Photochemical Cloud Processing of Primary Wildfire Emissions as a

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Characterization of Natural and Affected Environments

Photochemical Cloud Processing of Primary Wildfire Emissions as a Potential Source of Secondary Organic Aerosol Sophie Tomaz, Tianqu Cui, Yuzhi Chen, Kenneth G. Sexton, James M. Roberts, Carsten Warneke, Robert J. Yokelson, Jason Douglas Surratt, and Barbara J. Turpin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03293 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Environmental Science & Technology

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Photochemical Cloud Processing of Primary Wildfire Emissions as a

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Potential Source of Secondary Organic Aerosol

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Sophie Tomaz†, Tianqu Cui†, Yuzhi Chen†, Kenneth G. Sexton†, James M. Roberts‡,

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Carsten Warneke‡, Robert J. Yokelson§, Jason D. Surratt†,* and Barbara J.

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Turpin†,*

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†

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Health, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA

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‡

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USA

Department of Environmental Sciences and Engineering, Gillings School of Global Public

NOAA Earth System Research Laboratory, Chemical Sciences division, Boulder, CO 80305,

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§

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* Corresponding Authors:

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Email – [email protected]; Phone – 1-(919)-966-0470

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Email - [email protected]; Phone – 1-(919)-966-1024

Department of Chemistry and Biochemistry, University of Montana, Missoula, MT 59812, USA

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ABSTRACT

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We investigated the gas-phase chemical composition of biomass burning (BB) emissions and

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their role in aqueous secondary organic aerosol (aqSOA) formation through photochemical cloud

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processing. A high-resolution time-of-flight chemical ionization mass spectrometer using iodide

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reagent ion chemistry detected more than 100 gas-phase compounds from the emissions of 30

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different controlled burns during the 2016 Fire Influence on Regional and Global Environments

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Experiment (FIREX) at the Fire Science Laboratory. Compounds likely to partition to cloud

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water were selected based on high atomic oxygen-to-carbon ratio and abundance. Water

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solubility was confirmed by detection of these compounds in water after mist chamber collection

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during controlled burns and analysis using ion chromatography and electrospray ionization

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interfaced to high-resolution time-of-flight mass spectrometry. Known precursors of aqSOA were

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found in the primary gaseous BB emissions (e.g., phenols, acetate and pyruvate). Aqueous OH

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oxidation of the complex biomass burning mixtures led to rapid depletion of many compounds

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(e.g., catechol, levoglucosan, methoxyphenol) and formation of others (e.g., oxalate, malonate,

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mesoxalate). After 150 minutes of oxidation (approximatively a day of cloud processing), oxalate

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accounted for 13-16 % of total dissolved organic carbon. Formation of known SOA components

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suggests that cloud processing of primary BB emissions forms SOA.

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INTRODUCTION

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Secondary organic aerosol (SOA) composes a substantial fraction of the total organic

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aerosol mass,1-3 but the prediction of its concentration, spatial distribution and specific properties

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is still challenging since the sources and processes contributing to SOA formation are not fully

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understood.1,4-8 Besides gas-phase oxidation and vapor pressure-driven partitioning, SOA can

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also form in the atmospheric aqueous phase (referred to as aqSOA).9-15 Water-soluble organic

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gases (WSOGs) partition into cloud water, fog droplets and aerosol liquid water, and undergo

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radical and non-radical reactions forming highly oxidized, high-molecular weight compounds

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(HMWCs), and carboxylic acid salts that remain in the aerosol phase after water evaporation

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cycles.9,13-16 For instance, global in-cloud aqSOA production is considered as a substantial source

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of SOA in the atmosphere, as it could reach up to 20-30 Tg-y-1.17 Both field measurements and

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laboratory studies of single-component or complex solutions have provided insights into the

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potential sources and fate of aqSOA but mainly focusing on non-combustion sources.10,13,15-21

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Globally, biomass burning (BB) emissions, including those from wildfires, represent a

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large source of non-methane organic gases (NMOGs).22,23 Wildfires largely impact western U.S.

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air quality as the number of large wildfires (> 405 ha) has increased by a rate of seven fires per

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year from 1984 to 2011 and the exposure of wildfire smoke (i.e., fine particulate matter, PM2.5)

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has been related to acute and chronic health effects.24,25 Characterization of volatile (VOC) and

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semivolatile (SVOC) organic emissions from BB has been advanced, but far from complete.26,27

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Characterization of the water-soluble fraction and its impact on aqSOA formation is particularly

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elusive. Some evidence for cloud processing of BB plumes is provided by recent field studies.

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For example, decreasing concentrations of methanol and increasing concentrations of 3 ACS Paragon Plus Environment


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formaldehyde above clouds affected by savanna fires has been attributed to in-cloud oxidation of

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BB emissions.28,29 Phenolic dimers measured by aerosol mass spectrometry in samples collected

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from Fresno (CA, USA), San Pietro Capofiume and Bologna (Italy) have also been attributed to

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aqueous processing of BB.30,31 Gilardoni et al.30 highlighted the large potential for aqSOA

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formation from residential BB, estimating that it could lead to the formation of 0.1-0.5 Tg-y-1 of

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organic aerosol in Europe.30 Lin et al.32 reported high correlations between cloud chemistry

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tracers, such as oxalate and sulfate, and HMWCs in ambient aerosol affected by BB, suggesting

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that in-cloud secondary processing of BB emissions is potentially a large source of HMWCs.32

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Several primary organics emitted by BB (e.g. glycolaldehyde, acetic acid, levoglucosan, phenol,

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vanillin and guaiacol) have been studied in single-component solutions and identified as potential

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precursors of aqSOA through in-cloud OH-initiated oxidation.31,33-38 However, given the poor

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characterization of WSOGs in BB plumes, major aqSOA precursors may remain unidentified and

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the impact of complex primary BB emissions on aqSOA formation remains largely unstudied.

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The major goal of the present study was to improve our understanding of the emissions of

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primary WSOGs that might serve as aqSOA precursors in the western U.S. In order to address

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our study goal, WSOGs were measured in the gas phase and in water samples that contained

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complex mixtures of WSOGs scrubbed from primary BB emissions during the 2016 Fire

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Influence on Regional and Global Environments Experiment (FIREX) conducted at the U.S.

