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Terpenylic acid and nine-carbon multifunctional compounds formed during the aging of β-pinene ozonolysis secondary organic aerosol
Atmospheric Environment xxx (2015) 1e9
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Terpenylic acid and nine-carbon multifunctional compounds formed during the aging of b-pinene ozonolysis secondary organic aerosol Kei Sato a, b, *, Tianyu Jia b, Kiyoshi Tanabe a, Yu Morino a, Yoshizumi Kajii a, b, Takashi Imamura a a b
National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-8506, Japan Graduate School of Global Environmental Studies, Kyoto University, Nihonmatsucho, Sakyo-ku, Kyoto 606-8316, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SOA formed from b-pinene ozonolysis was exposed to OH radicals. Terpenylic acid and C9H14O5 in SOA particles increased after OH exposure. Nopinone oxidation will contribute to terpenylic acid and C9H14O5 formation. Formation mechanisms of terpenylic acid and C9H14O5 were suggested.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2015 Received in revised form 16 August 2015 Accepted 18 August 2015 Available online xxx
Recent field and laboratory studies suggest that forest aerosol particles contain more highly functionalized organic molecules than pinonic acid, a traditional molecular maker of secondary organic aerosol (SOA) particles. To investigate the reaction mechanisms during the aging of biogenic SOAs, the gases and particles formed from the ozonolysis of b- and a-pinene were exposed to OH radicals in a laboratory chamber. The particle samples were collected before and after OH exposure for analysis by liquid chromatography-negative electrospray ionization time-of-flight mass spectrometry. Pinic acid and terpenylic acid were abundant products in both b- and a-pinene ozonolysis SOA particles. Terpenylic acid and products with m/z 201.08 present in b-pinene SOA particles increased upon exposing SOA to OH radicals, whereas 3-methyl-1,2,3-butanetricarboxylic acid present in a-pinene SOA particles increased upon exposing SOA to OH radicals. The products with m/z 201.08 were suggested to be C9H14O5 compounds. Similar C9H14O5 compounds and terpenylic acid were also detected in SOA particles formed from the photooxidation of nopinone, a major first-generation product of b-pinene ozonolysis. The OHinitiated oxidation of nopinone will contribute to the formation of terpenylic acid and C9H14O5 compounds during the aging of b-pinene SOA. A formation mechanism for terpenylic acid via gas-phase diaterpenylic acid formation followed by self-dehydration in the condensed phase was suggested. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Biogenic volatile organic compound Secondary organic aerosol Environmental chamber Molecular marker Multiphase chemistry
Abbreviations: EIC, Extracted-ion chromatogram; FT-IR, Fourier transform-infrared spectroscopy; LCeTOF, Liquid chromatographyetime-of-flight mass spectrometry; MBTCA, 3-methyl-1,2,3-butanetricarboxylic acid; SMPS, Scanning mobility particle sizer; SOA, Secondary organic aerosol. * Corresponding author. National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-8506, Japan. E-mail addresses: [email protected] (K. Sato), [email protected] (T. Jia), [email protected] (K. Tanabe), [email protected] (Y. Morino), kajii.yoshizumi.7e@kyoto-u. ac.jp (Y. Kajii), [email protected] (T. Imamura). http://dx.doi.org/10.1016/j.atmosenv.2015.08.047 1352-2310/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Sato, K., et al., Terpenylic acid and nine-carbon multifunctional compounds formed during the aging of bpinene ozonolysis secondary organic aerosol, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.08.047
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1. Introduction Pinene is a major monoterpene emitted from vegetation into the atmosphere; the global emissions of a- and b-pinene are evaluated to be 66 and 19 Tg/yr, respectively (Guenther et al., 2012). The atmospheric oxidation of a- and b-pinene contributes to the formation of secondary organic aerosol (SOA) particles; these can adversely affect the climate, human health, and visibility (Kroll and Seinfeld, 2008; Hallquist et al., 2009). For better modeling atmospheric SOA particle formation, it is necessary to understand the aging of SOA (Volkamer et al., 2006). Szmigielski et al. (2007) reported that 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) formed from the OH-initiated oxidation of a-pinene is identified in forest aerosols. MBTCA is more highly functionalized than traditional SOA molecular markers such as pinonic acid and pinic acid; this suggests that the aging of SOA influences the molecular-level composition of ambient SOA particles. Early studies investigating the aging of a-pinene ozonolysis SOA reported little change in the chemical composition of SOA particles after the consumption of the reactant pinene (Qi et al., 2010), probably because gaseous and particulate oxidation products formed from monoterpenes with one double bond barely react with ozone (Ng et al., 2006). Recently, Donahue et al. (2012) conducted a chamber study by exposing monoterpene ozonolysis SOA to OH radicals. They used an online atmospheric pressure chemical ionization mass spectrometer and showed that the MBTCA signal increased significantly after exposing a-pinene SOA to OH radicals. Müller et al. (2012) suggested the formation mechanisms of MBTCA from the OH-initiated oxidation of pinonic acid in the gas phase. The difference in the gas-phase mechanisms between the oxidation of a- and b-pinene might affect the particulate products formed during the aging of SOA. The differences in the gas-phase mechanisms between a-pinene (endocyclic terpene) and bpinene (exocyclic terpene) have been studied extensively (Hatakeyama et al., 1989, 1991; Yu et al., 1999; Jaoui and Kamens, 2001, 2003; Docherty and Ziemann, 2003; Jenkin, 2004). A major first-generation product formed from a-pinene ozonolysis is pinonic acid, a ring-opening product,
(R1)
whereas a major first-generation product formed from b-pinene ozonolysis is nopinone, a ring-retaining product,
