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Aerosol Brown Carbon from Dark Reactions of Syringol in Aqueous Aerosol Mimics Jian Xu, Tianqu Cui, Brandon Fowler, Alison Fankhauser, Kai Yang, Jason Douglas Surratt, and V. Faye McNeill ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00010 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Earth and Space Chemistry

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Aerosol Brown Carbon from Dark Reactions of Syringol in Aqueous Aerosol

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Mimics

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Jian Xu,1,2 Tianqu Cui,3 Brandon Fowler,4 Alison Fankhauser,1 Kai Yang,1 Jason D. Surratt,3 and

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V. Faye McNeill1,*

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(1) Department of Chemical Engineering, Columbia University, New York, NY, USA 10027

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(2) Department of Environmental Science and Engineering, Fudan University, 220 Handan

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Road, Shanghai 200433, P.R. China (3) Department of Environmental Sciences & Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA 27599 (4) Department of Chemistry, Columbia University

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*to whom correspondence should be addressed: email: [email protected], phone: +1 (212)

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854-2869

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Abstract

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We performed a laboratory investigation of the chemical processing of syringol, a representative

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model phenolic compound emitted from wood burning, in concentrated aqueous salt solutions

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mimicking tropospheric aerosol particles. For solutions containing chloride salts, we observed the

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formation of light-absorbing organic products (‘brown carbon’), accompanied by a phase

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separation, within 10 hours under dark conditions. Products were characterized at the molecular

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level using ultra-performance liquid chromatography interfaced to diode array detection and high-

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resolution quadrupole time-of-flight mass spectrometry equipped with electrospray ionization

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(UPLC/DAD-ESI-HR-Q-TOFMS) and matrix-assisted laser desorption ionization interfaced to

ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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high-resolution TOFMS (MALDI-TOFMS). The UV-Vis spectra, together with high-resolution

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mass spectra results, suggest that syringol can be oxidized by dissolved oxygen, and the presence

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of Cl- promotes this reaction. Our results provide new insights into the evolution of aerosol optical

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properties during aging, specifically the formation of aerosol brown carbon in biomass burning

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plumes.

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Keywords

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aerosols, particulate matter, brown carbon, atmospheric chemistry, biomass burning

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Introduction

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Light-absorbing organic aerosol material, or aerosol ‘brown carbon’ (BrC), is an important but

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poorly quantified component of atmospheric aerosol emissions from biomass burning (BB).1-4

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While at long wavelengths light absorption by BrC is negligible compared to black carbon, its

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absorption increases with decreasing wavelength below ~400 nm. Previous ambient observations

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of aerosols in areas influenced by BB reported that BrC contributed 20–40% of total aerosol

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absorption at 350 nm,5 27% of total aerosol absorption at 404 nm, and almost negligible absorption

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at 532 nm.6 The optical properties of BB-derived BrC depend in a complex manner on the

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combustion conditions,7 fuel type,3, 8-9 ambient conditions,10 chemical functional groups, 11-12 and

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mixing state,6, 13 as well as on chemical aging processes.3, 14-16 For example, light absorption by

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primary BrC emissions showed a weaker wavelength dependence than did absorption by BrC

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formed via secondary processes.3 Aerosol acidity can also modify the absorptivity of BrC. Lin et

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al.17 reported that light-absorbing properties of BrC can be affected by the extraction solvent pH.

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Previous studies reported that aromatics, especially polycyclic aromatic hydrocarbon derivatives,

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polyphenols, and nitrogen-containing aromatics, are major contributors to BrC light absorption.11,


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BrC has been associated with extremely low-volatility organic compounds7 and large molecular

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weight compounds.18-19 In most cases, fresh BB-derived BrC is expected to lose its light-absorbing

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properties within one day due to chemical aging, however, certain BrC fraction are rather resistant

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to atmospheric aging.18-20 Additional studies suggested that the dark processing of BB-derived BrC

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may change the dominant chromophores from lignin fragments in the fresh emissions11, 21 to

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nitroaromatic species in the aged plumes.17

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It is becoming clear that BrC plays an important role in climate. Consideration of light absorption

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by BrC in models changes calculations of the net direct climate effect of organic carbon aerosols

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from scattering to close to zero, especially in areas heavily influenced by BB.22-23 Since BB was

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an important source of atmospheric aerosols prior to the industrial revolution, understanding the

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optical properties of BB aerosols and how they may evolve after emission is important for

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calculations of aerosol radiative forcing.24

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Methoxylated phenol compounds, including syringol and guaiacol, are products of lignin

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combustion, and have long been used as chemical tracers for BB in atmospheric aerosols.25-26 Field

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measurements reported that methoxy-phenols were enriched in the atmospheric aqueous phase,

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with concentrations up to three or four times higher than predicted based on Henry’s law.27 Several