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Forest Service Fire Science Laboratory in Missoula, MT. Organic gases were measured in real-

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time during all phases of each burn using a high-resolution time-of-flight chemical ionization

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mass spectrometer using iodide (I-) reagent ion chemistry (I-HR-TOF-CIMS). I-HR-TOF-CIMS

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was used because it is widely selective to oxygen and nitrogen containing organic molecules.39 In

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addition, water samples containing the complex mixtures of WSOGs scrubbed from the BB

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plume with mist chambers were also characterized at the molecular level by ion chromatography

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(IC) and high-resolution quadrupole time-of-flight mass spectrometry equipped with electrospray

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ionization (ESI-HR-QTOFMS). Many of the high oxygen-to-carbon (O:C) atom ratio-containing

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gases measured by the I-HR-TOF-CIMS were also found in the water samples analyzed by IC

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and ESI-HR-QTOFMS, confirming their water solubility. Photooxidation of the aqueous BB

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mixtures demonstrated the potential for BB gases to be transformed through cloud processing.

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Depletion of several primary WSOGs in the mixtures led to formation of compounds found

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primarily in the particle phase in the atmosphere, consistent with the expectation that aqSOA can

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form from cloud processing of BB emissions. The aim of this work is to improve our

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understanding of the potential precursors of aqSOA formed from primary BB emissions.

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EXPERIMENTAL SECTION

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2016 FIREX Campaign. Measurements of primary BB emissions characteristic of western U.S.

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fuels took place at the U.S. Forest Service Fire Sciences Laboratory (Missoula, Montana) as part

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of the 2016 FIREX campaign. Details about the laboratory40,41 and the controlled combustion

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experiments have been provided elsewhere.26,27 In this work, we provide results from the “stack

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burns,” during which primary BB emissions were pushed through a stack and sampled from a

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port on a platform about 17 m above the burn. Emissions were sampled after a residence time of

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approximately 5 seconds in the stack. I-HR-TOF-CIMS (Aerodyne Research, Inc.) measurements

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are reported for 30 fires, including twelve fuels characteristic of the western U.S. (e.g., lodgepole

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pine, Engelmann spruce, Douglas fir, ponderosa pine, excelsior, dung, loblolly pine, rice straw,

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bear grass, Jeffrey pine, sage and subalpine-fir) and aqueous oxidation experiments were

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conducted on samples from two fuels; ponderosa pine and Douglas fir. Douglas fir and ponderosa

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pine fuels were selected because they are major fuel types representing 18 and 11%, respectively, 5 ACS Paragon Plus Environment


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of the forested areas in the western U.S.42 The mean dry mass of fuel used was 1.29 kg (0.25 - 4.4

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kg) with a moisture content in the range of 0.009-1.226 (wet weight-dry weight/dry weight).

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Results from the 2016 FIREX campaign will be available in open access online43 at

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https://esrl.noaa.gov/csd/groups/csd7/measurements/2016firex/FireLab/DataDownload.

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Online Chemical Characterization of WSOGs. The I-HR-TOF-CIMS allows for the detection

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of a wide variety of O-, nitrogen (N)- and chlorine (Cl)-containing VOCs and SVOCs (including

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WSOGs).39,44,45 Emissions were pulled into the I-HR-TOF-CIMS via a ¼ inch unheated

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polytetrafluoroethylene (PTFE) tubing placed inside the stack (residence time 0.2 seconds),

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oriented perpendicular to the fuel bed and equipped with a virtual impactor (Teflon) to limit

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aerosol sampling. Stack emissions were diluted by a factor of 4-25 using a controlled UHP N2

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flow (Airgas), calibrated with a mass flow controller, before entering the instrument to avoid I-

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ion signal reduction. It is well known that relative humidity (RH) inside the instrument can affect

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the response factor of some compounds.39 In our case, the RH was low (because of the N2

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dilution) and the I(H2O)-/I- ratio was constrained to between 0.02 and 0.14 (Figure S1)

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(Supporting Information, SI). Details of I-HR-TOF-CIMS operation have been described

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previously and details are provided in SI and Figure S2.46,47 Compounds were detected using

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exact mass and relative ion abundance and considering ionization with I- only (no water cluster).

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Authentic standards were not used to positively identify and quantify compounds due to the

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complexity of BB emissions and the presence of multiple isomers having different ionization

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efficiencies. Note that compounds emitted by BB have various response factors; therefore the

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relative abundances do not provide exact mass concentrations in BB emissions.

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Mist Chamber Sampling of WSOGs During the 2016 FIREX Campaign. WSOGs were

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collected by scrubbing particle-free BB emissions into water using four Cofer mist chambers 6 ACS Paragon Plus Environment

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operated in parallel, as described previously (details in SI).48,49 Briefly, stack air was pulled

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through a pre-baked quartz fiber filter (QFF, Pall Life Science, 47 mm, baked at 550 °C for 4 h)

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to remove particles and then through a mist chamber, where gases were scrubbed by a fine

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refluxing water mist. Mist chambers operated at 25 L min-1 and sampled directly from the stack

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using a ¼ inch unheated PTFE line with a residence time of 0.04 seconds. Due to water loss by

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evaporation, water was added into the mist chambers during some burns to maintain the sampling

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volume. Details about mist chamber operation are reported in the SI and Figure S3. Note that the

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QFF filters can adsorb SVOCs, but calculations suggest that removal will be limited to the first

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few minutes of the sampling until equilibrium is achieved between the gas phase passing through

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the filter and the adsorbed phase on the filter’s fibers.20,50

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Aqueous-Phase OH

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Processing of WSOGs. Ponderosa pine (Total Organic Carbon, TOC=2405 µmol-C L-1) and

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Douglas fir (TOC=1338 µmol-C L-1) mist chamber samples were selected to study aqueous OH

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radical-initiated oxidation of primary WSOGs emitted by BB. Water samples comprised of

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WSOG mixtures scrubbed from the BB plume during the 2016 FIREX experiments were

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oxidized using a photochemical cuvette chamber as described previously and detailed in the

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SI.20,51 Control experiments were also conducted. Based on their TOC concentrations, samples

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were diluted by a factor of 2-4 before each experiment to reach 405 µmol-C L-1 and 601 µmol-C

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L-1 for Douglas fir and ponderosa pine, respectively. These are reasonable concentrations for BB-

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impacted cloud water as measured by Cook et al.52 who reported TOC concentrations in the range

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of 500‒1400 µmol-C L-1 for cloud water influenced by wildfires. OH radicals were generated in

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the complex aqueous mixtures by photolysis of H2O2 (500 µmol L-1) with a single wavelength

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lamp (254 nm). We used a single wavelength lamp because our aim was to study OH radical

Radical-Initiated

Oxidation

Simulating

Photochemical

Cloud



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reactions (not solar spectrum photolysis). Control experiments were used to evaluate potential

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artifacts induced by the photolysis of the 254 nm lamp alone and H2O2 alone.

