Role of liquid water in the formation of O3 and SOA particles from 1,2,3-trimethylbenzene

Atmospheric Environment 217 (2019) 116955

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Role of liquid water in the formation of O3 and SOA particles from 1,2,3trimethylbenzene

T

Hao Luoa, Long Jiab,∗, Quan Wana, Taicheng Ana, Yujie Wanga,∗∗ a

Guangzhou Key Laboratory of Environmental Catalysis and Pollution Control, School of Environmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou, 510006, PR China b State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China

ARTICLE INFO

ABSTRACT

Keywords: Photochemistry Ozone SOA 1,2,3-Trimethylbenzene Liquid water

Aromatic volatile organic compounds (VOCs) are highly reactive in the formation of ozone (O3) and secondary organic aerosols (SOA). 1,2,3-trimethylbenzene (123-TMB) is an aromatic VOCs which is directly emitted to the atmosphere by motor vehicle exhaust emissions and the evaporation of solvents. In this study, a series of smog chamber experiments were conducted to compare the formation of O3 and SOA particles from photooxidation of 1,2,3-TMB by UV radiation, under various humidity conditions and NOx control conditions, in the presence and absence of NaCl seed particles. At a relative humidity of 86%, the hygroscopic growth factor (GF) of SOA generated from 123-TMB was 1.044. Conversely, with increasing liquid water content (LWC) from 0 to 85.0 μg m−3, O3 concentrations decreased from 468 ppb to 323 ppb and the yield of SOA increased from 3.0 to 14.7%, with initial NaCl concentrations of 11.0 ± 0.7 μg m−3. FT-IR results show that with increasing LWC, the major contributors to SOA were identified as alcohols and hydrates. These findings indicate that liquid water is an important factor in the formation of O3 and SOA.

1. Introduction In many urban areas, the ground-level O3 generated from the reactions of VOCs with NOx causes serious air pollution and presents a health risk (Kampa and Castanas, 2008; Tagaris et al., 2009). SOA also has an important effect on air quality, climate change and human health (Finlayson-Pitts and Pitts, 1997; Tagaris et al., 2009; Du et al., 2014) and accounts for a significant component of ambient aerosols. Aromatic compounds are abundant VOCs in the urban atmospheric environment, contributing to more than 50% of the SOA and O3 (Odum et al., 1997; Yin et al., 2015). 1,3,5-trimethylbenzene (135-TMB) is one of the simplest trimethylbenzene components of aromatic hydrocarbons, which are directly emitted to the atmosphere by motor vehicle exhaust emissions and the evaporation of solvents (Lan and Minh, 2013). As emissions of 135-TMB are generally concentrated in urban areas, the formation of SOA in these environments can become an acute problem. Furthermore, 135-TMB conversion products of some aldehydes and ketones also lead to atmospheric O3 and SOA formation (Odum et al., 1997; Atkinson and Arey, 2003; Ziemann and Atkinson, 2012), which are known to be carcinogenic and mutagenic, harming

∗

both human and environmental health (Sorooshian et al., 2012; Zhao et al., 2013). Many previous studies have investigated the SOA photooxidation products of 135-TMB (Smith et al., 1999; Cocker et al., 2001; Healy et al., 2008; Wyche et al., 2009; Huang et al., 2015). The SOA components generated by gas–phase oxidation of 135-TMB generated in the first few hours after its emission, followed by continual ageing processes (Rudich et al., 2007; Andreae, 2009). Methylglyoxal was found to be one of the most important oxidation intermediate products, which is easily converted to SOA (Healy et al., 2008; Wyche et al., 2009; Rickard et al., 2010; Müller et al., 2012; Lim and Turpin, 2015; Ji et al., 2018). A recent study (Ji et al., 2017) reported that cresols were generated at a higher yield than ring-opening products (e.g., methylglyoxal) during toluene photooxidation. The generation of oxidized methyl products from the benzene ring, or ring–opened products such as dialdehydes, carboxylic acids, ketoaldehydes, ketocarboxylic acids and oxocarboxylic acids, have been observed following the photooxidation of aromatic compounds (Sato et al., 2012; Huang et al., 2014). Beardsley et al. (2013) found that higher SOA yields were formed in the presence of NaCl aerosol than under seedless conditions

Corresponding author. Corresponding author. E-mail addresses: [email protected] (L. Jia), [email protected] (Y. Wang).

∗∗

https://doi.org/10.1016/j.atmosenv.2019.116955 Received 16 May 2019; Received in revised form 2 September 2019; Accepted 4 September 2019 Available online 04 September 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved.