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recent studies suggested that aqueous phase chemistry of methoxy-phenols resulted in BrC

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formation.16,

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compounds, including syringol, guaiacol and phenol, serves as an important pathway to form

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yellow-brown-colored products with higher molecular weight.31 They found that the IR spectra of

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the colored products are similar to that of the humic-like substances (HULIS) found in ambient

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organic aerosol. They also reported enhanced formation of colored compounds from precursors

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with higher methoxy substitution. Kitanovski and co-workers examined the products from aqueous

28-31

Chang and Thompson showed that the aqueous OH oxidation of phenolic



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photonitration of guaiacol and a yellow-colored solution was observed.30 The reaction products

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and intermediates identified, including 4-nitroguaiacol and 4,6-dinitroguaiacol, were also

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confirmed in the ambient aerosol samples. In another recent study, vanillin, a guaiacol derivative

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and another proxy compound for BB emissions, was reported to form yellowish color and high

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molecular weight products at higher concentrations when it undergoes aqueous photolysis.29 The

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authors demonstrated that a large portion of the organic mass remained in the particle phase and

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non-volatile products were formed. More recently, Yu and coworkers performed experiments of

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BrC formation from aqueous reactions of phenol, guaiacol and syringol with two oxidants - the

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triplet excited states of an aromatic carbonyl (3C*) and hydroxyl radical. They observed that the

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product mixture absorbed light over the visible region and attributed this to conjugated double

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bonds formed during polymerization and functionalization. In a follow-up study,32 they

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investigated the chemical aging of the BrC products and showed that oligomerization is a major

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process in the first 2 hours, while functionalization and fragmentation dominate the continued

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aging processes. They proposed that aqueous chemistry of phenolics may contribute to the

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formation of extremely low-volatility organic compounds (ELVOCs).

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Here we report the formation of light-absorbing organic compounds via reactions of syringol in

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the absence of light in concentrated salt solutions containing Cl-. High salt concentrations were

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used in order to mimic aqueous atmospheric aerosols.33-34 We observed BrC formation,

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accompanied under some conditions by the formation of a precipitate phase also containing light-

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absorbing organics. Dimer and oligomer product compounds were identified by matrix-assisted

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laser desorption ionization interfaced to high-resolution time-of-flight mass spectrometry

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(MALDI-TOFMS), and light-absorbing products were characterized at the molecular level using

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ultra-performance liquid chromatography interfaced to diode array detection and high-resolution


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quadrupole time-of-flight mass spectrometry (UPLC/DAD-ESI-HR-QTOFMS). The chemistry

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reported here potentially represents previously unrecognized pathways for BrC formation from

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BB.

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Methods

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Solution preparation. All commercially available chemicals were used as received without

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further purification unless otherwise specified: syringol (Acros Organics; 99 %), guaiacol (Acros

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Organics; > 99 %), phenol (Alfa Aesar; > 99 %), NaCl (Fisher Scientific; > 99 %), (NH4)2SO4

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(Acros Organics; 99.5 %), NH4NO3 (Acros Organics; > 99 %), NH4Cl (Acros Organics; 99.5 %),

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KCl (Sigma Aldrich; > 99 %), NaNO3 (Acros Organics; > 99 %), Na2SO4 (Sigma Aldrich; > 99 %),

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CaCl2 (Sigma Aldrich; > 99 %), Fe2(SO4)3 (Sigma Aldrich; 97 %), dichloromethane (Sigma

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Aldrich; ≥ 99.8 %). For some experiments, the syringol was purified before use. In an effort to

101

purify the syringol sample, we prepared a saturated syringol solution (0.2 M). After syringol

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crystallized, we collected the crystals and used a desiccator to dry them before use.

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Salt solutions were prepared using Millipore water (18.2 MΩꞏcm) in a 100 mL or 250 mL flask at

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high, near-saturated concentrations in order to mimic atmospheric aerosol conditions to the extent

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possible in a bulk system.33-36 Fresh syringol solution (0.1 M) was made prior to each experiment.

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The reaction mixtures were prepared by introducing 0.1 M syringol into salt solution. Mixing time

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started count when the reaction mixture was transferred into a stirring (700 rpm), 250 mL Pyrex

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glass container. Typical reaction mixtures initially contained 5.1 M NaCl and 1 mM syringol. The

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syringol concentration used (1 mM) in this study is higher than the syringol concentrations in

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ambient aerosol (0.1-1 µM).25, 27, 37 The upper end of the concentration for total methoxyphenols

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can be as high as ~ 30 µM.27 In previous studies, the concentration of syringol employed varied



112

from 5 μM to 10 mM.31, 38-39 We used an initial syringol concentration of 1 mM in this study in

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order to quantify the formation of light-absorbing products. Results of control experiments

114

featuring lower initial syringol concentrations (0.1 mM and 0.01 mM) can be found in Figure S2.