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Aqueous-phase OH-initiated oxidation experiments were performed in triplicate. Samples

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were also subjected to both H2O2-only or UV-only control experiments. The OH production rate

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from H2O2 photolysis was calculated by fitting the H2O2 concentration decay during a H2O2+UV

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control experiment and was ~ 9.95 × 10-2 µM s-1.53 We estimated the OH production and

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consumption rate using the model developed by Lim et al.10. We added to this model the

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consumption of OH induced by organics using measured TOC values and the general scavenging

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rate constant, kC,OH= (3.8±1.9) × 108 L M-1 s-1 proposed by Arakaki et al.54 This general rate

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constant represents the reactivity of total WSOC with OH radical and was determined in various

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atmospheric samples (i.e. rain, fog, cloud, aerosols) and from different sites.54 This enabled

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estimation of the OH concentrations in our experiments ([OH] = 5 - 6.5 × 10-12 M). These values

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are somewhat higher than cloud droplet OH concentration values estimated for urban or maritime

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environments (10-15 to 10-12 M, respectively).55 However, the impact of BB on cloud droplet OH

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concentrations is highly uncertain, especially given the presence of light absorbing organics, and

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the potential for enhanced aqueous-phase OH production from photosensitized reactions and

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photolysis of HONO.

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Analytical procedures. The water-soluble TOC content of the mist chamber samples and blanks

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was measured after dilution and before each experiment using a TOC analyzer (Sievers M9, GE

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Analytical Instruments) that was calibrated daily with potassium hydrogen phthalate (Sigma

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Aldrich) (i.e., seven-point calibration curve in the range 10 - 2000 µmol-C L-1). A 25 µL aliquot

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of each sample was analyzed by IC (ICS 3000, Thermo Fisher) and organic acids were identified

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and quantified using authentic standards. IC method details are provided in SI and in Table S1. A 8 ACS Paragon Plus Environment

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10 µL aliquot of each sample was also analyzed by ESI-HR-QTOFMS (6520 Series, Agilent)

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without pre-separation over a mass range of 50-500 m/z operated in both the positive and negative

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ion modes. The mass resolution (m/∆m) was ~12,000 at the reference masses m/z 113 and 118 in

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negative and positive ion modes, respectively. ESI-HR-QTOFMS method details can be found in

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the SI. Total nitrogen and inorganic nitrogen (NH4+, NO3- and NO2-) concentrations were

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measured in ponderosa pine and Douglas fir mist chamber samples following the procedure

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described in Paerl et al.56 Organic nitrogen concentrations were calculated as the difference

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between total and inorganic nitrogen and detailed results can be found in the Table S3.

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RESULTS AND DISCUSSION

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Chemical Characterization of Primary WSOGs from Laboratory-Generated BB emissions

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During the 2016 FIREX Campaign. Across the 30 controlled stack burns, a total of 157 ions

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detected by I-HR-TOF-CIMS were assigned elemental formulas with a high degree of certainty

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(Figure 1). In all, 36 were N-containing (Table S2). C6H6O2 (benzenediol, 15% of the total ion

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count of the sum of the 30 controlled stack burns), HONO (nitrous acid, 11%), C2H4O2 (acetic

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acid, 9%), CH2O2 (formic acid, 9%) and C7H8O2 (7%) had the highest ion abundance (Figure 2).

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More than 90% of the I-HR-TOF-CIMS-detected ions were also detected by Koss et al.,26

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supporting the accuracy of formula assignments. Below we will use these measurements to

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develop insights into potential aqSOA precursors. To form SOA through cloud processing,

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aqSOA precursors must be water soluble and reactive, and they must form products that remain

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in the aerosol phase even after water evaporation. We focused our analyses on I-HR-TOF-CIMS-

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detected ions with O:C≥0.67 in the gas phase, because WSOGs typically have high O:C ratios

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(e.g. O:C=0.67 for methylglyoxal; O:C=1 for glyoxal, acetic acid and glycolaldehyde).13,57 We 9 ACS Paragon Plus Environment


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also examined N-containing compounds as potentially water-soluble. We verified the

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compounds’ presence in water samples (i.e., mist chamber samples) and demonstrated that the

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compounds react with OH. Ultimately, we show that authentic mixtures of BB-derived WSOGs

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form products that are commonly found in atmospheric aerosols, suggesting that aqSOA will

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form from cloud processing of BB emissions.

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The mean I-HR-TOF-CIMS-detected O:C, H:C and N:C ratios (unweighted) were ~0.49,

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1.44 and 0.11, respectively. I-HR-TOF-CIMS-detected gas-phase ions observed having an O:C ≥

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0.67 and a relative ion abundance ≥ 0.1% are listed in Table 1. C2H4O2 (acetic acid, 9%), CH2O2

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(formic acid, 9%), C6H10O5 (levoglucosan, 4%), C3H6O3 (3%), C5H8O4 (3%), C4H6O3 (2%),

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C6H8O4 (2%), C2H4O3 (glycolic acid, 1%), and HNCO (isocyanic acid, 1%) were the most

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abundant highly oxygenated compounds detected over the 30 fires (Figure 2). It is possible that

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some compounds with lower O:C ratios are also important aqSOA precursors, if they are present

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at high enough partial pressures. Given their ion abundance, this could be true for C6H6O2

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(benzenediol, 15%), C7H8O2 (7%), C6H6O3 (5%) and C5H8O3 (3%) (Table 1, Figure 2). CH2O2

