Effects of NOx, SO2 and RH on the SOA formation from cyclohexene photooxidation

Accepted Manuscript sEffects of NOx, SO2 and RH on the SOA Formation from Cyclohexene Photooxidation

Shijie Liu, Xiaotong Jiang, Narcisse T. Tsona, Chen Lv, Lin Du PII:

S0045-6535(18)32051-4

DOI:

10.1016/j.chemosphere.2018.10.180

Reference:

CHEM 22442

To appear in:

Chemosphere

Received Date:

27 July 2018

Accepted Date:

26 October 2018

Please cite this article as: Shijie Liu, Xiaotong Jiang, Narcisse T. Tsona, Chen Lv, Lin Du, sEffects of NOx, SO2 and RH on the SOA Formation from Cyclohexene Photooxidation, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.10.180

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ACCEPTED MANUSCRIPT

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Effects of NOx, SO2 and RH on the SOA Formation from

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Cyclohexene Photooxidation

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Shijie Liu, Xiaotong Jiang, Narcisse T. Tsona, Chen Lv, Lin Du*

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Environment Research Institute, Shandong University, Qingdao, 266237, China

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Correspondence to: Lin Du ([email protected])

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Abstract

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We performed a laboratory investigation of the secondary organic aerosol (SOA)

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formation from cyclohexene photooxidation with different initial NOx and SO2

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concentrations at low and high relative humidity (RH). Both SOA yield and number

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concentration first increase drastically and then, decreased when the [VOC]0/[NOx]0

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ratio changed from 30 to 10 and from 10 to 3. Though the presence of SO2 could

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increase the SOA number concentration, the SOA yield could only increase under

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[VOC]0/[NOx]0=10 and high RH, and [VOC]0/[NOx]0=3 and low RH experimental

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conditions, while decreasing under [VOC]0/[NOx]0=10 and low RH conditions. In the

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presence of SO2, the high RH and high NOx conditions were keys to efficient sulfate

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formation and could promote the SOA formation. The chemical composition of SOA

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was characterized using hybrid quadrupole-orbitrap mass spectrometer equipped with

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electrospray ionization (ESI-Q-Orbitrap-HRMS), and few organosulfates were

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identified. A visible enhancement of organosulfates and the formation of high

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molecular weight organic compounds were observed at high RH conditions, and this

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seemed to be the reason for the SOA yield increase at high RH.

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Keywords: cyclohexene; SOA yield; environmental factors; organosulfates;

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photooxidation

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Introduction

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Aerosols not only have significant impacts on air quality (Kanakidou et al., 2005;

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McFiggans et al., 2006; Jaoui et al., 2012), but are also thought to contribute to

35

climate change patterns (Adams et al., 2001; Hansen and Sato, 2001; Pokhrel et al.,

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2016). Aerosol particles can easily deposit in the lung through inhalation and have

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significant impacts on human health (Pope III and Dockery, 2006; Russell and

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Brunekreef, 2009). They also participate in heterogeneous chemical reactions,

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affecting the abundance and distribution of atmospheric trace gases (Hallquist et al.,

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2009). Secondary organic aerosols (SOA), which form from the oxidation of volatile

41

organic compounds (VOCs), have been receiving significant attention since recent

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years due to their large implication in the formation of atmospheric aerosols,

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accounting for a major fraction of the total atmospheric aerosols (Jimenez et al.,

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2009). Alkenes, widely emitted from biogenic and anthropogenic sources, are one of

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the most important components of VOCs in the atmosphere (Kesselmeier et al., 2002;

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Chin and Batterman, 2012). Cyclohexene, an example of alkenes with a ring structure,

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has been extensively studied for inferring photooxidation mechanism and aerosol

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formation characteristics due to its basic structural unit similar to that of

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monoterpenes and sesquiterpenes (Keywood et al., 2004b; Carlsson et al., 2012).

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Cyclohexene is mostly emitted from anthropogenic sources, and it was one of the

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most abundant cyclic alkenes in the automobile exhausted gas (13 mg L-1) in the

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urban areas (Fraser et al., 1998). Cyclohexene was also one of the first organic

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compounds investigated for its aerosol-forming potential (Kalberer et al., 2000;

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Rissanen et al., 2014).

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The anthropogenic-biogenic interactions in SOA formation have been

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highlighted in many field studies (Shilling et al., 2013; Cheng et al., 2015; Wang et

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al., 2018), and significant interest was given to understand how the photooxidation

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mechanisms are affected by anthropogenic co-pollutions in the atmosphere, such as 3

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NOx. The concentrations of main atmospheric oxidants (like OH, NO3 radicals and

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O3) were closely related to the concentration of NOx in the atmospheric environment

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(Rollins et al., 2012; Sarrafzadeh et al., 2016). The suppressing effect of NOx on SOA

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formation has been shown to be widely attributed to the effect of NOx on OH

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concentration. After eliminating this effect, the yield of the SOA formed from β-

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pinene photooxidation decreased moderately by 20-30% (Sarrafzadeh et al., 2016).

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The change of NOx concentration would lead to the competitive relationship between

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OH, NO3 radicals and O3, which would change the distribution of the VOCs oxidation

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products, and then influence the SOA formation (Rollins et al., 2012; Sarrafzadeh et

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al., 2016). NOx can also change the fate of peroxy radicals (RO2). Elevated

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concentrations of NOx tend to reduce the SOA formation by reaction of RO2 with NO

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to yield alkoxy radicals (RO) instead of RO2 or HO2 (Presto et al., 2005; Song et al.,

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2005; Lane et al., 2008). The fragmentation/decomposition of RO radicals producing

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higher volatility species justifies the decrease in SOA formation (Lim and Ziemann,

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2005, 2009). NOx can also suppress the new particle formation events, which would

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reduce the sink for condensation and then, decrease SOA formation under high NOx

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conditions (Lane et al., 2008; Wildt et al., 2014). The chemical composition of the

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photooxidation products was also related to the change of NOx concentration, e.g. the

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organic nitrates were formed at high NOx while organic hydroperoxide always

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formed at low NOx (Hallquist et al., 2009; Ziemann and Atkinson, 2012). In the last

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decade, the NOx concentration increased in China but decreased in the United States

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and the United Kingdom by more than 30% (Zhang et al., 2007; LaFranchi et al.,

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2011; Rollins et al., 2012). Hence, it is important to understand how the changes in

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NOx concentrations can affect SOA formation.

