Secondary organic aerosol formation from propylene irradiations in a chamber study

Accepted Manuscript Secondary organic aerosol formation from propylene irradiations in a chamber study Shuangshuang Ge, Yongfu Xu, Long Jia PII:

S1352-2310(17)30145-0

DOI:

10.1016/j.atmosenv.2017.03.019

Reference:

AEA 15230

To appear in:

Atmospheric Environment

Received Date: 13 January 2017 Revised Date:

3 March 2017

Accepted Date: 11 March 2017

Please cite this article as: Ge, S., Xu, Y., Jia, L., Secondary organic aerosol formation from propylene irradiations in a chamber study, Atmospheric Environment (2017), doi: 10.1016/j.atmosenv.2017.03.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Secondary organic aerosol formation from propylene irradiations in a chamber study Shuangshuang Ge 1, 2, Yongfu Xu 1,3∗, Long Jia 1

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1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric

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Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing

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100029, China. E-mail: [email protected]

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2. College of Earth Sciences, University of Chinese Academy of Sciences, Beijing

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100049, China

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3. Department of Atmospheric Chemistry and Environmental Sciences, College of

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Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

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Abstract

Some studies have shown that low-molecular-weight VOCs such as ethylene and

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acetylene can form SOA. However, so far propylene (C3H6) has not been studied. The

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current work systematically investigates irradiations of propylene in the presence of

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NOx (x=1, 2) in a self-made indoor chamber. Only a small amount of secondary

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organic aerosols (SOA) was formed under 5% and 80% RH conditions without

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sodium chloride (NaCl) seed particles or in the presence of solid NaCl. When NaCl

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was in the form of droplets, liquid water content (LWC) increased from 34.5 to 169.8

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µg m-3 under different initial NaCl concentrations, and correspondingly the amount of

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SOA linearly increased from 5.9 to 29.8 µg m-3 (SOA=0.0164×LWC+1.137, R2=0.97)

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at the C3H6/NOx ratio of 32.2-44.9. The initial C3H6/NOx concentration ratio

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(ppbC/ppb) considerably impacted the formation of SOA, in which the amount of

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SOA increased from 12.1 to 47.9 µg m-3 exponentially as the ratio decreased from

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46.5 to 6.3 with an important point of the ratio value of 11. At the ratio of less than 11

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in the regime under the control of C3H6, SOA concentrations decreased considerably

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Corresponding author. E-mail address: [email protected]

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with increasing ratio, whereas at the ratio value of larger than 11 in the NOx

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controlled regime, SOA slightly decreased with increasing ratio. From combination of

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the analysis of different functional groups of particles by IR spectra and

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ESI-Exactive-Orbitrap mass spectrometer, the constituents of SOA were identified to

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be

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CH2ClC(O)OCHClCHO),

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HO2CH(CH2Cl)C(O)OCCl(CH2Cl)C(O)OCHClCH2ONO2), etc.

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liquid-phase mechanism of SOA formation has been proposed in this study.

HOCH2CCl(CH3)OOH), organic

esters

(e.g.

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(e.g.

nitrates

(e.g.

Furthermore, a

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hydroperoxides

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Keywords: propylene, secondary organic aerosols, liquid water content, ratio of

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C3H6/NOx

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1. Introduction

Atmospheric pollution has aroused a growing amount of attention in recent years

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for its serious impact on climate change and human health. Fine particulate matter

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pollution, especially for the pollution of secondary organic aerosols (SOA),

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significantly affects the atmospheric visibility, air quality and oxidation capacity of

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the troposphere (Finlayson-Pitts and Pitts, 1997; Jacobson et al., 2000; Hurley et al.,

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2001; Pope et al., 2006). SOA accounts for about 80% of the total organic aerosol

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sources through the study of global model simulation (Spracklen et al., 2011). Besides

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that the relative high-molecular-weight (HMW) aromatic hydrocarbon compounds

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and alkenes are the precursors of SOA, recent studies have shown that the relative

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low-molecular-weight (LMW) volatile organic compounds (VOCs), such as ethylene

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and acetylene, also make contributions to the formation of SOA (Volkamer et al.,

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2009; Sakamoto et al., 2013; Ge et al., 2016; Jia and Xu, 2016). Although some

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review articles have a detailed introduction on the aqueous-phase chemistry in the

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troposphere and formation of fine particulate matter in urban (Herrmann et al., 2015;

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Zhang et al., 2015), the mechanism of SOA formation from LMW VOCs is still not

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very clear, and more research is needed. Propylene (C3H6) is very reactive in the troposphere. In previous work,

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propylene was often used to be the source of OH radicals in the study of SOA

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formation in a smog chamber, and the concentration of the added propylene ranged

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from 250 ppb up to 5.5 ppm (Odum et al., 1997a, 1997b; Edney et al., 2000; Jang and

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Kamens, 2001; Takekawa et al., 2003; Kleindienst et al., 2004). In these studies, it

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was presumed that addition of propylene into the reaction system did not affect the

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formation of SOA from parent hydrocarbon. Nevertheless, another study (Song et al.,

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2007) pointed out that the experiments with propylene could reduce OH

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concentrations in the system, leading to the decrease in the yield of SOA from

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m-xylene. However, whether propylene itself can form SOA or not under certain

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conditions has not been studied. Recent studies have shown that ethylene, which has

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the similar structure with propylene, can form SOA under photooxidation conditions

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through aqueous reactions in the presence of NaCl seed particles (Jia and Xu, 2016).

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Another LMW VOC, acetylene, also has been demonstrated to form SOA via aqueous

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reactions (Volkamer et al., 2009; Ge et al., 2016). They found that glyoxal is an

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important intermediate product from acetylene to form SOA, and the increased

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amount of liquid water content (LWC) in the seed aerosols can promote the formation

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of SOA. Model simulations show that oxidations of ethylene and acetylene can form

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SOA in the cloud droplets with a main component of oxalic acid (Warneck, 2003;

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Huang et al., 2011). For the further study of SOA formation from low weight

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molecule VOCs, it is interesting to investigate whether propylene can form SOA or

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not under some certain conditions.

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Many studies have shown that the VOCs/NOx (x=1, 2) ratios have a great

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influence on the SOA yields and the chemical compositions (Song et al., 2005; Kroll

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et al. 2006; Ng et al., 2007; Chan et al., 2010; Xu et al. 2014; Beardsley and Jang,

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hydrocarbon, isoprene and long-chain alkanes. The influence of VOC/NOx ratio on

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SOA formation from LMW propylene has not been studied. Previous work has

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indicated that the C3H6/NOx ratio impacted ozone production in the photochemical

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reactions (Hu et al., 2011). Thus, it is expected that the C3H6/NOx ratio will affect the

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yield of propylene SOA if SOA can be formed from propylene.