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(formic acid) and HNCO (isocyanic acid) are not considered aqSOA precursors since formic acid

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oxidation leads to the formation of CO2, and HNCO has a long lifetime in cloud water (from 10

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hours to 20 days), and will likely volatilize during cloud droplet evaporation.58

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I-HR-TOF-CIMS results above were confirmed with the ESI-HR-QTOFMS analysis of

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the chemical composition of two water-samples collected in mist chambers during controlled

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stack burns of ponderosa pine and Douglas fir fuels. As shown in Table 1, most of the WSOGs

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identified by I-HR-TOF-CIMS were also detected in these water-samples using IC and/or ESI-

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HR-QTOFMS in the positive and/or the negative ion modes. Only C3H8O3 and C8H8O were

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detected by I-HR-TOF-CIMS, but were not found in the water-samples. It should be noted that 10 ACS Paragon Plus Environment

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these compounds were also detected by Koss et al.26 confirming their presence in primary BB

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emissions. While C4H2O3 was also not detected in the water samples, this compound is likely to

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be maleic anhydride and can be expected to undergo hydrolysis in water, leading to the formation

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of maleic acid. Maleic acid was detected in water samples by negative-mode ESI-HR-QTOFMS.

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Table 1 includes several gases whose aqSOA formation potential has already been

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investigated and many others that have not. In addition, some of these observed compounds have

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been measured in the atmosphere in both the gas and particle phases. Acetic and pyruvic acids,

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detected by I-HR-TOF-CIMS, IC and negative-mode ESI-HR-QTOFMS, are considered

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precursors of aqSOA since their OH-initiated oxidation leads to the formation of the SOA

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constituents glyoxylate and oxalate under cloud-relevant conditions.15,36 Roughly 20% of acetic

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acid/acetate is found in the aerosol phase, compared to 60 to 90 % for oxalate.59-61 It should be

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noted that oxalate is the most abundant dicarboxylic acid in atmospheric aerosols and is found in

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the particle phase probably as a salt or complex.62-65 Because C4H6O4, C5H8O4 and C6H10O4 were

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detected in the negative-mode by ESI-HR-QTOFMS, we expect that they are organic acids, likely

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corresponding to succinic, glutaric and adipic acids, respectively. These compounds have also

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been detected in previous BB emission studies.65-67 Aqueous-phase OH oxidation of succinic,

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lactic, glutaric and adipic acids leads to the formation of malonic, glyoxylic and oxalic acids,

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which are found mostly in the particle phase in the atmosphere, and therefore these compounds

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are aqSOA precursors.68,69 C6H6O, C6H6O2, C7H8O2 and C8H8O3 were also detected by ESI-HR-

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QTOFMS operated in the negative ion mode; based on authentic standard injections and previous

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studies on BB emissions, these compounds could be identified as phenol, catechol, guaiacol

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and/or methylcatechol and vanillin, respectively.26,70,71 Aqueous-phase OH-initiated oxidation of

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these phenolic compounds could lead to the formation of small carboxylic acids such as maleic,



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succinic and oxalic acids through ring-opening pathways, but can also lead to the formation of

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phenolic dimers.31,34,35 Phenolic dimers can also be formed via reaction with the triplet excited

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state of an aromatic carbonyl (3C*) and in the dark at high solute concentrations in the presence

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of a salt.35,72 The detection of C6H10O5 by ESI-HR-QTOFMS operated in the negative ion mode

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indicates that this compound could correspond to levoglucosan (detectable in ESI negative ion

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mode as confirmed by injection of its authentic standard) or could be hydroxymethylglutaric acid.

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In the case of levoglucosan, used as a tracer for BB emissions,73 its aqueous-phase OH-initiated

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oxidation leads to the formation of functionalized ring-retaining products and small organic acids

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through bond scission.33 Some of these products will remain in the particle phase after droplet

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evaporation, whereas others will not. Certainly, cloud processing could affect the source –

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receptor relationship for this BB tracer by acting on the material that was originally in the gas

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phase forming new SOA but also affecting material originally in the particle phase, forming

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products that could be more or less volatile than the reactants.

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Water-soluble N-containing organic compounds detected during the burns by both I-HR-

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TOF-CIMS and ESI-HR-QTOFMS are presented in Table S2. Organic nitrogen comprised a

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large fraction of the total nitrogen content of mist chamber samples (44 to 54 % of total dissolved

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nitrogen for Douglas fir and ponderosa pine mist chamber samples; Table S3). Studies have

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previously highlighted the large contribution of N-containing organics to the total nitrogen budget

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in rainwater and aerosol (from 20 to 80 % of the total N), and also highlighted the important role

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of BB as a source of organic nitrogen in the atmosphere.74,75 Despite this, the role of N-containing

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organics in aqSOA is largely unexplored. Identification of reduced N-containing organics by ESI-

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HR-QTOFMS is somewhat difficult because organics frequently ionize by clustering with NH4+

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([M+NH4]+ ions) in the positive mode rather than by protonating ([M+H]+ ions). However, in our


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study, the ESI-HR-QTOFMS detection of these compounds by Na+ ion clustering and with I-HR-

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TOF-CIMS, helped us to positively confirm their molecular formulas. A few studies have

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explored the impact of amines on multiphase or aqueous chemistry through their reactions with

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carbonyls or their role as photosensitizers, leading to an enhancement of SOA formation, the

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formation of heterocyclic N-containing compounds and brown carbon formation.18,19,76-80

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However, the impact of N-containing organic compounds on aqSOA formation in clouds is still

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largely unexplored because the chemical structure and reactivity of these compounds have

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remained largely unknown. This work provides insights into potential precursors.