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(in the absence of NaCl) during 135-TMB photooxidation processes. Romonosky et al. (2015) found that SOA produced by photooxidation of 135-TMB-OH/NOx, was more resilient to photolysis than α-pinene and β-pinene, despite absorbing more light. O3 generated by 135-TMB were previously measured and evaluated against the relevant master chemical mechanisms (MCMv3 and MCMv3.1) (Bloss et al., 2005; Rickard et al., 2010). Although many studies have investigated the photooxidation of TMB, limited information is available on O3 formation during trimethylbenzene oxidation processes and the mechanism for heterogeneous SOA formation remains unclear. In a previous study in Tianjin, it was found that the emission contribution rate of TMB was not to be neglected (Yu et al., 2015). Therefore, the formation potential of SOA and O3 from 123-TMB should be considered in urban atmospheric environments. There is increasing evidence that a diverse range of particles (such as mineral dust, sea salts and organic particles) promote or inhibit SOA production (Huang et al., 2013; Liu et al., 2013; Ma et al., 2013), although research results differ widely due to the different capacity of particles in absorbing in water (Edney et al., 2000; Oh and Andino, 2000; Koehler et al., 2004). Sea salt aerosol is an important factor in many atmospheric chemical processes in coastal cities. NaCl is the major component of sea salt aerosol. The GF of pure NaCl particles is 2.25-fold at relative humidity (RH) of 85% (Hansson et al., 1998). The analysis of SOA was unaffected by NaCl because of the indistinctive absorption of NaCl in FT-IR(Beardsley et al., 2013). Jia and Xu (2015) found that NaCl in the aqueous phase could sharply reduce O3 formation and increase SOA formation from ethylene-NOx UV irradiation. Similar observations have also been reported following benzene irradiation (Wang et al., 2016), with the yield of SOA increasing in accordance with the LWC of the particle. Therefore, further research on the reaction mechanisms of heterogeneous aromatic hydrocarbons in the presence of NaCl seed particles is important for the transformation between organics and sea slat aerosol in coastal urban environment. The LWC has been shown to influence O3 and SOA formation in other active VOC irradiation systems, but 123-TMB has not been studied in this context to date. Therefore, in the present study, the heterogeneous photochemical oxidation of 1,2,3-TMB with NO2 was systematically investigated in the presence of NaCl seed aerosol. This study focuses on the influence of hygroscopic growth of NaCl on O3 and SOA formation during 123-TMB-NO2 irradiation, under different humidity and NaCl concentration conditions. The results of this investigation contribute to the understanding of aromatic compound photooxidation, as well as the formation mechanisms of SOA and O3 in the atmosphere.

was introduced into the reactor via a 20 L/min zero air stream. After all reagents were injected into the chamber, the reactor was kept in dark for 1 h until the reactants were well mixed. The concentrations of 123TMB and other VOC products were measured by GC/MS (7890A/ 5975C, Agilent, USA), which was added a thermal desorber (Master TD, Dani, Italy). The adsorption tube (TA60-80mesh, CAMSCO, USA) was used to enrich the VOCs in the chamber in 3 min with a flow rate of 0.03 L min−1. Helium (99.999%) was used as carrier gas with a flow rate of 1.0 mL min−1. After thermal desorption at 300 °C for 5 min, the GC was programed at 40 °C for 4 min, followed by heating at 20 °C min−1 to 300 °C, held for 5 min at 300 °C. Mass spectrometric measurement was performed with electron ionization (EI) at 70 eV. NOx and O3 were detected using a NOx analyzer (Model 42C, Thermo Scientific, USA) and a UV photometric O3 analyzer (Model 49C, Thermo Scientific, USA), respectively. A scanning mobility particle sizer (SMPS, with a CPC 3776 and a DMA 3080, TSI, USA) was used to measure particle size distribution and mass concentration. The wall loss of particles has been taken into account in this experimental system. The particles were collected by a Dekati low-pressure impactor (DLPI, Dekati Ltd., Kangasala, Finland) on a ZnSe disk, which were determined chemical properties by a FTIR spectrometer (Nicolet iS10, Thermo Fisher, USA). After FTIR analyzing, the ions of NO3− and Cl− in the collected particles were dissolved by ultrapure water (18.2 MΩ cm) and measured by ion chromatography (Dionex ICS-900, Thermo Fisher, USA). Particle sampling was performed according to the procedure previously reported in detail (Jia and Xu, 2014a, Jia and Xu, 2014b; Wang et al., 2016). LWC of SOA was measured according to the method of the reduced Dry-Ambient Aerosol Size Spectrometer (DAASS) (Engelhart et al., 2011), which was used in the Jia and Xu (2018). Then GF of SOA was calculated. The GF of ammonium sulphate ((NH4)2SO4) was applied according to the reported value by Meyer et al. (2009), of 1.15 at a RH of 85%, as this showed good agreement with Zdanovskii-Stokes-Robinson (ZSR) predictions, indicating only minor solute-solute interactions occur at 75% and 85% RH. Therefore, the ZSR Equation (1) based on the applied experimental conditions (Meyer et al., 2009). (GFparticle)3 = ε

NaCl(GFNaCl)

3

+ε

NaNO₃(GFNaNO₃)

3

+ε

SOA(GFSOA)

3

(1)

Where, the GF particle can be estimated from the GFs of all pure components and their respective volume fractions, ε; When the RH in each individual experiment (Table 1) is 8%, 28%, 50%, 61%, 73% and 90%, the GF of NaCl (NaNO3) is calculated by E-AIM model as 1.00 (1.00), 1.00 (1.14), 1.40 (1.27), 1.51 (1.36), 1.63 (1.48) and 2.12 (2.01) respectively (Biskos et al., 2006; Hu et al., 2010). According to the ZSR relationship (Stokes and Robinson, 1966; Gysel et al., 2004), Equation (1) was used to calculate the LWC based on the experimental conditions in the present study. The SOA yield is defined as the ratio of the mass concentration of SOA (ΔM(SOA)) divided by the reacted mass concentration of 123-TMB (ΔVOC(123-TMB)):