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Sulfur dioxide (SO2) is another important inorganic co-pollutant in urban areas,

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which is mainly emitted by coal-fired flue gases. By interacting with the organic

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particulate matter, SO2 plays an important role in SOA formation by enhancing the

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acid-catalyzed multiphase reactions (Wang et al., 2005; Lonsdale et al., 2012; Liu et 4

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al., 2016b). It was demonstrated that the seasonal variation of PM2.5 concentration is

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consistent with SO2 concentration in the atmosphere (Cheng et al., 2015). SO2 can be

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converted to sulfuric acid during oxidation by hydroxyl radicals (OH) in the presence

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of UV light (Somnitz, 2004), stabilized Criegee intermediates (sCI) (Mauldin III et

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al., 2012; Welz et al., 2012; Liu et al., 2017b) and atmospheric ions (Enghoff and

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Svensmark, 2008; Kirkby et al., 2011; Bork et al., 2013; Tsona et al., 2016). It was

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demonstrated that SOA formation is enhanced in the presence of SO2 under acidic

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conditions by increasing aerosol acidity and ammonium sulfate aerosol formation

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(Edney et al., 2005; Attwood et al., 2014; Liu et al., 2016b). Recent studies have

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shown that increased aerosol acidity is a key variable in enhancing SOA formation

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through acid-catalyzed reactive uptake and multiphase chemistry of oxidation

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products derived from VOCs (Iinuma et al., 2009; Surratt et al., 2010). SO2 can also

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impact new particle formation and SOA composition (Lonsdale et al., 2012). The

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formation of stable gas-phase products and SOA from the ozonolysis of cyclohexene

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in the presence of SO2 were investigated, and the collisional stabilization of initial

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clusters was an important aspect for the SOA formation processes involving sulfuric

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acid (H2SO4) and organic compounds (Carlsson et al., 2012). In the presence of SO2,

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organosulfates have been proven to be one of the important components in

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cyclohexene SOA, but their formation mechanism is still unclear.

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Organosulfates were first observed as new components in ambient aerosols in

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2005 (Romero and Oehme, 2005), and were properly identified two years later

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(Surratt et al., 2007). Recently, different kinds of organosulfates were observed in

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SOA around the world (Iinuma et al., 2007; Gomez-Gonzalez et al., 2008; Hawkins et

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al., 2010; Hatch et al., 2011; Kristensen and Glasius, 2011; Shalamzari et al., 2013;

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Hansen et al., 2014; Liao et al., 2015). Organosulfates have been proven to be an

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important contributor to SOA (Surratt et al., 2008; Froyd et al., 2010; Kristensen and

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Glasius, 2011; Tolocka and Turpin, 2012; Wang et al., 2015), accounting for almost

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one third of the total amount of SOA (Surratt et al., 2008; Tolocka and Turpin, 2012; 5

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Hettiyadura et al., 2017). Depending on the nature of the precursors and the

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complexity of atmospheric chemical reactions, the formation and transformation

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processes of organosulfates in SOA can be complex and varied. Extensive studies on

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the organosulfates formation have been simulated in a series of laboratory chamber

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studies through OH/NOx/O3-initiated reactions of BVOCs, such as isoprene, α-

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pinene, β-pinene, limonene and 2-methyl-3-buten-2-ol (MBO) (Surratt et al., 2007;

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Surratt et al., 2008; Hatch et al., 2011; Zhang et al., 2012; Zhang et al., 2014; Mael et

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al., 2015). Liu et al. investigated the SOA formation from the cyclohexene/NOx/SO2

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system and presented a new evidence that organosulfates can be formed from the

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anthropogenic VOCs photooxidation in the presence of SO2 (Liu et al., 2017a). Many

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unidentified organosulfates in the Arctic sites field data were also found previously

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among the products of cyclohexene photooxidation (Liu et al., 2017a). At present, the

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number of accurately identified organosulfates precursors is very limited, much lower

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than the number of organosulfates identified in field observations (Surratt et al., 2008;

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Wang et al., 2015; Kuang et al., 2016). Alkanes and aromatic hydrocarbons might

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also be important precursors for organosulfates not yet identified (Tao et al., 2014;

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Kuang et al., 2016). Organosulfates with the aromatic group could account for two-

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thirds of all organosulfates in Shanghai area (Ma et al., 2014). A large number of

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aliphatic organosulfates were detected in urban aerosols (Wang et al., 2015; Kuang et

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al., 2016). Although qualitative analyses of organosulfates have been gaining more

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attention and development since recent years (Lin et al., 2012; Shalamzari et al.,

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2013; Staudt et al., 2014), the lack of available authentic standards for identified

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organosulfates hinder their effective analysis. Accordingly, the identification of

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organosulfates precursors is necessary.

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The formation of organosulfates has been extensively studied and several

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possible formation mechanisms have been proposed. For instance, the acid-catalyzed

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heterogeneous reaction of sulfate or H2SO4 addition to protonated carbonyls was

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shown to be the main formation process of organosulfates (Surratt et al., 2007). 6

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Epoxide ring opening of epoxy-containing SOA constituent during photooxidation

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reactions was found to be an important intermediate process in this mechanism as

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well (Minerath et al., 2009; Darer et al., 2011; Hu et al., 2011; Mael et al., 2015).