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Aqueous-Phase OH-Initiated Oxidation of BB Mixtures. Figures 3 and 4 as well as S4-S7

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present the relative ion abundance of selected WSOGs (Table 1) during the OH-initiated

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oxidation of the water samples collected using mist chambers during the ponderosa pine and

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Douglas fir stack burn experiments. The relative ion abundances of the reactive compounds

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exhibited two types of distinct trends. The first trend is characterized by a continual decrease in

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ion abundance throughout the aqueous-phase oxidation experiment, indicating water-soluble OH-

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reactive BB compounds. This was observed for C6H6O2 (a), C6H7NO (c), C6H10O5 (d) and

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C3H4N2 (f) in Figure 3 and additional compounds in Figures S4-S7. The second trend is described

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by an increase in relative ion abundance, suggesting the formation of one or several isomers at

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this mass, followed by a subsequent decrease indicating further oxidation and formation of next

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generation products. This was observed for a few compounds present in the original sample, such

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as C6H6O3 (Figure 3,e) and others in Figure S4-S7. These compounds are both reactive and

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functionalized products.

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These experiments, which mimicked OH oxidation during cloud processing of primary

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BB emissions, demonstrated the formation of several organic acids (Figure 4), including some 13 ACS Paragon Plus Environment


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commonly found in atmospheric aerosols. The relative ion abundance of oxalic acid, a known

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component of aqSOA,13 increased by a factor 100 due to the OH-initiated oxidation of primary

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WSOGs (Figure 4). Other organic acids, such as C5H6O5, C3H6O3, C3H4O4, C4H6O4, C3H2O5,

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C2H2O3 (Figure 4) and C4H8O3 (Figure S4), detected by ESI-HR-QTOFMS operated in the

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negative ion mode, share a similar trend and likely correspond to ketoglutaric, hydroxypropanoic

300

(e.g. lactic), malonic, succinic, mesoxalic, glyoxylic, and hydroxybutanoic acids, respectively.

301

Many of these carboxylic acids are common components of atmospheric aerosols (e.g., malonic,

302

succinic, oxalic, glutaric and glyoxylic acids),60,61,81 suggesting that cloud processing of BB

303

emissions will lead to aqSOA formation.

304

Interestingly, the temporal trends for some compounds during the course of the

305

experiments appear to be different in the negative and the positive ion modes. This is the case for

306

the C6H6O2 (Figures 3,a and S6,f), C6H6O3 (Figures 3,e and S6,a), C6H8O3 (Figures S5,b and

307

S6,d), C5H10O4 (Figures S5,d and S6,e), C8H10O2 (Figures S5,f and S6,g), and C6H10O4 (Figures

308

S5,h and S6,i), indicating the likely presence of multiple isomers with different reactivities and

309

products. For example, C6H6O2 could correspond to benzenediol in the negative ion mode and the

310

sum of benzenediol and furan-like compounds (methylfurfural, acetylfuran) in the positive ion

311

mode. The increasing concentration of C6H6O2 in the negative ion mode in the first ten minutes of

312

the experiment could arise from the loss of NO2 during the oxidation of nitrocatechol (C6H5NO4),

313

leading to the formation of catechol (benzenediol) (Figures 3,a and S7,c). This illustrates the

314

chemical complexity of the BB emissions.

315

Control experiments suggest that OH oxidation was fast relative to 254 nm photolysis and

316

H2O2 reaction in almost all cases. Photolysis was observed for some compounds when exposed to

317

the UV lamp (254 nm) in the absence of OH. Photolysis mainly affected compounds having 14 ACS Paragon Plus Environment

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318

relatively high double bond equivalences (DBEs ≥4), such as C6H6O3, C7H6O2, C8H8O2 (Figure

319

S6,a,b and c). For most compounds, 254 nm photolysis degradation, which is an artifact of the

320

experiments, occurred at a rate that was slower than OH-initiated oxidation. For instance, C6H8O4

321

was not impacted by the UV light (Figures S5,g and S6,h), while C8H10O2 was moderately

322

photolyzed with a rate 8 to 30 times slower than OH radical oxidation (Figure S6,g). In contrast,

323

C6H6O3 was photolyzed in the Douglas fir sample at a rate comparable to that observed in the OH

324

radical experiment (Figure S6,a).

325

In most cases, ponderosa pine and Douglas fir experiments yielded the same temporal

326

trends, suggesting commonality in the composition of aqSOA precursors and products in

327

emissions from these fuels. Only C7H8O2 (Figure 3,b), C6H6O3 and C5H10O4 (Figure S6,a and e,

328

respectively) yielded different results. For example, an increase in ion abundance of C7H8O2 was

329

observed in the Douglas fir experiment, but not the ponderosa pine experiment (Figure 3,b),

330

indicating the presence of a C7H8O2 precursor in Douglas fir emissions that is not present in the

331

ponderosa pine emissions. In addition, the oxidation rate seems to be faster in the Douglas fir

332

water sample in comparison with the ponderosa pine sample. For instance, the maximum

333

concentration of pyruvic acid is reached after 60 minutes in the Douglas fir sample, while this

334

maximum is reached after 100 minutes of reaction time in the ponderosa pine sample (Figure

335

4,b). This might be caused by lower TOC concentration in the Douglas fir sample (405 µmol-C L-

336

1

337

part on the concentration of organics.

) compared to the ponderosa pine sample (601 µmol-C L-1) since the oxidation rate depends in

338

The relative ion abundance of NO3- also increased during the course of the OH-initiated

339

oxidation, and likely arises from the degradation of N-containing organic compounds (Figure

340

S7,a). Photooxidation of NO2-containing compounds could lead to the elimination of NO2- in the 15 ACS Paragon Plus Environment


Page 16 of 34

341

aqueous phase that could be further oxidized into NO3-.82-84 These reactions could be illustrated

342

by the photooxidation of C6H5NO4 taking place in the first 20 minutes, concomitantly with an

343

increase of NO2- concentration (Figure S7,b and c). Subsequently, NO2- ion abundance drastically

344

decreases and NO3- concentration increases in the water samples (Figure S7,a and b). Release

345

NO2 from nitrated aromatics can change the light absorption properties (bleaching).85 This

346

exemplifies the potential for cloud processing of N-containing BB emissions, potentially altering

347

the light-absorbing properties of the aerosol after cloud water droplet evaporation.18,76-80

348

Atmospheric Implications

349

In this study, we characterize BB emissions and identify compounds that we expect to

350

partition to cloud water and react. For some compounds, their impact on cloud photochemical

351

cycles have already been studied (e.g., acetic acid, guaiacol and vanillin) while other compounds

352

have not been studied yet (e.g., acrylic and propanoic acids, furan-like molecules and N-

353

containing compounds). Aqueous-phase oxidation of authentic BB mixtures provided evidence

354

for the aqueous OH oxidation of several compounds and the formation of compounds commonly

355

found in atmospheric aerosols, suggesting that cloud processing of primary BB emissions will

356

lead to aqSOA formation.

