Atmos. Chem. Phys., 17, 13329–13343, 2017 https://doi.org/10.5194/acp-17-13329-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Photooxidation of cyclohexene in the presence of SO2: SOA yield and chemical composition Shijie Liu1,2,3, Long Jia2, Yongfu Xu2, Narcisse T. Tsona1, Shuangshuang Ge2, and Lin Du3,1,2 1Environment Research Institute, Shandong University, Jinan, 250100, China 2State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China 3Shenzhen Research Institute, Shandong University, Shenzhen, 518057, China Correspondence to: Lin Du ([email protected]) and Yongfu Xu ([email protected]) Received: 12 January 2017 – Discussion started: 15 February 2017 Revised: 28 September 2017 – Accepted: 8 October 2017 – Published: 9 November 2017 Abstract. Secondary organic aerosol (SOA) formation from 1 Introduction a cyclohexene = NOx system with various SO2 concentra- tions under UV light was investigated to study the effects Alkenes are widely emitted from biogenic and anthropogenic of cyclic alkenes on the atmospheric environment in polluted sources (Kesselmeier et al., 2002; Chin and Batterman, urban areas. A clear decrease at first and then an increase 2012), and their gas-phase oxidation reactions with OH, in the SOA yield was found with increasing SO2 concen- NO3, or O3 are among the most important processes in the trations. The lowest SOA yield was obtained when the ini- atmosphere (Atkinson, 1997; Stewart et al., 2013; Paulson tial SO2 concentration was in the range of 30–40 ppb, while et al., 1999). Reactions of ozone with alkenes are an impor- higher SOA yield compared to that without SO2 could not tant source of free radicals in the lower atmosphere and thus be obtained until the initial SO2 concentration was higher highly influence the oxidative capacity of the atmosphere than 85 ppb. The decreasing SOA yield might be due to the (Paulson and Orlando, 1996). Some products of these reac- fact that the promoting effect of acid-catalysed reactions on tions have sufficiently low vapour pressure, allowing them to SOA formation was less important than the inhibiting effect condense with other gaseous species, and contribute to the of decreasing OH concentration at low initial SO2 concentra- secondary organic aerosol (SOA) mass (Sarwar and Corsi, tions, caused by the competition reactions of OH with SO2 2007; Sakamoto et al., 2013; Nah et al., 2016; Kroll and Se- and cyclohexene. The competitive reaction was an impor- infeld, 2008; Hallquist et al., 2009). SOA formation from the tant factor for SOA yield and it should not be neglected in oxidation of VOCs has been receiving significant attention photooxidation reactions. The composition of organic com- in recent years due to its large implication in the formation pounds in SOA was measured using several complemen- of atmospheric fine particulate matter (Jimenez et al., 2009). tary techniques including Fourier transform infrared (FTIR) SOA has significant impacts on human health (Pope III and spectroscopy, ion chromatography (IC), and Exactive Plus Dockery, 2006), air quality (Kanakidou et al., 2005; Jaoui et Orbitrap mass spectrometer equipped with electrospray in- al., 2012; McFiggans et al., 2006), and global climate change terface (ESI). We present new evidence that organosulfates (Hansen and Sato, 2001; Adams et al., 2001; Pokhrel et al., were produced from the photooxidation of cyclohexene in 2016). the presence of SO2. Although cyclic alkenes widely exist in the atmosphere, their gas-phase oxidation has received less attention than that of linear or branched alkenes (Sipilä et al., 2014). Cyclohexene is an important industrial chemical (Sun et al., 2013) and is also widespread in urban areas (Gros- jean et al., 1978). Cyclohexene has been extensively studied as a monoterpene surrogate for inferring oxidation mecha- Published by Copernicus Publications on behalf of the European Geosciences Union. 13330 S. Liu et al.: Photooxidation of cyclohexene in the presence of SO2 nisms and aerosol formation characteristics, because it has 2008; Hatch et al., 2011). Despite the well-recognized pres- the basic structural unit in abundant biogenic monoterpenes ence of organosulfates in SOA, their formation and trans- and sesquiterpenes (Carlsson et al., 2012; Keywood et al., formation processes can be complex and varied, depending 2004b). The rate constants for gas-phase reactions of cyclo- on the nature of the original organic compound involved. hexene with OH, O3, and NO3 were measured at room tem- Extensive studies on their formation have been performed perature to be (6.4 ± 0.1)× 10−11, (8.1 ± 1.8)× 10−17, and and several mechanisms based on a variety of reactions (5.4 ± 0.2)× 10−13 cm3 molecule−1 s−1, respectively (Stew- have been proposed. Using nuclear resonance techniques, art et al., 2013; Aschmann et al., 2012), and a correlation isoprene-derived epoxides formed during isoprene photoox- between the logarithm of the rate constants and the molecu- idation reactions were found to be important intermediates lar