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It has been shown that LMW species, such as ethylene and acetylene, can form

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SOA under the condition of the presence of seed particles with sufficiently high

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humidity levels (Volkamer et al., 2009; Jia and Xu, 2016; Ge et al., 2016). In our

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previous work, sodium chloride (NaCl) was chosen as seeds for the studies of

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formation of SOA from ethylene and acetylene (Jia and Xu, 2016; Ge et al., 2016; Ge

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et al., 2017). Sodium chloride is a main component of sea salt aerosols, which widely

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exists in the atmosphere, particularly near coastal regions.

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In this study, we still opted to use the NaCl as seed particles to study the

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photochemical process of C3H6 and NOx in a self-made smog chamber. The

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influences of different NaCl concentrations and C3H6/NOx ratios on the SOA

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formation are studied. Through the study, we verified the process of SOA formation

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from C3H6 and analyzed the chemical compositions. To our knowledge, this is the

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first time to identify the SOA formation from heterogeneous photochemistry of the

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C3H6-NOx-NaCl system.

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2. Experimental

The experiments of photochemical reactions of propylene with NOx were

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performed in a self-made indoor smog chamber. A pillow-shaped Teflon bag was

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housed in a 2×1.2×2-m enclosure, and the enclosure was equipped with two fans and

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two parallel banks of blacklamps (GE F40BLB). The thickness of Teflon bag reactor

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was 0.08 mm. The light intensity in the reactor was measured to be 0.44 min-1 based

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on the photolysis rate of NO2. The volume of Teflon bag reactor of experiments was

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1.3 m3, which had a minimum surface/volume (S/V) of 5.8 m-1. A similar description

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of the facility has been reported in our previous work (Du et al. 2007a, b; Jia et al.,

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2009; Jia and Xu, 2014; Ge et al., 2016), so a brief summary is given here. In all the experiments, the background gas was zero air. The zero air was

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produced by Zero Air Supply (Model 111 and Model 1150, Thermo Scientific, USA).

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To get further purified zero air, two additional hydrocarbon traps were used in our

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experiments. The reactant gas of C3H6 was from Beijing Haipu Gas Company Ltd.,

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China, with the purity of 99.5%. The 500 ppm NO2 gas was from Beijing Huayuan

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Gas Chemical industry Co. Ltd., China, which used N2 as the background gas. The

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reactants of prepared gas of C3H6 and NO2 in photochemical reactions were both

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introduced into the bag reactor directly by syringes.

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In order to control the RH of the background air, highly purified water was used

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by bubbling zero air through it. A hygrometer (Model 645, Testo AG, Germany) was

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used to measure the RH in the experiments. For the experiments with NaCl seed

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particles, NaCl aerosols were generated from a 150 mg L-1 aqueous solution of NaCl

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with a constant-rate atomizer (Model 3076, TSI, USA). The purity of NaCl reagent

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was 99.99%. The NaCl particles passed through an aerosol neutralizer (Model 3087,

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TSI) to get rid of the charge before being introduced into the reactor. When the

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reaction gases of propylene, NOx and NaCl seed aerosol were all introduced into the

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reactor, the reactor was maintained in the dark without any activities for 1 hour to

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make the reactants mixed completely. Then the lights were turned on for irradiation

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and meanwhile the two fans were opened to ensure homogeneous mixing of the air in

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the reactor.

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The concentrations of gas-phase species were measured every 30 mins. The

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concentrations of NOx and O3 were measured by Thermo Environmental Instruments

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(Model 42C-NOx analyzer and Model 49C-O3 analyzer, USA), respectively. The

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sampling flow rate was all 1 L min-1 for NOx, and O3 analyzers. The wall loss

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constants of NO2 and O3 were measured under different RH conditions. The rate

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ACCEPTED MANUSCRIPT constant for the wall loss of NO2 was obtained to be 3.0×10-7 s-1 at 9% RH and

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2.0×10-7 s-1 at 91% RH, and was 3.0×10-6 s-1 for O3 under both high and low RH

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conditions. The propylene concentration was determined by a long-path Fourier

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transform (FT) IR spectrometer (LP-FTIR, Thermo Fisher, Nicolet iS 10, Madison,

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USA). The Scanning Mobility Particle Sizer (SMPS, Model 3936, TSI) consisting of

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a differential mobility analyzer (DMA, Model 3081, TSI) and a condensation particle

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counter (CPC, Model 3776, TSI) was used to measure the concentration of particles.

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A constant density of 1.2 g cm-3 and 2.2 g cm-3 has been used to obtain the mass of the

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particles at ~80% RH and at 5% RH. The sampling flow rate of SMPS was 0.3 L

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min-1 and sheath flow rate was 3L min-1. The wall loss constant of particles was

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measured to be (3.0-5.0)×10-5 s-1. To remove the static electricity of Teflon bag, two

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ironing air blowers were around the reactor to get rid of the electric charge on the

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surface of the reactor (Ge et al., 2017).

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The particles formed in the experiments were collected on a ZnSe disk at the end

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of the experiment using a Dekati low-pressure impactor (DLPI, Dekati Ltd.,

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Kangasala, Finland). The particles ranged from 108-650 nm were collected on the

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third plate of the DLPI. The process of collecting particles by DLPI has been

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described by our previous work (Jia and Xu, 2016; Ge et al., 2016). The particles

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collected on the ZnSe disk were used for the analysis of the basic functional groups of

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the chemical composition with FTIR. After the analysis of FTIR, the particles on the

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ZnSe disk were dissolved with high-purity water and filtered through a 0.2-µm filter.

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The ion compositions of filtered solution were analyzed using ion chromatograph

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(DIONEX, ICS-900, Thermo Fisher Scientific). In addition, to analyze the organic

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compositions of SOA, the collected particles were analyzed by Exactive-Orbitrap

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mass spectrometer equipped with electro-spray interface (ESI) (Thermo Fisher

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Scientific). The ESI source was performed in the positive/negative ion mode under

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optimum conditions as follows: capillary voltage 3.2 kV, desolvation gas flow 200 µL

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min-1, and desolvation gas temperature 320 ℃. Mass spectra were acquired over a

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mass range of 50-500 Da. The element compositions of identified compounds were

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calculated by their accurate mass measurements.

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3. Results and discussion In order to determine the conditions of SOA formation from the

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propylene-NO2-hν system, a series of propylene photooxidation reactions was

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conducted in both absence and presence of NaCl seed particles at relatively low and

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high RH. The temperature of the reactor was controlled to be about 303 K during all

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the experiments. The initial experimental conditions are listed in Table 1.

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EXP.