357

In addition to their transfer and oxidation in cloud water, WSOGs can also be oxidized in

358

the gas-phase, and thus it would be informative to compare gas and aqueous lifetimes with

359

respect to OH. The aqueous lifetimes are highly uncertain due in large part to uncertainties in OH

360

concentrations in clouds. Cloudwater OH estimates range from 10-15 to 10-12 mol L-1.54 Estimates

361

typically account for OH uptake from the gas phase and aqueous OH production via photolysis of

362

HONO, NO3-, H2O2 and Fenton chemistry. Cloudwater organics are usually considered only as an

363

OH sink in these calculations, although there is an increasing understanding that organics can also 16 ACS Paragon Plus Environment

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364

be a source of oxidants and that organics can recycle OH radicals through autocatalysis.19,86,87

365

Assuming [OH]=106 molecules cm-3 in the gas-phase and [OH] = 10-13 mol L-1 in the aqueous-

366

phase, oxidation will be faster in the aqueous-phase than in the gas-phase for compounds such as:

367

acrylic acid (τg,OH=16 hours, τaq,OH=0.5 hours),88,89 phenol (τg,OH=1 day, τaq,OH=0.018 day)90,91 and

368

methylfurfural (τg,OH=5 hours, τaq,OH=0.4 hours).91,92 For these compounds, aqueous oxidation

369

will still be faster if [OH]= 10-14 mol L-1 (Table S4). At [OH] = 10-13 mol L-1, oxidation will also

370

be faster in the aqueous-phase than in the gas-phase for: acetic acid (τg,OH=15 days, τaq,OH=7.7

371

days),93,94 catechol (τg,OH=3 hours, τaq,OH=0.4 hours),95,96 benzoic acid (τg,OH=2 hours, τaq,OH=0.5

372

hours)97,98 and acetamide (τg,OH=3 days, τaq,OH=0.6 days).99,100 At [OH]= 10-14 mol L-1, the gas and

373

aqueous lifetimes of these compounds are comparable (within a factor of 5) with the exception of

374

acetic acid (τg,OH=15 days, τaq,OH=77 days). The presence of gas-phase OH concentrations 5-20

375

times higher in BB smoke, compared to background air, suggests that oxidation in both gas and

376

aqueous phases will be even faster in BB-impacted clouds.101,102

377

In the current study, after 150 minutes of oxidation (approximately one day of cloud

378

processing; cloud droplet lifetime ~ 10-20 minutes; 1 cycle/hour),68 we demonstrated that

379

aqueous-phase OH-radical initiated oxidation of BB mixtures led to the formation of compounds

380

that are commonly found in atmospheric aerosol. Thus, aqSOA formation from BB mixtures

381

warrants further study. An improved understanding of WSOG emissions and aqueous chemical

382

mechanisms combined with cloud parcel modeling may inform advances in the treatment of BB

383

emissions in global and regional models.

384 385

ASSOCIATED CONTENT



Page 18 of 34

386

Supporting Information

387

Details about experimental conditions; Organic acids quantification method (Table S1); List of

388

N-containing organic compounds identified (Table S2); Total, inorganic and organic nitrogen

389

concentrations (Table S3); Calculated gas and aqueous lifetimes for OH oxidation (Table S4); I-

390

HR-TOF-CIMS experimental details (Figures S1 and S2); TOC concentrations in mist chamber

391

samples (Figure S3); Ion abundance variation over time in mist chamber samples (Figures S4, S5,

392

S6 and S7).

393

AUTHOR INFORMATION

394

Corresponding author

395

* Corresponding Authors:

396

Email – [email protected]; Phone – (919) 966-1024

397

Email – [email protected]; Phone – (919) 966-0470

398

Notes

399

The authors declare no competing financial interest.

400

ACKNOWLEDGMENTS

401

This work was supported by the National Oceanic and Atmospheric Administration (NOAA)

402

Climate Program Office’s AC4 program, award number NA16OAR4310106. UNC Biomarker

403

Mass Spectrometry Facility is funded, in part, by the National Institute of Environmental Health

404

Sciences (grant #P30ES010126). We are thankful to Dr. Zhenfa Zhang for IEPOX synthesis and

405

to Leonard Collins for his help with the ESI-QTOF-MS. We are also thankful to Karen

406

Rossignol, Betsy Abare and Dr. Hans W. Paerl for the nutrient analyses. 18 ACS Paragon Plus Environment