(R2) In this study, we investigate the aging of a- and b-pinene ozonolysis SOA by employing a chamber technique similar to that used by Donahue et al. (2012). The offline SOA particle samples obtained before and after OH exposure were analyzed by liquid chromatographyetime-of-flight mass spectrometry (LCeTOF). This study aims to characterize the particulate products formed during the aging of b-pinene SOA and to suggest the reaction mechanisms during the aging of SOA. 2. Experimental methods 2.1. Chamber experiments The experimental procedures were similar to those described elsewhere (Sato et al., 2012). An evacuable Teflon-coated 6-m3
chamber was used for the experiments. The temperature of the chamber was maintained at 298 ± 2 K. Purified and filtered air was used for the experiments. The total pressure of purified air was 1019 ± 10 hPa. The relative humidity of purified air was <1%. The concentrations of the gaseous compounds were measured using a Fourier transform-infrared spectrometer (FT-IR; optical length: 221.5 m, Model Nexus 670, Thermo Fisher Scientific, Waltham, MA, USA). The particle size distribution was measured using a scanning mobility particle sizer (SMPS; Model 3934, TSI Inc., Shoreview, MN, USA). Table 1 summarizes the experimental conditions in each run. The ozonolysis experiments of a- and b-pinene (runs 1e5) were conducted by adding ozone (580e2770 ppb) to a pinene-air mixture (141e307 ppb) in the chamber. No hydroxyl radical scavenger was used. After the pinene concentration decreased below 20 ppb in each ozonolysis experiment, SOA particles formed from pinene ozonolysis were collected on a Teflon membrane filter (Fluoropore, diameter: 47 mm, pore size: 1 mm; Sumitomo Electric, Osaka, Japan). The flow rate for filter sampling was 16.7 L min1. The filter sampling duration was 12 min. Methyl nitrite (902e1024 ppb) was then added to the reaction mixture. Methyl nitrite was prepared by adding aqueous sulfuric acid to a mixture of sodium nitrite and methanol. The reaction mixture was irradiated using filtered light from xenon arc lamps (wavelength: >300 nm, total power: 19 kW). Hydroxyl radicals were produced by the following reaction sequence: CH3ONO þ hn / CH3O þ NO,
(R3)
CH3O þ O2 / CH2O þ HO2,
(R4)
HO2 þ NO / OH þ NO2.
(R5)
A second filter sampling was conducted 1 h after the start of irradiation. A photooxidation experiment of nopinone (run 6) was conducted by irradiating a mixture of nopinone (143 ppb) and methyl nitrite (1074 ppb). SOA particles formed by the photooxidation of nopinone were collected on a filter 1 h after the start of irradiation. 2.2. LCeTOF analysis A 20-mL aliquot of adipic acid-13C6 methanol solution (32 ng/mL) was added to each filter sample as an internal standard. The filter sample was then sonicated in 5-mL methanol for 30 min. The extract of the filter sample was concentrated to near dryness under a gentle stream of nitrogen. The concentrated extract was dissolved in 0.5-mL formic acid-water-methanol solution (v/v/v ¼ 0.05/ 99.95/100) and was used as an analytical sample. A 10-mL aliquot of each analytical sample was loop-injected into a LCeTOF instrument (Agilent Technologies, Palo Alto, CA, USA). An octadecyl column (Model ODS-3, inner diameter: 3.0 mm, length: 150 mm, particle size: 5 mm, GL Science Inc., Tokyo, Japan) was used for sample separation. The mobile phases used for liquid chromatography were 0.05% formic acid aqueous solution and methanol. The total flow rate of the mobile phases was 0.4 mL min1. The column temperature was maintained at 298 K. The methanol concentration was initially 5%, increased to 90% for 30 min, maintained at 90% for 15 min, and maintained at 5% for 15 min. The sample compounds were analyzed by a negative mode electrospray ionization time-of-flight mass spectrometer with a mass accuracy of <3 ppm. The extracted-ion chromatograms (EICs) of the twelve masses shown in Table 2 were investigated in this study. The peak intensity of each EIC was corrected using the internal standard intensity and was then converted into the pinonic
Please cite this article in press as: Sato, K., et al., Terpenylic acid and nine-carbon multifunctional compounds formed during the aging of bpinene ozonolysis secondary organic aerosol, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.08.047
K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
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Table 1 Experimental conditions and concentrations of aerosol produced in this study. Run no.
Reaction
1 2 3 4 5 6
b b b a a
a b c
[Reactant]0/ppb
-Pinene/O3c -Pinene/O3c -Pinene/O3c -Pinene/O3c -Pinene/O3c
144 141 284 307 154 143
Nopinone/CH3ONO þ light
[O3]0/ppb
620 2620 2770 670 580 e
[SOA]/mg m3 Period 1a
Period 2b
153 177 486 571 210 e
290 224 572 594 246 219
Filter sampling period before OH exposure. Filter sampling period after OH exposure. Methyl nitrite was added and irradiated after most pinene reacted with O3.
acid-equivalent concentration using calibration curves measured for adipic acid-13C6 and pinonic acid. Blank filters were prepared by sampling purified air in the chamber through Teflon filters. No impurities were identified by the LCeTOF analysis of the blank filters.
3. Results and discussion 3.1. Time series and OH exposure Fig. 1 shows the typical time series of the b-pinene, nopinone, and SOA particle concentrations (run 2). Ozone was added to the reaction mixture at 0 min. The b-pinene concentration decreased and the nopinone concentration increased by the reaction of bpinene with ozone. The FT-IR technique overestimates the nopinone concentration by a factor of 1.7e2.5 owing to interference from the absorptions of other products formed from b-pinene oxidation (Yu et al., 1999). The b-pinene concentration decreased to the detection limit (8 ppb) at 32 min. The first filter sampling was conducted between 56 and 68 min (period 1). Methyl nitrite was added and lamps were turned on at 74 min. The nopinone concentration decreased and the SOA particle concentration increased by the reactions of oxidation products with OH radicals. The second filter sampling was conducted between 134 and 146 min (period 2). Table 1 summarizes the SOA particle mass concentrations observed during the filter sampling periods. The SOA particle mass concentration was calculated from the SMPS volume concentration assuming a particle density of 1 g cm3. No wall loss correction was performed. The OH exposure in this study was estimated by using the methyl nitrite, formaldehyde, NO2, and nopinone concentrations
Fig. 1. Time series of gas and aerosol concentrations measured during run 2 with bpinene; periods 1 and 2 show filter sampling periods.