[C3H6]0 [NO2]0

No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

(ppm) 2.06 1.92 15.00 15.00 2.14 2.00 1.92 2.06 1.84 1.64 1.84 1.92 2.00 1.92 1.84 1.84

[NO]0

RH

[NaCl]0

C3H6/NOx

(ppb) 3.2 4.1 4.7 5.4 1.2 1.4 1.1 1.8 7.4 6.3 11.9 10.4 8.0 18.0 28.0 25.0

(%) 5 80 5 77 4 78 78 76 77 78 84 81 83 84 83 82

(µg m-3) ----35.3 15.3 27.2 42.2 56.5 67.8 38.5 43.3 40.6 41.5 42.2 44.5

(ppbC/ppb) 36.7 36.3 259.4 292.6 44.9 37.1 36.6 41.1 36.3 32.2 46.5 34.8 19.5 9.3 7.5 6.3

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(ppb) 165.0 154.7 168.8 148.4 141.7 160.2 156.4 148.6 144.5 146.6 106.9 154.3 298.0 611.0 717.0 853.0

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Table 1 Initial conditions in the C3H6-NO2-hν-NaCl system

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In the absence of NaCl particles, experiments (Exps. 1-4) with relatively low (~2

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ppm) and high concentration (~15 ppm) of C3H6 and ~ 150 ppb NO2 were conducted

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under both low (5%) and high (~80%) RH conditions. Exps. 1-4 were used to study

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the influence of different initial concentrations of C3H6 on the formation of SOA 7

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homogeneous

reactions.

When

adding

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propylene-NO2-hν system, one experiment with low RH (Exp. 5) and five

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experiments with high RH (Exps. 6-10) were conducted to study the effect of RH and

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NaCl on the formation of SOA. Exps. 11-16 studied the influence of variation of

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initial concentration C3H6/NOx ratios on the SOA yields and chemical compositions at

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the initial NaCl concentration of about 40 µg m-3 at high RH (~80% RH) and at

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~303K. The C3H6/NOx (ppbC/ppb) ratios ranged from 6.3-46.5.

189 3.1. Effects of NaCl particles on the formation of SOA

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NaCl

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The results from Exps.1-4 without NaCl seed particles show that a small number

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of particles could be formed during the experiments, in which after a reaction time of

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1-h under 5% and ~80% RH conditions the volume concentration of at most 0.013

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µm3 cm-3 particles were monitored from the experiment with about 2 ppm propylene

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and 150 ppb NO2, and 0.036 µm3 cm-3 particulates were obtained from the experiment

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with about 15 ppm propylene and 150 ppb NO2. Previous work by Sakamoto et al.

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(2013) found that ethylene SOA can be formed via Criegee oligomers to form

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oligomeric hydroperoxides under dry conditions (<1% RH), and the volume

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concentration of about 0.8 µm3 cm-3 was obtained through ethylene ozonolysis from

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the experiment with the initial reactant concentrations of 2.9 ppm for ethylene and 4.5

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ppm for ozone. Our experiments further demonstrate that the amount of the formed

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particles from homogeneous reactions of propylene is very small.

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In the presence of NaCl solid particles, SOA was also hardly formed under low

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RH condition (Exp. 5). Fig. 1 shows the time evolution of particle mass at different

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initial NaCl from 15.3 to 67.8 µg m-3 at 76-78% RH (Exps. 6-10), which clearly

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shows that particle amount increases with increasing initial NaCl concentrations. The

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variations of the increase in particle mass that is the quantity obtained from the

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subtraction of the mass of initial background NaCl droplets from total mass of

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particles were used to show the growth of particles due to the formation of new

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from the conversion of NaCl to NaNO3 and the change in LWC. It can be estimated

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that SOA accounts for over 50% of the increased particle mass. As RH rose to a

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certain level at which the NaCl seed particles were in the form of droplets, SOA was

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obviously formed, which can be clearly seen in the IR spectra. The amount of NaCl

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that reflects the LWC in the particles plays an important role in SOA formation (Fig.

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1), which is consistent with the results from LWM ethylene and acetylene (Jia and Xu,

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2016; Ge et al., 2016).

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with 15.3 µg m-3 NaCl with 27.2 µg m-3 NaCl -3 with 42.2 µg m NaCl with 56.5 µg m-3 NaCl -3 with 67.8 µg m NaCl

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Increased particle mass -3 concentration (µg m )

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0

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30

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50

60

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80

90 100 110

time (min)

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Fig. 1. Variations of particle mass concentrations with time after subtracting the

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initial background NaCl droplets under different initial NaCl seed conditions. All the

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reactions were conducted at high RH of 76-78% with the initial reactant gases of

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about 2 ppm for C3H6 and 150 ppb for NO2. The initial dry NaCl seed particles ranged

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from 15.3 to 67.8 µg m-3. The wall loss of particles has been corrected.).

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The yield of propylene SOA, defined as the ratio of the mass of SOA to the mass

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of parent propylene reacted, was calculated. The amount of SOA was calculated based

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on the mass balance. The total particles consist of NaCl seed particles, particle water,

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and the generated NaNO3 and SOA. The amounts of NaCl and NaNO3 were measured

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by ion chromatograph. The liquid water absorbed by NaCl and NaNO3 particles was 9

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obtained by the simulation of Extended Aerosol Inorganics Model (E-AIM Model III,

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available at http://www.aim.env.uea.ac.uk/aim/aim.php, accessed 20 August 2016)

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(Clegg et al., 1998; Jia and Xu, 2016; Ge et al., 2016). The results are listed in Table

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Table 2 Mass concentrations of different particle components and SOA yield in the

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propylene experiments at the end of the experiments.

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Exp. RH PM NaCl NaNO3 LWC SOA SOA -3 -3 -3 -3 -3 No. (%) (µg m ) (µg m ) (µg m ) (µg m ) (µg m ) yield (%) 6 78 60.7 4.2 16.1 34.5 5.9±1.6 0.3±0.08 7 78 109.7 5.7 31.2 59.2 13.6±2.9 0.6±0.13 8 76 164.0 19.2 33.4 95.8 15.6±4.6 0.7±0.20 9 77 231.3 29.5 39.2 138.1 24.5±6.5 1.2±0.32 10 78 283.3 32.7 51.0 169.8 29.8±8.0 1.6±0.43 Note: PM: the sum of NaCl, particle water, NaNO3 and SOA. NaCl: the amount of NaCl. NaNO3: the amount of NaNO3. LWC: liquid water content.

