Effect of Organic Coatings, Humidity and Aerosol Acidity on

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Effect of Organic Coatings, Humidity and Aerosol Acidity on Multiphase Chemistry of Isoprene Epoxydiols Matthieu Riva, David M. Bell, Anne-Maria Kaldal Hansen, Greg Drozd , Zhenfa Zhang, Avram Gold, Dan Imre, Jason Douglas Surratt, Marianne Glasius, and Alla Zelenyuk Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b06050 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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Environmental Science & Technology

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Effect of Organic Coatings, Humidity and Aerosol Acidity on Multiphase Chemistry of

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Isoprene Epoxydiols

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Matthieu Riva†, David M. Bell‡, Anne-Maria Kaldal Hansen§, Greg T. Drozd♯, Zhenfa

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Zhang†, Avram Gold†, Dan Imre&, Jason D. Surratt†, Marianne Glasius§,*, Alla Zelenyuk ‡,*

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

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

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‡ Pacific Northwest National Laboratory, Richland, WA, USA

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§ Department of Chemistry, Aarhus University, Aarhus, Denmark

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♯ Department of Environmental Science, Policy, and Management, University of California,

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Berkeley, CA, USA

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&

Imre Consulting, Richland, WA, USA

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* Corresponding Authors:

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Email: [email protected]; 3335 Innovation Blvd., Richland, WA 99354 USA

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Phone: (1) 509-371-6155; Fax: (1) 509-371-6139

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Email: [email protected]; Langelandsgade 140, DK-8000 Aarhus C, Denmark

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Phone: (45) 8715-5923 ; Fax : (45) 8619 6199

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Keywords: IEPOX, secondary organic aerosol, heterogeneous chemistry, single particle mass

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spectrometer

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TITLE RUNNING HEAD: Multiphase chemistry of IEPOX

ACS Paragon Plus Environment

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ABSTRACT

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Multiphase chemistry of isomeric isoprene epoxydiols (IEPOX) has been shown to be the

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dominant source of isoprene-derived secondary organic aerosol (SOA). Recent studies have

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reported particles composed of ammonium bisulfate (ABS) mixed with model organics

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exhibit slower rates of IEPOX uptake. In the present study, we investigate the effect of

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atmospherically-relevant organic coatings of α-pinene (AP) SOA on the reactive uptake of

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trans-β-IEPOX onto ABS particles under different conditions and coating thicknesses. Single

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particle mass spectrometry was used to characterize in real-time particle size, shape, density,

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and quantitative composition before and after reaction with IEPOX. We find that IEPOX

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uptake by pure sulfate particles is a volume-controlled process, which results in particles with

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uniform concentration of IEPOX-derived SOA across a wide range of sizes. Aerosol acidity

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was shown to enhance IEPOX-derived SOA formation, consistent with recent studies. The

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presence of water has a weaker impact on IEPOX-derived SOA yield, but significantly

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enhanced formation of 2-methyltetrols, consistent with offline filter analysis. In contrast,

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IEPOX uptake by ABS particles coated with AP-derived SOA is lower compared to that of

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pure ABS particles, strongly dependent on particle composition, and therefore on particle

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size.

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INTRODUCTION

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Isoprene (2-methyl-1,3-butadiene, C5H8) is the most abundant non-methane

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hydrocarbon emitted into the atmosphere with annual emissions estimated from 500 to 750

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Tg.1 Isoprene reacts primarily with hydroxyl radicals (OH) in the troposphere,2-6 but can also

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be oxidized by nitrate radicals (NO3) or ozone (O3).7-9 Although formation of secondary

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organic aerosol (SOA) from isoprene oxidation was previously considered negligible,10,11

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research over the last decade identified a large number of highly oxidized compounds (e.g., 2-

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methyltetrols, C5-alkene triols, and organosulfates) arising from the atmospheric oxidation of

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isoprene.5-7,9,12,13 High abundance of these compounds within fine organic aerosol (PM2.5,

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aerosol with aerodynamic diameter ≤ 2.5 µm) collected from isoprene-rich regions indicates

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that isoprene could be an important SOA precursor, thereby providing a stimulus to

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understanding the pathways from isoprene to SOA.12,13

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SOA formation from isoprene oxidation is now recognized as one of the major sources

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of SOA in the atmosphere.14 Laboratory studies over the last decade have revealed the critical

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role of aerosol acidity in forming SOA from the atmospheric oxidation of isoprene.9,15-18

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Recent studies have demonstrated significant formation of SOA from the reactive uptake of

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isomeric isoprene epoxydiols (IEPOX) in the presence of acidified sulfate aerosol and

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identified a large number of highly oxidized compounds within the aerosol phase, including

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organosulfates, 2-methyltetrols, C5-alkene-triols, and oligomers.17,19-21 IEPOX has been

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shown to be an important secondary oxidation product,22 and has a key role in SOA formation