measured by FT-IR. First, the time series of the OH concentration was estimated assuming a pseudo steady-state approximation:
½OH ¼ k1 ½CH3 ONO=ðk2 ½HCHO þ k3 ½NO2 þ k4 ½nopinoneÞ; (1) where k1 is the rate constant for the photolysis of methyl nitrite; k2, the rate constant for the OH þ HCHO reaction; k3, the rate constant for the OH þ NO2 reaction; and k4, the rate constant for the OH þ nopinone reaction. As the methyl nitrite concentration (~1 ppm) was higher than the initial b-pinene concentration (141e284 ppbv), the reactions of OH radical with b-pinene oxidation products other than nopinone will be negligible. The rate constant for the photolysis of methyl nitrite was determined to be (6.9 ± 2.7) 104 s1 under the present conditions. Literature
Table 2 Products investigated in this study; references: 1Yasmeen et al. (2010), 2Ma and Marston (2008), 3Claeys et al. (2009), 4Yu et al. (1999), 5this work, 6Kahnt et al. (2014), and 7Szmigielski et al. (2007). m/z
Molecular formula
Suggested product(s)
157.050 169.086 171.065 183.101 185.080 199.104 201.076
C7H10O4 C9H13O3 C8H12O4 C10H16O3 C9H14O4 C10H16O4 C9H14O5
203.055 217.071
C8H12O6 C9H14O6
343.139 357.156 367.175
C16H24O8 C17H26O8 C19H28O7
Terebic acid1 Pinalic-3-acid,2 pinalic-4-acid2 Terpenylic acid3 Pinonic acid4 Pinic acid,4 hydroxypinalic-3-acid2 10-Hydroxypinonic acid4 C9-carbonyl-dicarboxylic acid,5,6 C9-hydroxy-dicarbonyl-carboxylic acid5 3-Methyl-1,2,3-butanetricarboxylic acid (MBTCA)7 C9-tricarboxylic acid,5,6 C9-hydroxy-carbonyl-dicarboxylic acid5 Ester from terpenylic acid and diaterpenylic acid1 Ester from pinic acid and diaterpenylic acid1 Unknown1
Molecular structures and estimated saturated concentrations of compounds listed are shown in Fig. S1.
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values were used for the rate constants of k2, k3, and k4 (DeMore et al., 1997; Atkinson and Aschmann, 1993). The OH exposure was calculated by integrating the OH concentration over time. The OH exposure until the mean time of the second filter sampling was estimated to be (1.5 ± 0.6) 1011 molecule cm3 s; this corresponds to 1.7-days OH exposure at an OH concentration of 1 106 molecule cm3. If we reduce the nopinone concentration by a factor of 2, the OH exposure was estimated to be (1.6 ± 0.6) 1011 molecule cm3 s. The OH exposure was also estimated from the time series of the nopinone concentration and the rate constant for the reaction of nopinone with OH radical (Atkinson and Aschmann, 1993); the OH exposure was found to be (3.6 ± 0.4) 1011 molecule cm3 s using this method. The contribution from the nopinone SOA particle mass to D [SOA] was estimated; here, D[SOA] is the difference between the particle mass concentrations of periods 2 and 1. Nopinone produced by b-pinene ozonolysis in run 1 was evaluated to be 136 mg m3 using the FT-IR result of the initial b-pinene concentration and a literature value of the nopinone yield (17%; Yu et al., 1999). The SOA particle yield from nopinone in run 6 was calculated to be 0.28. As the SOA mass concentration was observed in a similar range in runs 1 and 6, the SOA particle yield value obtained from run 6 was applied to run 1, and the concentration of SOA particles formed from nopinone oxidation in run 1 was determined to be 27% of D[SOA]. A similar estimation was also performed using the data obtained from run 2; the concentration of SOA particles formed from nopinone photooxidation was estimated to be 78% of D[SOA]. The mass concentration of SOA particles formed form the photooxidation of nopinone will contribute to D[SOA] under the present conditions. 3.2. LCeTOF analysis of b- and a-pinene SOA particles Fig. 2a shows the pinonic acid-equivalent concentrations determined for the b-pinene SOA particles collected in run 1; particle wall loss corrections were not performed for the calculations of these concentrations. Two or more chromatographic peaks were observed in EICs with m/z 169.09, 185.08, 201.08, and 217.07. A single major chromatographic peak was observed in EICs with other masses. The total concentration of products was calculated for all twelve masses. Table 2 lists the products suggested for the twelve masses that were investigated (Yasmeen et al., 2010; Ma and Marston, 2008; Claeys et al., 2009; Yu et al., 1999; Szmigielski et al., 2007). As shown in Fig. S1, the retention times observed in this study for the peaks of terebic acid, terpenylic acid, pinonic acid, pinic acid, MBTCA, and the dimers of m/z 343.14, m/z 357.16, and m/ z 367.18 showed linear relationships with those observed with reverse-phase chromatography by Kristensen et al. (2014) and Yasmeen et al. (2010); these results support the present identifications for the eight products. The major products present in the sample collected before OH exposure were pinic acid, a product with m/z 357.16, and terpenylic acid (comprising 10.9%, 5.1%, and 4.2% of the total SOA particle mass, respectively). Fig. 2b shows the average period 2 to period 1 ratios determined using the results of runs 1e3 with b-pinene. The concentrations of terpenylic acid, C9H14O5, C9H14O6, MBTCA, and terebic acid increased after OH exposure. Terpenylic acid and C9H14O5 comprised 4.0% and 1.7% of the total aged particle mass, respectively. C9H14O6, MBTCA, and terebic acid each comprised less than 1% of the total aged particle mass. MBTCA is suggested to be formed from the OH-initiated oxidation of pinonic acid (Müller et al., 2012). Pinonic acid was also formed from b-pinene ozonolysis, although it was a minor product. The ratio of MBTCA to pinonic acid measured for aged bpinene SOA particles (1.60 ± 0.69) was similar to that measured for aged a-pinene SOA particles (1.30 ± 0.26). The OH-initiated
Fig. 2. Results of LCeTOF analysis for b-pinene SOA particles: (a) pinonic acidequivalent concentrations (run 1) and (b) period 2 to period 1 ratios (average of runs 1e3); dotted line is [period 1]/[period 2] ¼ 1.
