temperature inside the heated line was programmed to cycle through 7 different ◦ temperatures (25, 40, 65, 85, 100, 150, and 200 C) every hour. An automated 3-way valve switched the sample flow between bypass and TD modes every 5 min. By comparing the measurements between these two modes, we can determine the volatility profiles of the aqSOA. Between every two sample runs, Milli-Q water was atomized and analyzed as an analytical blank. The AMS data were analyzed using the AMS data analysis software (SQUIRREL v1.12 and PIKA v1.53 downloaded from http://cires.colorado.edu/jimenez-group/ ToFAMSResources/ToFSoftware/). The W-mode data was analyzed to determine the atomic ratios of oxygen-to-carbon (O / C) and hydrogen-to-carbon (H / C) and the organic mass-to-carbon ratio (OM / OC) of phenolic aqSOA (Aiken et al., 2008). V-mode data were analyzed for information of higher molecular weight ions with m/z > 300, + + such as syringol dimer C16 H18 O6 (m/z 306) and guaiacol trimer C21 H20 O6 (m/z 368). + + + Note that accurately quantifying the organic contributions to the H2 O , CO , and CO2 signals in an ensemble mass spectrum is critical to the determination of the O / C and H / C ratios of an organic aerosol (Aiken et al., 2008; Sun et al., 2009; Collier and Zhang, 2013). Since Ar was used as the carrier gas for atomization, N2 and CO2 did + + not interfere with the quantification of the organic CO and CO2 signals. In terms of + the H2 O signal, contribution from gaseous water molecules was negligible because the relative humidity measured at the AMS inlet was very low (< 2 %). In addition, since heating the aerosol to 40 ◦ C prior to AMS sampling led to almost no change in + the relative intensities of the H2 O signal in the mass spectra of aqSOA (Fig. S1 in the + Supplement), particles appeared to be completely dry. The organic portion of the H2 O + signal was thus determined as the difference between the measured H2 O signal and + the sulfate-associated H2 O signal estimated according to the known fragmentation pattern of sulfates (Allan et al., 2004).

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Prior to nano-DESI MS analysis, the blown-down samples of phenolic aqSOA were dissolved in Milli-Q water, atomized, and collected on Teflon membrane filters. The analyses were performed using a high-resolution LTQ-Orbitrap mass spectrometer (Thermo Electron, Bremen, Germany) with a resolving power (m/∆m) of 100 000 at m/z = 400. The instrument is equipped with a nano-DESI source assembled from two fused-silica capillaries (150 µm o.d./50 µm i.d.) (Roach et al., 2010b). Analyte molecules extracted into the liquid bridge formed between the two capillaries are transferred to a mass spectrometer inlet and ionized by nanoelectrospray. The analysis was performed under the following conditions: spray voltage of 3–5 kV, 0.5–1 mm distance from the tip of the nanospray capillary to the 300 ◦ C heated inlet of the LTQ-Orbitrap, and 0.3– 0.9 µL min−1 flow rate of acetonitrile : water (1 : 1 volume) solvent. The instrument was calibrated using a standard mixture of caffeine, MRFA (met-arg-phe-ala) peptide, and Ultramark 1621 (Thermo Scientific, Inc.) for the positive ion mode and a standard mixture containing sodium dodecyl sulfate, sodium taurocholate, and Ultramark 1621 (Thermo Scientific, Inc.) for the negative ion mode. Both positive and negative mode mass spectra were acquired using the Xcalibur software (Thermo Electron, Inc.). To analyze a sample, the nano-DESI probe was first placed on a clean area of the filter to record the background signal for ∼ 3 min and then positioned on the sample region to acquire data for an additional 4–5 min (Roach et al., 2010a). Peaks with S / N > 10 were selected using the Decon2LS software developed at the Pacific Northwest National Laboratory (PNNL) (Jaitly et al., 2009). Further data processing was performed with Microsoft Excel using a set of built-in macros developed by Roach et al. (2011). The background and sample peaks were aligned, and 13 the peaks corresponding to C isotopes were removed. Only peaks in the sample spectra that are at least 10 times bigger than the corresponding peaks in the background spectra were retained for further analysis. Peaks were segregated into

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different groups using the higher-order mass defect transformation developed by Roach et al. (2011). Specifically, the peaks were first grouped using a CH2 -based transformation and then an H2 -based second-order transformation. Formula Calculator v. 1.1 (http://www.magnet.fsu.edu/usershub/scientificdivisions/icr/icr_software.html) was then used to assign the molecular formula to each group using the following constraints: C ≥ 0, H ≥ 0, O ≥ 0 for the negative ion mode data and C ≥ 0, H ≥ 0, O ≥ 0, Na ≤ 1 for the positive ion mode data. Approximately 70 % of the peaks were assigned with molecular formula within these constraints. The formulas of neutral species were subsequently determined by removing the adduct ion (e.g., a proton or a sodium ion) from the positive ions or by adding a proton to the negative ions. Kendrick representation of high resolution mass spectral data can be used to search for potential oligomeric units (Hughey et al., 2001). In this study, O-based Kendrick diagram was used to investigate the degree of hydroxylation. The Kendrick mass (KM) and Kendrick mass defect (KMD) are calculated using the following two equations:

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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KMD = NM − KM

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Ion Chromatography (IC) and Total Organic Carbon (TOC) analysis

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Concentrations of inorganic/organic anions were measured using an ion chromatograph (Metrohm 881 Compact IC Pro, Switzerland) equipped with an autosampler, 21156

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where, 16 is the nominal mass of O, 15.9949 is the exact mass of the O, and NM is the KM rounded to the nearest integer. Plotting KMD vs. KM reveals homologous series of compounds differing only by the number of base units which fall on horizontal lines. Double bond equivalent (DBE) indicates the number of double bonds and rings in a closed-shell organic molecule (Pellegrin, 1983). For a molecule with a nominal formula of Cx Hy Oz (where x, y, z denote the number of C, H, and O atoms, respectively, in the molecule), DBE = 1 − y/2 + x.

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a Metrosep RP2 guard/3.6 column and a Metrosep A Supp15 250/4.0 column, and a conductivity detector. Details on the IC method are given in Ge et al. (2014). Briefly, anions were eluted at 0.8 mL min−1 using an eluent of 5 µmol L−1 Na2 CO3 and −1 0.3 µmol L NaOH in water. This method can separate and quantify 9 organic anions (glycolate, formate, acetate, pyruvate, oxalate, malate, malonate, maleate, and − − 2− 3− fumarate) and 7 inorganic anions (F− , Cl− , NO− 2 , Br , NO3 , SO4 and PO4 ). The IC results were evaluated in terms of reproducibilities of retention times and peak heights and linearity of the calibration curves. Analysis of external check standards, including a 7-anion standard mixture (Dionex) and 4 individual standards (Metrohm), always produced results that were within 10 % of certified values. Relative differences for replicate analyses were within 3 %. A Shimadzu TOC-VCPH analyzer was applied to measure TOC in the aqSOA samples. The instrument uses a combustion tube filled with oxidation catalyst to convert all ◦ carbon atoms into CO2 at 720 C under ultrapure air and quantifies the resulting CO2 using a non-dispersive infrared (NDIR) analyzer. Prior to combustion, inorganic carbon species (carbonates/bicarbonates and dissolved CO2 ) is transformed into CO2 by 25 % H3 PO4 , bubbled out, and determined by NDIR. TOC is determined as the difference. The TOC analyzer was calibrated using the standard solutions of NaHCO3 , Na2 CO3 , and potassium hydrogen phthalate (Sigma-Aldrich or Wako-Japan, ≥ 99.0 %). Results from external TOC check standards (Aqua Solutions) were always within 10 % of certified values.

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Results and discussion Overview of the chemical characteristics of phenolic aqSOA

The lifetime of phenols with respect to 3 C∗ and ·OH reactions in atmospheric fog and cloud water is on the order of minutes to hours during daytime (Smith et al., 2014). 3 ∗ Compared to ·OH, the reaction rates of C with phenols are faster, but the mass yields

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of aqSOA from both reactions are near 100 % for phenol, guaiacol, and syringol (Smith et al., 2014). As shown in Fig. 1 and summarized in Table 1, the aqSOA formed from all three phenols with both oxidants are highly oxygenated with average O / C ratios in the range of 0.85–1.23. These results are consistent with a previous study by Sun et al. (2010), where O / C ratios of phenolic aqSOA formed from direct photodegradation and ·OH oxidation were in the range of 0.80–1.06. The O / C of phenolic SOA from gas-phase ·OH oxidation are also near unity (Chhabra et al., 2011; Yee et al., 2013). Due to high oxygen contents, the organic mass-to-carbon (OM / OC) ratios of the aqSOA are high (average = 2.27–2.79; Table 1). Note that the OM / OC ratios determined by AMS agree well with those determined based on aqSOA mass measured gravimetrically and organic carbon mass measured by a TOC analyzer (Supplement Fig. S2). For the same oxidant, the O / C ratio of the aqSOA formed at t1/2 follows the order: phenol > guaiacol > syringol (Table 1). This trend is likely driven by precursor reactivity, which determines how long the solution needed to be illuminated to reach one half-life, and has the order: syringol > guaiacol > phenol. Longer illumination time increases the formation of highly oxygenated species and smaller ring-opening species. For the same reason, ·OH oxidation, which is slower than 3 C∗ reaction for the same phenol precursor, generally produces more oxidized aqSOA at t1/2 . Figures S3 and S4 in the Supplement show the nano-DESI MS spectra of the aq3 ∗ SOA of syringol, guaiacol, and phenol formed from C and ·OH reactions, respectively. Hundreds of species were identified, all of which are oxygenated with the median O / C ratios of the molecules varying from 0.33–0.55 in different aqSOA samples (Fig. 1). The signal-weighted average O / C ratios (Bateman et al., 2012) of phenolic aqSOA are in the range of 0.31–0.65 according to the negative ion mode nano-DESI results, which are significantly lower than the average O / C of bulk aqSOA measured by the AMS (Fig. 1). This discrepancy may be attributed to lower electrospray ionization efficiencies of some highly oxidized species or the dissociation of quasi-molecular ions which leads to the loss of highly oxygenated moieties (e.g., loss of CO2 for aromatic carboxylic acids) in nano-DESI analysis (Levsen et al., 2007). In addition, in order to provide better