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from the oxidation of isoprene.17,19 Formation of IEPOX has been identified from the

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photooxidation of isoprene under both low- and high-NOx conditions;22,23 however, the yield

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of IEPOX is greater under low-NOx conditions. Although gas-phase OH oxidation of IEPOX

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could occur, removal of IEPOX from the gas phase is likely dominated by acid-catalyzed

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uptake reactions onto acidic sulfate aerosols.24-27 Recent field campaigns have demonstrated


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the importance of IEPOX-derived SOA products in fine organic aerosol, especially in the

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southeastern U.S. where IEPOX-derived SOA tracers can represent up to 40% of the total fine

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organic aerosol mass.28-32

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Prior studies on SOA formation have utilized primarily polydisperse ‘pure’ inorganic

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seed aerosols. However, given that atmospheric sulfate particles often contain organic

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compounds, their presence could have an impact on the multiphase chemical processes of

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IEPOX.25 It is therefore important to investigate this aspect of the IEPOX chemistry. Indeed,

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recent studies indicate that coatings composed of model oligomeric organics significantly

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reduce the uptake rate of IEPOX by sulfate particles.21,25 However, as of yet, the effect of

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atmospherically relevant SOA coatings on IEPOX heterogeneous chemistry has not been

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studied. Additionally, the relationship between particle size and the reactive uptake rate of gas

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phase molecules like IEPOX provides mechanistic information on the processes involved.

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However, particle-size effect studies on multiphase chemistry of IEPOX have not been

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explicitly conducted. The relative rates between gas-phase mass transfer and particle surface

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controlled reactive uptake govern the evolution of particle size during IEPOX uptake by

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sulfate seed particles. If diffusion kinetics or the mass accommodation rate is the limiting

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factor, the size evolution will be surface area controlled, whereas when the chemical reaction

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kinetics is the limiting step, it means the size evolution will be volume controlled. While

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some recent studies suggest that IEPOX uptake kinetics on laboratory-generated inorganic

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aerosols is a volume controlled process,25,33 results from application of Henry's law constants

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to ambient aerosols vary by order of magnitude.29

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In the present work, we investigate the effect of particle acidity and relative humidity

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(RH) on reactive uptake of trans-β-IEPOX, which is the predominant IEPOX isomer,34 onto

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sulfate seed particles. We then expand on this work with a study of the IEPOX uptake into

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acidic sulfate particles containing SOA produced from α-pinene ozonolysis. In all cases, the


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reactions are characterized as function of particle sizes. Two single particle mass

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spectrometers (miniSPLAT and SPLAT II)35 were used to characterize in-situ and in real time

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size, composition, density, and shape of individual particles before and after the uptake

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reaction, and follow the changes in particle size and mass spectral peak intensities as function

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of reaction time.

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From the data, we estimate the density of the organic moiety of IEPOX products and

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determine the weight fraction of the IEPOX-derived SOA ( ) as a function of particle

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size. Note that isoprene-derived SOA density used in air quality models is derived from

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isoprene photooxidation experiments performed under low-NOx conditions in the absence of

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sulfate particles under dry conditions.5 However, under these conditions the reactive uptake of

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IEPOX does not occur or is significantly slower25,26 than other atmospheric sinks (e.g., OH

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oxidation or deposition).33,35 Therefore, the density of SOA produced from the multiphase

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chemistry of IEPOX and acidic seeds remains unknown.

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In addition to online single particle multidimensional analysis, aerosol bulk chemical

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composition was analyzed by offline ultra-performance liquid chromatography/electrospray

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ionization, high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-

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QTOFMS), and gas chromatography/electron ionization-mass spectrometry (GC/EI-MS)

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analysis.

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EXPERIMENTAL SECTION

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Chamber Experiments. Experiments were conducted under wet (50-60% RH) or dry ( 99%) ozonolysis prior to IEPOX injection. In these

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experiments approximately 300 ppb of ozone was added after ABS seed injection, then 2 or 5

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0.15 µL injections of AP were made to achieve thin (~20 nm) and thick (~40 nm) coating

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thickness, respectively. Nucleation of AP ozonolysis products was prevented by multiple

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injections of AP.39 At the end of each experiment particles were collected onto Teflon

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membrane filters (47 mm diameter, 1.0 µm pore size; Pall Life Science) at a flow rate of ~ 10

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L min−1 for 70-80 min. Filters were then stored in scintillation vials in the dark at −20 °C until

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analysis.