2. Experimental methods Experiments were performed in a 1.2 m3 Teflon FEP film smog chamber at the Institute of Atmospheric Physics, Chinese Academy of Sciences (Beijing, China). A detailed description of the smog chamber and instrumentation used have been described elsewhere (Wang et al., 2016; Ge et al., 2016; Jia and Xu, 2018). Briefly, a 1.2 m3 pillow shaped reactor housed in a chamber with dimensions of 2 m × 1.2 m × 2 m (height × width × depth) was constructed with 0.13 mm FEP-Teflon film, forming a surface-to-volume (S/V) ratio of 5.8 m-1. UV light intensity measured according to the NO2 photolysis rate constant was 0.44 min−1. The temperature in the chamber was controlled to be between 303 and 305 K. Prior to each experiment, the reactor was exposed to UV light and cleaned continuously for 12 h with purified compressed air. Zero air (Model 111 and Model 1150, Thermo Scientific, USA; ≤5% RH) aerated through ultrapure water (18.2 MΩ cm; Direct-Q 3 UV, Millipore Ltd., USA) was introduced into the reactor to obtain specific RH levels ranging from 8% to 90%. NaCl seeds were produced via an aerosol neutralizer (Model 3076, TSI, USA). All photochemical reactions were initiated under 1680 W (generated using 42 blacklights (GE F40BLB, 40 W)). 3 μL of liquid 1,2,3-trimethylbenzene (99.5%, Shandong xiya Chemical Industry Co. Ltd., Shandong, China)

γ(SOA yield) = ΔM(SOA) / ΔVOC(123-TMB)

(2)

Which γ is SOA yield; ΔM(SOA) is the mass concentration of SOA; ΔVOC(123-TMB) means the mass concentrations of reacted 123-TMB. 3. Result and discussion Table 1 presents the initial conditions of 123-TMB-NOx irradiation experiments. Experiments 1–6 studied the effects of RH on O3 and SOA formation in the absence of NaCl seed salt aerosol, while experiments 7–12 studied the effects of RH on O3 and SOA formation in the presence of 11.0 ± 0.7 μg m−3 NaCl seed salt aerosol. In all experiments, the initial 123-TMB concentration was set to 368.0 ± 28.0 ppb and more than half of the initial 123-TMB had reacted at the end of each experiment. 2

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Table 1 Initial experimental conditions for 123-TMB-NOx-NaCl irradiation. Exp.

VOC /ppb

NO /ppb

NO2 /ppb

VOC/NOx ratio

RH /%

T /K

LWC0 /μg·m−3

1 2 3 4 5 6 7 8 9 10 11 12

349.1 345.4 354.7 367.2 380.2 396.2 340.0 354.7 347.6 370.0 385.5 392.0

97.6 98.6 105.3 103.5 96.4 104.5 102.7 99.4 99.3 93.9 99.6 101.5

4.2 6.9 5.4 8.6 6.7 6.6 3.8 5.9 7.9 6.4 6.9 5.8

3.43 3.27 3.20 3.28 3.69 3.57 3.19 3.37 3.24 3.69 3.62 3.65

8 30 48 63 70 87 8 28 50 61 73 90

303 304 303 305 304 303 304 304 303 305 303 303

– – – – – – 0 0 16.8 25.9 47.4 85.0

Fig. 1. The variation in gas-phase concentrations of O3 generated from 123TMB-NOx photooxidation, throughout the reaction time, in the presence and absence of NaCl. Black solid diamond: O3 concentrations under high RH conditions; black hollow diamond: O3 concentrations under low RH conditions; black solid line: O3 concentrations without NaCl; black broken line: O3 concentrations with NaCl.

Aim Assess the effect of RH in the absence of NaCl seed salt aerosol

Assess the effect of RH in the presence of NaCl seed salt aerosol

Fig. 2. Variation in maximum O3 concentrations generated under different RH conditions. Diamonds: without NaCl; Triangles: with NaCl.

relationship between O3 and RH is illustrated in Fig. 2. The O3 peak decreased linearly in accordance with RH, while the fitting line slope in the presence of NaCl seed particles was 16% larger than that in the absence of NaCl. Many reactions can potentially affect O3 reduction with varying RH conditions. Chamber reactions include the wall loss of O3 under high RH, which can lead to a reduction in O3. To determine the effect of the wall loss on peak O3 concentrations, the experiments of 123-TMB-NOx-NaCl irradiations were simulated using MCM mechanism (MCM V3.1, which is available at http://mcm.leeds.ac.uk/MCM/, Jenkin et al., 1997) with our chamber conditions. The concentrations of maximum O3 were obtained using MCM simulation in the presence and absence of wall loss rate constant. In comparison to previous studies on the photooxidations of benzene with a reported 6.6% reduction in maximum O3 concentrations (Wang et al., 2016), a 7.3% reduction in maximum O3 concentrations were observed in the present study, with increasing RH from 8% to 90%. However, wall loss of O3 was not the main cause of the reduced maximum O3 concentrations in the present study. Therefore, water is the most important factor in heterogeneous reactions, significantly affecting the formation of O3. Maximum O3 concentrations observed in the present study were in the range of 323–468 ppb with a VOC/NOx ratio of 3.43, while maximum O3 concentrations reported in previous studies were ~230 ppb with a VOC/NOx ratio of 2.25 (Rickard et al., 2010), 291 ppb with a VOC/NOx ratio of 4.6 and 525 ppb with a VOC/NOx ratio of 1.0 (Bloss et al., 2005). Compared to the previous studies (Bloss et al., 2005; Rickard et al., 2010), there are similar O3 concentration levels generated by photooxidation of 123-TMB in this study. Furthermore, differences in UV light intensity may also lead to variation in maximum O3 concentrations. To examine the effect of water on variations in maximum O3 concentrations in each experiment, the LWC was established from the