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Control experiments with other salts and organics were performed as indicated in the text. In order

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to investigate the role of dissolved oxygen (O2) in the bulk solution, we bubbled O2 into one

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reaction mixture and bubbled N2 into another reaction mixture, and the two processes were done

118

simultaneously. The air flow rate for N2/O2 was regulated to an adequate amount in order to avoid

119

any splash of the solution. pH of the aqueous NaCl/syringol reaction mixtures was measured using

120

an electrochemical pH meter (Fisher) and found to be 5.9-6.3 immediately after mixing and 5.1-

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5.5 after 24 h. The pH of the bulk solution was not adjusted throughout the reaction process. The

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small decrease in pH that was observed may be due to the formation of uncharacterized organic

123

acid products. The pH of the 1 mM syringol control solutions was 5.9-6.3 and the pH of the

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solutions containing ammonium salts was between 5.0-5.5.

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The Pyrex glass container was wrapped with two layers of aluminum foil and all the experiments

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were performed under ambient temperature and pressure.

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UV-Vis experiments. Absorption spectra of the samples were determined using an HP 8453 UV-

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visible Spectrophotometer. The sample was pipetted into an open-top quartz cuvette having an

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optical length of 10mm.

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MALDI-TOFMS. MALDI-TOFMS was used to detect high molecular weight compounds in

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reaction mixtures after stirring 24 hours. The method is similar to that described by Shapiro et al.33

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Briefly, 0.4 µL of the mixture and 0.6 µL of the matrix were pre-mixed, spotted on a stainless steel

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sample plate, air dried before analyzed using a Bruker ultrafleXtreme MALDI TOF/TOF fitted


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with a frequency-tripled Nd:YAG laser (355 nm). The matrix solution was either 2,5-

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dihydroxybenzoic acid (DHB) or α-cyano-4-hydroxycinnamic acid (CHCA). Signal of the matrix

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solution alone was subtracted from the spectrum of the mixture.

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UPLC/DAD-ESI-HR-QTOFMS. The reaction mixtures were also analyzed by UPLC/DAD-ESI-

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HR-QTOFMS (6520 Series, Agilent) at the University of North Carolina. Phase-separated or

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DCM-extracted samples from these reaction mixtures after stirring 24 hours were dried and then

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dissolved and diluted by a factor of 15 in a 50:50 (v/v) solvent mixture of high-purity methanol

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(LC-MS CHROMASOLV-grade, Sigma-Aldrich) and water (Milli-Q, 18.2 Mꞏcm) prior to

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UPLC/DAD-ESI-HR-QTOFMS analysis. In addition, the aqueous phase of the reaction mixtures

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was diluted by a factor of 5 in the same solvent mixture used for the phase-separated samples. It

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should be noted that these phase-separated and aqueous-phase samples were filtered to remove

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any potential insoluble precipitates and transferred to amber UPLC auto-sampler glass vials prior

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to UPLC/DAD-ESI-HR-QTOFMS analysis. For comparison to the phase-separated and aqueous-

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phase samples obtained from the reaction mixtures, a 1 mM syringol standard prepared in the same

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solvent mixture and a solvent blank were also analyzed. Details of the UPLC/DAD-ESI-HR-

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QTOFMS operating conditions can be found elsewhere,40-41 and are also summarized in the

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Supporting Information. Briefly, the chromatographic separations were performed on a Waters

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ACQUITY UPLC HSS (high-strength silica) T3 column (2.1 × 100 mm, 1.8 µm particle size) by

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injecting 10 L of each sample. Each sample was analyzed only in the positive ion mode. For the

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positive ion mode, samples elute at a flow rate of 0.3 mL min-1 with a solvent mixture of methanol

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containing 0.1% ammonium acetate (LC-MS CHROMASOLV-grade, Sigma-Aldrich) and water

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containing 0.1% ammonium acetate (LC-MS CHROMASOLV-grade, Sigma-Aldrich), operated

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in the high-resolution mode (mass resolving power above 12,000).



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Computational chemistry. Geometry optimizations and vibrational frequency calculations were

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performed using Jaguar 9.2 (Schrodinger Inc.) with the Maestro graphical interface in order to

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predict and interpret UV-Vis absorption of potential products. Calculations were performed using

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density functional theory with the B3LYP or M06-2X functional and the 6-31G or 6-311G** basis

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set.34, 42 Calculations took minutes to hours when performed on a workstation with 2 8-core Intel

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Xeon E5-2630v3 2.4 GHz processors (Silicon Mechanics).