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Organosulfates may also result from nucleophilic substitution of nitrates of tertiary

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organonitrates by sulfates (Darer et al., 2011). Organosulfates could also be formed

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through sulfate radical-induced oxidations in aqueous aerosols. The sulfate radical

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could be added on the C=C double bond or abstracted the H atoms from the organic

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molecules to form the sulfate alkyl radicals. The sulfate alkyl radicals could then react

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by addition of oxygen or oxidation of OH radicals to form organosulfates with

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hydroxyl or carbonyl groups (Noziere et al., 2010; Schindelka et al., 2013;

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Szmigielski, 2016). However, the above mechanisms were studied in the presence of

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sulfate-containing seed particles, which may obscure the organosulfates formation

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pathways that are unique to SO2 chemistry (Shang et al., 2016). A direct

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heterogeneous reaction between SO2 and unsaturated compounds (alkene or

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unsaturated fatty acid) as another possible pathway for organosulfates formation in

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the troposphere has been presented (Passananti et al., 2016). The uncertainties in

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detailed reaction mechanisms lead to a substantial disconnection between known

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oxidation mechanisms and specific measurements of organosulfates.

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While some studies have focused on the SOA formation from cyclohexene

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photooxidation, a number of questions still remain elusive (Kalberer et al., 2000;

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Keywood et al., 2004a; Muller et al., 2008; Rissanen et al., 2014). There are few

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researches focusing on combined effects of both NOx and SO2 in SOA formation. In

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the current work, we conduct a laboratory study of the photooxidation of cyclohexene

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under different initial NOx and SO2 concentrations and RH conditions, highlighting

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the effects of these parameters on the cyclohexene SOA yield, particle number

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concentration, chemical composition and organosulfates formation. This study also

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proposes the formation mechanism of cyclohexene organosulfates. The results will

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provide a better understanding of SOA formation from cyclohexene photooxidation 7

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and, to a large extent, improve the understanding of complex air pollution in urban

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environments.

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Materials and methods

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Chamber description

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The cyclohexene photooxidation reaction was performed in a 1 m3 Teflon

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chamber at the Environment Research Institute of Shandong University. This reaction

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was driven by 10 black light lamps (GE F40BLB). The light spectrum of the black

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lamps ranged from 300 to 420 nm with peak intensity at 365 nm, which was similar to

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the irradiation of solar UV band. The chamber interior walls were covered by stainless

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steel to maximize and homogenize the interior light intensity. The effective light

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intensity, represented by the photolysis rate of NO2 (JNO2), presented a good linear

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relationship with the lamps number, and the average JNO2 was 0.126 min-1 at full light

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intensity. All experiments were performed at room temperature and the atmospheric

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pressure was maintained in the chamber at all times. The chamber was cleaned by

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purging with purified dry air for at least 3 times and prior to each experiment, and

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residual hydrocarbons, O3, NOx and SO2 could not be detected. In every experiment,

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the zero air, with no detectable non-methane hydrocarbons (NMHC < 1 ppb), NOx (<

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1 ppb), low O3 concentration (< 3 ppb), low particle numbers (< 5 cm-3) was used to

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fill the chamber. The flow rates of zero air were controlled by mass flow controllers

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(D08−8C/ZM, Beijing Sevenstar Electron Corporation). Initial concentrations of

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about 1 ppm of liquid cyclohexene (Aldrich, 99%, without further purification) were

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injected into a Teflon FEP tube firstly and dispensed into the chamber by purified dry

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air. Different volumes of NOx (Jinan Deyang Special Gas CO., LTD, 500 ppm NO in

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N2) and/or SO2 (Jinan Deyang Special Gas CO., LTD, 500 ppm SO2 in N2) were

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introduced into the chamber for the required concentrations, respectively. For the wet

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experiment, the zero air was humidified by bubbling the air through fritted glass in 8

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distilled water and RH>70 %. The RH was lower than 20% for experiments

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performed at dry conditions. RH was measured with a hygrometer (Model 645, Testo

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AG, Lenzkirch, Germany). The average value of the particles wall loss rate constant

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was 4.7×10-5 s-1, and the measured particle concentrations and SOA yield in this study

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were corrected in the same way as in our earlier study (Liu et al., 2017a). When using

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NO, the photooxidation reaction could not happen until it was oxidized to NO2, which

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means that both NO and NO2-initiated photooxidation reactions were actually

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triggered by NO2. Hence, the chemistry of SOA formation from both NO2 and NO

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processes is not different. The black lights were turned on and the photooxidation

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started when all cyclohexene, NO and/or SO2 were injected and well mixed. An

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overview of the experiment details is shown in Table 1.

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Table 1 An overview of the experiment conditions. Here, the VOC is cyclohexene No.

Initial

[VOC]0/

VOC

[NOx]0

(molecule cm-3)

[SO2]0

T

(ppb)

(K)

RH

SOA

Number

condition

yield

concentration

(%)

(%)

(104 cm-3)

1

10

0

Dry

4.15

1.4

2

10

50

Dry

3.53

1.6

3

10

100

Dry

3.25

17.3

4

10

200

Dry

2.70

22.3

5

10

0

Wet

3.68

1.1

6

10

50

Wet

3.63

5.7

7

10

100

302

Wet

5.58

36.4

10

200

~

Wet

6.73

31.3

9

3

0

306

Dry

2.55

1.2

10

3

50

Dry

2.80

6.5

11

3

100

Dry

4.40

25.6

12

3

200

Dry

5.63

39.6

13

30

0

Dry

0.53

0.0

14

30

50

Dry

0.34

0.1

15

30

200

Dry

0.40

0.2

8

2.97×1013

210 211

The concentrations of ozone, NOx and SO2 were measured online by the ozone

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analyzer (Model 49C, Thermo Electron Corporation, USA), NO-NO2-NOx analyzer 9

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(Model 42C, Thermo Electron Corporation, USA) and SO2 analyzer (Model 43i-TLE,

214

Thermo Electron Corporation, USA), respectively. We quoted the uncertainty on the

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measurements as 1% of the linear uncertainty as provided by the manufacturer in the

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user manual of the analyzers. Cyclohexene was sampled by a glass syringe before the

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UV lights were turned on. A gas chromatograph equipped with flame ionization

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detector (GC-FID; Agilent Technologies, 7890B) and a DB-624 capillary column (30

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m×0.32 mm, 1.8 μm film thickness) were used to measure the cyclohexene

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concentrations in the beginning and the end of each experiment. The same gas volume

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was introduced into the GC by the six-way valve mounted with a 0.5 mL stainless

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loop. Nitrogen was used as the carrier gas at a constant flow rate of 5 mL min-1. The

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temperatures of the inlet, oven and detector were 250, 100 and 300 °C, respectively.

