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Environmental Processes

Quantification of Particle-bound Organic Radicals in Secondary Organic Aerosol Steven Campbell, Svetlana Stevanovic, Branka Miljevic, Steven E. Bottle, Zoran Danil Ristovski, and Markus Kalberer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00825 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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Environmental Science & Technology

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Quantification of Particle-bound Organic Radicals in

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Secondary Organic Aerosol

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Steven. J. Campbell,†,‡* Svetlana Stevanovic,§◆ Branka Miljevic,§ Steven E. Bottle,¶

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Zoran Ristovski§ and Markus Kalberer†‡*

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† Centre

for Atmospheric Science, Department of Chemistry, University of Cambridge,

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Cambridge CB2 1EW, U.K.

‡

Department of Environmental Science, University of Basel, Klingerbergstrasse 27,

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Basel, 4056 Switzerland

§International

Laboratory for Air Quality and Health, Queensland University of

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Technology, Brisbane, QLD 4001, Australia

◆School

of Engineering, Deakin University, Vic 3126, Australia

ACS Paragon Plus Environment

1


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¶ARC

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Centre for Excellence for Free Radical Chemistry and Biotechnology, Queensland University of Technology, Brisbane, QLD 4001, Australia

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ABSTRACT: The chemical composition and evolution of secondary organic aerosol

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(SOA) in the atmosphere represents one of the largest uncertainties in our current

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understanding of air quality. Despite vast research, the toxicological mechanisms relating

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to adverse human health effects upon exposure to particulate matter are still poorly

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understood. Particle-bound reactive oxygen species (ROS) may substantially contribute

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to observed health effects by influencing aerosol oxidative potential (OP). The role of

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radicals in both the formation and ageing of aerosol, as well as their contribution to aerosol

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OP, remains highly uncertain. The profluorescent spin trap BPEAnit (9,10-bis-

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(phenylethynyl)-anthracene-nitroxide), previously utilized to study combustion-generated


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aerosol, has been applied to provide the first estimate of particle-bound radical

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concentrations in SOA. We demonstrate that SOA from different atmospherically

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important VOC precursors have different particle-bound radical concentrations, estimated

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for the ozonolysis of α-pinene (0.020 ± 0.0050 nmol/µg), limonene (0.0059 ± 0.0010

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nmol/µg) and β-caryophyllene (0.0025 ± 0.00080 nmol/µg), highlighting the potential

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importance of OH-initiated formation of particle-bound organic radicals. Additionally, the

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lifetime of particle-bound radical species in α-pinene SOA was estimated, and a pseudo-

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1st order rate constant of k = 7.3 ± 1.7 × 10-3 s-1 was derived, implying a radical lifetime

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on the order of minutes.

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INTRODUCTION

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Several epidemiological studies have shown a close correlation between exposure to

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ambient particulate matter (PM) and adverse human health effects, including increased

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morbidity from respiratory and cardiovascular disease as well as an increase in overall

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mortality.1–3 Secondary organic aerosol (SOA) constitutes a substantial fraction of

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ambient fine particulate matter.4 The formation of SOA in the atmosphere occurs via gas-


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to-particle conversion mechanisms, often initiated by the gas phase oxidation of VOCs5,6,

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and recently it has been shown that highly oxygenated multifunctional organic compounds

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could contribute significantly to SOA growth. 7–9. The chemical composition and evolution

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of SOA in the atmosphere represents one of the largest uncertainties in our current

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understanding of air quality and public health.10 Given the physical and chemical

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complexity of SOA in the atmosphere, it remains a challenge to determine the specific

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physical and chemical components that lead to observed adverse human health effects.11

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However, studies suggest that particle bound reactive oxygen species (ROS) are a major

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contributor to observed health effects associated with PM.11–13 Substantial quantities of

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ROS have been detected in biogenic SOA, and it has been demonstrated that radicals

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and related ROS can be produced in aqueous suspensions containing SOA.14,15

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ROS typically refers to a range of species including hydrogen peroxide, organic

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peroxides, oxidized organics and radicals such as the hydroxyl radical, superoxide and

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organic radicals.11 Atmospheric oxidation of biogenic VOCs, such as terpenes, leads to

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the formation of various radical species, such as Criegee Intermediates, hydroxyl radicals

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and peroxy radicals, as well as other ROS components such as organic hydroperoxides.