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Results

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We observed light-absorbing products form in aqueous solutions containing 5.1 M NaCl and 1

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mM syringol within 1 hours of mixing. Because all solutions were covered with two layers of

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aluminum foil until analysis, the reactions resulting in light-absorbing molecules are not

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photochemical in nature. The light absorption of the products varied with the initial NaCl

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concentration in the solution, with higher absorption corresponding to higher initial NaCl

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concentration or longer reaction time. The formation of light-absorbing products is accompanied

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by a phase separation under some conditions; a precipitate formed when the reaction mixture

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contained 1 mM syringol and more than 2 M NaCl.

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Light-absorbing products. An aqueous syringol solution containing no salt has a broad absorbance

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peak at 270 nm at ambient temperatures. Figure 1a shows that absorption at around 470 nm

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develops and increases with time for aqueous mixtures containing 1 mM syringol and 5.1 M NaCl,

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indicating the formation of one or more light-absorbing products. Figure 1b shows the time

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dependence of absorption at 470 nm. Error bars shown are the standard deviation in the measured

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absorbance. Note that data are shown in Figure 1 for up to 9 hours of reaction time, since precipitate

178

formed after 10 hours of reaction for these initial conditions, as discussed in the next section.


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Figure 2 shows the effect of initial NaCl concentration on the formation of light-absorbing

180

products. Measurements were made 24 hours after stirring. The absorption at 470 nm increases

181

with increasing initial NaCl concentration, as shown in Figure 2b. Precipitate formation was

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observed after 24 h of reaction for mixtures with initial NaCl concentrations greater than 2 M, as

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indicated by the shaded region on Figure 2b.

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Several control experiments were performed in order to elucidate the nature of the reaction. Figure

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S1a compares the absorption of a control sample containing 1 mM syringol with no salts after 24

186

h compared to 1 mM syringol in 5.1 M NaCl after 9 h. Absorption increases at 470 nm for the

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control solution but to a much lesser extent than the 5.1 M NaCl mixture, indicating an important

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role for salts in the BrC formation process.

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In order to confirm that the light-absorbing products were formed by syringol and not minor

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impurities in the syringol sample, we compare the UV-Vis spectra of the reaction mixtures formed

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from both the purified and unpurified samples, as shown in Figure S1c. They exhibited almost the

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same absorbance at the UV-Vis wavelengths, demonstrating that impurities play a negligible role

193

in the formation of the observed light-absorbing products.

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Another set of control experiments was performed with reaction mixtures initially consisting of

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aqueous solutions of 0.1 mM or 0.01 mM syringol and 5.1 M NaCl. BrC formation was also

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observed for these lower, more environmentally relevant initial concentrations of syringol. The

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data are shown in Figure S2.



Figure 1. UV-VIS absorption of solutions initially containing 1 mM syringol with 5.1 M NaCl as a function of time: (a) time series of UV-VIS spectra and (b) time dependence of absorption at 470 nm. The data points represent at least two measurements and the error bars represent the standard deviation.

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Figure 2. The effect of initial NaCl concentration on formation of light-absorbing products: (a) absorbance as a function of wavelength and (b) absorbance at 470 nm as a function of initial NaCl concentration. All samples contained 1 mM syringol. Measurements were taken after mixing for 24 hours. The shaded area indicate that precipitate was formed.

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In order to test the effect of dissolved O2 on the reaction, we compare the UV-Vis spectra from the

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O2-bubbling and N2-bubbling experiments (see Figure S1b). The solution that was bubbled with

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N2 exhibited minor light absorption in the visible region after 24 hours. For the solution bubbled

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with pure O2, the UV-Vis absorption develops significantly as time proceeds. The color of the

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solution changed within one hour, more rapidly than the mixtures that were in equilibrium with

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air. These results suggest that dissolved O2 drives the formation of light-absorbing products in the

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reaction mixtures.

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We also performed control experiments using a number of different salts in order to elucidate the

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role of NaCl: NaNO3, Na2SO4, (NH4)2SO4, NH4NO3, NH4Cl, CaCl2 and KCl. The solubility limit

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for Na2SO4 is close to 0.7 M, leading us to choose salt concentrations that would yield [Na+] ~ 1.4

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M for consistency (i.e., 1.4 M NaNO3). The results are compared in Figure S3 and Figure S4. KCl

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promotes the efficient formation of light-absorbing products by syringol, with higher absorbance

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at 470 nm than NaCl solutions with similar salt concentrations. A small amount of absorption

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(roughly 50% of that for an NaCl solution of similar concentration) was observed at 470 nm for 1

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mM syringol in 1.4 M NH4Cl or 1.4 M NaNO3 after 24 h. An increase in absorption at 470 nm

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was observed in aqueous solutions of syringol and CaCl2 only after 70 h. Absorption at 470 nm

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after 24 h for the (NH4)2SO4, Na2SO4 and NH4NO3 solutions is similar to absorption at 470 nm for

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the control solution with no salt. The remainder of our discussion is focused on the NaCl

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experiments.