3.1. Gas phase products 3.1.1. O3 formation Fig. 1 illustrates the variations in gas-phase concentrations of O3 with increasing reaction time, during 123-TMB-NOx photooxidation under different conditions. Under conditions of low or high humidity, the generation of O3 was rapid in the first hour of each experiment, with peak concentrations of O3 achieved at between 3 and 4 h under all experimental conditions tested. However, as the RH increased from 8% to 87%, the peak O3 concentration decreased by about 27.1% in the absence of a NaCl seeds aerosol, indicating that an increase in RH can reduce the maximum concentration of O3 formed. Previous studies have found the same result with benzene and ethylbenzene (Jia and Xu, 2014a; Wang et al., 2016), attributing the reduction in maximum O3 to the transfer of R–NO2 and ONO2− containing products into the particle phase. Fig. 1 shows the effect of NaCl particles on the gas-phase species at varying RHs of 8% and 90%. Similarly, to the trend observed for the effect of NaCl, the peak concentration of O3 reduced from 468 to 323 ppb as the relative humidity increased from 8% to 90%. Meanwhile, the trend in O3 concentrations under low RH conditions followed a similar pattern, in the presence or absence of NaCl seed aerosol, which was in contrast with the tendency under high RH conditions. Under the conditions of high RH, a delay in peak O3 concentration formation was observed in the reaction with NaCl. Therefore, it appears that NaCl seeds provide carriers that can generate heterogeneous reactions in the experimental system, with heterogeneous reactions influencing the generation of O3, including peak O3 concentrations and occurring time. To understand the role of water in the heterogeneous phase, the 3

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Fig. 3. The relationship between LWC and maximum O3 concentrations, in experiments 7–12. Solid and dashed line show the line of best fit.

calculated GF according to Equation (1). When the RH increased from 8% to 90%, the LWC increased from 0 to 85.0 μg m−3, in the presence of 11.0 ± 0.7 μg m−3 NaCl. As shown in Fig. 2, there is a negative linear relationship between maximum O3 concentrations and RH, indicating the changes of maximum O3 concentrations in the absence and presence of NaCl are due to a same factor liquid water. The relationship between LWC and maximum O3 concentrations is presented in Fig. 3. When LWC0 increased from 0 to 85 μg m−3, the O3 concentration reduced by 31% in an exponential trend. It indicates that the changes of LWC lead to O3 reduction. The result further shows a significant exponential tendency between LWC and maximum O3 concentrations. Hydroxyl radical formation rates were reported to decrease with increasing RH (Jia et al., 2011; Ge et al., 2016), inhibiting free radical reactions between VOCs and hydroxyl radicals, further reducing O3 formation. The increase in RH in the tropospheric atmosphere leads to hygroscopic growth of environmental aerosols, which ultimately affect the generation of O3. The LWC plays an important role in the heterogeneous reaction, inhibiting the production of O3. 3.1.2. GC-MS results Total ion chromatograms (TIC) and EI mass spectra for the products formed by the photooxidation of 123-TMB, are presented in Fig. 4. Unreacted 123-TMB is found at retention times (RT) of 8.10 min in Fig. 4(a), with mass spectra showing a parent ion mass m/z of 120, forming three ions C9H12 m/z 91 (C7H7+), m/z 77 (C6H5+) and m/z 105 (C8H9+). As shown in Fig. 4(b), one of the products in the gas phase is 2,3-butanedione. In the present study, 2,3-butanedione was observed at a RT of 2.04 min, with the mass spectra of 2,3-butanedione showing a parent mass m/z of 86 (C2H3O+ m/z 43) (Bethel et al., 2000). Another possible product was observed at the RT of 7.70 min in Fig. 4(c), which was found to have a parent mass m/z of 126 (C8H14O+) and was identified as 6-Methyl-5-hepten-2-one, with four product ions of m/z 111 (C7H11O+), m/z 69 (C5H9+), m/z 55 (C4H7+) and m/z 43 (C2H3O+). Due to the detection limit of the instrument, with the exception of these two products, no polar substances were detected in products. As shown in Fig. 5, the relationship between yield ratio of 2,3butanedione and initial LWC in the gas phase was established, while the yield of 2,3-butanedione increased with higher LWC. As a result, more open-ring products were produced, such as 2,3-butanedione, which has previously been established as a first-generation product formed from both 1,2,3- and 1,2,4-trimethylbenzene (Bethel et al., 2000). 2,3-butanedione is a ring-opening product of 123-TMB reacted with OH radical. The concentrations of 2,3-butanedione in the different LWC conditions have a little change. However, the amount of reacted 123-TMB decreased with the LWC0 from 242.2 ppb (low LWC0) to 176.2 ppb (high

Fig. 4. EI mass spectra for the chromatographic peaks identified as: (a) 1,2,3trimethyl benzene; (b) 2,3-Butanedione; (c) 6-Methyl-5-hepten-2-one.