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coverage of high-mass ions, nano-DESI mass spectra were analyzed only for ions with m/z > 100. Therefore, high O / C species with molecular weight lower than 100 dalton (Da), such as oxalate (O / C = 2), formate (O / C = 1), and pyruvate (O / C = 1), were not observed in our nano-DESI experiments. According to IC analysis, these small organic anions together represent 0.8–3.8 % of the TOC of aqSOA (Table 1). Figure 2 shows the AMS spectra of different aqSOA acquired after 50 % of the initial phenols had reacted (i.e., at t1/2 ). A prominent feature of these spectra is that + + + CO2 (m/z = 44), H2 O (m/z = 18), and CO (m/z = 28) are the largest peaks, similar to the spectral pattern of fulvic acid – a model compound representative of highly processed and oxidized organic particulate matter and humic-like substances (HULIS) (Zhang et al., 2005; Ofner et al., 2011). The AMS spectra of syringol aqSOA formed from different oxidants are almost identical (Fig. 2), indicating similar chemical compositions. Similarly, nano-DESI analysis shows the formation of a large number of common species, i.e., 883 species with common elemental composition (Table 1), through the reactions of syringol with 3 C∗ and ·OH, which account for 76 and 88 %, respectively, of the total number of molecules identified in the corresponding aqSOA. A similar overlap of common species was observed for guaiacol aqSOA. But the molecular compositions of phenol aqSOA are more different between the two oxidants (Table 1), which is consistent with the fact that the AMS spectra of the two phenol aqSOA are largely different at m/z ≥ 80 (Fig. 2l). The more distinct compositional differences of phenol 3 ∗ aqSOA between the ·OH and C reactions is probably due to the larger difference in reaction times (i.e., t1/2 = 672 min vs. 480 min; Table 1). Detailed discussions on the comparisons of aqSOA produced from the same precursor but different oxidants are given in Sect. 3.3. A total number of 149 common molecules were identified in all samples (Table 1). Figure 3 shows the Van Krevelen diagram of these common molecules. A majority of these molecules have molecular weight lower than 400 Da and DBE < 12 (Fig. 3), indicating that they contain two or less aromatic rings and that they were likely produced from ring-opening reactions. In addition, small carboxylate anions were observed in all

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Insights into aqSOA formation mechanisms

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In this section we synthesize the molecular composition and bulk chemistry results and interpret the formation mechanisms of phenolic aqSOA. A notable result is the large number of dimer and higher oligomers (up to hexamer) found in the aqSOA. As shown in Figs. S3 and S4 in the Supplement, the nano-DESI MS spectra of phenolic aqSOA contain clearly distinguished regions corresponding to monomers, dimers, trimers, tetramers, pentamers, and hexamers and their oxidation products. We therefore determine the distributions of phenolic aqSOA species based on the degree of oligomerization by summing signals in each region (Fig. 5). Oligomers and related 21160

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aqSOA samples, although they represent only a small fraction of the TOC (Table 1). It is interesting to point out that the number of molecules observed by nano-DESI decreases with increasing illumination time across all 3 phenols, suggesting that increased aging simplifies the products to a smaller set (Table 1). However, this trend could also be related to ionization efficiency of different types of phenolic aqSOA. For example, syringol aqSOA has more methoxy groups, thus is easier to get ionized by nano-DESI, compared to phenol aqSOA. Both flash-frozen (FF) and blown-down (BD) samples were chemically characterized and show almost identical AMS spectra (Supplement Fig. S5). This is a confirmation that the non-volatile components of these two sample types are chemically very similar. Since FF samples contain dissolved volatile species which should have evaporated during nebulization and drying, we estimated the amount of these species by examining the differences in the TOC concentrations between FF and BD samples after correction for the mass of unreacted precursors. Figure 4 shows the contributions of reactants (phenolic precursor and DMB) and products (dissolved volatile species and aqSOA) to the solution TOC after illumination to t1/2 . Dissolved volatile species formed during photolysis represent a small fraction (2.7–6.6 %; Table 1) of the total carbon originally present in the reactants, consistent with the high mass yields of phenolic aqSOA reported by Smith et al. (2014).

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derivative species clearly account for a significant fraction (24.2–92.6 %) of the total signals in the nano-DESI spectra of all phenolic aqSOA. Substituted monomers and smaller ring-opening species are also present in all aqSOA and they are particularly abundant in that of phenol + ·OH. Table 2 lists the 10 most abundant compounds identified in the aqSOA of syringol formed through reaction with ·OH or 3 C∗ . Among them 7 are common species, including syringol dimer (C16 H18 O6 ), hydroxylated syringol (C8 H10 O5 ), three dimer derivatives (C15 H14 O6 , C15 H16 O6 and C15 H16 O9 ), and two monomer derivatives (C15 H18 O7 and C12 H12 O7 ). Guaiacol aqSOA is also dominated by the dimer and related species whereas substituted monomers are more abundant in phenol aqSOA (Fig. 5). The presence of dimers and substituted monomers is also evident in the AMS spectra. As an example, Fig. 6 shows the AMS spectra of phenol aqSOA along with the NIST mass spectra of possible products. The spectra of guaiacol and syringol aqSOA are shown in 0 Supplement Figs. S6 and S7. Note that the AMS spectrum of biphenyl-4,4 -diol – a substituted phenolic compound – is very similar to the NIST mass spectrum (Supplement Fig. S8), indicating the validity of interpreting the AMS spectra of the phenolic aqSOA based on NIST spectra of possible products. The AMS spectra of phenol aqSOA show + + a prominent peak at m/z = 186 (C12 H10 O2 ), which is the molecular ion (M ·) of phenol + dimer (Fig. 6). Similarly, the molecular ion of guaiacol dimer (C14 H14 O4 ; m/z = 246; Fig. S6 in the Supplement) is also noticeable in the AMS spectra. These results indicate that oligomerization is an important aqueous-phase reaction pathway that leads to the formation of aqSOA from phenols. Hydroxylation is another important reaction pathway that forms and transforms phenolic aqSOA. As shown in Supplement Fig. S9, the O-based Kendrick diagram of syringol aqSOA clearly indicates the presence of a large number of species with different degrees of hydroxylation. Similarly, AMS analysis reveals the ubiquitous formation of hydroxylated products as well. For example, the AMS spectra of phenol aqSOA show + + prominent peaks at m/z = 110 (C6 H6 O2 ) and m/z = 202 (C12 H10 O3 ), suggesting the presence of hydroxylated phenol and hydroxylated phenol dimer, respectively (Fig. 6).

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In addition, signature ions representing 2-methoxyhydroquinone are detected in guaiacol aqSOA (Fig. S6 in the Supplement) and 3,4,5-trihydroxy benzoic acid is likely a product of syringol oxidation (Supplement Fig. S7) Both nano-DESI and AMS results further reveal the broad formation of aldehydes, esters, and carboxylated products. As shown in Table 2, C9 H10 O4 (MW = 182; DBE = 5), which was found to present at high abundance in the aqSOA of syringol + 3 C∗ , is + likely a syringol aldehyde. In addition, the pronounced C7 H5 O2 (m/z = 121) peak in the AMS spectra of phenol aqSOA (Fig. 6a) indicates the formation of phenol esters + such as methylparaben and ethylparaben (Fig. 6d) and the prominent C8 H7 O3 (m/z = 151) signal in the guaiacol aqSOA spectra (Fig. S6a in the Supplement) suggests the formation of a guaiacol ester – methyl vanillate (Supplement Fig. S6c). Small organic acid anions (i.e., formate, acetate, oxalate, malate, malonate, etc.) are observed in all samples and these species together account for less than 4 % of the TOC in aqSOA (Table 1). Note that the importance of organic acids is likely underestimated as IC only quantifies a limited number of low molecular weight aliphatic acids. Nano-DESI analysis further reveals the presence of a number of aromatic compounds with substituted carboxyl groups (e.g., aromatic esters; Supplement Fig. S10) and the formation of highly oxygenated C3–C5 aliphatic species in all samples, some of which (e.g., C3 H4 O4 , C4 H6 O4 and C5 H6 O5 ) are likely carboxylic acids based on DBE values. Furthermore, both nano-DESI and AMS analyses identify demethoxylated aromatic products (e.g., C15 H16 O6 and C15 H16 O9 in Table 2). These results together indicate that various fragmentation pathways, such as the cleavage of the aromatic rings and the losses of methoxy (−OCH3 ) groups, are also important during the aqueous-phase reactions of phenols. Based on these results, we propose a scheme in Fig. 7 of the main pathways of aqSOA formation through the reactions of phenols+ 3 C∗ . Briefly, phenols react with 3 C∗ and undergo multiple steps to eventually form HOOH (Anastasio et al., 1997), which is a source of ·OH via photolysis. The addition of ·OH to the aromatic ring, followed by O2 · addition and HO2 elimination, lead to the formation of hydroxylated products (Barzaghi

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and Herrmann, 2002). In the meantime, the ·OH-phenol adduct can undergo unimolecular elimination of H2 O to form a phenoxy radical (Atkinson et al., 1992; Barzaghi and Herrmann, 2002; Olariu et al., 2002), which then combines with another radical to form dimer and higher oligomers. Phenoxy radical may also form from the oxidation 3 ∗ of phenols by C via electron transfer coupled with proton transfer from the phenoxyl radical cation or from solvent water (Anastasio et al., 1997). Demethoxylation takes place through attachment of ·OH to ring positions occupied by methoxyl groups, followed by elimination of a methanol molecule to form semiquinone radicals (Steenken and O’Neill, 1977). Esterification of phenols can occur as a result of the reactions with organic acids (Offenhauer, 1964). Furthermore, the reactants and the products from all these pathways may undergo ring-opening process, forming ketones and carboxylic acids. Similar species, including oligomers, esters, carbonyls, carboxylic acids, and demethoxylated products, can be formed in ·OH-mediated reactions as well (Sun et al., 2010), although apparently with different reaction yields and rates.