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Phase separation. Phase separation was observed for the 1 mM syringol/5.1 M NaCl solution after

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10 hours of reaction, leading to an increase in the UV-Vis spectrum baseline due to scattering from

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precipitate suspended in the solution. The time of onset for precipitate formation varied with

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physical parameters such as the surface-to-volume ratio of the reactor and the rate of mixing, which


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likely influenced the kinetics of particle nucleation and growth. In the N2/O2 control experiments

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precipitate formed in solutions bubbled with O2 within 5 hours, more rapidly than in the mixtures

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that were in equilibrium with air. Precipitate did not form in solutions bubbled with N2.

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Figure 3 compares the UV-VIS absorption for the phase-separated samples. Figure S5 shows

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images of the control sample initially containing 1 mM syringol in the absence of salts, and the

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mixture sample initially containing 1 mM syringol and 5.1 M NaCl. There was no visible color

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change in the control sample after stirring for 24 hours (Figure S5a), and no precipitate was

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apparent after filtering (Figure S5f). The unfiltered 1 mM syringol/5.1 M NaCl reaction mixture

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has amber-color precipitates suspended in the solution after 24 h (Figure S5b). After removal of

239

the precipitates using a 0.2 μm PTFE filter, the filtered mixture sample still showed a darker color

240

than the control sample (Figure S5c). The precipitate exhibited a similar color to the unfiltered

241

mixture solution (Figure S5e). The mass of the precipitate greatly exceeded the initial mass of

242

organic material used in our experiments, indicating that the precipitate included both salt and

243

organic material. After washing with Millipore water several times to remove NaCl and water-

244

soluble organics, the precipitate was then dissolved in dichloromethane (DCM). The DCM

245

extracted solution was yellowish in color (Figure S5d). The corresponding UV-Vis spectra (Figure

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3 & Figure 6) suggest that the light-absorbing products are more soluble in DCM than in water.

247 248



249 250 251 252

Figure 3. The UV-Vis spectra of (a) the 1 mM syringol control, (b) the 1 mM syringol/5.1 M NaCl reaction mixture after 24 h of mixing, (c) the filtrate of the reaction mixture, (d) the DCM extract of the colored precipitate after filtration and washing with water. See text and Figure S3 for details.

253 254

Surface tension. The surface tension of a solution containing 1 mM syringol and 5.1 M NaCl and

255

a control solution containing only 1 mM syringol were measured immediately after mixing and

256

again after 24 hours. No change in surface tension was observed.

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Mass spectra characterization of the formed products. MALDI-MS has been used to verify the

258

presence of large organics with little fragmentation in previous studies.43-44 In this study, two types

259

of matrix (i.e. DHB and CHCA) were used to test the high molecular weight compounds in the

260

mixture solution. Though DHB has been frequently used in the detection of high molecular weight

261

organic compounds,33, 45 it has similar mass-to-charge ratio (m/z) to syringol, which resulted in

262

interference with the signal from the mixture solution. MALDI-TOFMS mass spectra using CHCA

263

as matrix are shown in Figure 4.

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265 266 267 268


Figure 4. MALDI-TOFMS mass spectrum (background subtracted) of the 1 mM syringol/5.1 M NaCl mixture solution after 24 h using CHCA as matrix. Only signals higher than 2 times of the matrix background are shown.

269 270

Weak signal was observed at m/z 154.08, which can be attributed to syringol (C8H10O3). A cluster

271

of peaks at m/z 304.1, 305.1, and 306.1 was consistently observed in the spectra. The largest peak

272

in the cluster appeared at m/z 306.1. This mass-to-charge ratio is consistent with a molecular

273

formula of C16H18O6. Additional significant peaks appeared at 172.04, 212.03, 234.02, and 385.13

274

amu. The identities of these species are not confirmed. Although most of the high intensity signal

275

was distributed in the 200 to 450 amu range, the m/z values of the high molecular weight

276

compounds in the spectra extends up to approximately 700 amu.

277

Figure 5 shows the results of UPLC/ESI-HR-QTOFMS analysis. For the solvent blank, the 1 mM

278

syringol/5.1 M NaCl solution, and the aqueous and DCM extracts, significant signal appeared in

279

the base peak chromatogram (BPC) after ~ 10.3 minutes elution time. The mass spectrum is shown

280

as a function of m/z in the inset. Syringol is observed at m/z 155 and 177 as the M + H+ and M +



281

Na+ ions, respectively. Multiple peaks are observed between 300 and 1000 amu, which is

282

consistent with the MALDI-TOFMS results. Signal is observed at m/z 305.1 in the product mixture,

283

consistent with the protonated form of a dimer product with mass 304 amu (C16H16O6). Extracted

284

ion chromatrogram (EIC) analysis shows that this peak is present in both the aqueous and DCM

285

extracts of the product mixture, with more abundance in the aqueous extract.