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Particle size distributions and the number concentrations were measured

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throughout each experiment with the scanning mobility particle sizer (SMPS), which

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consisted of a condensation particle counter (CPC model 3776, TSI Inc., USA) and a

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differential mobility analyzer (DMA model 3081, TSI Inc., USA). In the DMA, a

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sheath flow and aerosol flow used for the particle number concentrations and size

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distributions measurements were 3.0 and 0.3 L min-1, respectively. Estimates for

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SMPS were based on an aerosol density range of 1.0 -1.5 g cm-3. The SOA density

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(1.2 g cm-3), which was used to calculate the particle mass concentration from its

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volume concentration, was taken from our previous study (Liu et al., 2017a). The scan

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time of the SMPS was 240 s and the particle diameters were taken over a size range of

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13.6-736.5 nm. The SMPS data were recorded and analyzed using the TSI Aerosol

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Instrument Manager software v10.2.

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SOA collection and analysis

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SOA samples were analyzed offline using an Electrospray Ion Source Exactive

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Plus Orbitrap Mass Spectrometer (ESI-Orbitrap-HRMS, Thermo Fisher Scientific,

239

USA) and an Electrospray Ion Source Q Exactive Hybrid Quadrupole-Orbitrap Mass 10

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Spectrometer (ESI-Q-Orbitrap-HRMS, Thermo Fisher Scientific, USA). Both

241

techniques were used in the negative ion mode and the analyte molecules were

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detected as [M-H]-. The ESI-Orbitrap-HRMS identified organosulfates based on their

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exact mass measurements, exclusively. The tandem mass spectra (MS2) could be

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obtained from ESI-Q-Orbitrap-HRMS, and the tentative organosulfate structures were

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identified by the fragment ions of MS2. Aerosol sampling was started at the end of

246

each experiment after the aerosol concentration had reached a constant value.

247

Aerosols particles were collected by impaction onto ungreased aluminium foil

248

through a Dekati low pressure impactor (DLPI+, DeKati Ltd, Finland) for off-line

249

chemical analysis. The flow rate and sample pressure of DLPI+ were 10 L min-1 and

250

36.5 mbar, respectively. To analyze the SOA composition, the SOA collected on the

251

aluminium foil was extracted in a 1-mL vial with 1 mL acetonitrile in an ultrasonic

252

bath for 1 min to make sure the SOA was completely dissolved. Thereafter, 5 μL of

253

the extraction was injected directly into the HR-MS, not combined to liquid

254

chromatography, and the SOA was characterized using direct infusion. The observed

255

signals might serve as useful indicators for some of the major constituents and thus

256

the reaction mechanism leading to particle formation. Softer ionization with higher

257

mass resolution was used to unambiguously assign chemical formulae to the SOA

258

constituents. The formulae were calibrated using the manufacturer’s calibration

259

standards mixture allowing for mass accuracies <5 ppm in external calibration mode

260

by the Xcalibur 2.2 software. Exact operating conditions for ESI-Q-Orbitrap-HRMS

261

were 3.0 kV for the ionization voltage and 320 °C for the capillary. Both sheath gas

262

(30 U) and auxiliary gas (10 U) were ultra-pure N2.

11

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Results and discussion

264

Effects of NOx on SOA formation

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SOA formation is affected by the [VOC]0/[NOx]0 ratio, which could alter the

266

oxidation capacity in the photooxidation experiments (Ng et al., 2007a; Lane et al.,

267

2008; Sarrafzadeh et al., 2016; Edwards et al., 2017). To study the effect of NOx on

268

SOA formation, the initial levels of NOx were varied while all other experimental

269

conditions were kept constant. The SOA formation for the cyclohexene/SO2/NOx

270

system was measured with [VOC]0/[NOx]0 ratios ranging from 3 to 30. The SOA

271

yield, which is defined as the fraction of SOA mass to reacted VOC, was widely used

272

to represent the SOA formation potential in the photooxidation process. The

273

cyclohexene content was basically below the GC-MS detection at the end of each

274

experiment. The SOA yield under different NOx initial concentrations is shown in

275

Fig. 1(A), where it can be seen a slight decrease as the [VOC]0/[NOx]0 ratio decreased

276

from 10 to 3, and a strong decrease as the [VOC]0/[NOx]0 ratio increased from 10 to

277

30. The variation of the SOA yield with different initial NOx conditions observed in

278

Fig. 1(A) is very similar to that observed in some previous studies (Camredon et al.,

279

2007; Zhao et al., 2018). It was suggested that the increase in SOA yield with NOx

280

concentration could be due to the influence of OH levels. However, the OH

281

concentration was not measured in this study. The OH concentration has been proved

282

to increase rapidly at low-NOx conditions with increasing NOx and to decrease

283

gradually when it reached a maximum value at high-NOx conditions (Sarrafzadeh et

284

al., 2016; Zhao et al., 2018). The increase of OH at low-NOx conditions was

285

attributed to OH recycling through the NO + HO2 → NO2 + OH reaction. The

286

increasing O3 concentration, as the photolytic OH source through the O3 + hv → O2 +

287

O(1D) and O(1D) + H2O → 2OH reactions, also makes indirect contributions to the

288

concentration of OH. The OH radicals in the system could be consumed by NOx

289

through the NO2 + OH → HNO3 reaction, when the [VOC]0/[NOx]0 ratio was 3, 12

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which would slightly reduce the OH concentration in the chamber. Previous studies

291

have proved that a lower SOA yield was observed at high NOx concentrations than at

292

low NOx concentrations (Song et al., 2005). RO2 could be affected by NOx in VOCs

293

photooxidation directly (Lane et al., 2008). The increased fraction of RO2 + NO

294

reaction, which via fragmentation of the resultant RO radical results in the formation

295

of volatile organic products, would also be the reason of the suppression of SOA

296

formation at high NOx conditions (Kroll and Seinfeld, 2008; Lane et al., 2008).