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As discussed above, aqueous reactions of phenols produce a variety of low-volatility species including oligomers, functionalized monomer and oligomers (with varying numbers of carbonyl, carboxyl, ester, and hydroxyl groups), and small organic acids (e.g., formate, acetate, oxalate, and malate). Although aqSOA formed from the same precursor generally appear to be chemically similar, there are significant compositional 3 ∗ differences between the products from ·OH and C reactions. Overall, the molecular compositions of guaiacol and phenol aqSOA are more dependent on the oxidant than are syringol aqSOA (Fig. 5). Similarly, the AMS spectral patterns at m/z ≥ 80 exhibit more significant differences between ·OH and 3 C∗ for guaiacol (r 2 = 0.65; Fig. 2j) and phenol (r 2 = 0.42; Fig. 2l) whereas those for syringol are almost identical (r 2 = 0.97; Fig. 2h). Furthermore, a majority of the aqSOA molecules of phenol + ·OH contain only

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one benzene ring, whereas the C reaction produces more oligomers and substituted species based on the DBE values. Since the AMS results are quantitative, we further compare the relative abundances of signature ions in the AMS spectra of different aqSOA (Fig. 8). Details on the signature ions and their proposed precursors are given in Supplement Table S1. All these ions are odd electron ions, which usually have special mechanistic significances and are more indicative of the chemical identities of the precursors (McLafferty and Turecek, 1993). These ions can potentially be used to analyze ambient organic aerosol data for the presence of phenolic aqSOA. For instance, a previous study by our group observed + C16 H18 O+ 6 (m/z 306) and C14 H14 O4 (m/z 246) – signature ions representing syringol and guaiacol dimers, respectively, in ambient aerosols significantly influenced by wood combustion and fog processing (Sun et al., 2010). Similar to the nano-DESI results, 3 ∗ the AMS results also indicate that the aqSOA formed via C are more enriched of dimers and higher oligomers compared to ·OH for a given phenol. These observations suggest that more coupling of phenoxy radicals takes place during reactions initiated 3 ∗ by C than by ·OH. On the other hand, both IC and nano-DESI results indicate that ·OH-mediated reactions promote the formation of organic acids and other small ringopening species (see Sect. 3.2), consistent with the observations that ·OH reaction generally leads to more oxidized aqSOA as well as water-soluble volatile species (Table 1). A possible reason for the compositional differences observed in the aqSOA of the 3 ∗ same precursor but different oxidants is that C reacts faster with phenols (Smith et al., 2014) and thus takes shorter time to oxidize the same amount of phenols compared to ·OH. Longer illumination allows further oxidation and fragmentation of higher molecular weight species to happen, leading to the formation of smaller molecules with fewer aromatic rings. Indeed, the compositions of syringol aqSOA, which were produced after comparable illumination durations, are highly similar between the two oxidation conditions according to both nano-DESI and AMS results whereas the difference is the largest for phenol aqSOA whose illumination times are substantially different between

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C and ·OH oxidation (Figs. 2 and 5). However, the fact that there are more oligomers 3 ∗ and their derivatives in guaiacol + C condition compared to syringol + ·OH (Fig. 5) even though the guaiacol solution was illuminated longer (Table 1) suggests that coupling of phenoxy radicals is a more favored pathway through 3 C∗ reaction. 5

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Volatility profiles and UV-vis absorption spectra of phenolic aqSOA

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As discussed above, the chemical compositions of the phenolic aqSOA are complex. As a result, the volatilities of the aqSOA species span a broad range, from very low vapor pressure compounds such as oligomers to more volatile species such as low molecular weight acids. Figure 9 shows the volatility profiles of phenolic aqSOA formed 3 ∗ from C reactions measured by a thermodenuder coupled with the AMS. Ammonium sulfate and ammonium nitrate were analyzed simultaneously as references. On average, phenolic aqSOA are more volatile than ammonium sulfate, but less volatile than ammonium nitrate (Fig. 9). The fact that a significant fraction of the aqSOA mass remains in the particle phase even at 200 ◦ C (Fig. 9) is consistent with the presence of some very low volatility species such as oligomers. Compared to guaiacol and syringol aqSOA, phenol aqSOA show the slowest decay with increasing TD temperature, indicating that they are comprised of more low-volatility species. This is consistent with our chemical analyses which reveal that the aqSOA of phenol + 3 C∗ are composed of a larger fraction of species containing more than two aromatic rings, including trimer and higher oligomers (Fig. 5) and that the O / C ratios of phenol aqSOA are also highest among all for the same oxidant. These results together suggest that the volatility of phenolic aqSOA is strongly influenced by both polymer contents and average oxidation degree, as reported previously (Huffman et al., 2009). Recently, light-absorbing OA, also termed as “brown carbon”, has attracted much attention, due to their ability to absorb sunlight thus affect the radiative budget of the earth (Shapiro et al., 2009). Previous studies have shown that the aqueous-phase oxidation of phenols forms low-volatility oligomers, which absorb significant amounts

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We thoroughly characterized the chemical composition and studied the volatility and optical properties of phenolic aqSOA formed via reactions with two different oxidants: 3 ∗ C and ·OH. Elemental analysis of the AMS spectra indicates that all phenolic aqSOA are highly oxidized (O / C ratios: 0.85–1.23), despite the fact that some of the reactions were very fast (t1/2 < 1 h for syringol). For the same oxidant, the oxidation degree of the aqSOA formed at t1/2 follows the order: phenol > guaiacol > syringol. A large number of aqSOA molecules are identified, including oligomers (up to hexamers) and their derivatives with varying numbers of carbonyl, carboxyl, ester, and hydroxyl groups. A large number of ring-opening species including small organic acids (e.g., oxalate, formate, and acetate) are also identified. While the bulk compositions of the aqSOA are overall similar at t1/2 between the two oxidants for a given precursor, compositional differences are observed. Generally speaking, reactions mediated by ·OH produce more highly oxygenated species with a single aromatic ring, while oxidation by 3 C∗ enhances the formation of higher molecular weight species including oligomers and their oxygenated derivatives. The physical properties, such as volatility and light absorptivity,

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of UV-visible light and likely account for a significant portion of atmospheric HULIS (Gelencser et al., 2003; Chang and Thompson, 2010). In this study, we examined the optical properties of phenolic aqSOA using UV-vis spectroscopy. Figure 10 shows an 3 ∗ example of the UV-vis spectra of syringol aqSOA formed in the reactions with C and ·OH, respectively, at t1/2 . Both syringol aqSOA samples absorb in the tropospheric sunlight wavelengths (> 300 nm), while syringol does not. This enhancement is likely explained by the formation of conjugated structures as a result of polymerization and functionalization due to the additions of hydroxyl, carbonyl, and carboxyl functional groups to the aromatic rings. These results indicate that aqueous-phase reactions of phenols are likely an important source of brown carbon in the atmosphere, especially in regions influenced by biomass burning.

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of the phenol aqSOA depend on their chemical compositions. Our thermodenuder experiments indicate that the volatility profiles of phenolic aqSOA are influenced by both oligomer contents and average oxidation degree. In addition, the formation of aqSOA species with enhanced conjugated double bonds is probably responsible for the significant light absorption in the actinic region, suggesting that aqueous-phase reactions of phenols are an important source of brown carbon in the atmosphere. Overall, our results indicate that aqueous-phase processing of phenols represents an important pathway for the production of low-volatility, highly oxygenated and high molecular weight species, which remain in the particle phase after water evaporation. Since aqSOA formed from reactions of phenolic compounds are both water soluble and light absorbing, these reactions might significantly influence the chemical and physical properties, and thus the climatic and health effects, of atmospheric particles in regions influenced by biomass burning emissions. In this study, we also identified a number of AMS signature ions that are representative of phenolic aqSOA, e.g., + + C16 H18 O6 (m/z = 306) for syringol dimer, C14 H14 O4 (m/z = 246) for guaiacol dimer, + + C14 H14 O5 (m/z = 262) and C14 H14 O6 (m/z = 278) for hydroxylated guaiacol dimer, + C12 H10 O+ 2 (m/z = 186) for phenol dimer, C21 H20 O6 (m/z = 368) for guaiacol trimer, + and C18 H14 O3 (m/z = 278) for phenol trimer (Fig. 8). AMS has been broadly applied for chemical analysis of ambient aerosol and multivariate statistical approaches (e.g., positive matrix factorization) have been frequently used on organic aerosol mass spectral data to determine factors representing different sources and processes (Ulbrich et al., 2009; Zhang et al., 2011). An important criterion for validating the extracted factor is via examining the mass spectra of the factors for signature ions (Zhang et al., 2011). The signature ions identified in this study could be compared to ambient organic aerosol mass spectrometry data to investigate the impacts of phenolic aqSOA formation.