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Absorbing Oligomers Produces in Aerosol Particles Exposed to Methyglyoxal, Photolysis, and Cloud Cycling. Environ. Sci. Technol. 2018, 52, 4061-4071; DOI: 10.1021/acs.est.7b06105. Limbeck, A.; Puxbaum, H.; Otter, L.; Scholes, M.C. Semivolatile behavior of dicarboxylic acids and other polar organic species at a rural background site (Nylsvley, RSA). Atmos. Environ. 2001, 35, 1853-1862; DOI: 10.1016/S13522310(00)00497-0. Chen, B.; Yang, C.; Goh, N. K. Direct photolysis of nitroaromatic compounds in aqueous solutions. J. Environ. Sci. 2005, 17, 4, 598-604. Wang, J.; Song, M.; Chen, B.; Wang, L.; Zhu, R. Effects of pH and H2O2 on ammonia, nitrite, and nitrate transformations during UV254nm irradiation: Implications to nitrogen removal and analysis. Chemosphere. 2017, 184, 1003-1011; DOI: 10.1016/j.chemosphere.2017.06.078. Zhang, W.; Xiao, X.; An, T.; Song, Z.; Fu, J.; Sheng, G.; Cui, M. Kinetics, degradation pathway and reaction mechanism of advanced oxidation of 4-nitrophenol in water by a UV/H2O2 process. J. Chem. Technol. Biotechnol. 2003, 78, 788-794; DOI: 10.1002/jctb.864. Zhao, R.; Lee, A. K. Y.; Huang, L.; Li, X.; Yang, F.; Abbatt, J. P. D. Photochemical processing of aqueous atmospheric brown carbon. Atmos. Chem. Phys. 2015, 15, 6087-6100; DOI: 10.5194/acp-15-6087-2015. Li, W. Y.; Li, X.; Jockusch, S.; Wang, H.; Xu, B.; Wu, Y.; Tsui, W. G.; Dai, H. -L.; McNeill, V. F.; Rao, Y. Photoactivated Production of Secondary Organic Species from Isoprene in Aqueous Systems. J. Phys. Chem. A. 2016, 120, 9042-9048; DOI: 10.1021/acs.jpca.6b07932. Tong, H.; Arangio, A. M.; Lakey, P. S. J.; Berkemeier, T.; Liu, F.; Kampf, C. J.; Brune, W. H.; Pöschl, U.; Shiraiwa, M. Hydroxyl radicals from secondary organic aerosol decomposition in water. Atmos. Chem. Phys. 2016, 16, 1761-1771; DOI: 10.5194/acp-16-1761-2016. Teruel, M. A.; Blanco, M. B.; Luque, G. R. Atmospheric fate of acrylic acid and acrylonitrile: Rate constants with Cl atoms and OH radicals in the gas phase. Atmos. Environ. 2007, 41, 5769-5777; DOI: 10.1016/j.atmosenv.2007.02.028. Buxton, G.; Greenstock, C. L.; Philips Helman, W.; Ross, A. B. Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (.OH/.O-) in Aqueous Solution. J. Phys. Chem. Ref. Data. 1988, 17, 513-886, and references therein; DOI: 10.1063/1.555805. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frank, P.; Just, T.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Walker, R.W.; Warnatz, J. Evaluated kinetic data for combustion modelling. J. Phys. Chem. Ref. Data. 1992, 21, 411-429, and references therein; DOI: 10.1063/1.555908. Minakata, D.; Li, K.; Westerhoff, P.; Crittenden, J. Development of a Group Contribution Method To Predict Aqueous Phase Hydroxyl Radical (HO.) Reaction Rate Constants. Environ. Sci. Technol. 2009, 43, 6220-6227; DOI: 10.1021/es900956c. Bierbach, A.; Barnes, I.; Becker, K. H. Product and kinetic study of the OH-initiates gas-phase oxidation of furan, 2-methylfuran and furanaldehydes at ≈ 300 K. Atmos. Environ. 1995, 29, 2651-2660; DOI: 10.1016/1352-2310(95)00096-H. 26 ACS Paragon Plus Environment

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93. Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Rossi, M. J.; Troe, J. Evaluated kinetic, photochemical and heterogeneous data for atmospheric chemistry: supplement V, IUPAC subcommittee on gas kinetic data evaluation for atmospheric chemistry. J. Phys. Chem. Ref. Data. 1997, 26, 521-1011. 94. Monod, A.; Doussin, J. -F. Structure-activity relationship for the estimation of OHoxidation rate constants of aliphatic organic compounds in the aqueous phase: alkanes, alcohols, organic acids and bases. Atmos. Environ. 2008, 7611-7622; DOI: 10.1016/j.atmosenv.2008.06.005. 95. Tomas, A.; Olariu, R. I.; Barnes, I.; Becker, K. H. Kinetics of the reaction of O3 with selected benzenediols. Int. J. Chem. Kinet. 2003, 35, 223-230; DOI: 10.1002/kin.10121. 96. Smith, J. D.; Kinney, H.; Anastasio, C. Aqueous benzene-diols react with an organic triplet excited state and hydroxyl radical to form secondary organic aerosol. Phys. Chem. Chem. Phys. 2016, 17, 10227; DOI: 10.1039/c4cp06095d. 97. Wu, C.; De Visscher, A.; Gates, I. D. Reactions of hydroxyl radicals with benzoic acid and benzoate. RSC Adv. 2017, 7, 35776; DOI: 10.1039/c7ra05488b. 98. Richards-Henderson, N. K.; Hansel, A. K.; Valsaraj, K. T.; Anastasio, C. Aqueous oxidation of green leaf volatiles by hydroxyl radical as a source of SOA: Kinetics and SOA yields. Atmos. Environ. 2014, 95, 105-122; DOI: 10.1016/j.atmosenv.2014.06.026. 99. Barnes, I.; Solignac, G.; Mellouki, A.; Becker, K. H. Aspects of the Atmospheric Chemistry of Amides. ChemPhysChem. 2010, 11, 3844-3857; DOI: 10.1002/cphc.201000374. 100. Karpel Vel Leitner, N.; Berger, P.; Legube, B. Oxidation of Amino Groups by Hydroxyl Radicals in Relation to the Oxidation Degree of the α-Carbon. Environ. Sci. Technol. 2002, 36, 3083-3089; DOI: 10.1021/es0101173. 101. Yokelson, R.; Crounse, J. D.; DeCarlo, P. F.; Karl, T.; Urbanski, S.; Atlas, E.; Campos, T.; Shinozuka, Y.; Kapustin, V.; Clarke, A. D.; Weinheimer, A.; Knapp, D. J.; Montzka, D. D.; Holloway, J.; Weibring, P.; Flocke, F.; Zheng, W.; Toohey, D.; Wennberg, P. O.; Wiedinmyer, C.; Mauldin, L.; Fried, A.; Richter, D.; Walega, J.; Jimenez, J. L.; Adachi, K.; Buseck, P. R.; Hall, S. R.; Shetter, R. Emissions from biomass burning in the Yucatan. Atmos. Chem. Phys. 2009, 9, 5785-5812, 2009. DOI: 10.5194/acp-9-5785-2009. 102. Hobbs, P.V.; Sinha, P.; Yokelson, R. J.; Christian, T. J.; Blake, D. R.; Gao, S.; Kirchstetter, T. W.; Novakov, T.; Pilewskie, P. Evolution of gases and particles from a savanna fire in South Africa. J. Geophys. Res. 2003, 108, 8485, doi:10.1029/2002JD002352, 2003. 103. Fine, P. M.; Cass, G. P.; Simoneit, B. R. T. Chemical Characterization of Fine Particle Emissions from the Wood Stove Combustion of Prevalent United States Tree Species. Environ. Eng. Sci. 2004, 21, 6, 705-721; DOI: 10.1089/ees.2004.21.705. 104. Simoneit, B. R. T. Biomass burning - a review of organic tracers for smoke from incomplete combustion. Appl. Geochem. 2002, 17, 129-162; DOI: 10.1016/S08832927(01)00061-0. 105. Lim, H. J.; Carlton, A. G.; Turpin, B. J. Isoprene Forms Secondary Organic Aerosol through Cloud Processing: Model Simulations. Environ. Sci. Technol. 2005, 39, 4441-4446; DOI: 10.1021/es048039h. 27 ACS Paragon Plus Environment