286

Figure 6 shows the results of UPLC/DAD analysis at 470 nm. These UPLC/DAD chromatograms

287

have been corrected for dilution and background subtraction. For the solvent blank and the 1 mM

288

syringol control (in the absence of NaCl) solutions, no significant chromatographic peaks were

289

observed except for a small peak eluting at 10.12 minutes for the syringol control. Notably, three

290

substantial light-absorbing compounds at 470 nm were found to elute off the reverse-phase UPLC

291

column at 7.04, 9.42 and 10.06 minutes for the DCM extracts. Since the compound eluting at 10.06

292

min has a larger chromatographic peak area than the compound eluting at 10.12 minutes for the

293

syringol control sample, this slight retention time shift is likely explained by the difference in

294

abundance, and thus, are the same compound. As shown in Figure S6, the two earlier eluting light-

295

absorbing species at 7.04 and 9.42 minutes observed from the DCM extracts could not be observed

296

with the same retention times in the corresponding UPLC/ESI-HR-QTOFMS base peak ion

297

chromatogram (BPC). A single product, which had a mass of 191.1588 amu (C9H21NO3), in the

298

DCM extract could be observed at 7.57 minutes in the UPLC/ESI-HR-QTOFMS BPC for the

299

DCM extracts but not in the UPLC/DAD chromatograms at 470 nm. Since this DCM-extracted

300

product with mass 191.1588 amu does not overlap with the three compounds observed in Figure

301

6, the mass 191.1588 amu product does not contribute to the light-absorption properties of the

302

sample. For the aqueous extracts, only one distinct peak was observed to elute in the UPLC/DAD

303

chromatogram at 10.12 minutes, which corresponds exactly to the dimer product with mass 304


Page 16 of 32

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304

amu (C16H16O6) detected by the UPLC/ESI-HR-QTOFMS technique. We note that a small peak

305

was observed at the same retention time for the syringol control solutions, suggesting that small

306

amounts of the C16H16O6 are present in those solutions. Since the UPLC/DAD and UPLC/ESI-HR-

307

QTOFMS share the same retention time for this compound, this strongly supports that the dimer

308

product with mass 304 amu (C16H16O6) is contributing to the light-absorption of the aqueous and

309

DCM extracts of these samples. However, in comparison to the aqueous extracts, the DCM extracts

310

yields two additional light-absorption products resolved by the UPLC/DAD at retention times 7.04

311

and 9.42 minutes that are not observed by UPLC/ESI-HR-QTOFMS. This suggests that alternative

312

ionization techniques to ESI employed here may be required in the future in order to detect and

313

characterize them by mass spectrometry. Since these two light-absorbing products elute earlier on

314

the reverse-phase UPLC column than the light-absorbing dimer product with mass 304 amu, this

315

suggests they are more hydrophilic (or smaller, possibly monomeric) in nature.

316



317

318 319 320 321 322 323 324 325 326 327

Page 18 of 32

Figure 5. UPLC/ESI-HR-QTOFMS extracted ion chromatograms (EICs) of m/z 305 for different solutions after stirring 24 hours. ‘Solvent blank’ refers to 50:50 (v/v) solvent mixture of highpurity methanol and water (Milli-Q, 18.2 Mꞏcm) used for preparing samples for UPLC/ ESI-HRQTOFMS analysis. ‘Syringol control’ indicates an aqueous solution of 1 mM syringol in the absence of salt. ‘Water extracts’ refers to the water soluble fraction of the 1 mM syringol/5.1 M NaCl reaction mixture. ‘DCM extracts’ refers to the DCM extracted fraction of the 1 mM syringol/5.1 M NaCl reaction mixture. The inset figure shows the average mass spectra for the major chromatographic peaks eluting between 10.25 and 10.38 min, corresponding to the light purple shading area.

328


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329


330 331 332 333 334 335 336 337 338 339 340 341 342

Figure 6. UPLC/DAD chromatograms at 470 nm, which are corrected by background subtraction and dilution, for different solutions after stirring 24 hours. Note that these full UV-Vis spectra and UPLC/ESI-HR-QTOFMS mass spectra for these chromatophilic peaks shown here are provided in Figures S8-S12. ‘Solvent blank’ refers to 50:50 (v/v) solvent mixture of high-purity methanol and water (Milli-Q, 18.2 Mꞏcm) used for preparing samples for UPLC/DAD-ESI-HR-QTOFMS analysis. ‘Syringol control’ indicates an aqueous solution of 1 mM syringol in the absence of salt. ‘Water extracts’ refers to the water soluble fraction of the 1 mM syringol/5.1 M NaCl reaction mixture. ‘DCM extracts’ refers to the DCM extracted fraction of the 1 mM syringol/5.1 M NaCl reaction mixture. Light-absorbing products at 470 nm are observed to elute from the reverse-phase UPLC column for the DCM extracts at 7.04, 9.42, and 10.06 minutes. Light-absorbing products are also observed to elute at 10.12 and 10.15 minutes for the syringol control and the aqueous extracts, respectively.