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Aiken, A. C., DeCarlo, P. F., Kroll, J. H., Worsnop, D. R., Huffman, J. A., Canagaratna, M. R., Onasch, T. B., Alfarra, M. R., Prevot, A. S. H., Dommen, J., Duplissy, J., Metzger, A., Baltensperger, U., and Jimenez, J. L.: O/C and OM/OC ratios of primary, secondary, and ambient organic aerosols with a high resolution time-of-flight aerosol mass spectrometer, Environ. Sci. Technol., 42, 4478–4485, doi:10.1021/es703009q, 2008. Allan, J. D., Delia, A. E., Coe, H., Bower, K. N., Alfarra, M. R., Jimenez, J. L., Middlebrook, A. M., Drewnick, F., Onasch, T. B., Canagaratna, M. R., Jayne, J. T., and Worsnop, D. R.: A generalised method for the extraction of chemically resolved mass spectra from Aerodyne aerosol mass spectrometer data, J. Aerosol Sci., 35, 909–922, doi:10.1016/j.jaerosci.2004.02.007, 2004. Altieri, K. E., Carlton, A. G., Lim, H. J., Turpin, B. J., and Seitzinger, S. P.: Evidence for oligomer formation in clouds: reactions of isoprene oxidation products, Environ. Sci. Technol., 40, 4956–4960, doi:10.1021/es052170n, 2006. Anastasio, C., Faust, B. C., and Rao, C. J.: Aromatic carbonyl compounds as aqueous-phase photochemical sources of hydrogen peroxide in acidic sulfate aerosols, fogs, and clouds .1. non-phenolic methoxybenzaldehydes and methoxyacetophenones with reductants (phenols), Environ. Sci. Technol., 31, 218–232, doi:10.1021/es960359g, 1997.

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Acknowledgement. This work was supported by the US National Science Foundation, Grant No. AGS-1036675, the California Agricultural Experiment Station (Projects CA-D-ETX-2102-H and CA-D*-LAW-6403-RR). The nano-DESI measurements were performed at the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL) – a national scientific user facility located at PNNL, and sponsored by the Office of Biological and Environmental Research of the US PNNL is operated for US DOE by Battelle Memorial Institute under contract no. DEAC06-76RL0 1830. Additional funding was provided by a Jastro-Shields Graduate Research Award (UC Davis) and a graduate fellowship from the Atmospheric Aerosols and Health (AAH) program at UC Davis to L. Yu. We thank A. Dillner, K. George, and S. Collier for help with experiments.

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Ervens, B., Turpin, B. J., and Weber, R. J.: Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies, Atmos. Chem. Phys., 11, 11069–11102, doi:10.5194/acp-11-11069-2011, 2011. Fierz, M., Vernooij, M. G. C., and Burtscher, H.: An improved low-flow thermodenuder, J. Aerosol Sci., 38, 1163–1168, doi:10.1016/j.jaerosci.2007.08.006, 2007. Ge, X., Shaw, S. L., and Zhang, Q.: Toward understanding amines and their degradation products from postcombustion CO2 capture processes with aerosol mass spectrometry, Environ. Sci. Technol., 48, 5066–5075, doi:10.1021/es4056966, 2014. Gelencser, A., Hoffer, A., Kiss, G., Tombacz, E., Kurdi, R., and Bencze, L.: In-situ formation of light-absorbing organic matter in cloud water, J. Atmos. Chem., 45, 25–33, doi:10.1023/a:1024060428172, 2003. George, K. M., Ruthenburg, T. C., Smith, J. D., Anastasio, C., Yu, L., Zhang, Q., and Dillner, A.: FT-IR quantification of the carbonyl functional group in aqueous-phase secondary organic aerosol from phenols, Atmos. Environ., submitted, 2014. Hallquist, M., Wenger, J. C., Baltensperger, U., Rudich, Y., Simpson, D., Claeys, M., Dommen, J., Donahue, N. M., George, C., Goldstein, A. H., Hamilton, J. F., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M. E., Jimenez, J. L., Kiendler-Scharr, A., Maenhaut, W., McFiggans, G., Mentel, Th. F., Monod, A., Prévôt, A. S. H., Seinfeld, J. H., Surratt, J. D., Szmigielski, R., and Wildt, J.: The formation, properties and impact of secondary organic aerosol: current and emerging issues, Atmos. Chem. Phys., 9, 5155–5236, doi:10.5194/acp-9-5155-2009, 2009. Hawthorne, S. B., Krieger, M. S., Miller, D. J., and Mathiason, M. B.: Collection and quantitation of methoxylated phenol tracers for atmospheric-pollution from residential wood stoves, Environ. Sci. Technol., 23, 470–475, doi:10.1021/es00181a013, 1989. Hawthorne, S. B., Miller, D. J., Langenfeld, J. J., and Krieger, M. S.: PM-10 high-volume collection and quantitation of semi- and nonvolatile phenols, methoxylated phenols, alkanes, and polycyclic aromatic hydrocarbons from winter urban air and their relationship to wood smoke emissions, Environ. Sci. Technol., 26, 2251–2262, doi:10.1021/es00035a026, 1992. Huffman, J. A., Docherty, K. S., Mohr, C., Cubison, M. J., Ulbrich, I. M., Ziemann, P. J., Onasch, T. B., and Jimenez, J. L.: Chemically-resolved volatility measurements of organic aerosol from different sources, Environ. Sci. Technol., 43, 5351–5357, doi:10.1021/es803539d, 2009.

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McLafferty, F. W. and Turecek, F.: Interpretation of Mass Spectra, University Science Books, Mill Valley, California, 1993. Murphy, D. M., Cziczo, D. J., Froyd, K. D., Hudson, P. K., Matthew, B. M., Middlebrook, A. M., Peltier, R. E., Sullivan, A., Thomson, D. S., and Weber, R. J.: Single-particle mass spectrometry of tropospheric aerosol particles, J. Geophys. Res.-Atmos., 111, D23S32, doi:10.1029/2006jd007340, 2006. Offenhauer, R. D.: The direct esterification of phenols, J. Chem. Educ., 41, 39, doi:10.1021/ed041p39, 1964. Ofner, J., Krüger, H.-U., Grothe, H., Schmitt-Kopplin, P., Whitmore, K., and Zetzsch, C.: Physicochemical characterization of SOA derived from catechol and guaiacol – a model substance for the aromatic fraction of atmospheric HULIS, Atmos. Chem. Phys., 11, 1–15, doi:10.5194/acp-11-1-2011, 2011. Olariu, R. I., Klotz, B., Barnes, I., Becker, K. H., and Mocanu, R.: FT-IR study of the ringretaining products from the reaction of OH radicals with phenol, o-, m-, and p-cresol, Atmos. Environ., 36, 3685–3697, doi:10.1016/s1352-2310(02)00202-9, 2002. Pellegrin, V.: Molecular formulas of organic compounds: the nitrogen rule and degree of unsaturation, J. Chem. Educ., 60, 626–633, doi:10.1021/ed060p626, 1983. Roach, P. J., Laskin, J., and Laskin, A.: Molecular characterization of organic aerosols using nanospray-desorption/electrospray ionization-mass spectrometry, Anal. Chem., 82, 7979– 7986, doi:10.1021/ac101449p, 2010a. Roach, P. J., Laskin, J., and Laskin, A.: Nanospray desorption electrospray ionization: an ambient method for liquid-extraction surface sampling in mass spectrometry, Analyst, 135, 2233– 2236, doi:10.1039/c0an00312c, 2010b. Roach, P. J., Laskin, J., and Laskin, A.: Higher-order mass defect analysis for mass spectra of complex organic mixtures, Anal. Chem., 83, 4924–4929, doi:10.1021/ac200654j, 2011. Sagebiel, J. C. and Seiber, J. N.: Studies on the occurrence and distribution of wood smoke marker compounds in foggy atmospheres, Environ. Toxicol. Chem., 12, 813–822, doi:10.1002/etc.5620120504, 1993. Sander, R.: Compilation of Henry’s Law constants for inorganic and organic species of potential importance in environmental chemistry, available at: http://irs.ub.rug.nl/dbi/4581696d8b3ed (last access: 14 December 2006), 1999.