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Some of these species could directly partition into the particle phase, or be formed via

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secondary heterogeneous oxidation chemistry, potentially influencing aerosol OP (Figure

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1).

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Figure 1 – Simplified schematic illustrating potential mechanisms for the formation of

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particle-bound radicals from the ozonolysis of monoterpenes, such as α-pinene. The

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ozonolysis proceeds via Criegee chemistry, which can lead to the production of relatively

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stable products of sufficiently low volatility to undergo gas-to-particle conversion and form

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SOA. The hydroperoxide channel of the reaction subsequently produces both .OH and

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RO216, both of which could form particle-bound radicals via heterogeneous oxidation of

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SOA particles.


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Particle-bound ROS exogenously delivered to cells directly via inhalation of PM17 or

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formed via bio-transformation of particle components in vivo11 could be a key mechanism

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leading to observed health effects of PM. Several chemical assays are currently applied

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to study particle bound ROS, with varying sensitivities to certain components of ROS.

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18–22

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elucidated the presence of so-called environmentally persistent free radicals (EPFR) in

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combustion derived aerosol (i.e. soot)23–25, and have also shown such radical species to

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have a lifetime exceeding one day24. EPR has also been applied to study free radical and

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related ROS concentrations in ambient PM.26 However, the role of radicals in both the

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formation and ageing of aerosol, as well as their contribution to the health-relevant

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properties of ambient aerosol by influencing their oxidative potential (OP), at present

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remains highly uncertain.

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Recent studies using electron paramagnetic resonance (EPR) spectroscopy have

A study by Miljevic et al.27 introduced an assay based on the chemistry of 9-(1,1,3,3-

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tetramethylisoindolin-2-yloxyl-5-ethynyl)-10-(phenylethynyl)anthracene28.

This

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compound contains 9,10-bis-(phenylethynyl)-anthracene (BPEA) covalently linked to a

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nitroxide-containing ring (Figure S3), therefore given the acronym BPEAnit. The BPEAnit


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based chemical assay has been applied to quantify particle-bound radical concentrations

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in cigarette smoke27, smoke generated from combustion of diesel29, aerosol generated

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from biomass burning30 and gas-phase radicals produced from biofuel combustion31.

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Further characterization and mechanistic detail of the BPEAnit assay for particle-bound

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radical detection is detailed elsewhere.27,32

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The work presented here discusses the adaptation of the BPEAnit profluorescent assay

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and its application to the study of particle-bound radicals in SOA, formed from the

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ozonolysis of atmospherically important biogenic VOCs, a source of SOA with global

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importance. Previous studies focused on the particle-bound radical concentrations in

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combustion generated aerosol, Therefore, initial proof-of-concept experiments were

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performed, to determine whether the BPEAnit assay was sensitive enough to study

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particle-bound radicals in SOA, the composition of which is significantly different

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compared to combustion-generated aerosol in previous studies. We then applied the

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BPEAnit method to provide the first estimate of radical concentrations in SOA generated

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from the ozonolysis of α-pinene, an atmospherically important biogenic VOC.

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demonstrate that SOA formed form the ozonolysis of different VOC precursors, in this

We


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case α-pinene, β-caryopyllene and limonene, have different particle-bound radical

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concentrations, highlighting the potential importance of OH-initiated heterogeneous

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chemistry as a formation pathway to particle-bound organic radicals. Furthermore, the

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rate constant relating to the loss of organic radicals in α-pinene SOA, and the radical

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lifetime, was estimated for the first time, demonstrating the ability of this method to provide

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quantitative information about the role of organic radicals in SOA.

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MATERIALS AND METHODS

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Reagents. All reagents and solvents were acquired from Sigma-Aldrich unless

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otherwise stated. Dimethyl sulfoxide (DMSO, ≥99.9%, anhydrous) was used for the

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impinger solvent. The spin trap BPEAnit was synthesized as described by Fairfull-Smith

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et al.28. The VOCs used as precursors for SOA were α-pinene (98% (+/-)-α-pinene),

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limonene (≥98%) and trans-β-caryophyllene (≥99%), which were reacted with ozone

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produced by a UV lamp (185/254 nm, Appleton Woods®).