343

Discussion

344

The UV-Vis absorption spectra and the UPLC/DAD chromatograms indicate multiple light-

345

absorbing organic products (brown carbon) formed in the syringol/NaCl reaction mixtures.

346

Although we know that the reactions taking place are not photochemical since all samples were

347

shielded from light, the measured absorbance at 470 nm in our study is similar to the visible

348

absorption spectra of products observed from photochemical reactions of syringol.16, 31 Due to the

349

critical role of dissolved O2 that we infer from our control experiments, an oxidative process is

350

likely.

351

Methoxy-phenols (guaiacol and catechol) are known to undergo oxidation under dark conditions

352

in aqueous solutions in the presence of transition metal ions (TMI), forming unstable quinone

353

species which absorb at around 470 nm.46-49 We investigated the possibility that trace amounts of

354

TMI contamination in our salts could be causing the observed reactions and found that it is not

355

likely. All of the high-purity salts used in this study, including ammonium sulfate and other salts

356

observed to result in no reaction, are stated to contain at most a few ppm of heavy metals including

357

Fe, corresponding to roughly 10-5 M (see Supporting Information for full details). Further, a

358

mixture of 5×10-6 M Fe2(SO4)3, 1 mM syringol and 3.1 M (NH4)2SO4 yielded no BrC after 24

359

hours. Finally, control experiments involving solutions prepared with 1 mM phenol or 1mM

360

guaiacol and 5.1 M NaCl (Figure S7) did not yield light-absorbing products (recall that TMI-

361

catalyzed guaiacol oxidation is known to yield BrC46-49). Based on this evidence, we conclude that

362

the observed reaction is not due to catalysis by trace TMI contaminants.

363

Next, we discuss the possible role of the salt in the oxidation of syringol. Light-absorbing products

364

were formed in varying amounts from aqueous solutions of syringol with NaCl, KCl, CaCl2, and


Page 20 of 32

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365

NH4Cl. No reaction was observed with (NH4)2SO4, Na2SO4 and NH4NO3 solutions. The

366

implication is that chloride promotes the formation of light-absorbing products (although we note

367

that small amounts of BrC were also formed in aqueous syringol/NaNO3 solutions). Chloride salts

368

are known to decrease the solubility of O2 in aqueous solutions,50 which should inhibit oxidation,

369

rather than promote it. Therefore, we consider the interaction of the chloride salts with the organics

370

in solution.

371

Phenol is known to ‘salt out’ (in other words, its solubility decreases with increasing salt

372

concentration, with different salts reducing solubility to a different degree, as quantified by the

373

Setschenow constant, Ks).51-52 To our knowledge, this phenomenon has not been investigated for

374

syringol, but we expect its salting-out behavior to be similar to phenol due to their structural

375

similarities. While our experiments were not performed near the solubility limit for syringol, on a

376

molecular level, reduced solubility, corresponding to more hydrophobic behavior,53 may lead to

377

an increased encounter rate among the organics by increasing their local concentration. The trend

378

among the chloride salts (i.e., BrC production from KCl > NaCl > NH4Cl > CaCl2) is consistent

379

with an inverse relationship to the expected Setschenow constants of these salts for syringol. For

380

phenol, Ks,CaCl2 > Ks,NaCl > Ks, KCl.51 Ks,NH4Cl is often similar to Ks,NaCl.54 In other words, maximum

381

BrC formation is observed when Cl- is present, but salting-out (i.e., the hydrophobic effect) is

382

minimized.

383

We tentatively propose that, in the presence of chloride salts and dissolved O2, syringol may be

384

oxidized by O2 to form a quinone intermediate, which then reacts to form a dimer with a molecular

385

formula of C16H16O6 and molecular weight of 304.1 amu. Ionic liquid solvents have been observed

386

to enhance syringol oxidation by TMI catalysts, by stabilizing ionic reactive intermediates via

387

clustering interactions.55-56 While the fundamental chemistry of this system is quite different, it is



388

possible that the role of Cl- in this reactive system is to stabilize reactive intermediates. The

389

clustering behavior of Cl- is expected to differ from that of sulfate or nitrate due to steric

390

considerations. The fact that Cl- promoted reactions were not observed for phenol and guaiacol

391

(Figure S7), may be due to the fact that those species have fewer electron donating functional

392

groups, and therefore they and intermediates derived from them are expected to be both less

393

reactive than syringol and less capable of clustering with Cl-.