297 298

Figure 1 The cyclohexene SOA yield and number concentration with different initial NOx

299

concentrations.

300 301

Another way to examine the trend of SOA yield with different NOx

302

concentrations was the change of condensing surface for new formed low volatile

303

substances and the particle number concentration was the key factor affecting the

304

condensing surface. The particle number concentration at different [VOC]0/[NOx]0

305

ratios is shown in Fig. 1(B). There was no seed particle in all of our experiments. The

306

particle number concentration also increased drastically with initial NOx

307

concentration at low NOx and then decreased slightly along the further NOx

308

increment ([VOC]0/[NOx]0 ratio decreased from 10 to 3). The change in number

309

concentration with different initial NOx concentrations was consistent with the trend

310

of the SOA yield. The first formed particles in the photooxidation process were 13

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generated by the RO2 + RO2 reaction, which could form the products with higher

312

molecular weight for nucleation (Wildt et al., 2014; Kirkby et al., 2016). With the

313

continued increase of NOx concentration, the particle number concentration decreased

314

slowly and this decrease trend was due to the decreasing fraction of RO2 + RO2

315

reactions while the RO2 + NO reaction was becoming more and more important. The

316

suppression of new particle formation at high NOx is consistent with the results of

317

Wildt et al. (Wildt et al., 2014). Accordingly, these results indicate NOx promotes the

318

new particle formation at low-NOx conditions, while suppressing it under high-NOx

319

conditions. The trend in particle number concentration is similar to that of the SOA

320

yield. The new formed particles in the chamber could act as sink by providing the

321

condensing surface for low volatility substances. The change of SOA yield might be

322

due to the change of particle surfaces for the condensation (Zhao et al., 2018). This

323

explains why the SOA yield varied with the initial NOx concentrations.

324

Effects of SO2 on SOA formation

325

The SOA formation in the cyclohexene/NOx/SO2 photooxidation experiments

326

was investigated under different initial SO2 concentrations. Fig. 2 shows a remarkable

327

increase in the number of new formed particles with the increase of SO2 concentration

328

regardless of the initial NOx conditions. The high particle number concentration was

329

attributed to sulfate formation from SO2 oxidation as the level of sulfate has been

330

proven to be the key factor affecting nucleation (Sipilä et al., 2010; Liu et al., 2016b).

331

The particle number concentration was significantly increased by NOx at fixed SO2

332

concentration, indicating the importance of NOx in affecting the particle number

333

concentration. Nucleation was directly related to the sulfate level (Liu et al., 2016b;

334

Sipilä et al., 2010), whereas the oxidation of SO2 to sulfate formation could be

335

enhanced in the presence of NOx at dry conditions.

336

14

ACCEPTED MANUSCRIPT

337 338

Figure 2 The particle number concentration at different initial concentrations of NOx and SO2 and

339

different RH.

340 341

Fig. 3 presents the effect of SO2 on SOA yield from cyclohexene photooxidation.

342

A reverse trend of the change of SOA yield with increasing initial SO2 was observed

343

when the [VOC]0/[NOx]0 ratios were 3 and 10. At [VOC]0/[NOx]0=10, with SO2

344

concentration increasing, the SOA yield decreased gradually, from 4.2% to 2.7%.

345

Conversely, the SOA yield at [VOC]0/[NOx]0=3 was increased with SO2

346

concentration. Although the SOA yield at [VOC]0/[NOx]0=3 only accounts for 60%

347

of that at [VOC]0/[NOx]0=10 without SO2, it was about 2 times higher than that at

348

[VOC]0/[NOx]0=10 and 200 ppb initial SO2. At [VOC]0/[NOx]0=3 conditions, SO2

349

has a positive impact on SOA formation and enhances the SOA yield, while

350

suppressing SOA formation at [VOC]0/[NOx]0=10. A previous study has pointed out

351

that when [VOC]0/[NOx]0=5~6, the SOA yield decreased at first and then increased

352

with increasing SO2 concentrations, and the decreased SOA yield was caused by the

353

competitive reaction of OH with SO2 and that with the VOC (Liu et al., 2017a). Some

354

studies have found that the SOA yield is not significantly affected by the particle

355

acidity at low-NOx conditions, while being enhanced at high-NOx conditions 15

ACCEPTED MANUSCRIPT 356

(Offenberg et al., 2009; Eddingsaas et al., 2012). The enhancement of SOA through

357

acid-catalyzed heterogeneous reactions could be only observed when the sulfate in the

358

particle-phase reaches a certain concentration (Offenberg et al., 2009). The sulfate

359

formation could be enhanced in the presence of NOx and, thus, more sulfate would

360

form in the particle-phase at [VOC]0/[NOx]0=3. This means that sulfate formation

361

could be enhanced at high NOx concentration in the photooxidation process, despite

362

the OH concentration and the reaction intensity of SO2 + OH were lower at

363

[VOC]0/[NOx]0=3 than at [VOC]0/[NOx]0=10.

364

Compared to the increase of particle number concentration with SO2 addition, the

365

increase of SOA yield was not significant. This was because the surface for further

366

condensation of organic vapors was sufficient and the condensation surface was no

367

longer the limiting factor for SOA formation in the presence of SO2.

368

369 370

371

Figure 3 The cyclohexene SOA yield at different initial NOx and SO2 concentrations.