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Flow Tube Set-up for SOA Production and Sampling. SOA is produced by reacting VOC

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precursors with ozone. The ozonolysis reaction took place in a flow tube reactor


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maintained at ambient temperature (~16°C) and pressure under dry conditions (relative

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humidity approximately < 2%) (Figure 2). The experimental set-up comprised a 2.5 L

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glass flow tube in which a VOC reacts with ozone, an activated charcoal denuder

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downstream of the flow tube to remove gas phase VOCs and an impinger to capture and

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sample SOA. The removal efficiency of the activated charcoal denuder with respect to O3

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was measured to be > 99.9% at [O3] = 290 ppm in air.

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Figure 2 – Schematic of the flow tube set-up to produce SOA and facilitate the capture of

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particle-bound radicals. The set-up comprises of a 2.5 L flow tube where the VOCs reacts

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with ozone, an activated charcoal denuder which removes oxidized gas phase organics


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as well as O3, an impinger containing a 4 µM solution of BPEAnit in 40 mL DMSO, and

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an SMPS to monitor the particle mass concentration.

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The impinger contains 40 mL of a 4 µM solution of BPEAnit in DMSO, where particle-

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bound radicals are scavenged by BPEAnit/DMSO assay forming a fluorescent product

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that can be detected via fluorescence spectroscopy. A HEPA filter (HEPA cap in-line,

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Whatman, 90406AA) was added in-line to remove particles and perform gas-phase

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control measurements. The VOC precursor was contained in a 25 mL pear-shaped flask,

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and N2 (oxygen-free nitrogen, BOC) carrier gas regulated via a 20-2000 cm3/min mass

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flow controller (MKS 1179A Mass-Flo® controller) was flowed over the VOC at 200

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cm3/min to introduce the VOC precursor into the flow tube in the gas phase. Ozone was

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produced by flowing synthetic air (Zero grade, BOC) at 200 cm3/min (20-2000 cm3/min

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MKS 1179A Mass-Flo® controller) through a UV lamp (185/254 nm, Appleton Woods®).

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For β-caryophyllene, the pear-shaped flask was submersed in a water bath at 60 oC to

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volatilize the VOC.


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Ozone was measured using a UV photometric ozone analyzer (Thermo Scientific model

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49i) and particle concentration was measured using a TSI scanning mobility particle sizer

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(SMPS) composed of TSI 3080 electrostatic classifier (X-ray neutralizer and differential

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mobility analyzer TSI model 3081) and a condensation particle counter (TSI model 3775)

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in the size range of 19 – 897 nm. SOA mass concentrations were estimated assuming a

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particle density of 1.2 g cm-3. Gas phase concentrations of α-pinene, β-caryophyllene and

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limonene were measured using a proton transfer reaction time-of-flight mass

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spectrometer (PTR-ToF-MS 8000, Ionicon Analytik, Innsbruck, Austria) in the m/z range

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10-500, with a time resolution of 10 s and a mass resolution m/Δm of 5000 (full width at

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half maximum) at the mass of protonated acetone. (See “Gas Phase Measurements” in

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the Supplementary material). Using this method, gas phase concentrations of

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-caryophyllene, α-pinene and limonene were measured to be 21, 158 and 135 ppm

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respectively.

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The collection efficiency of SOA particles in DMSO in the liquid impinger was estimated

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to be ~ 9% for an impinger containing 40 mL of DMSO at a sampling flow rate of 400

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cc/min, using the same method described by Miljevic et al.33 (see section “Collection


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Efficiency of the Impinger in Supplementary material). An impinger is advantageous

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despite low collection efficiency (relative to that of a traditional filter), as it allows the direct

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scavenging of radicals in SOA, improving the time resolution compared to classical

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aerosol collection methods such as filter sampling. During the impinger collection

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experiments, SOA masses generated were stable over the sampling time, with a

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variability of 5-10% observed over multiple experiments for different SOA precursor

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regimes.

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Fluorescence Measurements. Fluorescence measurements of the sampled BPEAnit

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solution were conducted in a modified cuvette holder (Ocean Optics model CUV). The

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solution was excited at 430 nm by an LED (Roithner APG2C1-435 435 nm, 380 mW at

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350 mA) connected to an optical fiber (Ocean optics 00S-003948-07) that was

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subsequently coupled to an aspheric lens (Thorlabs, type C230TMD-A) to focus the light

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from the optical fiber into the cuvette holder. The same lens and optical fiber are then

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connected to a UV-Vis spectrometer (Ocean Optics UV2000+).