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Single Particle Characterization. For all experiments, miniSPLAT and SPLAT II were used

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to characterize the size, density, shape, and chemical compositions of individual particles

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during the IEPOX uptake into the seed aerosol. Both instruments have been previously

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described in detail.35,40-42 Briefly, single particles are sampled through a 100 µm flow-

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calibrated critical orifice at a flow rate of 0.1 L min-1 and focused into a collimated particle

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beam in the aerodynamic lens inlet before transmission to the sizing region. Each particle is

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detected by light scattering at two optical detection stages to yield individual particle velocity,

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which is used to determine particle vacuum aerodynamic diameter (dva) with precision better

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than 0.5%.35,41 The two detection events are also used to time the triggering of a pulsed

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desorption/ionization excimer laser, operated at 193 nm with an output energy of 1.5-2.0 mJ

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per pulse, and acquire individual particle mass spectra. Each data point reported in this work

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represents the analysis of ~ 2000 individual particle mass spectra.

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In addition to sampling of the polydisperse particle flow, miniSPLAT and SPLAT II

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were used to characterize dva and mass spectra of mobility- and mass-selected particles to

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yield quantitative information on particle density or effective density, shape, and composition

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as described in detail elsewhere.41,43-45 In addition, characterization of mobility- and mass-

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selected particles over a wide range of sizes and masses makes it possible to evaluate the

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relationship between particle size and reactivity. Measurements on classified particles were

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performed at the end of each step: injection of seed particles, AP-derived SOA coating

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process, and IEPOX reactive uptake. In all the experiments conducted at elevated RH,

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particles were dried by passing through two inline diffusion dryers prior to classification and

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characterization.

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Filter Extraction and Chemical Analysis. Chemical characterization of SOA from IEPOX

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uptake was also performed by UPLC/ESI-QTOFMS (6520 Series, Agilent) operated in both

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negative and positive ion modes and by GC/EI-MS. Operating conditions have been described

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in detail elsewhere.21 Particles collected from experiments were extracted from filters with 22

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mL of high-purity methanol (LC-MS CHROMASOLV-grade, Sigma-Aldrich) by 45 min of

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sonication. Methanol extracts were then blown dry under a gentle N2(g) stream at room

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temperature. Dried extracts were reconstituted in 2 mL methanol, divided into two equal

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portions and then blown dry.

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For UPLC/ESI-QTOFMS analysis, one dried extract was reconstituted with 150 µL of

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a 50:50 (v/v) solvent mixture of methanol (LC-MS CHROMASOLV-grade, Sigma-Aldrich)

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and high-purity water (Milli-Q, 18.2 MΩ). 5 µL and 10 µL aliquots were injected onto the

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UPLC column (Waters ACQUITY UPLC HSS T3 column, 2.1 × 100 mm, 1.8 µm particle

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size) and eluted at a flow rate of 0.3 mL min-1 with a solvent mixture of methanol containing

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0.1% acetic acid or 0.1% ammonium acetate (LC-MS CHROMASOLV-grade, Sigma-

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Aldrich) and water containing 0.1% acetic acid or 0.1% ammonium acetate (LC-MS

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CHROMASOLV-grade, Sigma-Aldrich) for negative and positive mode, respectively. A

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mixture of 2-methyltetrol sulfate esters (C5H11O7S−) was synthesized in-house for use as an

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authentic standard.29

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The second portion of dried filter extract was trimethylsilylated by addition of 100 µL

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N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) + trimethylchlorosilane (99:1 (v/v),

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Supleco) and 50 µL pyridine (Sigma-Aldrich, 98%, anhydrous). The mixture was heated for 1

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h at 70 °C and analyzed within 24 hours following trimethylsilylation. Analyses were

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performed by GC/EI-MS at 70 eV (Hewlett 5890 Packard Series II Gas Chromatograph

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interfaced to a HP 5971A Series Mass Selective Detector, Econo-CapTM-ECTM-5 column,

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30 m × 0.25 mm × 0.25 µm). Operating conditions and temperature program of the GC/MS


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were as described previously.17 Authentic 2-methyltetrols and cis- and trans- 3-

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methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) were synthesized in-house and used to

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quantify identified products.19,29

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RESULTS AND DISCUSSION

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In this study we investigate reactive uptake of trans-β-IEPOX by sulfate aerosol of

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varying acidity, and by ABS seed particles coated with AP-derived SOA of different effective

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coating thicknesses. Experiments were carried out under wet and dry conditions. Table S1

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summarizes the different experimental conditions, which were chosen to be consistent with

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those used in previous chamber experiments.21,26

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We begin by examining results from IEPOX uptake into ABS particles under dry

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conditions. Figure 1 displays a mass spectrum of IEPOX-derived SOA obtained by

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subtracting ABS mass spectral peaks from the mass spectrum of reacted ABS particles. The

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inset in Figure 1 presents the time series of the relative mass spectral intensities of the

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identified characteristic fragment ions (m/z=31 (CH3O+), 59 (C3H7O+), 61 (C3H9O+), 71

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(C4H7O+), 75 (C3H7O2+), 83 (C5H7O+), 87 (C4H7O2+), 101 (C5H9O2+), and 105 (C4H9O3+)).