Effects of RH on SOA formation

372

For experiments performed at high RH, both the particle number concentration

373

and the SOA yield were slightly lower than those at dry conditions as shown in Fig. 1. 16

ACCEPTED MANUSCRIPT 374

Although the active uptake of highly soluble compounds such as glyoxal can produce

375

additional organic compounds at high RH (White et al., 2014; Faust et al., 2017), the

376

slight decrease in SOA yield was due to the reduced formation of oligomers at high

377

RH (Nguyen et al., 2011; Zhang et al., 2011; Lewandowski et al., 2015). Jia and Xu

378

also pointed out that the SOA yield and formed oligomers decreasing with RH was

379

due to the obstruction of the oligomerization reaction of sCIs at high RH, which then

380

hindered the SOA formation (Jia and Xu, 2018).

381

In the presence of SO2, the particle number concentrations at high RH conditions

382

were significantly higher than those at dry conditions as shown in Fig. 2. Sulfate was

383

the key factor for nucleation in the photooxidation (Sipilä et al., 2010; Liu et al.,

384

2016b). The increase in the number concentration of new formed particles at high RH

385

conditions suggests that the sulfate formation was seemingly promoted at high RH.

386

This is in agreement with the results of a previous study which also pointed out that

387

sulfate was produced rapidly from SO2 in the presence of NOx in a high RH

388

environment (Wang et al., 2016). At high RH and [VOC]0/[NOx]0=10 conditions, with

389

the initial SO2 concentration increasing from 100 ppb to 200 ppb, the particle number

390

concentration decreased whereas the SOA yield continuously increased (Fig. 3). This

391

indicated that condensation became significant in SOA formation. The decrease in

392

number concentration is likely due to the increased condensation of new formed

393

particles at high RH. While the particle number concentration at 200 ppb SO2 was

394

lower than that at 100 ppb SO2 at high RH, it was still higher than those at dry

395

conditions. The new particle formation from cyclohexene photooxidation in the

396

presence of SO2 was seemingly promoted at high RH.

397

Fig. 3 shows that the change in SOA yield with SO2 concentrations at high RH

398

displays opposite trend to that at low RH when the [VOC]0/[NOx]0=10. The particle

399

acidity may affect the SOA yield at high RH by promoting the H+ formation via the

400

dissolution of sulfuric acid. The high concentration of NOx can increase the sulfate

401

concentration in the particle-phase and increase the SOA yield, but the increase in 17

ACCEPTED MANUSCRIPT 402

SOA yield at [VOC]0/[NOx]0=10 and high RH conditions is more important than that

403

at [VOC]0/[NOx]0=3 and low RH conditions. A previous study showed that the

404

presence of acid seed aerosol had no influence on SOA yields at dry conditions (Ng et

405

al., 2007b). Other studies showed that at high RH, the sulfate formation is not only

406

greatly facilitated but, also, a large amount of HONO is formed from the NOx

407

oxidation in the presence of SO2 (SO2 + 2NO2 + 2H2O → 2H+ + SO42− + 2HONO),

408

which provides additional OH radicals for the photooxidation and improves the SOA

409

yield (Wang et al., 2016). It follows that the change of RH could also influence the

410

SOA formation by changing the oxidation capacity of the system. The increase of

411

both RH and NOx can increase the particle acidity, with the effect of RH being more

412

significant. Wong et al. indicated that the existence of particle-phase water at high RH

413

enhanced the absorption of photooxidation products, e.g., organic acids (Wong et al.,

414

2015). This comparison suggests that the enhancement of particle acidity by

415

increasing the H+ concentration from the dissolution of sulfuric acid at high RH is

416

more important than the increase of sulfate at dry conditions. This has atmospheric

417

implications in the significance of SOA enhancement in the pollution events where

418

high SO2 and NOx concentrations and high RH scenarios often coexist.

419 420

Chemical composition

421

The chemical composition of SOA was important for analyzing the degree of

422

cyclohexene oxidation, and for evaluating the transformation from gas-phase to

423

particle-phase. The aerosol-phase chemical composition of cyclohexene SOA was

424

analysed on the basis of HR-MS data of negative ion mode ESI and the mass spectra

425

were recorded at a resolution of 105. The mass spectra of products formed in the

426

presence and the absence of SO2 are shown in Fig. 4. The products were quantified

427

without chromatography. Although the degree of ionization of different substances in

428

ESI-HRMS was different, the relative change of mass spectra peak heights could be 18

ACCEPTED MANUSCRIPT 429

used to analyze the change of SOA chemical composition (Blair et al., 2017). The

430

mass spectra peak of nitric acid (m/z 62) in ESI-HRMS was decreased with SO2

431

addition, which meant that the nitric acid formation was suppressed with the presence

432

of SO2. Both NO2 and SO2 could be oxidized by OH to form nitric acid and sulfuric

433

acid, respectively, and the reaction for nitric acid formation was restrained due to

434

competition. The field observation data also showed that the NO3- content in aerosols

435

always decreased when the SO42- content increased (Liu et al., 2016a; Wang et al.,

436

2016). The competition between SO2 + OH and NO2 + OH reactions might be one of

437

the important factors influencing the photooxidation reactions. It was observed that

438

the presence of SO2 could decrease the oxidation degree of organic aerosols and this

439

decrease might result from the reduction of OH due to competitive reactions with SO2

440

and NO2 (Liu et al., 2016b). In addition to the possible pathway for the formation of

441

organonitrates from the RO2 + NO reaction, which likely competed with the RO2 +

442

HO2/RO2 reaction and the RO2 autoxidation (Perring et al., 2013), the cyclohexene

443

reaction via NO3-initiated oxidation was also an effective pathway for the formation

444

of organonitrates in our system (Edwards et al., 2017; Wang et al., 2018). However,

445

there was no evidence of the presence of N-containing compounds from the main

446

peaks of HR-MS spectra, indicating low to no formation of nitrogen compounds in the

447

cyclohexene photooxidation. It is also suggested that if formed, the volatility of

448

organonitrates was too high to allow their condensation into the particle-phase. This is

449

consistent with our previous results showing that the intensities of the 1622 and 1230

450

cm-1 FTIR bands assigned to the -ONO2 group in cyclohexene SOA formation were

451

very low (Liu et al., 2017a). The presumed low amount of organonitrates might be

452

due to the low concentration of NO3 radicals and NO in the photooxidation process,

453

and the fact that the formed organonitrates compounds might decompose through OH

454

oxidation or photolysis (He et al., 2011; Suarez-Bertoa et al., 2012). Previous studies

455

also reported that organonitrates have a very short lifetime (Lee et al., 2016), as they

456

likely undergo nucleophilic attack by sulfate and form organosulfates (Hu et al., 2011; 19

ACCEPTED MANUSCRIPT 457

He et al., 2014). The formation of organonitrates is then of little importance in the

458

cyclohexene photooxidation in the presence of NOx and SO2.