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fluorescence response at the detector, a 10 nm peak integration window was used, from

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480 – 490 nm. The amount of BPEAnit reacting with particle-bound radicals scavenged

To quantify the


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in the impinger were estimated by plotting known concentrations of the fluorescent methyl

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adduct of BPEAnit (BPEAnit-CH3) vs. fluorescent intensity at 485 nm.29,30,32 (Figure S4).

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RESULTS AND DISCUSSION

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Quantification of Particle-Bound Radicals in SOA Generated from α-pinene Ozonolysis.

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In order to probe the fluorescence response of the BPEAnit assay to SOA, SOA formed

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by α-pinene ozonolysis, was chosen as a model SOA system, given its ease of use and

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the extensive characterization of its chemistry. SOA was generated in a flow tube and

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sampled in an impinger containing BPEAnit in DMSO, using the set-up depicted in Figure

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2. Initially, an experiment with a high mass concentration of SOA at 34 mg m-3 was

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conducted as a proof-of-principle, to ascertain whether or not the BPEAnit assay was

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sensitive to particle-bound radicals in SOA, the results of which are displayed in Figure

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3.

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1 mL aliquots of the BPEAnit solution are taken every 20 minutes, and the fluorescent

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response measured. There is an observed linear response over time as more SOA is

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sampled into the impinger. The fluorescent compounds observed have been shown to be


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stable over time periods of several hours 34, and therefore it can be assumed that as more

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SOA is sampled (and therefore more particle bound radicals are sampled) in the impinger,

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the concentration of fluorescent products will also accumulate, allowing a quantitative

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measure of radicals in the SOA captured in the impinger. A linear response is observed

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as more SOA is captured in the impinger over time, with an r2 = 0.976 correlation, and a

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clear increase in the BPEAnit-R fluorescence signal after 100 minutes of sampling (Figure

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3A) is observed, compared to the background BPEAnit-R peak prior to sampling.

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Figure 3 – (A) Example raw data, showing smoothed spectra of unreacted BPEAnit

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solution (prior to sampling, black) and after 100 minutes of sampling SOA into the

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BPEA/DMSO solution (red). Both characteristic peaks at 485 nm and 510 nm increase in

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intensity upon sampling increasing mass concentrations of SOA34. (B) Example proof-of-


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principle experiment where a large SOA mass concentration of 34 mg m-3 was sampled

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through the impinger containing BPEAnit and DMSO (black), and a gas-phase control

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experiment was performed by adding a HEPA filter in-line (red). Samples of the BPEAnit

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solution are taken every 20 minutes, and the increase in fluorescence was observed over

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time. Error bars represent the variability observed over three repeats.

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Further to this, a control experiment to account for gas phase interference was

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conducted by passing the SOA mixture through a HEPA filter upstream of the charcoal

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denuder (see Figure 3B), to remove the particle phase material and determine any gas

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phase contribution to the signal. Despite having an activated charcoal denuder in-line, it

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is possible for some gas phase constituents to pass through into the impinger. The radical

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content due to aerosol is therefore corrected by subtracting the gas phase contribution,

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with the gas phase (HEPA, red) also illustrated in Figure 3(B). The linearity observed over

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the sampling period indicates that the fluorescent BPEAnit adducts formed upon sampling

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of SOA are relatively stable over time, consistent with previous results. 34,29,30


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Following on from preliminary tests, a series of experiments were performed where the

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SOA mass concentration in the flow tube was altered, and subsequently sampled into the

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BPEAnit impinger, to determine the BPEAnit/DMSO assay’s response to lower, more

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atmospherically relevant SOA mass concentrations. The SOA mass concentrations were

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altered by changing the O3 concentration mixed into the flow tube, whilst the α-pinene

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concentration remained constant. Figure 4A illustrates the background-subtracted

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BPEAnit/DMSO assay’s response to four different mass concentrations of SOA (100 g

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m-3 to 30 mg m-3) sampled.

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It should be noted that the data presented from now on is for the particle phase

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component only, determined by subtracting the gas phase only fluorescent signal (HEPA)

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from the total observed fluorescent response. Figure 4B shows the fluorescent product

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concentration increase, expressed as BPEAnit-CH3 min-1 equivalents as a function of

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increasing SOA mass concentration. An r2 > 0.99 is observed, indicating that the BPEAnit-

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CH3 is sufficiently stable over time to allow pre-concentration. This has potential

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implications for ambient studies; if the fluorescent product is stable over time, then


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ambient aerosol mixtures could be sampled for longer, which could potentially allow the

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quantification of radicals at low ambient aerosol concentrations.