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Some fragment ions have been previously observed using other types of mass spectrometers

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operated in positive ion mode in field studies and laboratory IEPOX uptake

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experiments.19,21,28,46 For example, Lin et al.21 observed the same major fragment ions as those

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shown in Figure 1 (m/z 83, 101, 105) using ESI-MS operated in the positive ion mode. Figure

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1 shows that while the relative intensity of the mass-spectral peak at m/z 87 attains a

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maximum value nearly instantaneously, the intensity of the peak at m/z 101 increases at the

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slowest rate. The different temporal evolutions suggest that fragment ions at m/z 87 and 101

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originate from different molecules. The rapid increase in the relative intensity of the ion at m/z

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87 is often followed by a decrease, suggesting it could be a fragment of IEPOX that is


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reacting in the particle, consistent with the mass spectrum of the authentic IEPOX standard

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(Figure S1). Formation of IEPOX-derived SOA is also apparent in the changes in particle

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size, an example of which is illustrated in Figure S2, showing that the particles mean mobility

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diameter (dm) increases with time (half-life of the rise of ~15 min) in response to the addition

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of IEPOX, consistent with previous observations.19,26 Filter measurements reveal the presence

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of the known IEPOX-derived SOA tracers (2-methyltetrols, C5-alkene triols, organosulfates,

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dimers, and trimers), which have been previously observed and reported from reactive uptake

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of IEPOX.17 Relative contributions of each tracer from all experiments (Table S1 and Figure

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2) reveal that experimental conditions, such as humidity, strongly impact relative

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concentrations of the identified tracers.47 The differences will be discussed in more detail in

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the following sections.

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Determination of dva of either mobility- or mass-selected particle yields information on

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the particle shape (sphericity/asphericity) and density, or effective density.41 Figure 3a

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illustrates the changes in dva distributions of mobility-selected particles (at 80, 116 and 151

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nm) due to the reaction of ABS particles with IEPOX. It shows that addition of IEPOX-

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derived SOA to the ABS seed particles results in significant decrease of dva for particles with

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the same dm, indicating a decrease in particle density. The narrow line widths of all the dva

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distributions shown in Figure 3a indicate that the particles are spherical before and after the

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reaction, and therefore that these data provide direct information on particle density.41,45

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Figure 3b shows that the density of ABS particles after the reaction with IEPOX under dry

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conditions is 1.48 ± 0.02 g cm-3, independent of particle size suggesting volume-controlled

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reactive uptake of IEPOX. It should be noted that ABS particles, even at extremely low RHs,

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are not crystalline,40 which explains their high reactivity with IEPOX under dry

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conditions.17,25-27 The measured densities of the ABS particles before and after the reaction

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with IEPOX (Figure 3b) together with the observed changes in particles size due to the


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addition of IEPOX-derived SOA (Figure S1) make it possible to estimate density of IEPOX-

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derived SOA and calculate the weight fraction of IEPOX-derived SOA in reacted sulfate

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aerosol.44 The data indicate that prior to the addition of IEPOX, ABS particles had a mean

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diameter of 122 nm (Figure S1) and a density of 1.77 ± 0.02 g cm-3 (Figure 3b). After the

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addition of IEPOX, particle mean diameter became 155 nm and the particle density decreased

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to 1.48 ± 0.02 g cm-3 (Figure 3b). Assuming volume additivity, we calculate a density of 1.2 g

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cm-3 for IEPOX-derived SOA, which then can be used to calculate the weight fraction of

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IEPOX-derived SOA = 0.41 ± 0.01 for all particle sizes.

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To evaluate the limiting reaction parameters (IEPOX and sulfate) on the reactive

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uptake of IEPOX, multiple IEPOX injections were performed. Each addition of IEPOX

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results in a decrease in particle density and with it a decrease in dva for mobility- or mass-

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selected particles (Figure S3a). Here, as well, the dva distributions of mobility- and mass-

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selected particles indicate that all particles are spherical and the density of particles, and

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therefore their composition, is independent of their size. We find that after three identical and

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successive injections of IEPOX, the weight fraction of IEPOX-derived SOA in particles

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gradually increases from of 0.41 to 0.54, with only a negligible change following the

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fourth IEPOX addition (Figure S3b). In other words, after the first IEPOX injection, a fraction

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of sulfate aerosol remains unreacted, indicating competition between kinetics of IEPOX

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reactive uptake on ABS particles and IEPOX wall loss.