LWC0). Therefore, the yield of products 2,3-butanedione increased directly caused by the decrease of the amount of reacted 123-TMB. In the present study, the variation in gas phase products under different LWC conditions showed a logarithmic growth trend with the product yield increasing with the LWC and then stabilizing once the LWC reaches a certain value. Therefore, it is likely that the enhancing effect of increasing LWC reaches a definite saturation point. As the photooxidation reaction proceeds, a higher LWC will contribute to the production of 4

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Fig. 5. The relationship between the 2,3-butanedione yield ratio and initial LWC in the gas phase. Black circle: 2,3-butanedione yield ratio; Black broken line: line of best fit for 2,3-butanedione yield ratio.

Fig. 7. Volume concentration variations of particles with time under different RH in the presence of NaCl system.

hydrocarbon products, which undergo further oxidation in the gasphase. After exposure to UV radiation, 123-TMB undergoes ring opening activity and increasing LWC promotes this reaction process.

RH > 50%, which could be explained by the hygroscopic growth of NaCl. The initial LWCs were obtained using the E-AIM model according to the initial NaCl concentration and the initial RH. The LWC during the reaction process was determined on the basis of NaCl, NaNO3 and SOA concentrations. The GF for 123-TMB-NaCl (1.15) was calculated according to the ZSR Equation (1) to be lower than that of 135-TMBaqueous (NH4)2SO4 (1.31) (Cocker et al., 2001). However, the effects of organic compounds on inorganic aerosols appear to be species-specific. Cruz and Pandis (2000) found that glutaric acid and pinonic acid can promote or inhibit water sorption in the presence of ammonium sulphate ((NH4)2SO4) or NaCl, respectively. Therefore, in the present study, the organic acids presented in the product result in a lower GF than that in the other studies using ammonium sulphate as a seed particle. The relationship between each component of particles with LWC0 at the time of maximum O3 concentrations is shown in Fig. 8. Particulate matter (PM) was established as the particle mass concentrations, measured by SMPS, calculated according to Equation (3).

3.2. Particle products 3.2.1. Hygroscopic growth of particles The observed relationship between the hygroscopic GF of SOA generated from 123-TMB photooxidation and RH, is illustrated in Fig. 6. At an RH of 86%, the water content of SOA generated from 123TMB photooxidation was measured 3 h after light exposure was initiated, showing a hygroscopic GF of 1.044. The experimental data showed an exponential GF increase. With the increase in relative humidity, the hygroscopic GF growth rate increased. Wex et al. (2009) found that the hygroscopic GF of SOA generated from α-pinene was less than 1.05 in the absence of seed particles, at a RH of < 90%, while hygroscopic growth increased under RH conditions of > 98% (Wex et al., 2009). The GF established for 123-TMB in the present study was similar to that of pure 135-TMB (1.05–1.06 at a RH of 0–85%) (Cocker et al., 2001). In the present study, the GF measurement error is ± 0.02, and the GF value is similar to the previous studies, which are approximately below 1.05 (Varutbangkul et al., 2006; Wex et al., 2009; Massoli et al., 2010). Volume concentration variations of particles with time under different RH in the presence of NaCl system are presented in Fig. 7. The volume concentration was obtained after wall loss correction. As the reaction progresses, the volume concentration of particles produced from 123-TMB under different RH increased in different degrees. Meanwhile, the volume concentration increased significantly when

PM = SOA + NaCl + NaNO3+LWC

(3)

Where, concentrations of SOA were obtained by mass conservation; NaCl and NaNO3 represent the mass concentrations of NaCl and NaNO3, respectively, calculated by the measured concentrations of Cl− and NO3−; LWC represent liquid water content of particle product components. Fig. 8 shows the total particle components, including NaCl, NaNO3, SOA and LWC. The concentrations of NaCl reduced slightly, due to the

Fig. 6. The relationship between the hygroscopic growth factor of SOA generated from 123-TMB photooxidation and RH conditions.