A

B

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Figure 4 – (A) Observed signal increase over time for several different SOA mass

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concentrations sampled, corrected for the contribution of gas phase components to the

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fluorescent signal (Figure 3). Aliquots of the BPEAnit/DMSO assay are sampled every 20

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minutes and the fluorescence spectrum analyzed. A linear response is observed in each

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case (r2 >0.95), highlighting the stability of the fluorescent product over time.

(B)


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BPEAnit-CH3 equivalent fluorescent product production per minute (nmol min-1) as a

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function of increasing SOA mass concentrations sampled, derived from the average

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increase of signal over time.

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The limit of detection (LOD), achieved with the current set-up, was calculated to be 85

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± 10 µg m-3, using the 3σbl method which considers three times the standard deviation of

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the background signal. As demonstrated in a recent study by Brown et al.35, increasing

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the air flow rate or collection time by using a particle-into-liquid-sampler (PILS)-type

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instrument for particle collection would likely facilitate a 20 times increase in sensitivity.

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The BPEAnit/DMSO method presented here could therefore be used to determine

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ambient particle-bound radical concentrations in polluted urban environments.

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The linear increase in the SOA concentration as a function of mass allows the estimate

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of the particle-bound radical concentration present in α-pinene SOA, using the calibration

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curve presented in Figure S4. This was calculated to be 0.02 ± 0.0050 nmol/µg of α-

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pinene SOA, corrected for gas phase interference and collection efficiency, as well

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assuming a 1:1 reaction stoichiometry between particle-bound radicals in SOA and


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BPEAnit. Assuming an average molecular mass of 200 g mol-1, considering the typical

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molecular weight of first-generation products of α-pinene ozonolysis, the radical

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concentration can be expressed as 4 ± 1 ng radicals/ µg SOA, indicating ~ 0.4% of the

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total SOA particle mass contributes to the radical related OP of the SOA. To the author’s

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knowledge, this represents the first time that particle-bound radical concentrations have

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been estimated for α-pinene SOA.

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Estimating the Lifetime of Particle-Bound Radicals in α-pinene SOA. The residence time

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of fresh SOA prior to sampling was altered using different sized reaction vessels, with a

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variety of reaction volumes leading to a variety of reaction times, between 27-500 s, for

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SOA formed from α-pinene ozonolysis in the reaction system before being sampled in the

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impinger. A decrease in the fluorescent response was observed as the residence time in

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the flow tube was increased (Figure S5). This is likely due to the reaction and therefore

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loss of radicals in the particle phase. The ozonolysis reaction, which in turn produces

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OH, was estimated to have reached completion after 20 s using the Master Chemical

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Mechanism (MCM v3.2) (https://atchem.leeds.ac.uk).36,37 Therefore, there should be

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limited SOA ageing in the flow tube after this reaction has reached completion. Ozone is


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indeed in excess in the flow tube, but assuming that all of the α-pinene is consumed in

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this initial 20 seconds, there should be limited secondary chemistry occurring with respect

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to heterogeneous oxidation by ozone. Therefore, we assume that the decrease in the

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fluorescent signal as a function of residence time in the flow tube can be interpreted as

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reactive loss of radicals in SOA. The mass normalized fluorescence intensity obtained

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from Figure S5 decays exponentially with residence time (Figure 5), indicating a pseudo-

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first order decay of radicals in SOA.

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Figure 5 – Plot showing the average signal decrease (fluorescent counts/g SOA) as a

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function of residence time, with an exponential decay fitting and an r2 of 0.994. Error bars

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represent variability in the normalized intensity observed over three repeats.


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A pseudo-first order radical loss rate coefficient in freshly generated SOA particles was

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derived and estimated to be k = 7.3 ± 1.7 × 10-3 s-1, which therefore infers an average

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pseudo first-order radical lifetime in this SOA regime of approximately 137 ± 31 s. To the

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authors’ knowledge, this represents the first time that the rate constant relating to radical

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loss and the radical lifetime in α-pinene SOA has been determined. Given the short

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lifetime of particle-bound radicals in α-pinene SOA, they may represent a short-lived

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component of ROS, contributing to aerosol OP, which needs to be investigated further.