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Schauer, J. J., Kleeman, M. J., Cass, G. R., and Simoneit, B. R. T.: Measurement of emissions from air pollution sources. 3. C-1-C-29 organic compounds from fireplace combustion of wood, Environ. Sci. Technol., 35, 1716–1728, doi:10.1021/es001331e, 2001. Shapiro, E. L., Szprengiel, J., Sareen, N., Jen, C. N., Giordano, M. R., and McNeill, V. F.: Lightabsorbing secondary organic material formed by glyoxal in aqueous aerosol mimics, Atmos. Chem. Phys., 9, 2289–2300, doi:10.5194/acp-9-2289-2009, 2009. Simoneit, B. R. T., Schauer, J. J., Nolte, C. G., Oros, D. R., Elias, V. O., Fraser, M. P., Rogge, W. F., and Cass, G. R.: Levoglucosan, a tracer for cellulose in biomass burning and atmospheric particles, Atmos. Environ., 33, 173–182, doi:10.1016/s1352-2310(98)00145-9, 1999. Smith, J. D., Sio, V., Yu, L., Zhang, Q., and Anastasio, C.: Secondary organic aerosol production from aqueous reactions of atmospheric phenols with an organic triplet excited state, Environ. Sci. Technol., 48, 1049–1057, doi:10.1021/es4045715, 2014. Steenken, S. and O’Neill, P.: Oxidative demethoxylation of methoxylated phenols and hydroxybenzoic acids by the hydroxyl radical. An in situ electron spin resonance, conductometric pulse radiolysis and product analysis study, J. Phys. Chem., 81, 505–508, doi:10.1021/j100521a002, 1977. Sun, Y., Zhang, Q., Macdonald, A. M., Hayden, K., Li, S. M., Liggio, J., Liu, P. S. K., Anlauf, K. G., Leaitch, W. R., Steffen, A., Cubison, M., Worsnop, D. R., van Donkelaar, A., and Martin, R. V.: Size-resolved aerosol chemistry on Whistler Mountain, Canada with a highresolution aerosol mass spectrometer during INTEX-B, Atmos. Chem. Phys., 9, 3095–3111, doi:10.5194/acp-9-3095-2009, 2009. Sun, Y. L., Zhang, Q., Anastasio, C., and Sun, J.: Insights into secondary organic aerosol formed via aqueous-phase reactions of phenolic compounds based on high resolution mass spectrometry, Atmos. Chem. Phys., 10, 4809–4822, doi:10.5194/acp-10-4809-2010, 2010. Ulbrich, I. M., Canagaratna, M. R., Zhang, Q., Worsnop, D. R., and Jimenez, J. L.: Interpretation of organic components from Positive Matrix Factorization of aerosol mass spectrometric data, Atmos. Chem. Phys., 9, 2891–2918, doi:10.5194/acp-9-2891-2009, 2009. Yee, L. D., Kautzman, K. E., Loza, C. L., Schilling, K. A., Coggon, M. M., Chhabra, P. S., Chan, M. N., Chan, A. W. H., Hersey, S. P., Crounse, J. D., Wennberg, P. O., Flagan, R. C., and Seinfeld, J. H.: Secondary organic aerosol formation from biomass burning intermediates: phenol and methoxyphenols, Atmos. Chem. Phys., 13, 8019–8043, doi:10.5194/acp13-8019-2013, 2013.

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Zhang, Q., Alfarra, M. R., Worsnop, D. R., Allan, J. D., Coe, H., Canagaratna, M. R., and Jimenez, J. L.: Deconvolution and quantification of hydrocarbon-like and oxygenated organic aerosols based on aerosol mass spectrometry, Environ. Sci. Technol., 39, 4938–4952, doi:10.1021/es048568l, 2005. Zhang, Q., Jimenez, J. L., Canagaratna, M. R., Allan, J. D., Coe, H., Ulbrich, I., Alfarra, M. R., Takami, A., Middlebrook, A. M., Sun, Y. L., Dzepina, K., Dunlea, E., Docherty, K., DeCarlo, P. F., Salcedo, D., Onasch, T., Jayne, J. T., Miyoshi, T., Shimono, A., Hatakeyama, S., Takegawa, N., Kondo, Y., Schneider, J., Drewnick, F., Weimer, S., Demerjian, K., Williams, P., Bower, K., Bahreini, R., Cotrell, L., Griffin, R. J., Rautiainen, J., Sun, J. Y., Zhang, Y. M., and Worsnop, D. R.: Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced northern hemisphere mid-latitudes, Geophys. Res. Lett., 34, L13801, doi:10.1029/2007GL029979, 2007 Zhang, Q., Jimenez, J., Canagaratna, M., Ulbrich, I., Ng, N., Worsnop, D., and Sun, Y.: Understanding atmospheric organic aerosols via factor analysis of aerosol mass spectrometry: a review, Anal. Bioanal. Chem., 401, 3045–3067, doi:10.1007/s00216-011-5355-y, 2011.

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Sample information Precursor

Oxidant

a

t1/2

OM / OC

O/C

AMS results P c b m/z ≥ 80 OSC

(min)

Nano-DESI MS results # of common

Organic acids

Dissolved volatile

(%)

(%)

moleculese

molecules  

(% of TOC)f

speciesg (%)

0.8

6.6

             149           

0.7

5.9

0.8

4.6

2.2

2.7

2.1

2.8

3.8

5.7

Syringol

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C

16

2.29

0.85

1.66

0.04

13.3



1156

(C8 H10 O3 )

·OH

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2.27

0.86

1.64

0.08

12.3



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C∗

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2.37

0.92

1.79

0.05

14.7

0.70

827

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·OH

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2.72

1.18

1.85

0.51

9.8

0.15

871

Phenol

3

C∗

480

2.63

1.11

1.70

0.52

8.8

0.51

721

(C6 H6 O)

·OH

672

2.79

1.23

1.72

0.74

6.8

0.01

445

883

643

209

t1/2 is the time when approximately half of the phenolic precursor was reacted (as monitored by HPLC/UV-vis).

b

OSC indicates the oxidation state of the carbon atom (= 2 × O / C − H / C) % of total ion signal at m/z ≥ 80 in the AMS spectra. d Estimated based on the signal contribution of the molecular ions of the dimers in the AMS spectra and the NIST spectra. NIST spectrum of syringol dimer is not available. e Total number of molecules identified in the (+) ion mode and (−) ion mode nano-DESI MS spectra. f % of organic carbon mass in aqSOA accounted for by the sum of 8 organic acids (formate, acetate, pyruvate, malate, oxalate, malonate, fumarate, and mealate). g Dissolved volatile species is calculated as the differences in TOC between flash-frozen and blown-down samples after correction for the mass of unreacted precursors. c

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Table 1. Summary of the chemical characteristics of phenolic aqSOA formed under different experimental conditions.

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3 OH OCH3 OH OCH3 OCH3 OCH CO OH OCH C15HH C H O6 C15H16 18OO 7 OCH OCH3 CO 7 OH H O C15HHH18 O363 CO OHOCH 4 C15 H416 O6 15 16 8 8 OCH3 C H COOH HOHO OCH 7OH 15 18 7 OCH H CO H O OCH 8 4 C H O H O C OCH 3 15HO 16 6 3 15 18 OCH OCH3 (310.1052) OCH H CO H CO 7 H CO (292.0946) OH HO (292.0946) (310.1052) HO (292.0946) C15(310.1052) H188O7 (310.1052) C15HH164OO6 (292.0946) OH HOCH O HO OH H CO OH C15H18O7 OH HO 15 16 6 OCH33 8 7 4 4 CC H OCO OH O CC1515HH1818OO77 C15H18HO HH OO OCH HO O 8OCH 7OH OH (310.1052) 1515 1616 6 3 3 OCH C H C 7 OCH H OCO CO H CO 3 3 (310.1052) 3 OH 8OCH 77 4 4 (292.0946) (292.0946) O 8 OCH 8 4 6 15HOHO16 6 OH HOH OH 3 (292.0946) (310.1052) O OH OH (292.0946)(292.0946) OCH (310.1052) (310.1052) OH OH OH O 3OH OH OH H O C8H10O5 C HH3CO C OCH 3 CO OCHOCH OH OCH 8 10 5 3OH OH OCH 3 H 3CO O OCH O C HHOO10 O H 3 CO 3 3 5 4OCH 33 OCH 3 3H CO OCH4OCH O55OH C88H1010O55OH C88H10OH 3 H 3 CO 5 4 OH 3 (186.0528) 3 (186.0528) OH OH OH 5 4 H O OH OH (186.0528) (186.0528) H O OH H O H O C C OCH H CO OCH H CO 8 H 10 5 8C 10 5O (186.0528) OH 3 HO OH 3 H O OH 3 CO OH 3 OH OH OH OH H O C OCH H OCH H CO OH 5 C85H10 O5 C 4 H 8 10O5 (186.0528) 8 10 5 3 3 3 3 44OCH HO OH OCH HO OH OH OO CC88HH81010O10 OCH H10 5 CC 44OCHOH3 HH3O CO5 O55 5 C8H10 OCH OCH H5 CO CO OCH 8H 1010 5 5 C8H 33 H 33CO 3HCO H 3 CO OH 8H 3 3O 3 CO 3 (186.0528) OCH (186.0528) H CO OH OCH H3OCO 5 5 (186.0528) 4 4 (186.0528) (186.0528) (186.0528) OH 4 3 OH 5 4 HO OH HO OH OCH3 H3CO OCH H CO HO OH OH (186.0528) (186.0528) (186.0528) (186.0528) OH HHOO OH OH OH HHO3CO HO OHOCH3 C15H18O7(186.0528) C12H12O7(186.0528) HHO CO OH OCH OH HO OHOH H CO OH C15HH C12H O7OHOH OH H12OOOH 6 7OH 7OH OCH 18O 7OH H CO H CO OH OCH3 H3CO OH C C OH 6 7 OCH 15 18 7 12 12 7 H CO (310.1052) (268.0583) OCH 3 H H COCO 3 OCH H HCOCO OCH OH 6O OH (310.1052) (268.0583) OCH HH33CO OCH CO OCH3 H333CO OH CC C HH1818 H127O 1515 H O CO C12 O7 7 (310.1052) O77 (268.0583) OOH 12H12 H CO OHOH ∗ 6 6 ofCC 7 7 O OH OH H O O 7 7 C H O H O Molecular formula top 10 most abundant compounds with their exact mass in the parenthesis. C H O H O O OH 1515 1818 7 7 C15H18 12 12 12 12 77 C12H12O7 HO COCO H 7 HO CO HO O OH OH OH O (310.1052) (268.0583) 6 6 (310.1052) 77 (268.0583) OH 6 7 OH HO O 7 OCH OH O (310.1052) (268.0583) (310.1052) (268.0583) OH (310.1052) (268.0583) OCH OH CO OCH O HO OH O OCH H O O OH HO OCH3 O O OH O H3CO HO O O O OH OCH3 H3CO O OH C12H12O7 C15H16O9 HO OCH HOCH OO O HO O OH OH OH OHOH H HCO OOH HOH16 O93 O C15H 8 OCH 7 7OH OCH 12O 7OH OCH O CC1212H C H3H12 OH HO 7 15 16 9 OCH OH 3 H O OH 7 7 (268.0583) (340.0794) OCH OCH3 OH OCH OH 3CO HO HO OCH OH 7 7 H O OH OH (268.0583) (340.0794) OCH H CO OH OCH H CO OCH 33CO H 3 H O 3 3 H O 3 H O H O H O CC C HOOH OH 1212H 1212O 7 7 (268.0583) OH 15 16 9 (340.0794) OH OCH3 OH O OH H O C OCH 15 16 9 OCH 21176 H O OH O HO O OH 8OCH 7 7 CC 77O HHO 8OCH HH OO CC1515HH1616OO99 C15H OCH 1212 1212 7 7 C12H12O7OCH OHOH 16OOH 9 OCH HO OOOH O HOCH 8 7 (340.0794) HHOO OH (268.0583) (340.0794) O HO O7 7 7 (268.0583) 7 7 H O OH OCH HOH O OH HO O (268.0583)(268.0583) (340.0794)(340.0794) (268.0583) (340.0794) OH O OH OOOH HO OCH OCH3 OH OH OH HHOO OCH HO