Fig. 8. Relationship between corrected particle mass and initial LWC0. 5

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volatilization of gaseous ClNO2 generated from N2O5 and NaCl droplets in the heterogeneous reaction through the reaction (4): N2O5(g) + NaCl (aq) = ClNO2(g) + NaNO3 (Leu et al., 1995; Ravishankara, 2009). SOA and LWC were the major particle components, while NaNO3, SOA and LWC increased in accordance with LWC0 to different degrees. Under low LWC0 conditions (RH < 48%), less NaNO3 was formed, as NaNO3 was generated from the heterogeneous reaction between liquid NaCl with N2O5 or HNO3. At a RH of 48%, the specific surface area of NaCl increased and promoted the heterogeneous reaction between liquid NaCl and N2O5, as well as allowing more water-soluble substances to be transformed into SOA. Although the majority N2O5 was consumed by thermal decomposition and only a small part was consumed by hydrolysis and photolysis under dry conditions, hydrolysis become remarkable in the presence of liquid water under humid conditions. The remarkable growth of NaNO3 indicates that the surface area of aerosols increased from 0.0015 to 0.0046 m2 m-3 according to the LWC of NaCl particles, encouraging the reaction between N2O5 and liquid NaCl. Aromatic ring cleavage produces several low-volatility compounds, including glyoxal and methylglyoxal, which are widely considered to be important contributors to SOA formation (Zhang et al., 2015; Ji et al., 2017). With increasing LWC0 from 0 to 85 μg m−3, the mass concentration of SOA increased from 41.5 to 152.8 μg m−3. These results show that the amount of reacted 123-TMB was between 176.2 and 242.2 ppb, representing an increased SOA yield from 3.0% to 14.7%. In this study, the concentration of OH radical was not directly measured, but we can indirectly indicate the concentration of OH radical by the amount of reacted 123-TMB. At high humidity condition, the amount of reacted 123-TMB (176.2 ppb) is less than that at low humidity (242.2 ppb). Meanwhile, by comparing the yield of SOA at high humidity and low humidity, we found that the yield of SOA at high humidity (14.7%) was higher than that at low humidity (3.0%). The yield of SOA in the present study was at the same level as reported in previous studies on trimethylbenzene of 2.8%–7.9% (Cocker et al., 2001) and 4.1%–7.0% (Rodigast et al., 2017). The SOA yield was increased according to humidity conditions in the photooxidation of benzene (Wang et al., 2016), toluene and 135-TMB (Beardsley et al., 2013). The effect of LWC in the present study and relative humidity in previous studies, are both reflected in the water content of particles in the reaction system, indicating that the amount of SOA generated per unit reactant increases with LWC. In contrast, Cocker et al. (2001) found that the aerosol yield did not depend on the presence or absence of seed particles, the physical state (aqueous or water-free) of the seed, or RH, proposing that electrolytes in solution inhibit the formation of hemiacetals in aerosol-phase. The inhibitory effect of NaCl in aqueous solution is less than that of other seed particles under acidic hydrolysis conditions, which can be attributed to the phenomenon that the yield of SOA increases according to the LWC. Moreover, the increased LWC can dissolve water-soluble products and promote the formation of SOA in the aqueous reaction. Therefore, LWC can be considered a key factor in the growth of SOA in heterogeneous systems.

Fig. 9. Comparison of FTIR spectra under low and high LWC0 conditions and peak fitting of O–H absorption from 3800 to 2000 cm−1. Black and red lines show experimental data; green lines show four separate fitting absorption peaks. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

with NO (Hallquist et al., 1999; Barket et al., 2004; Paulot et al., 2009; Lockwood et al., 2010). RO2 is one of the most common intermediate products in VOC oxidation. It indicated that ON groups could be important products that lead to SOA formation. The other chamber studies found high molecular weight VOCs having higher ON molecular yields in particle phase (Fry et al., 2009; Matsunaga and Ziemann, 2009). But in our results, as shown in Fig. 9, the band of R–NO2 in 1636 cm−1 was increased with LWC0, which in LWC0 = 85 μg m−3 is several times greater than in LWC0 = 0 μg m−3. This indicates that the hydrolysis of R–NO2 is not dominant in the reaction process. The hydrolysis of R–NO2 is actually closely related to the functional groups it connects. R–NO2 on the tertiary butyl can rapidly hydrolysis (Liu et al., 2012). The tertiary butyl does not exist in 123-TMB in our experiment system, so there is no R–NO2 rapid hydrolysis channel. Therefore, we believed that hydrolysis of R–NO2 in our experimental system is not important . The peak for NO3− ion was almost non-visible with low LWC0, while it sharply increased with a LWC0 of 85 μg m−3. A heterogeneous reaction between liquid NaCl and N2O5 or HNO3 can explain this phenomenon, which is in agreement with the ion chromatography results for NaNO3 (section 3.2.1). The absorption peak areas for other groups increased with higher LWC0 by more than 3-fold. The initial concentration of NaCl is similar, indicating that LWC is the main factor in the increase of organic functional groups and NO3−. LWC is also a key factor in the promotion of SOA products generated from photooxidation of 123-TMB and therefore, the effect of LWC on various functional groups in SOA is discussed. In order to distinguish the stretching vibration of O–H from carboxylic acid and alcohol, the 3800-2000 cm−1 peak was fitted using a Gauss curve. Four peaks were obtained at 3437, 3264, 3002 and 2605 cm−1. The bands at 3437, 3264 and 3002 cm−1 are from the hydroxyl group of different alcohols and 2605 cm−1 is from the hydroxyl of carboxylic acid. Fig. 10 shows the relationship between the absorption peak and LWC0 for O–H (3264 cm−1), COOH (2605 cm−1), C]O (1721 cm−1), NO3− (1358 cm−1) and C–OH groups (1075 cm−1), showing that all functional groups increased according to LWC0. The FT-IR results in the present study confirm that particle phase R–NO2 and ONO2− products are the main factors explaining the reduction in O3 peak concentration. N2O5 hydrolysis was enhanced by increased RH conditions, resulting in an increase in NO3− (Fig. 10) and a reduction in O3 concentrations. As LWC0 increased from 0 to 85 μg m−3, the relative intensity of O–H groups increased by 3.2-fold, while COOH, C]O, NO3− and C–O were