Estimations of LWC by E-AIM, which neglects any contribution of the organics

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to aerosol water content, make the amount of SOA overestimated slightly. In order to

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compare the E-AIM method, some additional experiments were conducted for the

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direct measurement of LWC using the method of the reduced Dry-Ambient Aerosol

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Size Spectrometer (DAASS) (Engelhart et al., 2011). The experimental conditions

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were similar with those in Exp. 10. At the end of the experiment, the total particles

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containing water were measured by SMPS first, and then the sample gas was dried for

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the removal of particle water with the method of DAASS. Using the measured masses

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of total particles, LWC, NaCl and NaNO3, the amount of SOA was obtained to be

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23.4 µg m-3 after wall loss correction, which is 21% less than that from Exp. 10

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obtained by E-AIM (29.8 µg m-3 ), demonstrating that both DAASS and E-AIM

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methods are basically consistent. It should be pointed out that the amount of SOA was

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underestimated through the method of DAASS, because the dissolved species, which

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belong to volatile/semivolatile compounds, would evaporate into the gas phase again

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after the removal of aerosol water. It is known from Table 2 that the yield of propylene SOA from the experiment

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with initial NaCl seed particles of 67.8 µg m-3 (Exp. 10) increased by 5.3 times as

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compared to that with 15.3 µg m-3 NaCl particles (Exp. 6). Similarly, LWC increased

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by a factor of 4.9 for these two experiments when initial NaCl particle concentrations

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rose from 15.3 to 67.8 µg m-3. The number concentrations of NaCl increased from

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8.46×104 to 1.98×105 cm-3, as the initial dry NaCl concentration rose from 15.3 to

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67.8 µg m-3 at the beginning of the experiments. More LWC would exist under the

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condition of higher NaCl seed number concentrations at the given relative humidity.

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LWC rather than the number of seed particles was the critical factor on SOA

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formation in our study, because SOA was hardly formed when NaCl was in the form

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of solid. Thus, the increased LWC in the aerosols with more NaCl seed particles is the

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main reason that leads to the increase of SOA. A linear increasing relationship

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between SOA and LWC can be obtained (Fig. 2). The existence of NaCl droplets

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served as nuclei provided the favorable conditions for SOA formation. Because of the

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strong hygroscopic properties of NaCl, LWC greatly increased with increasing NaCl

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seed particles, leading to the increase of the surface of NaCl droplets. The larger

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particle surface area provided a greater reaction interface, which promotes the

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processes of adsorption and dissolution of SOA precursors (intermediate products).

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Thus, the amount of SOA increased with increasing LWC. It should be pointed out

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that the amount of SOA was likely underestimated in our study. Because some

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intermediate products, which serve as SOA precursors, can be lost onto the smog

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chamber wall surfaces. According to the simulation results from oxidation of alkane

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and alkene, La et al. (2016) found that the mass reduction due to gas-wall partitioning

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for n-octane alkyl nitrates was 2% in the gas phase. The wall loss of intermediate

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products increased with increasing oxidation degree and volatility of the products

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(Zhang et al., 2015). Currently, we cannot measure the losses of intermediate products

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in our study. The work on the wall losses of intermediate products should be studied

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in the future work.

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Fig. 2. Relationship between SOA and LWC. (All the experiments were carried out at

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~303K and ~78% RH.)

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3.2. Effects of initial concentration ratios C3H6/NOx on the formation of SOA

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It has been considered that the initial concentration ratio of VOC/NOx affects

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SOA formation. In order to investigate the influence of initial concentration ratios

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C3H6/NOx (ppbC/ppb) on the SOA formation from propylene, a series of experiments

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were conducted (Exps. 11-16). The results show that the amount of SOA

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exponentially decreases with increasing C3H6/NOx ratio. It seems that there is an

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important point of the ratio value, which is about 11. At the ratio values of less than

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11, SOA decreases considerably with increasing ratio, whereas at the ratio value of

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larger than 11, SOA slightly decreases with increasing ratio, which is associated with

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the formation mechanism of SOA.

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ACCEPTED MANUSCRIPT 50 Experimental data Fitting line

-3

SOA (µg m )

40

20

10 5

10

15

20

25

30

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[C3H6]/[NOx]

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Fig. 3. Variations of SOA concentrations with initial C3H6/NOx (ppbC/ppb)

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concentration ratios. The initial concentration of C3H6 was about 2 ppm, and the

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initial NOx concentration varied from 119 ppb to 878 ppb. The concentration of NaCl

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seed particles was about 40 µg m-3. All the experiments were conducted at 81%-84%

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RH.

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Propylene can react with O3, NO3 and OH radicals. Ozone is formed through the

308

NO2 photolysis, and NO3 and OH radicals are formed via the further reactions of

309

ozone, including its reactions with NO2 and the reaction of its photolytic product with

310

water vapor. It is estimated from MCM simulation that the amount of C3H6 consumed

311

by ozone, NO3 and OH radicals accounts for 45.3%, 1.5% and 53.2% of total

312

consumption of C3H6 at the C3H6/NOx ratio of 46.5, respectively. With the decrease in

313

C3H6/NOx ratio from 46.5 to 6.3, the amount of consumed propylene increases from

314

1.14 ppm to 1.70 ppm. Meanwhile, according to the simulation of MCM, the amount

315

of C3H6 reacted by OH radicals increased by 37% at the C3H6/NOx ratio of 6.3

316

compared with that at the C3H6/NOx ratio of 46.5. Similarly, the amount of C3H6

317

reacted by O3 increased by 28% as the C3H6/NOx ratio decreased from 46.5 to 6.3. As

318

a result, the reaction of propylene with OH radicals become slightly more important

319

under lower C3H6/NOx ratio. The first-generation products of the reaction of C3H6

320

with O3 are mainly Criegee intermediate, HCHO and CH3CHO, and the reactions of

AC C

EP

TE D

307

13

ACCEPTED MANUSCRIPT 321

C3H6 with NO3 and OH radicals produce peroxy radicals O2NOCH2CH(CH3)OO/

322

O2NOCH(CH3) CH2OO and HOCH2CH(CH3)OO/HOCH(CH3)CH2OO, respectively. According to the ozone formation isopleths from the simulation of the C3H6-NOx

324

photochemical reactions, it is known that when the ratio value of C3H6/NOx is larger

325

than 11.1, formation of ozone is controlled by initial NOx concentrations, whereas the

326

ratio value is smaller than 11.1, ozone formation is controlled by initial C3H6

327

concentrations. Experimental results show that the concentration of ozone increased

328

with decreasing C3H6/NOx ratio (increasing NOx concentration at relatively constant

329

C3H6 concentrations). When the photochemical system was under control of NOx, the

330

maximum O3 concentration has a slight increase of 73 ppb as the ratio value of

331

C3H6/NOx decreased from 46.5 to 34.8. However, in the regime of under control of

332

C3H6, the maximum O3 concentration greatly increased by 95 ppb as the ratio

333

decreased from 9.3 to 6.3. Thus, the influence of C3H6/NOx ratio on O3 concentrations

334

in the C3H6 control regime is much more important than that in the NOx control

335

regime. Production of OH is mainly through the reactions of ozone. Thus, formation