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DBEa Molecular Proposed structure DBE No. a Molecular C* reaction •OH reaction structure DBE Molecular Proposed No. Molecular a 3 a structure 3 structure Proposed C* reaction •OH reactionDBE formula formula structure formula formula structure C* Proposed reaction •OH reaction DBEa Proposed structure DBE No.a a a structure DBE Molecular Proposed DBE No. Molecular Molecular Proposed OCH OCH formula formula 8 C H O Proposed structure DBE aa Molecular 16 18 6 Proposed structure Molecular HO OCHDBE HO formula DBE Proposed structure DBE aOCH 3 structure formula Proposed DBE a formula (306.1103) H O H O C OCH H O C HO 18OCH 16 6 16 18 6 formula structure OCH formula OCH OCH OCH OCH 3 CO OCH H CO OCH 8 8O 8 OCH 1 8 OCH HCO O O6 OCO O6 3 HOCH C16HHH C16HHHH O16 (306.1103) C(306.1103) C(306.1103) O 18 OCH O 18 OCH (306.1103) 16H181 16H18 6 CO 3 HHOCH H 3 8 8 O HOCO CO CO H H OH OH H O H O OH OH 8 1 8 H O H O C C 3 3 16 18 6 (306.1103) 16 H18 O6 OCH OCH (306.1103) H O C C 16 18 6 16 18 6 OCH OCH CO CO H H 88OCH 3 OCH 3 OCH C H18O6 OH(306.1103) OH CO18O6 H 3H C16H118O6 C16 C16H OCH OCH 1 1 (306.1103) 88OCH OCH OCH OH OH (306.1103) (306.1103) H CO H CO 8 8 18O6 16HH3CO CO H CO OH 8OCH 1 (306.1103) 8OCH (306.1103) OH OH OCH 3OH OCHOH (306.1103) 3 OH OH (306.1103)(306.1103) OH (306.1103) H O OH H O OH OH OCH OH OCH OH OH H 3 CO OH OCH H CO OCHOCH OCH O 5 OCH 3 CC C9 H10 O4 C9H10O4 C9H10O4OH OCH3 33 9 OCH OCH OH HH OO O6 OCHOH3OCH 1515 1414 6 6 C15 HOOH14 OCH3 3 OH OH OCHH 3 CO 5 3 (290.0790) 2 9 OCH 5 2 9 OH OH6 3 C15HHOH C9HHH3CO O4 OH (182.0579) 10CO 14O O 3 O OH (290.0790) OCH C C(182.0579) H O H O (182.0579) (290.0790) OH 3 9 10 2 4 15 14 O HO OCH3 5 6 9 HO OCH 33 H 3 CO OH C C O4 (182.0579) OH 5 2 9 9H10 15H 14O 6 O OCH 3 H 3 CO OCH (290.0790) OCH H CO 3 O 3 C C H O H O 3 O 5 9 H 10 4 14 6 C H O 99OCH 3 OCHOCH O OH C1515H C 3 OCH 9 10O4 C9H10O4 OCH 55 (290.0790) 2 22 (182.0579) 3 (182.0579) (290.0790) O OH 5 14O6 15 O 14 6OCH O OH 2 9 9 33 (182.0579) (290.0790) OH OCH OCH3 OOH (290.0790) (182.0579)(182.0579) (290.0790) OCH OH 3 O HO HO HO OH HO OCH OH OCH OH O 3 OH O OCH HOCH O O HO OCH3 OCH 15HH 16O6 O OH6 O H OCH O6 OCH OCH33OCH 9 9 OCH CC 8 C15 H314 O6 C15H14O6 C15 O OCH HOH14 HOHO 16 15 9 16 O6 C15 8 OCH OH OCH HO 3 8 H C C OH HO14O OH HO 15H 6 15 16O 6 O OCH (292.0946) OCH HO OCH (290.0790) (290.0790) (292.0946) (290.0790) (292.0946) HO HO O H O CC C 9 8 15H143 6 15 16 6 OHOHO OCH OCH OH OH H O H O 3 OH OH C15H16O6 H O HH OO 3 3 C1515 9 8 (290.0790) (292.0946) 14 6 O OCH OCH OCH OCHOCH HH OO CC1515HH1616OO66 C15H OCH HO HO O OH 8OCH 1414 6 OCH3 OCHOH (292.0946) C OH O C15 16 6 OCH OCH OCH HO 3 3 (290.0790) 99OCH 88OH OH HOH (290.0790) (292.0946) O OCH HO 3 6 15 O14 6OCH3 OOHOH 9 8 9 OCH (292.0946) OH OH (290.0790) H CO OCH3 (292.0946) OCHOCH (290.0790)(290.0790) (292.0946) OCH3OH OH OCH H CO OH OCH OH OCH OH OH OCH OCH 3 OCH 3 Proposed formula structure structure CNo. H18 OMolecular 8 DBE No. a Molecular 16 6formula DBE Molecular HProposed O OCH HOProposed formula structure 3 OCH No.a a 3 structure DBE No. formula (306.1103) H O structure H O C16 OCH C16H18Oa 6 formula HO 18 6 3

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Table 2. Top 10 most abundant compounds in syringol SOA formed in 3 C∗ - and ·OH-mediated 3 reactions with (–) nano-DESI Table Topabundant 10 most abundantMS. compounds syringol SOA 654 2.identified C*- and •OH-mediated Top 10 2. most compounds in syringolinSOA formed in3 3formed 654 Table C*- andin•OH-mediated 3 Table 2. Top 10 most abundant compounds in syringol SOA formed in 654 C*- and •OH-mediated Table 2. Top 10 most abundant compounds in syringol SOA formed in 654 C*and •OH-mediated 33 655 reactions identified with (-) nano-DESI MS. 655 reactions identified with (-) nano-DESI MS. 3 ∗ Table Top most abundant compounds in syringol syringol formed in C*654 Table •OH-mediated 3 2.2.Top 1010most abundant compounds in SOA formed 654 C*-and and •OH-mediated C 3nano-DESI reaction reaction 3·OH 655 reactions identified with (-)3C* nano-DESI MS.inSOA 655 reactions with (-) MS. Table Top 10 most abundant compounds syringol SOAin inreaction 654 C*- and •OH-mediated Table 2.identified Top 102.most abundant compounds in syringol SOA formed informed 654 C*-•OH and •OH-mediated reaction C* reaction •OH reaction 655 reactions reactions identified with (-) nano-DESI MS. 3 MS. ∗ (-) ∗ 3 nano-DESI 655 identified with reaction reaction formula DBE Molecular•OH formula Proposed DBE 655Molecular reactions identified with (-) C* nano-DESI MS.Molecular C* reaction reaction•OH 655 No. reactions identified with 3(-) nano-DESI MS. 3 Proposed Molecular Molecular Proposed Molecular Proposed C* reaction •OH reaction