459 460

Figure 4 Negative ion mode ESI mass spectrum of SOA formed from the cyclohexene

461

photooxidation in the presence and absence of SO2. The mass resolution is 105.

462 463

Compared to the peaks formed in the cyclohexene/NOx photooxidation system

464

in the absence of SO2, new peaks at m/z larger than 150 were observed in the HR-MS

465

data in the presence of SO2. Some of the new peaks were considered to be the

466

organosulfates according to the calculated masses. Table 2 lists the accurate mass

467

fittings for new peaks observed in ESI negative ion mode in the presence of SO2. The

468

organosulfates listed in Table 2 have also been detected in a previous study, with

469

relatively higher mass spectra peak intensities than in the current study (Liu et al.,

470

2017a). The abundance of organosulfates was lower at high [VOC]0/[NOx]0 ratio

471

(≈10) than that at lower [VOC]0/[NOx]0 ratio (≈5). High NOx concentrations do not

472

only promote the reactions of RO2 with NO and NO2 instead of RO2 or HO2 (Lane et

473

al., 2008), but also change the OH concentration and the [HO2]/[OH] ratio in the

474

chamber (Sarrafzadeh et al., 2016). Wang et al. pointed out that sulfate was produced

475

rapidly from SO2 in the presence of NOx and at high RH. Finally, NOx promotes the 20

ACCEPTED MANUSCRIPT 476

organosulfates formation by affecting the formation of sulfate, the precursor for

477

organosulfates formation.

478 479

Table 2 The products of the photooxidation of the cyclohexene/NOx/SO2 system Measured a

Proposed Ion

Delta b

Formula

(ppm)

195.03322

C6H11O5S-

-0.243

1

211.02828

S-

0.464

1

S-

1.171

2

S-

1.738

2

m/z

226.98641

Ion

M-Z

241.00278

480 481 482

a Sort

C5H7O8 C6H9O8

by abundance intensity.

b Delta: c

C6H11O6

RDB c

label the peak with the difference between the theoretical and measured m/z.

RDB: ring and double bond equivalent.

483 484

The secondary mass spectra (MS2) of product ions were obtained to identify the

485

structures of parent ions, [M−H]−. The new peaks were identified by MS2 and the MS2

486

spectrum for the new peaks is shown in Fig. 5. The product ions with m/z 96.96

487

(HSO4-) and m/z 79.96 (SO3−) in MS2 were the characteristic fragment ions for

488

organosulfates (Surratt et al., 2007; He et al., 2014). The organosulfates were

489

successfully identified in the particle-phase from the cyclohexene photooxidation with

490

the addition of SO2. The presence of the m/z 96.96 peak indicates that the hydrogen

491

atom is present on the carbon atom next to the one bearing the HO-SO2-O- group

492

(Attygalle et al., 2001). The loss of neutral SO3 in the m/z 211.03, 226.99 and 241.00

493

MS2 organosulfates illustrates that each of these three products has a labile proton at

494

the β position (Attygalle et al., 2001).

495

21

ACCEPTED MANUSCRIPT

496 497

Figure 5 The MS2 spectrum of the parent ion at m/z=195 (A), m/z=211 (B), m/z=227 (C),

498

m/z=241 (D).

499 500

In order to further understand the opposite trend of SOA yield with SO2 under

501

dry and wet conditions, the aerosol-phase chemical composition of the photooxidation

502

of cyclohexene at different RH values was also analysed on the basis of ESI-HRMS

503

data. The mass spectra of SOA formed under dry and wet conditions in the presence

504

of SO2 are shown in Fig. 6. The increased aerosol acidity enhanced the acid-catalyzed

505

reactive uptake and multiphase chemistry of cyclohexene oxidation products, and

506

further enhanced the SOA formation. The increase in RH resulted in a visible

507

enhancement of organosulfates peak intensities in the RH-MS spectrum. This proofed

508

that at high RH, the water content increased in the particle-phase by the hygroscopic

509

absorption of sulfuric acid, and the increased hydrogen ion provided more acidity for

510

the organosulfates formation. Especially the peak intensities of organosulfates with

511

high molecular weight or more oxygen increased greatly, demonstrating the

512

increasing importance of heterogeneous processes at high RH. The increase in SOA 22

ACCEPTED MANUSCRIPT 513

yield with SO2 under high RH conditions might be explained by the increased amount

514

of organosulfates with increasing RH. The relative change of MS peak intensities

515

could characterize the change of SOA chemical composition. As shown in Fig. 6,

516

compared to dry conditions, the fraction of compounds with low molecular weight

517

was decreased when increasing the RH in the cyclohexene/NOx photooxidation

518

system. Because the products with high molecular weight are those with low

519

volatility, they are mostly formed at high RH, which explains the increased SOA yield

520

at high RH. The increased intensity of the products with more oxygen might be due to

521

the increased OH concentration in the chamber, while the increased formation of

522

HONO from NOx oxidation at high RH might explain the increase in OH

523

concentration (Wang et al., 2016).