394

Results of ab initio simulations indicate that the quinone dimer is likely to absorb at visible

395

wavelengths. The energies of the highest occupied molecular orbital (HOMO) and lowest

396

unoccupied molecular orbital (LUMO) were calculated using Jaguar, then the absorption

397

wavelength was derived from the HOMO-LUMO energy difference using Planck’s equation. The

398

energy calculations were performed using either the B3LYP functional or M06-2X, and either the

399

6-31G or 6-311G** basis set. The calculated absorption wavelength for the quinone dimer ranges

400

between 348 nm and 483 nm depending on the functional and basis set used. Detailed results can

401

be found in Table S2. We note that this calculation method does not yield information about the

402

oscillator strength (molar absorptivity).

403

Based on the UPLC/ESI-HR-QTOFMS mass spectra (Figure 5), UPLC/DAD (Figure 6), and the

404

UV-Vis absorption (Figure 3), a higher concentration of the quinone dimer was in the aqueous

405

phase than in the DCM phase, but we observed higher light absorption from the DCM phase

406

(Figures 3, as well as Figure 6) than the filtrate (Figures 3). Therefore, we infer that additional

407

light-absorbing species were present in the product mixture.

408

Slikboer and coworkers 48 reported that the formation of light-absorbing products during the dark

409

reaction of guaiacol and catechol with Fe(III) was accompanied by the formation of colloidal


Page 22 of 32

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410

organic particles and a phase separation, similar to our observations of precipitate formation. They

411

proposed that the phase separation was due to the formation of hydrophobic polymer products in

412

their system. Lavi et al. also observed the formation of water-insoluble light-absorbing products

413

from the oxidation of guaiacol, catechol and o- and p-cresol by Fe (III) under acidic conditions.49

414

Based on our observation that surface tension stays constant after 24 hours of reaction, we infer

415

that the reaction products are not necessarily more hydrophobic (water-insoluble) than the

416

reactants. This is supported by the fact that light-absorbing products appeared in both aqueous and

417

DCM extracts, as evidenced by the UV-Vis, UPLC/DAD, and UPLC/ESI-HR-QTOFMS analysis.

418

The quinone intermediate (C16H16O6) has a similar O:C ratio to syringol (C8H10O3), 0.375. We

419

note that aqueous inorganic/organic mixtures with similarly low O:C ratios have been observed to

420

undergo liquid-liquid phase separation in aerosols.57-58 Phase separation initiated by the organics

421

may lead to chemical potential-driven repartitioning of the salt and the salt/organic co-precipitation

422

that we infer from comparing the mass of the precipitate to the initial mass of organics in the

423

system.

424

Atmospheric implications

425

A previously unrecognized mechanism for the in situ formation of aerosol BrC has been identified

426

which involves dark reactions of syringol in aqueous media containing chloride salts. Biomass

427

burning aerosols may contain significant chloride.59 This mechanism may be especially active in

428

air masses impacted by both biomass burning and sea salt (e.g. southern West Africa60 and the

429

north coast of Australia61) and should be further investigated in field measurements.

430

The formation of light-absorbing organic products was accompanied by phase separation in our

431

experiments and those of Slikboer et al.48 Aerosol-phase experiments are necessary in order to



432

determine if this could happen under the chemical and geometric conditions of an aerosol particle.

433

If this does occur in atmospheric aerosols, such a phase change may have additional implications

434

for the aerosol physicochemical properties beyond light absorption, such as aerosol heterogeneous

435

chemistry (e.g., impeding reactive uptake of organics or trace pollutants), CCN ability, and gas-

436

particle partitioning. BrC in atmospheric aerosols has been observed to exist as amorphous brown

437

carbon spheres, or “tar balls”,62-65 as well as water soluble forms.66-68

438

Acknowledgements

439

We are grateful to Prof. Avram Gold and Prof. Zhenfa Zhang for helpful discussions. The authors

440

acknowledge support of the National Science Foundation (CHE–1506789). Jian Xu acknowledges

441

a scholarship fund from China Scholarship Council (File No. 201506100076) for participation in

442

this research. Tianqu Cui and Jason D. Surratt were supported in part by NOAA (awards

443

NA13OAR4310064 and NA16OAR4310106).

444

Supporting Information

445

Results of control experiments. Photos of reaction solutions, filters, filtrate, and filter extracts. UV-

446

Vis and UPLC/ESI-HR-QTOFMS mass spectra corresponding to individual chromatophilic peaks.

447

Specifications for purity and Fe/heavy metal content of chemicals used in the experiments. Results

448

of DFT calculations. Method description for pendant drop tensiometry and UPLC/DAD-ESI-Q-

449

TOFMS.


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450 451

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