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3 OH OCH C15H14O C H10OOH4OO 5 C15H16O6 (290.0790) 2 C15H14O6 (182.0579) 93 OCH O 3 OH OCH OCH OH3 OCH 5 2 9 OCH O6OCH OCH C15O9H C15 HO14O6 HH O (182.0579) O O H3OOCH 3 OH O H OCH OH HO 16 6 3 9 OCH3 (290.0790) 8OCHOCH HOOH (182.0579) (290.0790) 3 9 8 3 H O H O OH H O H O C C OCH (290.0790) (292.0946) H CO15HH C O 14OO6 H O 16O6 15 14 6 (290.0790) 15 16 6 (292.0946) OCH OH OCH 15 O 3 OH OCH OCH 3 OCH OCH OH OH 3 9 OCH 8 OCH 3HO O 9 O C15HHO16HOO 6 8 OH OH HO6 H14O O OCH O HOCH H C153H143O6C15(290.0790) C 3 3 (292.0946) O (290.0790) H O 15 16 6 OCH OCH (292.0946) OH OCH OCH H O OCH 3 OCH 8 HO OH OH OCHOH 3 93 OH C915H16O(292.0946) 8 OH C15H14O(290.0790) OCH 6 C HO 6 C H OCH OH 3 H14 O O6 HO O OH H O (290.0790) (292.0946) 15 15 16 6 O OCH OCH 3 93 3 8 OCH HO OCH3 HO OCH OCH OCH3 OCH OH OH OCH HOH H CO OH CO 8 OH 3 OCH (290.0790) (292.0946) C15H314O6 (290.0790) C15H916O6 (292.0946) OH OCH3 OCH OH OCH OCH OCH OH HO O63 OCH CH15OHO 14OCH C15HH16 OO 3 8OCH O OH OCH3 39 OCH OCH OH H CO 6 OH OCH H COOH OH H O 3 9 8 OH OCH OCH3 OH C(292.0946) C(290.0790) HH O O6 15H16O6 15H18O7 (292.0946) OCH OCH HOH O C15HH18COO C OH 3 OCH H HCO 3 (290.0790) 15 7 3 CO Table 2.4Continued. 3 3 H CO OH HO 16 OCH 8OCHOCH 7OCH OH OCH HO 8 O7 7 OH OH OH OH O OCH3 HOCH OCHOCH C H18 C15H416O6 (292.0946) HOH16O CO (292.0946) (310.1052) 3 C H C OCH COHO H 18 15 6 15 7 (310.1052) 3 OCH3 OCH OCH3 15 CO OCH HCO H OHOCH HO HO 8 7 4 OHHO OH OH 8 7 4 OH C H O H O C O 6 OCH3 15 18 7CO C H O C154H16O6 315(292.0946) OCH3 (310.1052) HOOCH3 HO CO OH (292.0946) 15 18 7 CO H (310.1052) OCH H ∗ 16 OH HO OCH 7 O OCH OCH 3 OH 83 OH C815H18O(310.1052) 7 4 OH OCH H CO C HH reaction C15H16O(292.0946) 6 C 7 C H OCH3 3 H O O H CO ·OH reaction O O OH OH (292.0946) (310.1052) O 15 16 6 15 18 7 OCH H O H CO 8 7 4 OH OH 3 3 HO OOH OH OCH3H 8 7 4 OH OH OCH ∗ OH OH (292.0946) (310.1052) O OH 15H16O∗6 (292.0946) 15H18O7 (310.1052) OCH3 OH CO OH No. Molecular formula Proposed Proposed DBE C H O OCHDBE C8CH CC OCHH33 CO CO H 3 CO OH H O16 18O 8 OCH 7OCH 3 10O5 8HMolecular 10O5 3 3 3H OO5 6 OH OCH H CC815HHH C158formula 310 OCH CO CO 10 57 OH OCH H 3 3 3 OH OH OOH OH H O OH 8 7 4 5 4 4 4 (292.0946) (310.1052) 5 4 4 O OCH3 structure C C H OCH structure HH CO (292.0946) (310.1052) (186.0528) (186.0528) H O O C C OH OCHOCH 8H10O5 (186.0528) 8H10O5 (186.0528) OCH 3 3H 3 CO H 3 CO OH 3 CO OH OCHOH 8 10 5 8 10 5 3 3 OH OH H O OH HO OH HO O 5 4OCH 4OCH HO OH OH 5 5 C8H10O 4 OHOH C(186.0528) HOCH OCH O H104H O5 O C8H10O C84C 8H10O H 35 CO OH OH OCH H 5 CO 3 OH H 3 CO 3 CO 3 3 (186.0528) (186.0528) OH OHOCH OH 4OH 74 33 (186.0528) 6 C155H18 O H O OH OH OH 47 HHO OH OCH H O OH 7 C5 1210O 125 7 OCH COCO C8H OCHOH HO OCHOH H CO 8H10O 5 C8HH10 3HO 3 3 3 CO (186.0528) (186.0528) O H C OCH H CO OCH H CO OH OCH OH (186.0528) (186.0528) H CO 5 8 10 5 3 OH 3 OH 3H 3 OCH CO 5 43 H3O OH H O OH OH HO (310.1052) OHOH4 HCO O OH 4 OHOH OCH OH (186.0528) (186.0528) H5O H410O5 (186.0528) CH818 C8(268.0583) OCH 3 H CO HH3 CO OCH HCO CO OH OCH 10O5 (186.0528) OH C15 H5O OHOCHOH 3 CO OCH3 HH3O COOH H18H10 O O OCH 3 C12H12O7 H 3HCO OCH H7O H OH 7 CC158H CC128H O HH10312 O 5 OH OCH CO 4OH 7 HCO O OCH4 H CO OH OH H CO OH 6 5 C(186.0528) 7 7 5 4 4 OH OH 6 7 7 OH OCH3 H3CO OCH OH C(186.0528) 3 (268.0583) OO7 OH OH 15H18O7 (310.1052) 12H12O7 (268.0583) C15HH 18 C HHH3CO (186.0528) (186.0528) CO OH H OO OCH OH H CO7 OH HO OH 6 (310.1052) 7 OCH 7OH HHOHCOCO OH OH OCH H3CO 7 O7 C12H1212O12 7 H18OOH7H COCO OH OH OCH H33CO C15H186O7C15(310.1052) C12H12 7 OH (310.1052) (268.0583) HOCO OH O (268.0583) OH7 3 OCH OH O OH OH 6 7 7 6 7 H OCO O OH OCH H CO H O O C C H O H O OCH OH H CO H 15 18 7 12 12 7 O 3 OH C15H18O7OH C12H123 O7 O OHH3CO (310.1052) (268.0583) (268.0583) OCH HOCO 3 6 (310.1052) 7 7 OOH OH OH HOCH O 7 O OCH HO O OH O C15H618O7 (310.1052) C12H712O7 (268.0583) OH O (310.1052) (268.0583) OH H CO H CO O O OH C15H OCH O 12O7 18O 7 OH OCH H CO HOHO 7 7 H CO O OHOH OCH OH7 HO OH 7 C126H12 O7(310.1052) C715 H16 OC912HH 8 O OCH OH OOH 6 7 O (268.0583) O OCH H CO C12H12O7 C O OHO O OCH C15H16O9 H CO C15HHH OH (310.1052) (268.0583) O OCHHOOCH 12H12O7OH OHOH OCHOH (268.0583) (340.0794) O 8OHOCH O 16 9 OH 7 7 HO HO OH O 8 7 7 OCH H CO C12H12O7 (268.0583) C15H16O9 (340.0794) O H O OCH (340.0794) C12HH 12COO7 OHOHOCH C HHOOHHHOOOOCH OH 9 O OOH OCH O OCH OH 7 (268.0583) 7 OCH OH HOHOOH OH HO O 8 OH 8 7 O9 C15HH15O16HOO16 OCH3OH CO H12OH73OH C12H127O7C12(268.0583) C15H16 9 (340.0794) OCH HOCH O OH OOH OH H CO OCH OH (340.0794) OOH7 OH OCH H O HO HOCH OH O OH 8 7 7 7 (268.0583) HO O OCHHO OH8 OH HO O H O H O C C OCH H CO 12 12 7 15 16 9 OCH H O H O H O C C (340.0794) (268.0583) (340.0794) OH OH 12 12OCH7 O 15 H16 9 O OCH OCH OCH OHH CO OCH O O H O OH OCH 8 7 (268.0583) 7 OH O OH HO HO O HOH O HO 8 HO OH O C H712O7 (268.0583) C15H716O9 (340.0794) (268.0583) (340.0794) HO OH O OH O OH OCHOCH OCH OCH O C HOH12OCH C15HH16 OCH OH OH O O9 HOO OH O8 HO7 OH 7 C15H1216O 7 HO O O HO OH OHOH HO O C 8 7 9 C1512HH HO O 7HC 67 5 6 O5 C7H6O5OH OCH HOCH 8 C815 H16 O 8 5 H O O (268.0583) (340.0794) 16 9 OCH OCH OCH OH 9 7 O O OH OH O5 OCH O (268.0583) (340.0794)HO OH HO O HO HO 5 8 O8 OH C15H816O9 (340.0794) C7H(170.0215) OH HO OH O OH O HO 6O5 OCH (170.0215) O OCH CH15OHH C7H (340.0794) OCH 16 9 OH (170.0215) OCH OOH OH O HO OCH 8 HO6 5 OH 5 8 (340.0794) O HHOO OH HO HO OHO O OCH OH OH 5 8 8 OH C15H16O9C15(340.0794) C7H6O5 C(170.0215) H O H6O HO O OCH OCH (170.0215) H O HO OCHOCHH OH OH OCH HO5 OH O OH OH HO OH 5OH O OH8 HO OH 8 (340.0794) 5 8 H16O9 C 16HHHOO 9OH 87H6O5 7C H OCH HO OH O OH C15 C O O OCH OCH OH (340.0794) (170.0215) (340.0794) 15 H16 9 OCH 7 6 HO5 OH OHOCH O OH H O OH OH OCH HO OH OHOH5 8 8OCH C H8 O (170.0215) HO OH H O HO 5 OHOH OH HO O O HO C15H816O9 (340.0794) OCH OH OH (340.0794) (170.0215) 7 6 5 (170.0215) OH OH HOCH OCH OH OH OCH C15HHOH16 C7HH OCH 6OH 5 O OH OHHO OH O O9 HO OCH OH OH 5OHOH 8 C16 8 HO OH C12H10 H O H O H O 18O 9 7 H O H O C C OCH 5 8 8 OH HOCH O 16 18 9OH 12HO10 7OH OH OCH OH OCH (170.0215) HOOH (340.0794) H CO OCH 8 9 8 OH H CO OCH OH H O 88 8 OH7 O (340.0794) (170.0215) O HO OH OH C12HC10 HO OHO 18O9 (354.0950) OCH OH OH HO OH H O HO 9 C16 OC9 16H9 (266.0426) HHOHOH O7OHOH OH CH 16 H O 9 OCH OCH OH OHOH 12 10 (266.0426) 7C12 COH18 OCH OH 8 HO 10 9 H18(354.0950) 88 OH OCH HO OH HO OH H CO HO OH 8 8 O7 C12 OH OCH OH (266.0426) H10 C16H189O9C16(354.0950) C12(266.0426) OH HH18CO OHO9OH H10HO HO7 HOCH O OCH HOH O OHOH OH OH (354.0950) OCH (266.0426) OCH OH OH OCH OH O OH H O OH 8 9 (354.0950) 8 H CO OH OH H O 8 9 H18O9 C HHOOCH 812H10O7 C H O OCH OH OH HO OH C16 C O (354.0950) OCH H10 O OH OH 16 H 18 9 12 7 (266.0426) (354.0950) OCH OCH (266.0426) OH 3 OCH OH OH OH OH OH 3 H O H O OH OH CO HO OCH OH HO HO H CO 8 9 8 OH HOH O 8 OH OHOH OCH OH C16H918O9 (354.0950) C12H810O7 (266.0426) HO OCH OH OCH (266.0426) OCH OH OCHOCH O93OHHOH O OH CHOH18OCH C12HHH10CO OH H CO OH OH OH 8 OH 3OH 8 OCH O O 7 9 C(354.0950) OH OH OH H COOH HO 8 9 8 C1516HHOH 14CO 8 OCH HO H CO OCH C H O OCH H O OCH 3 (266.0426) (354.0950) OH OCH OH 15H14O 12 12 6 H CO C O H O OCH 3 OH 3 OCH OH 8 HO 12 12 6OH OCH OH 3 OH (354.0950) HO OH HO OH OH 7OCH 10 OH OH 7 9 O6 (266.0426) HO OH C12H12 HO 9 HOOCH3 OCH OH C15H10 H CO OCH3 OH 14O8 (322.0688) CH15 C12HH12CO O6 OHOHOCH (252.0634) 3 OCH OH OH7 O HOH 14O 8 (252.0634) 3 10 (322.0688) 9OCHOHOCH HO OH OH OH 7 10 9 H O H O C C H O H O OCH OCH H CO OCH H CO 15 14 8 12 12 6 C C H O H O OCH OCH3 (252.0634) 3 (322.0688) OH 15(322.0688) 14 8 12 12 6 3 O OH OH OH H O OCH O (252.0634) OH H O 3 7 10 9 OH OH 10 C15 H14 O8 10 9 OH O 77 9CH HO O 12 HO 12 OC 6 HHHOO O HO O OCH O CO6 O C15H14O(322.0688) C OCH3 (252.0634) (322.0688) OH OOCH 8 C15H 12 12 6 14 8 12 12 3 OCH H CO (252.0634) OH H OH O H O H O O HOH O OHOH 7 10 9 7 10 O O HO O C15H C12(252.0634) H912O6 (252.0634) 656 14 8 (322.0688) 656 (322.0688) 3OH (322.0688) (252.0634) C15HHO14O8OH O HOOCH C12H12OH6O O O HO O OO 7 10 9 OCH OH O OH 3 a 7 10 9 656 a (322.0688) (252.0634) 656 O O OH 657 Molecular Molecular formula of(322.0688) top 10 of most abundant compounds with their exactHOtheir mass in themass in the 657 formula top 10 O most abundant (252.0634) compounds exact HO O with O OH OH O OH 656 a 656 a formula of top 10 most 657 Molecular abundant compounds with theirO exact mass in the 3