524

The RH condition and particle-phase water were likely key parameters

525

influencing the formation of organosulfates. The relative change of MS peak

526

intensities of organosulfates (m/z 195, 211, 226 and 241) could characterize the

527

change of the ratio of the formed organosulfates in SOA. As shown in Fig. 6, the

528

fraction of organosulfates increased at high RH in the cyclohexene/NOx

529

photooxidation system. The relative peak intensity of organosulfates with high

530

molecular weights increased with increasing RH. It suggests that the proportion of

531

organosulfates with higher molecular weights would be higher in all the formed

532

organosulfates. The fact that the major peaks are dissimilar between the low- and

533

high-RH samples suggests that the major mechanism of cyclohexene photooxidation

534

is sensitive to RH.

23

ACCEPTED MANUSCRIPT 535

536 537

Figure 6 The negative ion mode ESI mass spectrum of cyclohexene SOA at low- and high-RH

538

and in the presence of SO2.

539 540

The relative ratios of organosulfates with m/z 227 and 241 increase obviously, as

541

shown in Fig. 6. The peaks at m/z 147 and 161, which are 80 m/z lower than the

542

peaks at m/z 227 and 241, respectively, also increase at high RH. Kalberer et al.

543

showed that the species at m/z 147 and 161 are 3-hydroxy glutaric acid

544

[HOC(O)CH2CH(OH)CH2C(O)OH]

545

[HOC(O)CH2CH2CH(OH)CH2C(O)OH], respectively (Kalberer et al., 2000). Both 3-

546

hydroxy glutaric acid and 3-hydroxy adipic acid contain one hydroxyl group on the

547

secondary carbon atoms. Sulfate esterification of alcohols could also be the pathway

548

leading to the formation of m/z 227 and m/z 241 organosulfates. The sulfate was

549

added to the alcohol group by nucleophilic substitution, releasing a molecule of water

550

and forming a carbocation and the bisulfate ion (Surratt et al., 2007). The resulting

551

carbocation becomes a nucleophilic site for the lone pair of electrons on the bisulfate

552

ion. The molecular structures of the organosulfates are shown in Scheme 1. The

553

fragmentation of both m/z 227 and 241 orgnasulfates to form m/z 96.96, 79.96

and

24

3-hydroxy

adipic

acid

ACCEPTED MANUSCRIPT 554

suggested that the sulfate group was on a secondary or primary carbon atom rather

555

than the tertiary carbon atom and the tentative organosulfates structures also

556

corroborated (Pathak et al., 2004; Surratt et al., 2008). We detected a new

557

organosulfate with m/z 213 at high RH, and its formation heavily relied on water.

558

Aschmann et al. showed that 6-oxohexanoic acid [HC(O)CH2CH2CH2CH2C(O)OH]

559

was one of the cyclohexene ozonolysis products in the presence of water (Aschmann

560

et al., 2003). The aldehyde compound first oxidized to the acyl radical (Kwok and

561

Atkinson, 1995; Wang et al., 2006), which then reacted with RO2/HO2 to form the

562

acyloxy radical [R(O)O]. Hydroperoxyperoxy radicals [ROO] are subsequently

563

formed by decarboxylation of acyloxy radicals (Chacon-Madrid et al., 2013). The H-

564

shift and OH elimination would form an epoxy group (Orlando and Tyndall, 2012),

565

and the ring opening through acid-catalyzed reaction with sulfate will finally form the

566

organosulfate with m/z 213. The structural identification of cyclohexene SOA was

567

tentatively made.

25

ACCEPTED MANUSCRIPT

568 569 570

Scheme 1 Mechanism of organosulfates formation from cyclohexene photooxidation.

Conclusion

571

The combined effects of NOx, SO2 and RH on cyclohexene SOA formation was

572

investigated in laboratory chamber studies. The SOA yield and number concentration

573

increased as the [VOC]0/[NOx]0 ratio decreased from 30 to 10 and decreased as the

574

[VOC]0/[NOx]0 ratio decreased from 10 to 3. The following sequence of experimental

575

conditions was observed to enhance the new particle formation: [VOC]0/[NOx]0=10

576

and wet condition > [VOC]0/[NOx]0=3 and dry condition > [VOC]0/[NOx]0=10 and

577

dry condition. The SO2 could prompt the SOA yield under [VOC]0/[NOx]0=10 at wet

578

conditions and [VOC]0/[NOx]0=3 at dry conditions. However, SOA formation was 26

ACCEPTED MANUSCRIPT 579

suppressed at [VOC]0/[NOx]0=10 and dry conditions in the presence of SO2, due to

580

the competition between OH + SO2 and cyclohexene + OH. The RH is likely one key

581

factor affecting the nucleation and SOA formation in the presence of SO2 or sulfate.

582

High RH and high NOx conditions can favour more H+ or sulfate formation,

583

respectively and both species can promote SOA formation through acid-catalyzed

584

reactions. The increased dissolution of sulfuric acid and, consequently, more H+

585

formation at high RH seems to be more important on SOA acid-catalyzed reactions.

586

Three tentative organosulfates structures were inferred from the cyclohexene

587

photooxidation. A visible enhancement of organosulfates formation was observed at

588

high RH conditions, indicating that the acid-catalyzed formation of organosulfates at

589

high RH was more effective. The current results suggest that the increased formation

590

of organic compounds with high molecular weight would explain the increase in SOA

591

yield at high RH.

592

Acknowledgements

593

This work was supported by National Natural Science Foundation of China

594

(91644214), and Shandong Natural Science Fund for Distinguished Young Scholars

595

(JQ201705). We thank Prof. Long Jia and Prof. Yongfu Xu of the State Key

596

Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry,

597

Institute of Atmospheric Physics for ESI-Q-Orbitrap-HRMS analysis.

598 599 600

27

ACCEPTED MANUSCRIPT

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Ziemann, P.J., Atkinson, R., 2012. Kinetics, products, and mechanisms of secondary organic aerosol formation. Chem. Soc. Rev. 41, 6582-6605.

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ACCEPTED MANUSCRIPT Highlights 1. Combined effects of NOx, SO2 and RH on cyclohexene SOA formation were studied. 2. High RH and NOx promote organosulfates formation. 3. Organosulfates formation mechanism from cyclohexene was proposed.