oxidation of pinonic acid will contribute to the formation of MBTCA present in aged b-pinene SOA particles. Terebic acid is suggested to be formed from the OH-initiated oxidation of terpenylic acid (Yasmeen et al., 2010). The total concentration of m/z 185.08 increased slightly after OH exposure. Among these products, pinic acid decreased by 16%e27% after OH exposure, whereas another minor product (probably hydroxypinalic-3-acid) increased after OH exposure. Fig. 3a shows the pinonic acid-equivalent concentrations determined for the a-pinene SOA particles collected in run 5. The major products present in the sample collected before OH exposure were pinic acid, 10-hydroxypinonic acid, and terpenylic acid (comprising 8.7%, 4.6%, and 3.6% of the total SOA particle mass, respectively). Fig. 3b shows the average period 2 to period 1 ratios determined using the results of runs 4 and 5. The concentrations of MBTCA, terebic acid, C9H14O5, and C9H14O6 increased after OH exposure. MBTCA comprised 1.0% of the total aged particle mass. Terebic acid, C9H14O5, and C9H14O6 each comprised only <1% of the total aged particle mass. As discussed previously, MBTCA and terebic acid will be formed from the OH-initiated oxidation of pinonic acid and terpenylic acid, respectively. Kahnt et al. (2014) suggested that C9H14O5 as well as C9H14O6 formed from the ozonolysis of campholenic aldehyde were C9-carbonyl-dicarboxylic acid and C9-tricarboxylic acid, respectively. As campholenic aldehyde is formed from the heterogeneous oxidation of a-pinene, C9carbonyl-dicarboxylic acid and C9-tricarboxylic acid will also be formed from the ozonolysis of a-pinene. 3.3. Comparisons with nopinone products Fig. 4 shows a comparison of the EICs observed for the b-pinene SOA particles with those observed for the a-pinene and nopinone SOA particles. Fig. 4a shows the EICs with m/z 171.07 observed for the b-pinene SOA particles. A peak appearing at 9.7 min and a
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K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
Fig. 3. Results of LCeTOF analysis for a-pinene SOA particles: (a) pinonic acidequivalent concentrations (run 5) and (b) period 2 to period 1 ratios (average of runs 4 and 5); dotted line is [period 1]/[period 2] ¼ 1.
minor peak appearing at 13.2 min are attributed to terpenylic acid and norpinic acid, respectively (Yasmeen et al., 2010). Terpenylic acid was present in the b-pinene SOA particles collected before OH exposure, suggesting that terpenylic acid is formed from b-pinene ozonolysis. The peak intensity of terpenylic acid increased after OH exposure. On the other hand, the peak intensity of terpenylic acid present in a-pinene SOA particles barely increased after OH exposure (Fig. 4b). Fig. 4c shows the EIC measured for nopinone SOA particles. Terpenylic acid was also formed from the photooxidation
5
of nopinone; this suggests that the OH-initiated oxidation of nopinone contributes to terpenylic acid formation during the aging of b-pinene SOA. Terpenylic acid formation from the OH-initiated nopinone oxidation is newly found in this study. Fig. 4d shows the EICs with m/z 201.08 observed for the bpinene SOA particles. Several chromatographic peaks were observed for the b-pinene SOA particles, suggesting that several isomers of C9H14O5 are present in these samples. The total signal of the C9H14O5 compounds increased after OH exposure, as discussed previously. The number of chromatographic peaks measured for the a-pinene SOA particles was smaller than that measured for the b-pinene SOA particles (Fig. 4e). The C9H14O5 compounds formed during the aging of b-pinene SOA contain not only the C9H14O5 compounds formed from a-pinene ozonolysis but also other C9H14O5 isomers. The C9H14O5 compounds formed from a-pinene ozonolysis include C9 carbonyl-dicarboxylic acids (Kahnt et al., 2014). The chromatographic peaks measured for nopinone photooxidation were similar to those measured for b-pinene oxidation (Fig. 4f). The OH-initiated oxidation of nopinone contributes to the formation of C9H14O5 products during the aging of b-pinene SOA. The EIC with m/z 217.07 measured for b-pinene SOA particles was also compared with those measured for a-pinene and nopinone SOA particles. The results of C9H14O6 compounds were very similar to those of C9H14O5 compounds. The C9H14O6 compounds formed during the aging of b-pinene SOA contain not only the C9H14O6 compounds formed from a-pinene ozonolysis but also other C9H14O6 isomers. The C9H14O6 compounds formed from apinene ozonolysis include a C9 tricarboxylic acid (Kahnt et al., 2014). The OH-initiated oxidation of nopinone contributes to the formation of C9H14O6 compounds during the aging of b-pinene SOA. The products with m/z 185.08 were also formed from nopinone photooxidation. However, pinic acid was barely formed from nopinone photooxidation. The product identified to be hydroxypinalic-3-acid was mainly formed from nopinone photooxidation. 3.4. Suggested formation mechanism for terpenylic acid Fig. 5 shows the suggested formation mechanism for terpenylic acid. Nopinone undergoes H abstraction from C-a carbon by OH
Fig. 4. Extracted-ion chromatograms (EICs) of m/z 171.07 measured for (a) run 1, (b) run 5, and (c) run 6, and EICs of m/z 201.08 measured for (d) run 1, (e) run 5, and (f) run 6.