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O 657 a Molecular formula O of top 10 most abundant compounds with their exact mass in the HO O parenthesis. 658 parenthesis. HO O 656 O Molecular formula formula of top 10 most10abundant compounds with their exact the in the 657 Molecular of top most abundant compounds with theirmass exactin mass 658 parenthesis. a a parenthesis. 656 657 Molecular formula top 10 most abundant compounds their exact Molecular formula of top 10of most abundant compounds with theirwith exact mass in mass the in the parenthesis. 658 parenthesis. 27 a 27 ∗ a Molecular formula of top compounds 10ofmost abundant compounds with theirwith exact mass in the Molecular of top 10 most abundant with their exact abundant mass in the parenthesis. 657 Molecular formula top 10 most compounds their exact mass in the 27formula 658 parenthesis. parenthesis. 27 27 parenthesis. 27 658 parenthesis. 27 27 27 27

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21177

L. Yu et al.

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ACPD 14, 21149–21187, 2014

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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Figure 1. The average O / C ratios of aqSOA formed from the reactions of syringol (SYR), guaiacol (GUA), and phenol (PhOH) with 3 C∗ and ·OH, respectively determined by AMS (blue bars) and the average O / C of organic acids determined by IC (pink squares). The distributions of the O / C of individual molecules in the aqSOA determined by nano-DESI MS are shown in box plots, in which the whiskers above and below the boxes indicate the 95th and 5th percentiles, the upper and lower boundaries of the boxes indicate the 75th and 25th percentiles, and the lines in the boxes indicate the median values and the cross symbols indicate the mean values. The O / C of the precursors are shown as black circles.

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HyO1

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(a) SYR + C* (t1/2=15min)

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OM/OC=2.29 O/C=0.85 H/C=1.66

30 (g) Slope = 0.97 ± 0.00

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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21179

14, 21149–21187, 2014

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Figure 2. AMS spectra of aqSOA formed from the reactions of (a–b) syringol (SYR), (c–d) guaiacol (GUA), and (e–f) phenol (PhOH) with 3 C∗ and ·OH, respectively. The peaks are colorcoded according to four ion categories: Cx H+y , Cx Hy O+1 , Cx Hy O+z , and Hy O+1 (x ≥ 1; y ≥ 0; z ≥ 2). The ion signals at m/z ≥ 80 are enhanced by a factor of 20 for clarity. The photoreaction time and the elemental ratios of the aqSOA are shown in the legends. Scatter plots that compare the mass spectra of aqSOA formed from two different oxidants for all m/z 0 s (g, i, k) and for m/z ≥ 80 (h, j, l) were performed using the orthogonal distance regression (ODR). The linear regression slopes and correlation coefficients are shown in the legends.

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ACPD 14, 21149–21187, 2014

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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Figure 3. (a) Van Krevelen diagram of common molecules identified in every phenolic aqSOA via nano-DESI MS analysis. Each data point is colored by its DBE value. (b) A frequency histogram of the molecular weight of these common molecules.

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ACPD 14, 21149–21187, 2014

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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Figure 4. Contributions of reactants (phenolic precursor and DMB) and products (dissolved volatile species and aqSOA) to the solution TOC after illumination to t1/2 . TOC amounts are expressed relative to the TOC in the initial solution prior to illumination (i.e., at t0 ).

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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Figure 5. The signal weighted distributions of syringol (SYR), guaiacol (GUA) and phenol 3 ∗ (PhOH) aqSOA formed in C - and ·OH-mediated reactions, respectively, based on the degree of oligomerization. The data are from the (−) nano-DESI MS spectra. Note that hexamer and derivatives are only found in (+) nano-DESI MS spectrum for GUA aqSOA initiated with 3 ∗ 3 ∗ C and (−) nano-DESI MS spectrum for PhOH aqSOA initiated with C . The numbers indicate the contributions of individual categories to the total signals for each sample.

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ACPD 14, 21149–21187, 2014

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Figure 6. Comparisons between (a) the AMS mass spectra (in integer m/z) of phenol (PhOH) 3 ∗ aqSOA formed via reactions with C and ·OH, respectively, and the NIST mass spectra of (b) biphenol-4,4’-diol and 4-phenoxyphenol, (c) 4,4’-dihydroxydiphenyl ether, (d) methylparaben and ethylparaben, (e) catechol and hydroquinone, and (f) phenol. The chemical structures for each compound are shown and the molecular ions (M+ ·) are marked.

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ACPD 14, 21149–21187, 2014

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Figure 7. A schematic illustrates the formation of hydroxylated species, dimers and higher oligomers, esters, and demethoxylated products from aqueous photooxidation of phenolic compounds. Species produced via pathways (a–e) may undergo further ring-opening processes to form ketones and carboxylic acids. Phenol: R1 = H, R2 = H; Guaiacol: R1 = OCH3 , R2 = H; Syringol: R1 = OCH3 , R2 = OCH3 . Note that while radical coupling here is shown through the carbon opposite (para) the phenoxy group, other geometric isomers will also be formed during these reactions.

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ACPD 14, 21149–21187, 2014

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Figure 8. Comparisons of the relative abundances of signature ions in the AMS spectra of the aqSOA of (a) syringol (SYR), (b) guaiacol (GUA), and (c) phenol (PhOH) produced from 3 C∗ and ·OH-mediated reactions. The signal contributions of certain signature ions are enhanced for clarity. The m/z values of the signature ions are shown in front of the ion formula in the x axes. Identities of possible parent compounds are shown to the right. 2OH represents 2 additional hydroxyl groups attached to the aromatic ring.

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Discussion Paper

ACPD 14, 21149–21187, 2014

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SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

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Figure 9. Mass thermograms of ammonium sulfate ((NH4 )2 SO4 ), ammonium nitrate (NH4 NO3 ), syringol (SYR), guaiacol (GUA) and phenol (PhOH) aqSOA formed in 3 C∗ -mediated aqueousphase reactions.

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Discussion Paper

ACPD 14, 21149–21187, 2014

| Discussion Paper

SOA from aqueous reactions of phenols with two oxidants L. Yu et al.

Title Page Introduction

Conclusions

References

Tables

Figures

J

I

J

I

Back

Close

|

Abstract

| Full Screen / Esc

Discussion Paper |

21187

Discussion Paper

Figure 10. UV-vis spectra of syringol and syringol aqSOA formed in 3 C∗ - and ·OH-mediated aqueous-phase reactions. The aqSOA spectra were corrected for absorbance contributions from unreacted reactants (syringol and DMB).

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