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The impact of RH on IEPOX reactive uptake was evaluated by conducting the

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experiment under wet (~50% RH) conditions. Particle density and weight fraction of IEPOX-

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derived SOA were determined in the same manner as in the experiments conducted under dry

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conditions. In all experiments under wet conditions, particles were sent through a diffusion

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drier before they were characterized by the single particle mass spectrometers to avoid

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potential measurement artifacts from water.48 The data indicate that the density of particles


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reacted under wet conditions is slightly lower (1.42 ± 0.02 g cm-3) than that under dry

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conditions (1.48 ± 0.02 g cm-3). Here, as well we find that particle density, and therefore

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composition, is independent of particle size (Figure S4), suggesting volume-controlled

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reactive uptake of IEPOX. As before, particle density is used to calculate the weight fraction

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of IEPOX-derived SOA under wet conditions: = 0.49 ± 0.01, which is slightly higher

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than under dry conditions (0.41 ± 0.01). Gaston et al.25 and Riedel et al.26 reported a slight

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decrease in the reactive uptake coefficient (γ) of IEPOX uptake under comparable conditions,

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which was attributed to dilution of acidity by additional particle water. Kinetics does not

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likely explain the difference in . Moreover, the composition of IEPOX-derived SOA

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in the dry and wet experiments is found to be noticeably different. As shown in Figure 2, 2-

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methyltetrol formation is significantly enhanced in the presence of water, consistent with the

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known acid-catalyzed epoxide ring-opening mechanisms.17,47 Similarly, composition analysis

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by single particle mass spectrometry indicates that relative intensity of peaks at m/z 75 and

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105, characteristic of 2-methyltetrols (Figure S2), are ~ 3 times higher in the mass spectra of

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IEPOX-derived SOA produced under wet conditions.

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Experiments with non-acidified deliquesced ammonium sulfate (AS) seed aerosol

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under wet conditions were performed to investigate the effect of acidity on IEPOX uptake. As

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reported in Table S1, aerosol acidity differs significantly between experiments performed

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with ABS and AS seed aerosol. Unlike ABS particles, AS particles are crystalline below 35%

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RH,40 therefore in these experiments we start with wet AS particles generated at nearly 100%

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RH and reduce the RH down to ~50% RH to assure that during exposure to IEPOX the AS

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seed particles are liquid. As reminder, we note that before characterization by miniSPLAT or

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SPLAT II wet particles are always dried by passing through two inline diffusion dryers. The

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data show that following IEPOX uptake onto liquid AS seeds and subsequent drying, the

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reacted particles are spherical and have density of 1.54 ± 0.02 g cm-3 for all particle sizes


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(Figure S5). In sharp contrast, when pure AS particles are dried they effloresce to form

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aspherical crystalline AS particles, whose effective density decreases with particle size, as

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their dynamic shape factor increases with size (Figure S5), consistent with previous studies.48

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The weight fraction of IEPOX-derived SOA in this case is lower = 0.31 ± 0.01,

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which is consistent with the slower IEPOX uptake kinetics. Under comparable conditions,

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Riedel et al.26 have reported a γ two orders of magnitude lower than the γ determined from

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IEPOX uptake on wet acidified AS seed aerosol (6.5 ± 6.4 × 10-4 vs 1.9 ± 0.2 × 10-2,

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respectively). This is also consistent with previous work demonstrating that aerosol acidity

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significantly enhances IEPOX-derived SOA yields.19,21,27 Comparison of the time profiles of

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characteristic ions presented in Figures 1 and S6 shows that formation of fragment ions is

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approximately 3 times slower for AS seed. Loss of IEPOX to the walls could explain the

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lower IEPOX-derived SOA content in the particles due to slower uptake kinetics on the AS

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particles. In addition, we find that the relative intensities of peaks at m/z 87 and 101 in the AS

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seed experiments increase at nearly the same rates (Figure S6), in contrast to their temporal

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evolution observed for ABS seed aerosol (Figure 1).

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The largest mass fraction of atmospheric sub-micron aerosol is comprised of organic

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compounds that are dominated by SOA formed from the gas-phase oxidation of volatile

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organic compounds (VOCs).49 While recent studies have shown that the presence of organic

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coatings in the aerosol phase suppresses the uptake and multiphase kinetics of IEPOX,21,25

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they utilized model organics, which are not found in the atmosphere. In contrast, in the

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present study we use SOA produced from ozonolysis of α-pinene (AP-derived SOA) as a

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representative biogenic SOA commonly found in the atmosphere. ABS particles were

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exposed to oxidation products arising from AP ozonolysis prior to injection of IEPOX into

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chamber. In separate experiments, two different quantities of AP-derived SOA were deposited

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onto the ABS particles to vary the organic coating thickness (Table S1). A typical mass


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spectrum of AP-derived SOA (Figure S7) is dominated by fragment ions at m/z 43, 59 and 73.