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radicals. Lewis et al. (2005) conducted transition state theory calculations and found that the branching ratio for H abstraction from C-a carbon is 44%. The formed radical subsequently reacts with oxygen and NO to form the alkoxy radical a2. The branching ratios from the alkoxy radical a2 were determined using the rate constants for the decomposition of alkoxy radicals (Kroll and Seinfeld, 2008) and the H shift isomerization of alkoxy radicals (Vereecken and Peeters, 2010). The available pathways from a2 are the C(O)R dissociation (9.5 107 s1), CH2R dissociation (4.7 104 s1), 1,5-H shift (3.3 106 s1), and reaction with O2 (4.7 104 s1). 98% of the alkoxy radical a2 undergoes C(O)R dissociation, namely, the ringopening reaction, to form the acyl radical a3. The acyl radical a3 reacts with oxygen and NO to form the acyloxy radical a4. 4% of a4 decomposes to CO2 and the alkyl radical a5. The alkyl radical a5 reacts with oxygen and NO to form the alkoxy radical a6. 44% of a6 again undergoes ring-opening to form the tertiary alkyl radical a7. The formed tertiary alkyl radical a7 reacts with oxygen and NO to form the alkoxy radical a8. 98% of a8 undergoes the 1,5-H shift to form the acyl radical a9. The acyl radical a9 reacts with oxygen and NO to form the acyloxy radical a10. 15% of a10 undergoes the 1,6-H shift to form the acyl radical a11. The acyl radical a11 reacts with oxygen to form the acylperoxy radical a12. The acylperoxy radical a12 reacts with HO2 to form diaterpenylic acid (a13) and ozone (Moortgat et al., 1989). Diaterpenylic acid a13 undergoes selfdehydration to form terpenylic acid (a14). The saturation concentrations of a13 and a14 were evaluated using SPARC on-line calculator (Hilal et al., 2003) to be 6.8 and 45 mg m3, respectively (Table S1), suggesting that substantial fractions of a13 and a14 are present in the condensed phase. The dehydration of diaterpenylic acid likely occurs on the particle surface or in the liquid phase. Yasmeen et al. (2010) suggested that the dehydration from the hydroxyl group of diaterpenylic acid and carboxylic acid takes place in particle phase; this supports the self-dehydration mechanism suggested in this study for diaterpenylic acid. Of the reaction mechanism mentioned in this paragraph, the formation pathway for the alkoxy radical a8 from the oxidation of nopinone is the same as that suggested by Master Chemical Mechanism version 3.2 (MCM; Jenkin et al., 1997; Saunders et al., 2003; Jenkin, 2004). The formation mechanism for terpenylic acid from the reaction of the
alkoxy radical a8 is newly suggested in this study. The yield of diaterpenylic acid from nopinone photooxidation was estimated to be 0.11% at most; this maximum value was calculated by multiplying all the branching ratios referred and estimated here. The estimated maximum yield value was lower than the yield of terpenylic acid from nopinone photooxidation determined in run 6 (1.6%), suggesting that other formation pathways for terpenylic acid are available or the structure activity relationship rate constants for the alkoxy unimolecular reactions underestimate the yield of terpenylic acid. The present results showed that terpenylic acid is formed from the ozonolysis of both a- and b-pinene. Claeys et al. (2009) suggested that terpenylic acid is formed from a-pinene ozonolysis via the heterogeneous reaction of a-pinene oxide. However, no formation mechanism of terpenylic acid from b-pinene ozonolysis has been suggested. The acyloxy radical a4 is suggested to be produced during the ozonolysis of a- and b-pinene by the MCM model. The formation mechanism for terpenylic acid suggested in this study will also explain the formation of terpenylic acid from the ozonolysis of a- and b-pinene. The formation mechanisms for the acyloxy radical a4 from the ozonolysis of a- and b-pinene, as shown in Fig. 5, are the same as those proposed by the MCM model. The acyloxy radical a4 leads to terpenylic acid formation by a similar reaction pathway as that described for nopinone photooxidation. Diaterpenylic acid is a key monomer of ester dimer formation during the ozonolysis of a- and b-pinene (Yasmeen et al., 2010). The direct formation of diaterpenylic acid consistently explains the formation of ester dimers from a- and b-pinene ozonolysis under the present dry conditions. 3.5. Predicted formation pathways for C9 multifunctional compounds As the molecular structures of C9H14O5 and C9H14O6 were not suggested from the present experimental results, the probable formation pathways for C9H14O5 and C9H14O6 are predicted based on the rate constants for the unimolecular reactions of alkoxy radicals (Kroll and Seinfeld, 2008; Vereecken and Peeters, 2010). Fig. 6 shows the predicted formation pathways for C9
Fig. 5. Suggested formation mechanisms for terpenylic acid from the OH-initiated nopinone oxidation and the ozonolysis of a- and b-pinene.