3.2.2. FT-IR results for SOA The infrared spectra of SOA produced from the photooxidation of 123-TMB-NOx-NaCl under low and high humidity conditions shows an increase in LWC0 from 0 to 85 μg m−3, which are presented in Fig. 9. The broad peak observed between 3800 and 2000 cm−1 was attributed to O–H stretching of carboxylic acids and alcohols (Jang and Kamens, 2001; Coury and Dillner, 2008); The band at ~1721 cm−1 was assigned to the stretching of C]O in carboxylic acids, ketones and aldehydes; The band at 1636 cm−1 was possibly be attributed to R–NO2; The band at 1358 cm−1 and the sharp peak at 825 cm−1 may be attributed to NO3− ion (Miller and Wilkins, 1952); The band at 1075 cm−1 can possibly be attributed to the C–OH group of alcohols (Jang and Kamens, 2001; Jang et al., 2002). The organonitrate (ON) molecules are important production oxidized from VOCs in the presence of NOx. ON groups (ONO2) form in an important reaction of peroxy radicals (RO2) 6

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Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 41603072, No. 41375129, No. 41875166), the Guangdong Natural Science Foundation (No. 2016A030313695) and the National Key R&D Program of China (2017YFC0210005). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.atmosenv.2019.116955. References Andreae, M.O., 2009. A new look at aging aerosols. Science 326, 1493–1494. Atkinson, R., Arey, J., 2003. Atmospheric degradation of volatile organic compounds. Chem. Rev. 103, 4605–4638. Barket, D.J., Grossenbacher, J.W., Hurst, J.M., Shepson, P.B., Olszyna, K., Thornberry, T., Carroll, M.A., Roberts, J., Stroud, C., Bottenheim, J., Biesenthal, T., 2004. A study of the NOx dependence of isoprene oxidation. J. Geophys. Res. 109, D11310. Beardsley, R., Jang, M., Ori, B., Im, Y., Delcomyn, C.A., Witherspoon, N., 2013. Role of sea salt aerosols in the formation of aromatic secondary organic aerosol, yields and hygroscopic properties. Environ. Chem. 10, 167–177. Bethel, H.L., Atkinson, R., Arey, J., 2000. Products of the gas-phase reactions of oh radicals with p-xylene and 1,2,3- and 1,2,4-trimethylbenzene: effect of NO2 concentration. J. Phys. Chem. A 104, 8922–8929. Biskos, G., Russell, L.M., Buseck, P.R., Martin, S.T., 2006. Nanosize effect on the hygroscopic growth factor of aerosol particles. Geophys. Res. Lett. 33, L07801. Bloss, C., Wagner, V., Bonzanini, A., Jenkin, M.E., Wirtz, K., Martin-Reviejo, M., Pilling, M.J., 2005. Evaluation of detailed aromatic mechanisms (MCMv3 and MCMv3.1) against environmental chamber data. Atmos. Chem. Phys. 5, 623–639. Cocker III, D.R., Mader, B.T., Kalberer, M., Flagan, R.C., Seinfeld, J.H., 2001. The effect of water on gas–particle partitioning of secondary organic aerosol: II. m-xylene and 1,3,5-trimethylbenzene photooxidation systems. Atmos. Environ. 35, 6073–6085. Coury, C., Dillner, A.M., 2008. A method to quantify organic functional groups and inorganic compounds in ambient aerosols using attenuated total reflectance ftir spectroscopy and multivariate chemometric techniques. Atmos. Environ. 42, 5923–5932. Cruz, C.N., Pandis, S.N., 2000. Deliquescence and hygroscopic growth of mixed inorganicorganic atmospheric aerosol. Environ. Sci. Technol. 34, 4313–4319. Du, Z.J., Mo, J.H., Zhang, Y.P., 2014. Risk assessment of population inhalation exposure to volatile organic compounds and carbonyls in urban China. Environ. Int. 73, 33–45. Edney, E.O., Driscoll, D.J., Speer, R.E., Weathers, W.S., Kleindienst, T.E., Li, W., Smith, D.F., 2000. Impact of aerosol liquid water on secondary organic yields of irradiated toluene/propylene/NOx/(NH4)2SO4/air mixtures. Atmos. Environ. 34, 3907–3919. Engelhart, G.J., Hildebrandt, L., Kostenidou, E., Mihalopoulos, N., Donahue, N.M., Pandis, S.N., 2011. Water content of aged aerosol. Atmos. Chem. Phys. 11, 911–920. Finlayson-Pitts, B.J., Pitts, J.N., 1997. Tropospheric air pollution: ozone, airbornetoxics, polycyclic aromatic hydrocarbons, and particles. Science. 276, 1045–1052. Fry, J.L., Kiendler-Scharr, A., Rollins, A.W., Wooldridge, P.J., Brown, S.S., Fuchs, H., Dubé, W., Mensah, A., dal Maso, M., Tillmann, R., Dorn, H.-P., Brauers, T., Cohen, R.C., 2009. Organic nitrate and secondary organic aerosol yield from NO3 oxidation of beta-pinene evaluated using a gas-phase kinetics/aerosol partitioning model. Atmos. Chem. Phys. 9, 1431–1449. Ge, S.S., Xu, Y.F., Jia, L., 2016. Secondary organic aerosol formation from ethyne in the presence of NaCl in a smog chamber. Environ. Chem. 13, 699–710. Gysel, M., Weingartner, E., Nyeki, S., Paulsen, D., Baltensperger, U., Galambos, I., Kiss, G., 2004. Hygroscopic properties of water-soluble matter and humic-like organics in atmospheric fine aerosol. Atmos. Chem. Phys. 4, 35–50. Hallquist, M., Wangberg, I., Ljungstrom, E., Barnes, I., Becker, K.H., 1999. Aerosol and product yields from NO3 radical-initiated oxidation of selected monoterpenes. Environ. Sci. Technol. 33, 553–559. Hansson, H.-C., Rood, M.J., Koloutsou-Vakakis, S., Hämeri, K., ORSINI, D., Wiedensohler, A., 1998. NaCl aerosol particle hygroscopicity dependence on mixing with organic compounds. J. Atmos. Chem. 31, 321–346. Healy, R.M., Wenger, J.C., Metzger, A., Duplissy, J., Kalberer, M., Dommen, J., 2008. Gas/particle partitioning of carbonyls in the photooxidation of isoprene and 1,3,5trimethylbenzene. Atmos. Chem. Phys. 8, 3215–3230. Hu, D.W., Qiao, L.P., Chen, J.M., Ye, X.N., Yang, X., Cheng, T.T., Fang, W., 2010. Hygroscopicity of inorganic aerosols: size and relative humidity effects on the growth factor. Aerosol and Air Quality Research 10, 255–264. Huang, M.Q., Hao, L.Q., Gu, X.J., Hu, C.J., Zhao, W.X., Wang, Z.Y., Fang, L., Zhang, W.J., 2013. Effects of inorganic seed aerosols on the growth and chemical composition of secondary organic aerosol formed from OH initiated oxidation of toluene. J. Atmos. Chem. 70, 151–164. Huang, M.Q., Hu, C.J., Guo, X.Y., Gu, X.J., Zhao, W.X., Wang, Z.Y., Fang, L., Zhang, W.J., 2014. Chemical composition of gas and particle–phase products of OH–initiated oxidation of 1,3,5–trimethylbenzene. Atmos. Pollute. Res. 5, 73–78. Huang, M.Q., Lin, Y.H., Huang, X.Y., Liu, X.Q., Guo, X.Y., Hu, C.J., Zhao, W.X., Gu, X.J., Fang, L., Zhang, W.J., 2015. Experimental study of particulate products for aging of 1,3,5trimethylbenzene secondary organic aerosol. Atmos. Pollute. Res. 6, 209–219. Jang, M., Kamens, R.M., 2001. Characterization of secondary aerosol from the