336

of OH radicals increases with increasing ozone. Indeed, the maximum concentration

337

of OH radicals decreased with increasing C3H6/NOx ratio (Fig. 4), which is similar to

338

the variation of O3 with the ratio. In the NOx control regime with the ratio values of

339

46.5 to 19.5, the increase of OH radical concentrations was small, while it is large as

340

the ratio decreased from 9.3 to 6.3 in the C3H6 controlled regime. It is obvious that the

341

more OH radicals were produced, the more propylene was consumed, and the more

342

intermediate products were produced. HOCH2CH(CH3)OO/HOCH(CH3)CH2OO from

343

the reaction of C3H6 with OH radicals is the most important SOA precursor, which

344

accounts for half of the total reacted C3H6 (ppbC). From the simulated results of

345

HOCH2CH(CH3)OO/HOCH(CH3)CH2OO with MCM (Fig. 4), we can see that the

346

concentration of HOCH2CH(CH3)OO/HOCH(CH3)CH2OO has an increase as the

347

ratio value decreases from 46.5 to 19.5, but has a much obvious increase as the ratio

348

value decreases from 9.3 to 6.3. This trend of change with C3H6/NOx is quite similar

AC C

EP

TE D

M AN U

SC

RI PT

323

14

ACCEPTED MANUSCRIPT 349

to that in OH radicals. Therefore, the increase of both oxidants and SOA precursors

350

promotes the formation of SOA, as the C3H6/NOx ratio decreases, particularly in the

351

C3H6 control regime.

800

OH

0.6

0.5

700

0.4

SC

600

Concentration (ppt)

RO2

500

0.3

400 300 5

10

15

M AN U

Concentration (ppb)

900

RI PT

352

20

25

30

35

40

0.2

45

C3H6/NOx

353 Fig.

355

HOCH2CH(CH3)OO/HOCH(CH3)CH2OO (RO2) with initial C3H6/NOx concentration

356

ratios (ppbC/ppb) from 46.5 to 6.3 from MCM simulations.

357 358

4.

Variations

of

concentrations

of

OH

and

TE D

354

3.3. Chemical compositions of SOA

The particles collected at the end of the experiments were analyzed by the FTIR.

360

The IR spectra (Fig. 5) show that the different functional groups, whose wavenumbers

361

have been discussed specifically in our previous work (Jia and Xu, 2016; Wang et al.,

362

2016; Ge et al., 2016; Ge et al., 2017), increase with increasing NaCl seed particles,

363

reflecting the role of LWC. The formed SOA contains organic compounds with

364

functional groups of C-H, O-H, C=O, C-O, C-Cl and -ONO2.

AC C

EP

359

365

15

ACCEPTED MANUSCRIPT 0.020

0.020

-

NO3

0.015

ONO2

C-Cl

0.010

0.015

C-O-C

0.005

C-O C-OH

0 1800 1600 1400 1200 1000 800 -1

O-H

-

Wavenumber (cm ) C-H NaCl (µg m-3):

67.8 56.5 42.2 27.2 15.3 0

0.005

NO3

C=O C-Cl

ONO2

0 3500

3000

366

2500 2000 Wavenumber (cm -1 )

1500

RI PT

0.010

1000

Fig. 5. IR spectra of particles with different initial NaCl concentrations at 76-78%

368

RH.

M AN U

SC

367

369

To examine the role of LWC, additional experiments with a different seed of

371

ammonium sulfate (AS, (NH4)2SO4) that is one of the major components of ambient

372

submicron aerosols in urban atmospheres were conducted. Similar to the results from

373

the experiments with NaCl, propylene SOA can also be formed in the experiments

374

with AS droplets, which has been clearly detected by both FTIR and ESI-Exactive

375

-Orbitrap MS (Fig. not shown). An analysis also shows that SOA contains sulfur,

376

indicating that ammonium sulfate, just like NaCl, participates into the formation of

377

SOA. It was calculated that the amount of SOA from the experiment with AS was

378

about 13.4 µg m-3 at the LWC of 80.3 µg m-3, which is 7% lower than that from the

379

experiment with NaCl under the same LWC. It is considered from the results of

380

experiments with both NaCl and AS droplets that LWC is a key factor for the

381

formation of SOA from propylene, rather than the different species of seed particles.

AC C

EP

TE D

370

382

The relative absorption intensity of different functional groups at different

383

C3H6/NOx ratios in the experiments with NaCl, which was defined as the ratio of the

384

absorption intensity at the lower ratios of C3H6/NOx to that at the ratio of 46.5, was

385

shown in Fig. 6. The increased functional groups with decreasing ratio of C3H6/NOx

386

from 46.5 to 6.3 also demonstrated that the low ratio of C3H6/NOx can promote the

16

ACCEPTED MANUSCRIPT 387

formation of SOA, which is consistent with the results from the measured particles by

388

SMPS.

389 3.0 C3H6/NOx=19.5

C3H6/NOx=7.5

C3H6/NOx=6.3

RI PT

C3H6/NOx=34.8

C3H6/NOx=9.3

2.0 1.5 1.0 0.5 0.0

O-H

C=O

ONO2

M AN U

390

SC

Relative intensities

2.5

C3H6/NOx=46.5

391

Fig. 6. Variations of relative intensities of different functional groups with C3H6/NOx

392

ratios from 46.5 to 6.3.

In order to identify the chemical compositions of the SOA, the particles from the

394

C3H6/NOx ratio of both 46.5 and 6.3 were analyzed by ESI-Exactive-Orbitrap MS

395

under positive (Fig. 7) and negative ion mode conditions (not shown), respectively.

396

The intensities of some peaks (m/z values) were different between the ratio of

397

C3H6/NOx 46.5 and 6.3, whereas the compositions of SOA were almost the same. The

398

ratio of the intensities of these peak heights at m/z=148.99339, 192.94461, 228.14961,

399

277.92205, 362.8994 and 447.8766 is about 1.0:1.1:1.5:0.6:0.7:0.4 at the C3H6/NOx

400

ratio of 46.5, and about 1.0:1.0:1.4:0.7:0.9:0.6 at the C3H6/NOx ratio of 6.3,

401

respectively. Thus we consider that the C3H6/NOx ratio does not affect the types of

402

SOA components, but slightly affects their relative concentrations.

EP

AC C

403

TE D

393

17

ACCEPTED MANUSCRIPT 1.5x106 107.97

C3H6/NOx=46.5

6

9.0x105

228.15 192.94 148.99

6.0x105

277.92

362.90

5

3.0x10

447.88

0.0 50

100

150

200

250

300

107.97

1.5x10

Intensity

500

192.94 148.99

5

9.0x10

362.90

277.92

6.0x105

447.88

3.0x105 0.0 100

150

200

250

TE D

50

300

350

400

450

500

m/z

Fig. 7. ESI-Exactive-Orbitrap MS results of SOA in the positive ion mode from propylene-NOx-NaCl experiments under low and high C3H6/NOx ratios.