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Studies by Fuller et al (2014)18, and Gallimore et al (2017)37 have observed a short-lived

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component of ROS as measured using a dichloroflurescein based chemical assay, which

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may influence aerosol OP and is underestimated using classical filter-based studies.

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Furthermore, the loss of radicals may be indicative of a SOA ageing process, where the

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particle-bound radicals contribute to composition change of the SOA shortly after fresh

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SOA formation.

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The exponential fit has a non-zero intercept, which implies that there is a fraction of

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radicals being detected by the BPEAnit assay that have a longer lifetime and lower


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concentration in α-pinene SOA than the short lived fraction. Previous studies, such as

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Gehling et al.24 ,have also observed certain components of particle-bound

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environmentally persistent free radicals with a lifetime of more than one day. The previous

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calculation estimates the rate coefficient associated with the decay of radicals in the

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condensed phase by assuming pseudo first order kinetics.

300

However, given the relatively high concentration of reactants in this system, there could

301

be substantial loss of radical species in the particle phase via self-reaction with other

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organic radicals. Thus the decay of the radical concentration could also follow second-

303

order kinetics, which can be approximated as: 𝑹. + 𝑹.→𝐏𝐫𝐨𝐝𝐮𝐜𝐭𝐬

304

The rate of loss of radical species R. can then be expressed as:

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306

307

𝑑[𝑅.] = ― 2𝑘[𝑅.]2 𝑑τ Which leads to the integrated rate law:

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1 .

[𝑅 ]

=

1 [𝑅.]0

+ 𝑘τ

310 311

Thus, a plot of 1/[R.] vs. τ should yield a straight line, with a slope of k. A plot of 1/[R.]

312

vs τ is given in Figure 6 with a linear fit of r2 = 0.949. [R.] at the time of sampling (τ) is

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estimated using the same method described previously, by considering the increase in

314

fluorescent signal over time and calibrating using the BPEA-CH3 fluorescent standard.

315

An estimate of the 2nd-order rate coefficient can be obtained from the linear fit, where k =

316

0.171 ± 0.022 nmol-1 µg s-1. Given that the pseudo-first order and second order fits have

317

similar r2 correlations, 0.994 and 0.949 respectively, it is difficult to resolve, statistically

318

from the given data whether pseudo-first order kinetics or second order kinetics best

319

describes the loss of radicals in the SOA.


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Figure 6 – Plot of 1/[R.] vs τ, describing the second order loss of radical species in the

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condensed phase. A linear fit yields an r2 correlation coefficient of 0.949, and an estimate

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of the rate coefficient k = (0.171 ± 0.022) nmol-1 s-1. Error bars are derived from the

324

variability of the fluorescent signal observed over 3 repeats.

325

Quantification of Particle-Bound Radicals in SOA Generated from Various VOC

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Precursors. The radical concentrations in SOA generated from a variety of VOCs was

327

investigated, to expand on the proof-of-principle study on α-pinene. α-pinene, limonene

328

and β-caryophyllene were the VOC precursors chosen for comparison, given the fact that

329

they produce SOA with differing physical (e.g. viscosity) and chemical properties. Using

330

the flow tube set-up and fluorescent cuvette apparatus described previously (Figure 2),


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the fluorescence response of the assay over time was measured for SOA generated from

332

the ozonolysis of each precursor, the results of which are illustrated in Figure 7.

333 334

Figure 7 – Concentration of radicals calculated per unit mass of SOA (ng/µg SOA),

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corrected for differing mass concentrations in the flow tube as well as the collection

336

efficiency of the impinger. Error bars represent the observed standard deviation of

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fluorescent signal per 20-minute sample analysis observed over 3 repeats.

338

The data has been normalized to account for varying SOA mass concentrations

339

generated from the different VOC precursors, and corrected for gas phase (i.e.