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multifunctional compounds from the OH-initiated oxidation of nopinone. The branching ratios for H abstraction from the C-a, Cb, C-c, C-d, and C-e carbons of nopinone were evaluated to be 44%, 13%, 10%, 3%, and 23%, respectively (Lewis et al., 2005). Only the most probable pathway for C9-hydroxy-dicarbonyl-carboxylic acid or C9-carbonyl-dicarboxylic acid from each H abstraction pathway was depicted, although we found two or more formation pathways from each H abstraction pathway. Note that the formation of other isomers that are not depicted here are also possible. (a) If we assume the H abstraction from C-a carbon, only very minor pathways were found for C9H14O5 products. The products formed by the H abstraction from C-a carbon mainly undergo dissociation pathways. (b) The H abstraction from C-b carbon leads to the formation of the alkyl radical b1. The formed radical reacts with oxygen and NO to form the alkoxy radical b2. 4% of b2 undergoes ring-opening to form alkyl radical b3. The formed alkyl radical b3 reacts with oxygen and NO to form the alkoxy radical b4. 80% of b4 undergoes ring-opening to form tertiary alkyl radical b5. The tertiary alkyl radical b5 reacts with oxygen and NO to form the alkoxy radical b6. 75% of b6 undergoes the 1,5-H shift to form the acyl radical b7. The acyl radical b7 reacts with oxygen to form the acylperoxy radical b8. This radical reacts with HO2 to form the C9-hydroxyldicarbonyl-carboxylic acid b9. The b9 product isomerizes to the cyclic hemiacetal b10 in the condensed phase (Lim and Ziemann, 2009). The yield of the b9 product from nopinone photooxidation was estimated to be 0.29% at most. (c) The H abstraction from C-c carbon leads to the formation of the tertiary alkyl radical c1. The formed radical reacts with oxygen and NO to form the alkoxy radical c2. 97% of c2
7
undergoes ring-opening to form the tertiary alkyl radical c3. This radical reacts with oxygen and NO to form the alkoxy radical c4. 77% of c4 undergoes 1,5-H shift to form the alkyl radical c5. The formed alkyl radical reacts with oxygen and NO to form the alkoxy radical c6. 97% of the resulting radical undergoes ring-opening to form the acyl radical c7. This acyl radical reacts with oxygen to form the acylperoxy radical c8. The acylperoxy radical c8 reacts with HO2 to form the C9hydroxyl-dicarbonyl-carboxylic acid c9. This product undergoes cyclization to form the cyclic hemiacetal c10 in the condensed phase. An aldehyde group of the cyclic hemiacetal c10 is oxidized in the gas phase or in the condensed phase to form the cyclic hemiacetal c11. A ring-opening product would also be formed from the decomposition of c11 in the condensed phase, although this product is not shown. The yield of the c9 product from nopinone photooxidation was estimated to be 7.2% at most. (d) The H abstraction from C-d carbon leads to the formation of the alkyl radical d1. The formed radical reacts with oxygen and NO to form the alkoxy radical d2. 10% of the alkoxy radical d2 undergoes ring-opening to form the alkyl radical d3. This radical again reacts with oxygen and NO to form the alkoxy radical d4. 94% of d4 undergoes ring-opening to form the acyl radical d5. The acyl radical d5 reacts with oxygen and NO to form the acyloxy radical d6. 5% of d6 undergoes the 1,6-H shift to form the acyl radical d7. The formed radical reacts with O2 to form the acyl peroxy radical d8. The acyl peroxy radical d8 reacts with HO2 to form the C9-carbonyldicarboxylic acid d9. The radical d9 oxidizes to form the C9tricarboxylic acid d10. The yield of the d9 product from nopinone photooxidation was estimated to be 0.014% at most.
Fig. 6. Predicted formation pathways and structures of C9 multifunctional products based on the rate constants for unimolecular reactions of alkoxy radicals (Kroll and Seinfeld, 2008; Vereecken and Peeters, 2010).
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K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
(e) The H abstraction from C-e carbon leads to the formation of the tertiary alkyl radical e1. The formed radical reacts with oxygen and NO to form the alkoxy radical e2. 94% of e2 undergoes ring-opening to form the acyl radical e3. The formed radical reacts with oxygen and NO to from the acyloxy radical e4. 5% of e4 undergoes the 1,6-H shift to form the alkyl radical e5. The formed radical reacts with oxygen and NO to form the alkoxy radical e6. The formed radical mainly undergoes ring-opening to form the acyl radical e7. This radical reacts with oxygen to form the acylperoxy radical e8. The resulting radical reacts with HO2 to form the C9-carbonyldicarboxylic acid e9. The oxidation of the e9 product also forms the C9-tricarboxylic acid d10. The yield of the e9 product from nopinone photooxidation was estimated to be 1.1% at most. Of the reaction mechanisms shown in Fig. 6, the formation pathways for the alkoxy radicals b6, c4, and d2 from the oxidation of nopinone are the same as those suggested by the MCM model. The oxidation mechanisms of the alkoxyl radicals b6, c4 and d2 and the oxidation mechanism of nopinone via the tertiary alkyl radical e1 are newly suggested in this study. Among the products with m/z 201.08, the b10, c10, d9, and e9 products had the saturated concentrations in a range 0.1e2.4 mg m3 as listed in Table S1, suggesting that these products exist mainly in the condensed phase. On the other hand, the b9 and c9 products had the saturated concentrations in a range 61e102 mg m3, suggesting that these products exist mainly in the gas phase under ambient conditions although these products exist mainly in the particle phase under present experimental conditions. The products with m/z 217.07, i.e., the d10 and c11 products, had the saturated concentrations lower than 102 mg m3, suggesting that these products are low-volatility organic compounds which exist basically in the condensed phase.
4. Conclusions In this study, the gases and particles formed from the ozonolysis of b- and a-pinene were exposed to OH radicals in a laboratory chamber. The total OH exposure in this study was estimated to be 1.5e3.6 1011 molecules cm3 s. As a result of OH exposure, the SOA particle concentration increased. SOA particle formation from the photooxidation of nopinone, formed from the ozonolysis of bpinene, contributes to the increase in the particle concentration during the OH exposure. Pinic acid and terpenylic acid are abundantly present in the b- and a-pinene ozonolysis particles. Terpenylic acid and C9H14O5 products present in b-pinene SOA particles increased upon exposing SOA to OH radicals, whereas MBTCA present in a-pinene SOA particles increased upon exposing SOA to OH radicals. Terpenylic acid and similar C9H14O5 products were also formed from the photooxidation of nopinone. The OH-initiated oxidation of nopinone contributes to terpenylic acid and C9H14O5 formation during the aging of b-pinene SOA. The formation mechanism for terpenylic acid from nopinone oxidation was newly suggested. Diaterpenylic acid formed from the oxidative doublering opening of nopinone undergoes self-dehydration to form terpenylic acid in the condensed phase. The formation of terpenylic acid from the ozonolysis of a- and b-pinene can also be explained by similar processes. The formation pathways for C9H14O5 from nopinone photooxidation and the molecular structures of C9H14O5 were predicted based on the rate constants for the unimolecular reactions of alkoxy radicals. The predicted structures of C9H14O5 were C9-carbonyl-dicarboxylic acid and the acyclic and cyclic forms of C9-hydroxyl-dicarbonyl-carboxylic acid.