EP

408

M AN U

228.15 1.2x106

450

C3H6/NOx=6.3

6

407

400

SC

1.8x106

406

350

m/z

404

405

RI PT

Intensity

1.2x10

The Thermo Scientific Xcalibur software was used to analyze the MS results.

410

The element compositions of compounds were calculated by their accurate mass

411

measurements. The identification of chlorinated organic compounds was based on the

412

characteristics of natural abundances of chlorine isotopes (35Cl and 37Cl, ~3:1) in mass

413

spectra. In the positive ion mode, based on the calculation of the accurate mass from

414

different elements, an [M+Na]+ ion at m/z =148.99339 is assigned as a molecular ion

415

formula of C3H7O3ClNa+ that has a mass difference of ∆=-4.2 mDa (148.99339-

416

molecular ion weight of C3H7O3ClNa+). According to the mechanism of C3H6

417

oxidation in the gas phase in MCM and possible pathways of formation of SOA, the

AC C

409

18

ACCEPTED MANUSCRIPT proposed structure of this identified compound C3H7O3Cl is HOCH2CCl(CH3)OOH.

419

The calculated result of molecular ion at m/z=192.94461 is an [M+Na]+ molecular ion

420

formula of C4H4O3Cl2Na+ with the difference of ∆=1.64 mDa. The proposed structure

421

is CH2ClC(O)OCHClCHO. The processes of formation of the detected compounds

422

will be discussed later. In the negative ion mode, some carboxylic acids and esters

423

compounds were also measured in the particles. The peak measured at

424

m/z=231.94186 is simulated to have an [M-H]- molecular ion formula of

425

C4H4O6NCl2-

426

O2NOCHClCO(O)CHClCH2OH. Similarly, the peak measured at m/z=316.92085 is

427

assigned as an [M-H]- molecular ion formula of C6H3O9N2Cl2- (∆=-0.71 mDa) that

428

has a molecular structure of ONOCHClC(O)C(O)OCCl(CH2ONO)COOH. The

429

detected compounds are summarized in Table 3. These detected Cl-containing

430

compounds by ESI-Exactive-Orbitrap MS contain different functional groups, such as

431

–OH, C=O, –ONO2, etc., which is consistent with the results of IR spectra. Thus, it is

432

concluded that hydroperoxides, organic nitrates, carboxylic acids and esters

433

compounds make a great contribution to SOA, although some compounds still cannot

434

be identified.

mDa),

and

its

proposed

molecular

structure

is

TE D

M AN U

SC

(∆=0.26

RI PT

418

435

Table 3 Plausibility of different types of compounds with elemental formulae

437

measured by ESI-Exactive-Orbitrap MS.

AC C

EP

436

No

Ion mode

1

[M+Na]

2 3 4

+ +

[M+Na]

+

[M+Na]

Ion Formula

+

C3H7O 3ClNa

+

C4H4O 3Cl2 Na

C6H3O 6NCl2 Na +

Predicted (m/z)

Error (mDa)

Proposed Structure

148.99339

148.99759

-4.2

HOCH2CCl(CH3)OOH

192.94461

192.94297

1.64

CH2ClC(O)OCHClCHO

277.92205

277.92296

-0.91

ONCHClC(O)C(O)OCHClC(O)CHO

144.98215

144.98176

0.39

CH3CCl(OH)CHClOH

+

C3H7O 2Cl2

+

C3H5O 4NCl2

+

188.95776

188.98901

1.25

HOCHClCH(CH2Cl)ONO 2

C4H4O 6NCl2

-

231.94186

231.94212

0.26

O 2NOCHClC(O) OCHClCH2OH

316.92085

316.92156

-0.71

ONOCHClC(O)C(O)OCCl(CH2ONO)COOH

401.89633

401.89532

1.01

HO 2CH(CH2Cl)C(O)OCCl(CH2Cl)C(O)OCHClCH2 ONO 2

[M+H]

5

[M+H]

6

-

[M-H]

7

-

[M-H]

C6H3O 9N 2Cl2

8

-

-

[M-H]

+

Measured (m/z)

C8H8O 9NCl4

-

438 439 440

3.4. Formation mechanism of SOA from C3H6-NO2-NaCl irradiation system 19

ACCEPTED MANUSCRIPT In the system of C3H6-NO2-hν experiments without NaCl seed aerosols,

442

propylene SOA can be rarely formed at both low and high RH with relatively low and

443

high initial concentrations of propylene. The result is the same even in the

444

experiments with solid NaCl seed particles. However, SOA can be formed obviously

445

under high RH conditions in the presence of NaCl droplets, which illustrates that

446

aqueous processes are important for propylene SOA formation.

RI PT

441

We take some detected compounds (Table 3) by ESI-Exactive-Orbitrap MS as

448

examples to illustrate the processes of SOA formation (Fig. 8) in the

449

C3H6-NO2-NaCl-hν experiments. The intermediate products from the propylene-NOx

450

gas-phase reactions play an important role in the formation of SOA. Peroxyl radicals

451

(RO2) are important intermediate products during photooxidation of C3H6. One of the

452

formed

453

HOCH2CH(CH3)OO/HOCH(CH3)CH2OO, can further react with HO2 and RO2 to

454

form HOCH2CH(CH3)OOH (pathway 1) and CH3CHOHCHO (pathway 2). The

455

Henry’s law constant of HOCH2CH(CH3)OOH is not available in the literature.

456

Nevertheless, HOCH2OOH, which has a similar structure with HOCH2CH(CH3)OOH,

457

has a high Henry’s law constant of 106 M atm-1 (Lee et al., 2000; O'Sullivan et al.

458

1996). Moreover, the half-life of HOCH2OOH is about several tens of minutes under

459

neutral pH and at least 100 min under acidic conditions (pH≤5.4) (Chen et al., 2008).