Fig. 10. Relationship between major absorption peaks from FT-IR spectra and LWC0 under different RH conditions, in the presence of NaCl.

enhanced by 6-, 3.9-, 11.9- and 5.2-fold, respectively. The increments in NO3− and COOH with increasing LWC0 were most significant, which are the main components of SOA. As shown in Fig. 10, the growth rate continued to decline as the LWC0 increased, as the water soluble organic products, such as glyoxal, do not increase with a higher LWC0. Soluble products continuously dissolve, causing glyoxal in the gas phase to decrease with increasing LWC0, though glyoxal can easily dissolve in particle water, affecting the growth rate of NO3−. The strength of NO3− functional groups grows slowly, which is consistent with the trend in O3 concentrations shown in Fig. 4. It verifies that N2O5 and NaCl in the liquid reaction are the main cause of O3 reduction with increased LWC0. These findings show that product variations in the gas phase (section 3.1) are comparable to those observed in the particle phase, further supported by the similar phenomenon observed for SOA generation by Wang et al. (2016). 4. Conclusion In this study, LWC was found to be the critical factor leading to a decrease in peak O3 concentrations under different RH conditions. The peak O3 concentration decreased exponentially by 31.0% as the LWC0 increased from 0 to 85.0 μg m−3, due to the hydrolysis reaction of N2O5 with liquid NaCl being the major process under high LWC conditions. Furthermore, LWC appears to also be a key factor in the enhanced formation of SOA. As the LWC0 increased from 0 to 85.0 μg m−3, it was observed that the relative intensities of organic groups O–H, C]O, C–OH and NO3− ion were all increased. The band representing O–H groups formed from COOH showed the biggest increase of all groups with increasing LWC, as alcohols and hydrates are the main contributors to SOA under high LWC conditions. The yields of SOA from 123-TMB were increased from 3.0 to 14.7% as the LWC0 increased from 0 to 85 μg m−3. Due to the complexity of the atmospheric environment, it is difficult to estimate the environmental relevance of the experiments results for the mechanism of SOA and O3 formation in the presence of particle water and precursor seed particles. Therefore, it is necessary to combine these findings with the field observations. In addition, the impacts of seed precursors with different properties such as hygroscopicity and acidity, on the generation of aerosols, require further exploration. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7

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