340

background) contributions to the fluorescent signal. Observed radical concentrations for

341

α-pinene, limonene and β-caryophyllene were 0.020 ± 0.0050 nmol/µg, 0.0059 ± 0.0010


25


Page 26 of 47

342

nmol/µg and 0.0025 ± 0.00080 nmol/µg respectively. Assuming an average molar mass

343

of 200 g/mol for α-pinene and limonene SOA, and 240 g/mol for β-caryophyllene SOA,

344

considering the typical molecular weights of first-generation oxidation products produced

345

from VOC ozonolysis, the radical concentrations for α-pinene, limonene and β-

346

caryophyllene can be expressed as 4 ± 1 ng/µg, 1.18 ± 0.24 ng/µg and β-caryophyllene

347

0.59 ± 0.18 ng/µg respectively. This indicates that particle-bound organic radicals

348

constitute about 0.4%, 0.1% and 0.05% of the total SOA particle mass for α-pinene,

349

limonene and β-caryophyllene, respectively. It is evident that fluorescent response over

350

time upon sampling α-pinene SOA is substantially larger than that of both β-caryophyllene

351

and limonene SOA regimes (Figure S6).

352

The higher concentration of radicals observed in α-pinene SOA could be attributed to

353

the higher .OH yield associated with this ozonolysis oxidation scheme compared to that

354

of β-caryophyllene and limonene.36,37,39,40 OH yields reported in the literature are 0.83 ±

355

0.2341, 0.28 ± 0.0842 and 0.14 ± 2.343 for α-pinene, limonene and β-caryophyllene

356

ozonolysis, respectively. A greater .OH yield in the α-pinene ozonolysis system could

357

therefore lead to more heterogeneous uptake of .OH, which can react on the particle


26

Page 27 of 47


358

surface to yield a larger concentration of organic radicals in the SOA. The .OH uptake to

359

α-pinene SOA has been extensively studied before44–46, altering the chemical composition

360

of SOA as well as increasing the SOA mass concentration and volatility.47

361

Further studies are required to fully elucidate the mechanisms leading to the greater

362

radical concentrations observed in α-pinene SOA compared to SOA produced from

363

limonene and β-caryophyllene. There could be other products formed in large quantities

364

in the α-pinene O3 system that lead to the production of BPEAnit-R (i.e. fluorescent

365

product), although limited literature data is available to estimate the extent of this effect,

366

and more detailed studies are required. One could age the SOA before introducing known

367

quantities of OH radicals into a flow tube and monitor the fluorescent response as a

368

function of OH concentration, therefore indicating whether OH variability can account for

369

the observations.

370

Furthermore, a study by Shiraiwa et al.46 showed that the ageing of organic aerosol by

371

reactive species could be limited by bulk-phase diffusivity, which in turn reduces the

372

extent of oxidation in the particle by species such as OH radicals. It is assumed the

373

viscosity of SOA used in this study likely follows the trend β-caryophyllene > limonene >


27


Page 28 of 47

374

α-pinene, given the fact that the general volatility of the oxidized products formed from

375

ozonolysis follows the trend α-pinene > limonene > β-caryophyllene. A combination of

376

both the diffusion-limited kinetics upon reactive uptake of OH, as well as the increased

377

OH yield from the α-pinene reaction could lead to the observation of a larger radical

378

concentration in α-pinene SOA compared to both limonene and β-caryophyllene SOA.

379

Comparison with Literature Data. Calculated concentrations of organic particle-bound

380

radicals in this work are compared with literature values for particle-bound radical

381

concentration estimates presented in Table 1. The concentration range of particle-bound

382

radicals in SOA observed in this work (0.02 - 0.002 nmol µg-1) is in reasonable agreement

383

with literature values obtained for a range of ambient PM2.5 and combustion generated

384

aerosol using BPEAnit (0.001 – 10 nmol µg-1) and EPR (4×10-5 – 0.4 nmol µg-1) (Table 1).

385

Typically, observed particle-bound radical concentrations for ambient PM2.5 and

386

combustion-generated PM, quantified with EPR or BPEAnit, are higher than those

387

observed for SOA.


28

Page 29 of 47


388

Table 1 – Comparison of literature values for radical concentrations observed using the BPEAnit

389

assay and EPR. Samples include SOA (this work), ambient PM2.5, combustion generated aerosol

390

and radical formation upon PM2.5 extraction into water.