Acknowledgments This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 25340021, FY2013-2015) and Global Environmental Research Fund of the Ministry of the Environment of Japan (5-1408, FY2014-2016). K.S. thanks Yoshikatsu Takazawa and Akio Togashi of the National Institute for Environmental Studies for providing technical support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2015.08.047. References Atkinson, R., Aschmann, S.M., 1993. Atmospheric chemistry of the monoterpene reaction products nopinone, camphenilone, and 4-acetyl-1-methylcyclohexene. J. Atmos. Chem. 16, 337e348. Claeys, M., Iinuma, Y., Szmigielski, R., Surratt, J.D., Blockhuys, F., Van Alsenoy, C., € ge, O., Sierau, B., Go mez-Gonza lez, Y., Vermeylen, R., Van der Veken, P., Bo Shahgholi, M., Chan, A.W.H., Herrmann, H., Seinfeld, J.H., Maenhaut, W., 2009. Terpenylic acid and related compounds from the oxidation of a-pinene: implications for new particle formation and growth above forests. Environ. Sci. Technol. 43, 6976e6982. DeMore, W.B., Sander, S.P., Golden, D.M., Hampson, R.F., Kurylo, M.J., Howard, C.J., Ravishankara, A.R., Kolb, C.J., Molina, M.J., 1997. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. Evaluation number 12. JPL Publication 97-4, 1e266. Docherty, K.S., Ziemann, P.J., 2003. Effects of stabilized Criegee intermediate and OH radical scavengers on aerosol formation from reactions of b-pinene with O3. Aerosol Sci. Technol. 37, 877e891. Donahue, N.M., Henry, K.M., Mentel, T.E., Kiendler-Scharr, A., Spindler, C., Bohn, B., Brauers, T., Dorn, H.P., Fuchs, H., Tillmann, R., Wahner, A., Saathoff, H., € hler, O., Leisner, T., Müller, L., Reinnig, M.-C., Hoffmann, T., Naumann, K.-H., Mo Salo, K., Hallquist, M., Frosch, M., Bilde, M., Tritscher, T., Barmet, P., Praplan, A.P., vo ^t, A.S.H., Baltensperger, U., 2012. Aging of DeCarlo, P.F., Dommen, J., Pre biogenic secondary organic aerosol via gas-phase OH radical reactions. In: Proceedings of the National Academy of Sciences of the United States of America, vol. 09, pp. 13503e13508. Guenther, A.B., Jiang, X., Heald, C.L., Sakulyanontvittaya, T., Duhl, T., Emmons, L.K., Wang, X., 2012. The model of emissions of gases and aerosols from nature version 2.1 (MEAGAN2.1): an extended and updated framework for modeling biogenic emissions. Geosci. Model Dev. 5, 1471e1492. Hallquist, M., Wenger, J.C., Baltensperger, U., Rudich, Y., Simpson, D., Clayes, M., Dommen, J., Donahue, N.M., George, C., Goldstein, A.H., Hamilton, J.F., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M.E., Jimenez, J.L., Kiendler-Scharr, A., Maenhaut, W., McFiggans, G., Mentel, Th F., Monod, A., Prevot, A.S.H., Seinfeld, J.H., Surratt, J.D., Szmigielski, R., Wildt, J., 2009. The formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmos. Chem. Phys. 9, 5155e5236. Hatakeyama, S., Izumi, K., Fukuyama, T., Akimoto, H., 1989. Reactions of ozone with a-pinene and b-pinene in air: yields of gaseous and particulate products. J. Geophys. Res. 94, 13013e13024. Hatakeyama, S., Izumi, K., Fukuyama, T., Akimoto, H., Washida, N., 1991. Reactions of OH with a-pinene and b-pinene in air: estimate of global CO production from the atmospheric oxidation of terpenes. J. Geophys. Res. 96, 947e958. Hilal, S.H., Karickhoff, S.W., Carreira, L.A., 2003. Prediction of the vapor pressure, boiling point, heat of vaporization and diffusion coefficient of organic compounds. QSAR Comb. Sci. 22, 565e574. Jaoui, M., Kamens, R.M., 2001. Mass balance of gaseous and particulate products analysis from a-pinene/NOx/air in the presence of natural sunlight. J. Geophys. Res. 106, 12541e12558. Jaoui, M., Kamens, R.M., 2003. Mass balance of gaseous and particulate products from b-pinene/O3/air in the absence of light and b-pinene/NOx/air in the presence of natural sunlight. J. Atmos. Chem. 43, 101e141. Jenkin, M.E., 2004. Modelling the formation and composition of secondary organic aerosol from a- and b-pinene ozonolysis using MCM v3. Atmos. Chem. Phys. 4, 1741e1757. Jenkin, M.E., Saunders, S.M., Pilling, M.J., 1997. The tropospheric degradation of volatile organic compounds: a protocol for mechanism development. Atmos. Environ. 31, 81e104. €ge, O., Claeys, M., Herrmann, H., 2014. CamKahnt, A., Iinuma, Y., Mutzel, A., Bo pholenic aldehyde ozonolysis: a mechanism leading to specific biogenic secondary organic aerosol constituents. Atmos. Chem. Phys. 14, 719e736. Kroll, J.H., Seinfeld, J.H., 2008. Chemistry of secondary organic aerosol: formation and evolution of low-volatility organics in the atmosphere. Atmos. Environ. 42, 3593e3624. Kristensen, K., Cui, T., Zhang, H., Gold, A., Glasius, M., Surratt, J.D., 2014. Dimers in apinene secondary organic aerosol: effect of hydroxyl radical, ozone, relative
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Terpenylic acid and nine-carbon multifunctional compounds formed during the aging of b-pinene ozonolysis secondary organic aerosol Kei Sato a, b, *, Tianyu Jia b, Kiyoshi Tanabe a, Yu Morino a, Yoshizumi Kajii a, b, Takashi Imamura a a b
National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-8506, Japan Graduate School of Global Environmental Studies, Kyoto University, Nihonmatsucho, Sakyo-ku, Kyoto 606-8316, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SOA formed from b-pinene ozonolysis was exposed to OH radicals. Terpenylic acid and C9H14O5 in SOA particles increased after OH exposure. Nopinone oxidation will contribute to terpenylic acid and C9H14O5 formation. Formation mechanisms of terpenylic acid and C9H14O5 were suggested.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2015 Received in revised form 16 August 2015 Accepted 18 August 2015 Available online xxx