460

Thus, we consider that the solubility of HOCH2CH(CH3)OOH is also large and it is

461

stable enough to undergo further reactions in our study. The detected composition of

462

chlorine-containing hydroperoxides (proposed structure HOCH2CCl(CH3)OOH) by

463

ESI-Exactive-Orbitrap MS shows that substitution of one hydrogen atom on

464

HOCH2CH(CH3)OOH

465

HOCH2CCl(CH3)OOH, which belongs to one of SOA compositions.

from

the

reaction

of

propylene

with

OH

and

O2,

AC C

EP

TE D

RO2

M AN U

SC

447

by

a

Cl

atom

in

the

aqueous

phase

produced

466

Halogenation of dissolved organic compounds can take place in the aqueous

467

phase in the presence of Cl atoms. According to the Henry’s law and the simulation of

468

MCM, the dissolved OH radicals (~10-12 M) can convert Cl- (~8 M) in the NaCl

20

ACCEPTED MANUSCRIPT 469

droplets to Cl atoms, and Cl atoms can rapidly react with dissolved organic

470

compounds. The photolysis of ClNO2 that is from the reaction of N2O5 in the NaCl

471

droplets, can also release Cl atoms under the UV irradiation condition. These

472

reactions are listed below (Behnke et al., 1997; Fang et al., 2014): N2O5(gas)

N2O5(aq)

(R1)

474

N2O5(aq) + H2O → 2NO3- +2H+

475

N2O5(aq)

476

NO2+ + H2O → NO3- +2H+

477

NO2+ + Cl-

478

ClNO2

479

Cl- + OH → Cl + OH-

(R7)

480

Cl + VOC→ products

(R8)

NO2++ NO3-

(R3)

M AN U

Cl+NO2

(R2)

(R4)

(R5)

SC

ClNO2

RI PT

473

(R6)

It can be estimated that the ClNO2 concentration of less than 10-1 M was obtained

482

from R5 with the rate constant of ~1010 M-1 s-1 (Behnke et al., 1997). According to the

483

comparison of the rates of formation of Cl atoms from R7 with a rate constant of ~109

484

M-1 s-1 and from R6 (the photolysis of ClNO2) with a rate constant of ~10-4 s-1 (Mielke

485

et al., 2011), the amount of Cl atoms from R7 was much larger than that from the

486

ClNO2 photolysis. In addition, the rate constant for the reaction of Cl with benzoic

487

acid was 1010 M-1 s-1 (Fang et al., 2014). Thus, we consider that the dissolved organic

488

compounds in our study should react with Cl immediately after Cl atoms emerged.

EP

AC C

489

TE D

481

Besides that the dissolved OH radicals convert Cl- to Cl atoms in the bulk

490

aqueous phase (von Gunten, 2003; Fang et al., 2014), the OH radicals in the gas phase

491

can react with Cl- at the interface to form a OH…Cl- intermediate, and two OH…Cl-

492

intermediates undergo a self-reaction to release Cl2 to the gas phase (Knipping et al.,

493

2000). Under the condition of without any VOCs dissolved in the NaCl droplets, the

494

formed Cl2 from both interfacial and aqueous reactions release to the gas phase to

495

form Cl atoms via its photolysis in the gas phase. When the pH value of the aqueous

496

phase is about 4, the contribution to Cl atoms from interfacial and aqueous reactions 21

ACCEPTED MANUSCRIPT is 2:3, and becomes 1:4 at pH 3.2 (Knipping et al., 2000). According to the simulation

498

of MCM and the Henry’s low constant of HCOOH, the pH of NaCl droplets in our

499

study was about 4 after 15-mins of the reaction. Much more acids were formed and

500

dissolved into the NaCl droplets over the course of the experiments. As a result, the

501

pH level would maintain below 4, so that the aqueous-phase process was a main Cl

502

source. The dissolved VOCs in our reaction systems quickly reacted with Cl atoms in

503

the aqueous phase, leading to consumption of Cl atoms, which considerably reduced

504

the release of Cl2 to the gas phase. Nevertheless, the Cl atoms from the source of

505

interfacial reactions were able to release into the gas phase, where they reacted with

506

gaseous VOCs. Therefore, the chlorinated organic gases in the gas phase can also

507

make a contribution to SOA through their dissolution into the aqueous phase.

M AN U

SC

RI PT

497

Other RO2, such as HCOCH2OO and CH3C(O)OO can also undergo further

509

reactions to form HCOCH2ONO2, HOCH2CHO and CH3C(O)OH (pathways 3, 4 and

510

5). In the aerosol phase, both the HOCH2CH(CH3)OOH formed from pathway 1 and

511

the CH3CHOHCHO from pathway 2 dissolve into the aqueous phase directly and then

512

are

513

CH2ClCClOHC(O)OH via halogenation or/and oxidation subsequently. The dissolved

514

HOCH2CH2ONO2 from pathway 3 is also converted to HOCHClCH2ONO2 and then

515

undergoes

516

CH2ClCClOHC(O)OH to form high molecular weight ester, for which one of the

517

proposed

518

HO2CH(CH2Cl)C(O)OCCl(CH2Cl)C(O)OCHClCH2ONO2.

519

HOCH2CHO and CH3C(O)OH from pathways 4 and 5 are converted to

520

HOCHClCHO and CH2ClC(O)OH in the aerosol phase, and then they can react with

521

each other to form an ester that is proposed as CH2ClC(O)OCHClCHO. The other

522

detected compounds of SOA by MS can also be explained in a similar way. Therefore,

523

the halogenated products from directly dissolved products and/or the products via

converted

HOCH2CCl(CH3)OOH,

further

AC C

EP

the

to

TE D

508

reaction

structure

with

HOC(O)CH(CH2Cl)OOH

HOC(O)CH(CH2Cl)OOH

is

and

and

probably The

dissolved

22

ACCEPTED MANUSCRIPT 524

oxidation reactions in the aqueous phase generally result in the formation of SOA that

525

was detected by FTIR and MS. Moreover, the dissolved products in aqueous phase can react with OH to form

527

radicals by H-atom abstraction. Previous work pointed out that two radical species

528

can form a dimer that may undergo further reactions to form a trimer or higher carbon

529

number compounds (Lim et al., 2010). This pathway may also exist in the SOA

530

formation from propylene, because only parts of the SOA compositions were

531

measured and analyzed in our study. SOA formation is a complex process, and the

532

analysis of SOA compositions deserves further study.

…

…

CH3C(O)OO* ⑤ HO2

④ RO2

CH3C(O)OH

HOCH2CHO

HOCH2CH(CH3)OO*/HOCH(CH3)CH2OO* ② RO2 ①HO

③ NO

HCOCH2ONO2

CH2ClC(O)OH HOCHClCHO HOCH CH ONO 2 2 2

Aerosol phase

535 536 537 538

HOCH2CH(CH3)OOH

CH3CHOHCHO

HR

HR OH/HO2

ClOH/HO2 HR

HOC(O)CH(CH2Cl)OOH

OH Cl*

CH2ClCClOHC(O)OH

HO2CH(CH2Cl)C(O)OCCl(CH2Cl)C(O)OCHClCH2ONO2 d

…

Fig. 8. Mechanisms for SOA formation from C3H6 (*: radicals; HR: halogenated

AC C

534

CH3CHOHCHO

HOCH2CCl(CH3)OOHd

HOCHClCH2ONO2

EP

CH2ClC(O)OCHClCHOd

HOCH2CH(CH3)OOH

TE D

OH

HR

…

2

HCOCH2OO*

HR

HR

M AN U

CH3CH=CH2+NO2+NaCl+hv OH

SC

533

RI PT

526

reactions; d detected compound. ①②③④⑤: reaction pathways.).