Study

Method

Particle Type

[Radicals] (nmol ug-1)

This work

BPEAnit

α-pinene SOA

0.020 ± 0.0050

This work

BPEAnit

Limonene SOA

0.0059 ± 0.0010

This work

BPEAnit

β-caryophyllene SOA

0.0025 ± 0.00080

Crilley (2012)48

BPEAnit

Roadside PM2.5

0.1-10

Pourkhesalian

BPEAnit

Biodiesel combustion

0.001-1

Stevanovic (2013)30

BPEAnit

Diesel combustion

0.04

Stevanovic (2013)30

BPEAnit

SOY biodiesel

1.5

Miljevic (2010)27

BPEAnit

Side Stream Cigarette Smoke

0.02-0.05

Hedayat (2016)29

BPEAnit

Biodiesel Combustion

0.05-0.4

Arangio (2016)26

EPR

PM2.5 EPFRb

0.0002-0.001a

Arangio (2016)26

EPR

PM2.5 Water Extract

4.0×10-5 a

Tong (2018)15

EPR

Napthalene SOA EPFRb

0.02-0.05a

Dellinger (2013)24

EPR

PM2.5 EPFRb

0.05-0.4a

(2015)49


29


391

a

Page 30 of 47

The concentration of radicals is converted to nmol µg-1 by considering the number

392

concentration of spins (i.e. radicals) per µg aerosol mass and dividing by 6.02×1023.

393

EPFR:

Environmentally

persistent

free

b

radicals

394 395

Furthermore, the particle-bound radical concentration in SOA is two orders of magnitude

396

greater than observed radical production upon sampling PM2.5 into water (Table 1,

397

Arangio et al., 201626), a pathway which was suggested to have important consequences

398

for particle toxicity. Recently, a study by Tong et al.15 detected no particle-bound radicals

399

in SOA generated from the OH-initiated oxidation of isoprene and β-pinene, in contrast to

400

our observations for SOA produced from the O3-initiated oxidation of α-pinene, limonene

401

and β-caryophyllene. Our estimated radical lifetime for α-pinene SOA is on the order of

402

137 s, indicating that offline filter sampling methods, e.g. used prior to EPR analysis, may

403

not be fast enough to capture this relatively short-lived radical component, highlighting

404

the need for rapid online quantification of particle-bound radicals in order to elucidate their

405

influence on aerosol composition and hence toxicity. These experiments have

406

demonstrated that the spin trap BPEAnit is capable of quantifying radicals in SOA

407

generated from the ozonolysis of a variety of VOC precursors. Radical chemistry in


30

Page 31 of 47


408

organic aerosol represents a large area of uncertainty regarding the toxicological effects

409

and chemical evolution of organic aerosol; further development of this technique could

410

result in a valuable assay that in principle could be applied to probe various chemical

411

processes involved in the initial formation, as well as ageing, of SOA. Furthermore, the

412

demonstrable stability of the fluorescent BPEAnit-R adduct will allow the study of radicals

413

at low concentrations in a variety of atmospherically relevant aerosol.

414 415

ASSOCIATED CONTENT

416

Supporting Information

417

The supporting information is available free of charge.

418

Figure S1 – An example of SOA mass stability in the flow tube, as measured with an

419

SMPS. In this particular example. Figure S2 - Number size distribution of particles with

420

and without an impinger containing 40 mL of acetonitrile. Removal of particles > 570 nm

421

is inefficient in this impinger. Figure S3 – Schematic showing the reaction of the

422

profluorescent nitroxide probe BPEAnit. Calibration curve generated for varying

423

concentrations of BPEA-CH3, with an r2 = 0.999 correlation coefficient upon linear fitting.


31


Page 32 of 47

424

Figure S4 – Calibration curve generated for varying concentrations of BPEA-CH3, with an

425

r2 = 0.999 correlation coefficient upon linear fitting. Figure S5 - Fluorescence response of

426

the BPEAnit assay as a function of residence time of SOA in the flow tube normalized for

427

particle mass and corrected for gas phase contributions. Figure S6 – Increase in the

428

fluorescent response upon sampling SOA generated from the ozonolysis of limonene, β-

429

caryophyllene and α-pinene.

430

AUTHOR INFORMATION

431

Corresponding Author

432

*(S.J.C) E-mail: [email protected]

433

*(M.K.) E-mail: [email protected]

434

Notes

435

The authors have no competing financial interests.

436

ACKNOWLEDGEMENTS


32

Page 33 of 47


437

This work was funded by the European Research Council (ERC grant 279405) and

438

Natural Environment Research Council (NERC) (NE/K008218/1). This work has also

439

received funding from the European Union’s Horizon 2020 research and innovation

440

programme through the EUROCHAMP-2020 Infrastructure Activity under grant

441

agreement No. 730997.

442

TOC Graphic

443 444

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