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Figure 1. Elemental ratios of gas-phase organics measured from the 30 different controlled stack

829

burns using I-HR-TOF-CIMS: (a) H:C versus O:C ratios; and (b) H:C versus N:C ratios. The

830

color map represents the average % abundance of each primary BB compound measured from the

831

30 controlled stack burns. The yellow shading corresponds to an O:C ratio ≥ 0.67.

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Figure 2. Relative ion signal abundance of all the compounds detected during the 30 controlled

837

stack burns using the I-HR-TOF-CIMS.

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Figure 3. Relative ion abundances of selected potential precursors of aqSOA emitted by BB

843

during aqueous-phase OH-initiated oxidation experiments (H2O2+UV) and during control

844

experiments (UV or H2O2 only) for Douglas fir (405 µmol-C L-1) and ponderosa pine (601 µmol-

845

C L-1) samples. All compounds presented here were detected using ESI-HR-QTOFMS operated

846

in the negative ion mode for C6H6O2, C7H8O2, C6H10O5, C6H6O3, and in the positive ion mode for

847

C6H7NO and C3H4N2. Standard deviations are calculated over replicates of three different

848

experiments. Zero relative ion abundance was attributed to non-detected compounds.

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852 853

Figure 4. Relative ion abundances of acetic + glycolic acids, pyruvic acid, oxalic acid, C2H2O3,

854

C3H2O5, C3H6O3, C3H4O4, C4H6O4 and C5H6O5 during the course of aqueous-phase OH-initiated

855

oxidation (H2O2+UV), UV and H2O2 control experiments for Douglas fir (405 µmol-C L-1) and

856

ponderosa pine (601 µmol-C L-1) samples.. Acetic+glycolic, pyruvic and oxalic acids were

857

quantified by IC, while all other compounds were identified using ESI-HR-QTOFMS operated in

858

the negative ion mode. Standard deviation are calculated over replicates of three different

859

experiments.

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862 863

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Table 1. Potential precursors of aqSOA based on measurements by I-HR-TOF-CIMS, detection in water samples, their O:C ratio, molecular mass, potential structure and whether previously studied. Compound O:C Molecular mass Detection in water-samples aqSOA potential Potential compound h formula (g mol-1) studied IC ESI (-)a ESI(+)b 26,c,d C2H4O2 1.00 60.05 x x acetic acid acetic acid36 26 C3H4O2 0.67 72.06 x acrylic acid C3H6O2 0.67 74.08 x propanoic acid C2H4O3 1.5 76.05 x glycolic acid c,d glycolic acid105 C3H4O3 1.00 88.06 x x pyruvic acidc pyruvic acid15 C3H6O3 1.00 90.08 x x hydroxypropanoic acidc lactic acid69 C3H8O3 1.00 92.09 glycerol C6H6O 0.17 94.11 x phenol26,70,71 phenol35 C4H2O3 0.75 98.06 maleic anhydride C4H4O3 0.75 100.07 xNH4+e hydroxy furanone, dihydro furandione26 C4H6O3 0.75 102.09 x oxobutanoic acid66, methyl-oxopropanoic acid C4H8O3 0.75 104.10 x hydroxybutyric acid C6H6O2 0.33 110.11 x xH+ benzene diol26,70, acetylfuran, methyl furfural26,71 catechol34 C5H8O3 0.60 116.12 x oxopentanoic acid, methyl oxobutanoic acid C4H6O4 1.00 118.09 x succinic acid66 succinic acid69 70 26,70,71 C8H8O 0.13 120.15 acetophenone , tolualdehyde g + C7H6O2 0.29 122.12 x xNa benzoic acid, hydroxybenzaldehyde26,70,71 C7H8O2 0.29 124.14 x methylbenzenediol70, methoxyphenol50,26 guaiacol35 71 26,70 methyl furoic acid , hydroxymethyl furfural , hydroxymethyl C6H6O3 0.50 126.11 x xH+ pyranone C6H8O3 0.50 128.13 x xH+ C5H8O4 0.80 132.11 x glutaric acid66 glutaric acid68,69 C5H10O4 0.80 134.13 x xNa+ g C8H8O2 0.25 136.15 x xH+ phenyl acetic acid71, methyl benzoic acid26 + C8H10O2 0.25 138.16 x xH (hydroxyethyl)phenol70, methylguaiacol26,70 C6H8O4 0.67 144.13 xg xH+ xNa+ 1,4:3,6-Dianhydro-β-d-glucopyranose103 + C6H10O4 0.67 146.14 x xNa adipic acid66, isosorbide adipic acid68 103 26,70 C8H8O3 0.38 152.15 x hydroxyphenyl acetic acid, methoxybenzoic acid , vanillin vanillin34 C6H8O5 0.83 160.12 x ketoadipic acid C6H10O5 0.83 162.15 x hydroxy methylglutaric acid, levoglucosan104 levoglucosan33 a In the negative ion mode, compounds are detected as deprotonated ion ([M-H]-). bIn the positive ion mode, ions could be protonated or cluster with Na+ or NH4+. The cluster ion are indicated as follows xclustered ion. cIdentified by the IC using an authentic standard dAcetic and glycolic acids elute at the same retention ACS Paragon Plus Environment

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time. eNot positively identified since the detected ion could be C4H4O3 clustered with NH4+ or C4H7NO3 protonated by H+.g Same time trend during OH oxidation in both positive and negative ion modes.h O:C as present in the gas phase.

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Photochemical Cloud Processing of Primary Wildfire Emissions as a

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