4. Atmosphere implications

539

Propylene is an important chemical raw material and exists widely in the

540

troposphere as one of primary pollution gases. The concentration of propylene in the

541

atmosphere is in the 10-1～101 ppb levels (Chang et al., 2005), which is similar to

542

other LMW VOCs, such as ethylene and acetylene. Previous work has demonstrated 23

ACCEPTED MANUSCRIPT that ethylene and acetylene can form SOA under some certain conditions (Volkamer

544

et al., 2009; Ge et al., 2016; Jia and Xu, 2016). Through the study of

545

propylene-NO2-NaCl irradiation experiments, we confirm that propylene SOA can be

546

formed in heterogeneous reactions, and make a contribution to the total particles in

547

the atmosphere. The amount of SOA formed from propylene was greatly influenced

548

by atmospheric humidity. It is obtained that the amount of SOA increases with

549

increasing LWC, with the yield being 5 times higher at 169.8 µg m-3 of LWC than that

550

at 34.5 µg m-3 of LWC. The NaCl seed aerosols, which greatly exist in coastal

551

atmosphere, not only serve as a liquid water holder, but also participate into the

552

formation of SOA.

M AN U

SC

RI PT

543

The ratio of VOCs/NOx varies all the time in the atmosphere, such as the ratio of

554

4-18 ppbC/ppb in Guangzhou, China (Zou et al., 2015) and 7-150 ppbC/ppb in

555

Tianjin, China (Ran et al., 2011). Our study shows that the amount of SOA is

556

sensitive to the ratios of C3H6/NOx, and that low ratio of C3H6/NOx in the C3H6

557

controlled regime enhance formation of SOA. The concentration of propylene in the

558

atmosphere was in the level of ppb (Chang et al., 2005), but NOx was dozens or up to

559

hundreds of ppb (Shi et al., 2009). Both the C3H6 concentration and the C3H6/NOx

560

ratio in the atmosphere were low. Owing to the sensitivity limitations of our

561

instruments, we used high concentrations of reactants in the experiments, because too

562

less SOA could not be detected in our study. On the other hand, the high ratio of

563

C3H6/NOx in our experiments was used to serve as a representative of the total VOCs

564

to NOx ratio in the atmosphere, since the VOCs/NOx ratio of 6.3-46.5 (ppbC/ppb)

565

widely existed in the atmosphere in some cities of China. In this way, to understand

566

the potential of SOA formation from different VOC species, we can use the same

567

approach to compare the relative contribution of their formation of SOA. It is obvious

568

that the low C3H6/NOx ratio is a normal situation in the atmosphere in cities. The

569

effect of the VOCs/NOx ratio must be taken into account when estimating the factors

570

of SOA formation in the atmosphere by model study. Although the concentrations of

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propylene in our experiments were high, we believe that the chemical processes do

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exist and particles can be formed under some certain conditions to make contribution

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to the ambient particles in the atmosphere. Our study provides some insights in the formation of SOA from light non-

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methane hydrocarbon, propylene. The study of propylene SOA is important for the

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cognitive update of the contribution of LWM compounds to atmospheric particles

577

under high humidity and different VOCs/NOx ratio conditions, which is helpful in the

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atmospheric SOA models. Moreover, through our study, we find that the process of

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aqueous phase reactions from LWM alkenes is the main process of their SOA

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formation. Thus, the contribution of propylene SOA to environmental pollution

581

should be considered.

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582 583

5. Conclusions

The propylene-NOx-NaCl irradiations experiments were conducted in a

585

self-made smog chamber. Experimental results show that without NaCl seed particles

586

only a small number of particles could be formed during the experiments, in which

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the volume concentration of particles was monitored to be at most 0.013 µm3 cm-3 and

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0.036 µm3 cm-3 from the experiment with about 2 ppm propylene and 150 ppb NO2,

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and from the experiment with about 15 ppm propylene and 150 ppb NO2 under 5%

590

and ~80% RH conditions, respectively. Propylene SOA was hardly generated even in

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the presence of artificial NaCl seed particles that were in the form of solid. SOA can

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be obviously formed under high relative humidity (RH) conditions through

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heterogeneous reactions in the presence of sodium chloride (NaCl) droplets. Liquid

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water content is a critical factor for the formation of SOA. When LWC increased

595

from 34.5 to 169.8 µg m-3, the amount of SOA linearly increased from 5.9 to 29.8 µg

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m-3 with a regression equation of SOA=0.0164×LWC+1.137 (R2=0.97) at the initial

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C3H6/NOx concentration of 32.2-44.9. The amount of SOA increased exponentially

598

from 12.1 to 47.9 µg m-3 when the initial C3H6/NOx ratio (ppbC/ppb) decreased from

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ACCEPTED MANUSCRIPT 46.5 to 6.3 with the increase of NOx concentrations from 118.8 ppb to 878.0 ppb at

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relatively constant concentration of 2 ppm for propylene. According to the ozone

601

formation isopleths, formation of ozone controlled by initial NOx or C3H6

602

concentrations has a great influence on the formation of SOA. When the C3H6/NOx

603

ratio is smaller than 11 in our experiments, which is in the C3H6 controlled regime,

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SOA decreases considerably with increasing ratio, whereas at the ratio value of larger

605

than 11 in the NOx controlled regime, SOA slightly decreases with increasing ratio.

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Peroxyl radicals (RO2) are important intermediate products from photooxidation of

607

C3H6, which serve as important SOA precursors. RO2 radicals undergo further

608

oxidation, and their products dissolve into aerosol water. Some of the dissolved

609

products undergo further halogenation and esterification reactions to form high

610

molecular weight esters. It is obtained that propylene SOA contains hydroperoxides,

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organic nitrates, carboxylic acids and esters. Therefore, it is concluded from our study

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that the processes of liquid-phase reactions are most important for SOA formation

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from propylene.

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Acknowledgments

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This work was supported by the National Natural Science Foundation of China (No.

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41375129 and No. 41105086) and the "Strategic Priority Research Program (B)" of

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the Chinese Academy of Sciences (Grant No. XDB05010104).

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ACCEPTED MANUSCRIPT Highlights: 1. Formation of SOA from propylene is confirmed for the first time. 2. An exponential relation of SOA with initial C3H6/NOx ratio is explained. 3. Main chemical compositions of propylene SOA are determined.

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4. Participation of Cl atoms in the formation of SOA is determined.

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5. A liquid-phase mechanism for SOA formation is proposed.