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It should be noted that the main IEPOX-derived SOA fragment ions (m/z 75, 83, 87 and 101)

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are observed here in very low abundance. The density of AP-derived SOA coated ABS

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particles, and therefore weight fraction of AP-SOA ( ), depends on particle size (Figure

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S8). In the experiment with thinly coated ABS particles, the decreases from ~ 75 % for

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80 nm particles to ~ 15% for 500 nm particles (Figure S8b). Alternatively, the AP-derived

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SOA particle content can be expressed as an equivalent coating thickness.44 Note that since

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the exact morphology of these particles has not been established, we use the term “equivalent

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coat thickness” as a convenient measure of how much AP-SOA is present. The thin coating

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case yields 18 ± 2 nm for all particle sizes. In other words, while the coating thickness

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remains constant at 18 nm, the rapidly decreases with particle size.

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After exposure to IEPOX, the average particle mass spectrum of AP-derived SOA

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coated ABS particles exhibits high intensities of the IEPOX-derived SOA marker fragment

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ions at m/z 87 and 101 (Figure S9). The inset in Figure S9 shows the time-series of different

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fragment ions identified from reactive uptake of IEPOX onto coated acidified AS. Although

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the curves of the temporal evolution of the fragment ions are qualitatively similar to those

334

observed using AS particles (Figure S6), formation of IEPOX-derived SOA is approximately

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2 times faster on the thinly AP-derived SOA-coated ABS particles. However, compared to

336

experiments performed using pure ABS particles (Figure 1), both the kinetics and the

337

temporal evolution curves are different, demonstrating that AP-derived SOA coatings impact

338

IEPOX reactive uptake kinetics (Figure S9). After IEPOX injection, the changes in particle

339

density exhibit strong size dependence, with the particle density of the smallest particles

340

remaining unchanged, while the density of larger particles decreases significantly (Figure

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S10a). This size-dependent pattern indicates that the IEPOX-derived SOA content increases

342

with particle size. Similarly, the relative intensities of IEPOX-derived SOA fragment ions


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also clearly show that the concentrations of the IEPOX-derived SOA products increase with

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particle size (Figure S10b). The measured particle densities can be used to calculate the

345

and as function of particle size presented in Figure 4a. This figure shows that

346

the of the smallest particles appears negligible, increasing to ~ 20% for larger

347

particles.

348

In order to compare these results with those from the pure ABS particles, the

349

with respect to ABS seed was also calculated (Figure 4b). It increases rapidly from 0 to 30%,

350

as the ABS seed aerosol diameter increases from 60 to 100 nm, and then remains nearly

351

constant at ~30% for diameters > 100 nm. As discussed above, the data on pure ABS particles

352

show that ABS core size has no effect on the reactive uptake of IEPOX, suggesting that the

353

reactive uptake of IEPOX by the coated particles is affected by the quantity of organic present

354

in the particle.

355

Figure S11 displays plotted as a function of with respect to ABS seed

356

particle. The data show that once drops below ~ 50 %, the remains constant at

357

~ 30 % with respect to ABS seed particle. Similar experiments with AP-derived SOA coated

358

ABS particles were also performed under wet conditions. As before, ABS particles were

359

coated by AP-derived SOA to form an equivalent coating thickness that was determined to be

360

22 ± 3 nm. The coated particles were then exposed to IEPOX. Both, particle density and mass

361

spectral data indicate that while smaller particles contain undetectable amounts of IEPOX-

362

derived SOA, the reaches ~ 35 % with respect to ABS seed particle when

363

drops below ~ 50 % (Figure S12), which is very similar to experiment performed under dry

364

conditions presented above. These findings imply that when particle organic content increases

365

under either dry or wet conditions, reactive uptake of IEPOX could be significantly limited

366

compared to pure ABS particles.


15


Page 16 of 34

367

Finally, ABS particles were coated with a larger quantity of AP-derived SOA to

368

produce a thicker (38 ± 2 nm) organic coating (Table S1). Figure S13 shows the measured

369

densities of both AP-derived SOA coated ABS particles prior to the addition of IEPOX and

370

after the reaction, yielding that are higher than 50% for particles smaller than ~ 300 nm.

371

When these particles are exposed to IEPOX, their average mass spectra indicate very low

372

intensities for fragment ions of IEPOX-derived SOA products. Similarly, the densities of

373

these particles before and after exposure to IEPOX are nearly indistinguishable, with small

374

differences observed only for the largest particles (Figure S13), indicating that a 40 nm

375

coating of AP-derived SOA virtually halts the reactive uptake of IEPOX. Here again, IEPOX

376

uptake occurred only when dropped below ~ 50 % with respect to ABS seed particle,

377

which is consistent with other experiments described above. We conclude that the amount of

378

organics on the ABS particles plays a key role on reactive uptake of IEPOX.