orbital energies for simple cyclic alkenes was observed. for organonitrate and organosulfate formation via potential The effect of pressure and that of the presence of SO2 on SOA reactions (Darer et al., 2011; Hu et al., 2011). The au- the formation of stable gas-phase products and SOA from thors further found that organonitrates could easily be trans- the ozonolysis of cyclohexene were investigated (Carlsson formed to organosulfates during hydrolysis in the presence et al., 2012). It was found that the collisional stabilization of sulfate. Some studies also showed that 2-methyl-3-buten- of initial clusters was an important aspect for SOA forma- 2-ol (MBO), due to its emissions that are larger than isoprene tion processes involving sulfuric acid (H2SO4/ and organic in some regions (Baker et al., 1999), is an important precur- compounds. The effect of the structure of the hydrocarbon sor for organosulfates and SOA in the atmosphere through parent molecule on SOA formation was investigated for a se- its reactions with OH under NO and aerosol acidity condi- ries of cyclic alkenes and related compounds (Keywood et tions and from acid-catalysed reactive uptake of MBO-based al., 2004b), and the SOA yield was found to be a function of epoxides formed during MBO photooxidation (Mael et al., the number of carbons present in the cyclic alkenes ring. The 2015; Zhang et al., 2012, 2014). Organosulfate formation relative SOA yields from ozonolysis of cyclic alkenes can be was also found from oxidation of hydroxyhydroperoxides quantitatively predicted from properties of the parent hydro- (Riva et al., 2016) and from heterogeneous reactions of SO2 carbons, like the presence of a methyl group and an exocyclic with selected long-chain alkenes and unsaturated fatty acids double bond. (Passananti et al., 2016). SO2, one of the most important inorganic pollutants in ur- Reactions with sulfates or H2SO4 were the main for- ban areas, plays an important role in SOA formation (Wang mation processes of organosulfates. Qualitative analyses of et al., 2005; Lonsdale et al., 2012; Liu et al., 2016). Seasonal organosulfates in SOA have been gaining more attention and variations of SO2 concentrations were found to be consis- development in recent years (Lin et al., 2012; Shalamzari tent with seasonal variations of PM2:5 concentration (Cheng et al., 2013; Staudt et al., 2014). Riva et al. (2015) investi- et al., 2015). Smog chamber simulations have indicated that gated the formation of organosulfates from photooxidation of SO2 could enhance the formation of SOA from the oxidation polycyclic aromatic hydrocarbons and found that, in the pres- of VOCs under acidic conditions by increasing aerosol acid- ence of sulfate aerosol, this photooxidation was a hitherto un- ity and ammonium sulfate aerosol formation (Edney et al., recognized source of anthropogenic secondary organosulfur 2005; Liu et al., 2016; Attwood et al., 2014). Anthropogenic compounds (Riva et al., 2015). A more complete structural SO2 emissions can impact new particle formation, and SOA characterization of polar organosulfates that originate from composition (Lonsdale et al., 2012). isoprene SOA was performed (Shalamzari et al., 2013), and Although the existence of organosulfates in ambient an organosulfate related to methyl vinyl ketone and minor aerosols was first observed in 2005 (Romero and Oehme, polar organosulfates related to croton aldehyde were identi- 2005), proper identification of these aerosols was done 2 fied. However, there have been no reports on the yield and years later. In a series of laboratory chamber studies, it was chemical composition of SOA obtained from photooxidation shown that organosulfates present in ambient aerosols col- of cyclohexene in the presence of SO2. lected from various locations mostly originate from acid- In the present work, we investigated the yields and chemi- catalysed reactions of SOA formed from photooxidation of cal composition of SOA during cyclohexene photooxidation α-pinene and isoprene (Surratt et al., 2007). Recently, dif- under different SO2 concentrations conditions. A better un- ferent kinds of organosulfates have been observed in SOA derstanding of the magnitude and chemical composition of around the world, and organosulfates have been identified SOA from different SO2 concentrations will contribute to a as a group of compounds that have an important contribu- more accurate SOA prediction from anthropogenic sources tion to the total amount of SOA in the atmosphere (Sur- and provide valuable information related to air pollution in ratt et al., 2008; Froyd et al., 2010; Kristensen and Glasius, urban environments. 2011; Tolocka and Turpin, 2012; Wang et al., 2016). Labo- ratory chamber studies showed that OH = NOx = O3-initiated reactions of BVOCs, such as isoprene, α-pinene, β-pinene, and limonene with sulfates or sulfuric acid are the main processes for organosulfate formation (Surratt et al., 2007, Atmos. Chem. Phys., 17, 13329–13343, 2017 www.atmos-chem-phys.net/17/13329/2017/ S.