379

Presence of organics on ABS particles could lead to a liquid-liquid phase separation,

380

especially at high organic loadings, as recently demonstrated.50-53 Phase separation can

381

significantly affect overall physical and chemical aerosol properties, and decrease the rate of

382

heterogeneous chemistry. For instance, organic coatings on aqueous aerosols can suppress

383

heterogeneous reactions, such as N2O5 reactive uptake.54 Similarly, semi-solid or glassy-phase

384

organics can inhibit or significantly slow down the partitioning of semivolatile products.55

385

Changes in particle composition and morphology in the presence of high amounts of organics,

386

such as phase separation, have previously been shown to have strong effects on the uptake

387

and heterogeneous chemistry of epoxides.25,56 In the present study we showed that IEPOX

388

uptake by pure inorganic particles, both AS and ABS, is a volume-controlled process, which

389

results in particles with uniform composition across a wide range of sizes. In contrast, IEPOX

390

uptake by ABS particles coated with AP-derived SOA exhibits strong dependence on the

391

, and therefore on particle size. As shown in Figure 4 and discussed above, IEPOX


16

Page 17 of 34


392

uptake occurred only when the of thin or thick coatings of AP-derived SOA on sulfate

393

aerosol dropped below ~ 50 % under both dry and wet conditions, effectively limiting IEPOX

394

uptake to larger particles with lower . Similarly to the laboratory-generated particles used

395

in this study, atmospheric particles composed of SOA and sulfates often exhibit size

396

dependent composition, with small particles being dominated by SOA.57,58 It should be also

397

noted that liquid-liquid phase separation within organic coatings is highly impacted by the

398

oxidation state of aerosol components, i.e. oxygen-to-carbon (O:C) ratios.51 In general, phase

399

separation occurred for O:C < 0.5-0.6 and not for O:C ratios above 0.8. In the intermediate

400

regime (0.6-0.8), liquid-phase separation depends on composition and organic functional

401

groups.59 Ozonolysis of AP produces SOA with O:C ratios from 0.3 to 0.4 60 and liquid-liquid

402

phase separations most likely occurred in the present study, which was already reported in

403

previous studies.

404

the key role of organic coatings on SOA formation arising from reactive uptake of IEPOX.

405

However, further studies are needed to fully resolve the impacts of varying organic coatings

406

and phase separation on IEPOX uptake and multiphase chemistry. Indeed, gas-phase

407

oxidation of other SOA precursors, such as isoprene or aromatic compounds, yield SOA with

408

O:C ratios higher than 0.6.60 Therefore, organic coatings resulting from the oxidation of the

409

other SOA precursors could adversely impact IEPOX reactive uptake, and may need to be

410

considered in explicit models of IEPOX-derived SOA formation.61, 62

51-53

This work provides new insight on isoprene SOA chemistry, showing

411 412

Supporting Information Available

413

The table provides a summary of experimental conditions. Figures show typical temporal

414

evolution of particle mean diameter during IEPOX uptake; typical particle mass spectra; time

415

evolution of the relative intensity of the major fragment ions from IEPOX uptake on seed


17


Page 18 of 34

416

particles; measured density of seed and reacted particles as a function of particle size;

417

calculated weight fraction of organics and IEPOX-derived SOA as function of particle size

418

and AP-derived SOA weigh fraction. This material is available free of charge via the Internet

419

at http://pubs.acs.org/.

420 421

ACKNOWLEDGMENTS

422

The authors wish to thank the Camille and Henry Dreyfus Postdoctoral Fellowship Program

423

in Environmental Chemistry for their financial support. The authors wish also to thank Joel

424

Thornton (University of Washington) for useful discussions. This work is also funded in part

425

by the National Science Foundation under grants CHE 1404644, the Carlsberg Foundation

426

and the Centre of Excellence Cryosphere-Atmosphere Interactions in a Changing Arctic

427

Climate (CRAICC) funded by NordForsk. Work by A. Z. and the development of the

428

advanced single particle analysis methods (A.Z.) were supported by the US Department of

429

Energy, Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical

430

Sciences, Geosciences & Biosciences. The part of the research was performed using EMSL, a

431

DOE Office of Science User Facility sponsored by the Office of Biological and

432

Environmental Research and located at Pacific Northwest National Laboratory.

433


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Page 19 of 34

434


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of Ammonium Sulfate. Aerosol Sci. Technol. 2011, 45, 244-261.

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54. Gaston, C.J.; Thornton, J.A.; Ng, N.L. Reactive uptake of N2O5 to internally mixed

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inorganic and organic particles: The role of organic carbon oxidation state and inferred

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organic phase separations. Atmos. Chem. Phys., 2014, 14 (11), 5693-5707.

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55. Zhou, S.; Shiraiwa, M.; McWhinney, R.D.; Pöschl, U.; Abbatt, J. Kinetic limitations

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in gas-particle reactions arising from slow diffusion in secondary organic aerosol.

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56. Drozd, G.T.; Woo, J.L.; McNeill, V.F. Self-limited uptake of α-pinene oxide to acidic

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aerosol: The effects of liquid-liquid phase separation and implications for the

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formation of secondary organic aerosol and organosulfates from epoxides. Atmos.

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Chem. Phys. 2013, 13 (16), 8255-8263.

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57. Vaden, T.D.; Imre, D.; Beránek, J.; Shrivastava, M.; Zelenyuk, A. Evaporation

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kinetics and phase of laboratory and ambient secondary organic aerosol. Proc. Natl.

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58. Zaveri, R.A.; Shaw, W.J.; Cziczo, D.J.; Schmid, B.; Ferrare, R.A.; Alexander, M.L.;

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Alexandrov, M.; Alvarez, R.J.; Arnott, W.P.; Atkinson, D.B.; Baidar, S.; Banta, R.M.;

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Barnard, J.C.; Beranek, J.; Berg, L.K.; Brechtel, F.; Brewer, W.A.; Cahill, J.F.; Cairns,

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B.; Cappa, C.D.; Chand, D.; China, S.; Comstock, J.M.; Dubey, M.K.; Easter, R.C.;

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Erickson, M.H.; Fast, J.D.; Floerchinger, C.; Flowers, B.A.; Fortner, E.; Gaffney, J.S.;

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Gilles, M.K.; Gorkowski, K.; Gustafson, W.I.; Gyawali, M.; Hair, J.; Hardesty, R.M.;

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Harworth, J.W.; Herndon, S.; Hiranuma, N.; Hostetler, C.; Hubbe, J.M.; Jayne, J.T.;

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Jeong, H.; Jobson, B.T.; Kassianov, E.I.; Kleinman, L.I.; Kluzek, C.; Knighton, B.;

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Kolesar, K.R.; Kuang, C.; Kubátová, A.; Langford, A.O.; Laskin, A.; Laulainen, N.;

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K.; Tomlinson, J.; Volkamer, R.; Wallace, H.W.; Wang, J.; Weickmann, A.M.;

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59. Song, M.; Marcolli, C.; Krieger, U.K.; Zuend, A.; Peter, T. Liquid-liquid phase

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separation in aerosol particles: Dependence on O:C, organic functionalities, and

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compositional complexity. Geophys. Res. Lett. 2012, 39 (19), L19801.

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60. Chhabra, P.S.; Ng, N.L; Canagaratna, M.R.; Corrigan, A.L.; Russell, L.M.; Worsnop,

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D.R.; Flagan, R.C.; Seinfeld, J.H. Elemental composition and oxidation of chamber

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organic aerosol. Atmos. Chem. Phys. 2011, 11 (17), 8827-8845.

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61. McNeill, V.F.; Woo, J.L.; Kim, D.D.; Schwier, A.N.; Wannell, N.J.; Sumner, A.J.;

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62. Pye, H.O.T.; Pinder, R.W.; Piletic, I.R.; Xie, Y.; Capps, S.L.; Lin, Y.-H.; Surratt, J.D.;

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681 682 683

Figure 1. Mass spectrum of IEPOX-derived SOA formed as a result of IEPOX uptake onto

684

ABS seed aerosols under dry conditions. Inset shows the temporal evolution of the relative

685

intensities of the characteristic mass-spectral peaks, including two major peaks at m/z 87 (red)

686

and 101 (blue). t=0 min indicates the injection of IEPOX into the chamber.

687


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688 689

Figure 2. Relative contributions (% by mass) of IEPOX-derived SOA tracers by both GC/EI-

690

MS and UPLC/ESI-QTOFMS in filters collected from different experiments. ABS

691

corresponds to the experiment performed using ammonium bisulfate seed particle under dry

692

(< 5%) and wet conditions (~ 50-55%). AS represents experiments performed using

693

ammonium sulfate seed particles under wet conditions. AP-thin and AP-thick correspond to

694

experiments using α-pinene (AP)-derived SOA with 18 ± 2 nm and 38 ± 2 nm equivalent

695

coatings on ABS seed particles, respectively.

696


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697 698 699

Figure 3. (a) The dva distributions of ABS seed particles (blue) and ABS + IEPOX particles

700

(red) classified at dm=80 nm. Note that multiply charged particles are also present. The dva of

701

ABS + IEPOX particles with the same dm are smaller because of their lower density; (b) The

702

measured densities of ABS seed particles (blue) and ABS + IEPOX particles (red) as a

703

function of particle size.

704 705


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706 707 708

Figure 4. (a) The calculated weight fractions for α-pinene (AP)-derived SOA ( , green),

709

the organic moiety of IEPOX-derived SOA ( , red), and total organics (

710

as a function of particle size; (b) The calculated weight fractions of AP-derived SOA (green)

711

and the organic moiety of IEPOX-derived SOA (red) with respect to (wrt) the ammonium

712

bisulfate (ABS) seed vs. the ABS seed diameter. Open and solid circles represent repeated

713

experiments.

, yellow)

714


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TOC Art 84x47mm (300 x 300 DPI)


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Effect of Organic Coatings, Humidity and Aerosol Acidity on

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