Recent field and laboratory studies suggest that forest aerosol particles contain more highly functionalized organic molecules than pinonic acid, a traditional molecular maker of secondary organic aerosol (SOA) particles. To investigate the reaction mechanisms during the aging of biogenic SOAs, the gases and particles formed from the ozonolysis of b- and a-pinene were exposed to OH radicals in a laboratory chamber. The particle samples were collected before and after OH exposure for analysis by liquid chromatography-negative electrospray ionization time-of-flight mass spectrometry. Pinic acid and terpenylic acid were abundant products in both b- and a-pinene ozonolysis SOA particles. Terpenylic acid and products with m/z 201.08 present in b-pinene SOA particles increased upon exposing SOA to OH radicals, whereas 3-methyl-1,2,3-butanetricarboxylic acid present in a-pinene SOA particles increased upon exposing SOA to OH radicals. The products with m/z 201.08 were suggested to be C9H14O5 compounds. Similar C9H14O5 compounds and terpenylic acid were also detected in SOA particles formed from the photooxidation of nopinone, a major first-generation product of b-pinene ozonolysis. The OHinitiated oxidation of nopinone will contribute to the formation of terpenylic acid and C9H14O5 compounds during the aging of b-pinene SOA. A formation mechanism for terpenylic acid via gas-phase diaterpenylic acid formation followed by self-dehydration in the condensed phase was suggested. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Biogenic volatile organic compound Secondary organic aerosol Environmental chamber Molecular marker Multiphase chemistry
Abbreviations: EIC, Extracted-ion chromatogram; FT-IR, Fourier transform-infrared spectroscopy; LCeTOF, Liquid chromatographyetime-of-flight mass spectrometry; MBTCA, 3-methyl-1,2,3-butanetricarboxylic acid; SMPS, Scanning mobility particle sizer; SOA, Secondary organic aerosol. * Corresponding author. National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-8506, Japan. E-mail addresses: [email protected] (K. Sato), [email protected] (T. Jia), [email protected] (K. Tanabe), [email protected] (Y. Morino), kajii.yoshizumi.7e@kyoto-u. ac.jp (Y. Kajii), [email protected] (T. Imamura). http://dx.doi.org/10.1016/j.atmosenv.2015.08.047 1352-2310/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
1. Introduction Pinene is a major monoterpene emitted from vegetation into the atmosphere; the global emissions of a- and b-pinene are evaluated to be 66 and 19 Tg/yr, respectively (Guenther et al., 2012). The atmospheric oxidation of a- and b-pinene contributes to the formation of secondary organic aerosol (SOA) particles; these can adversely affect the climate, human health, and visibility (Kroll and Seinfeld, 2008; Hallquist et al., 2009). For better modeling atmospheric SOA particle formation, it is necessary to understand the aging of SOA (Volkamer et al., 2006). Szmigielski et al. (2007) reported that 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) formed from the OH-initiated oxidation of a-pinene is identified in forest aerosols. MBTCA is more highly functionalized than traditional SOA molecular markers such as pinonic acid and pinic acid; this suggests that the aging of SOA influences the molecular-level composition of ambient SOA particles. Early studies investigating the aging of a-pinene ozonolysis SOA reported little change in the chemical composition of SOA particles after the consumption of the reactant pinene (Qi et al., 2010), probably because gaseous and particulate oxidation products formed from monoterpenes with one double bond barely react with ozone (Ng et al., 2006). Recently, Donahue et al. (2012) conducted a chamber study by exposing monoterpene ozonolysis SOA to OH radicals. They used an online atmospheric pressure chemical ionization mass spectrometer and showed that the MBTCA signal increased significantly after exposing a-pinene SOA to OH radicals. Müller et al. (2012) suggested the formation mechanisms of MBTCA from the OH-initiated oxidation of pinonic acid in the gas phase. The difference in the gas-phase mechanisms between the oxidation of a- and b-pinene might affect the particulate products formed during the aging of SOA. The differences in the gas-phase mechanisms between a-pinene (endocyclic terpene) and bpinene (exocyclic terpene) have been studied extensively (Hatakeyama et al., 1989, 1991; Yu et al., 1999; Jaoui and Kamens, 2001, 2003; Docherty and Ziemann, 2003; Jenkin, 2004). A major first-generation product formed from a-pinene ozonolysis is pinonic acid, a ring-opening product,
(R1)
whereas a major first-generation product formed from b-pinene ozonolysis is nopinone, a ring-retaining product,
(R2) In this study, we investigate the aging of a- and b-pinene ozonolysis SOA by employing a chamber technique similar to that used by Donahue et al. (2012). The offline SOA particle samples obtained before and after OH exposure were analyzed by liquid chromatographyetime-of-flight mass spectrometry (LCeTOF). This study aims to characterize the particulate products formed during the aging of b-pinene SOA and to suggest the reaction mechanisms during the aging of SOA. 2. Experimental methods 2.1. Chamber experiments The experimental procedures were similar to those described elsewhere (Sato et al., 2012). An evacuable Teflon-coated 6-m3
chamber was used for the experiments. The temperature of the chamber was maintained at 298 ± 2 K. Purified and filtered air was used for the experiments. The total pressure of purified air was 1019 ± 10 hPa. The relative humidity of purified air was <1%. The concentrations of the gaseous compounds were measured using a Fourier transform-infrared spectrometer (FT-IR; optical length: 221.5 m, Model Nexus 670, Thermo Fisher Scientific, Waltham, MA, USA). The particle size distribution was measured using a scanning mobility particle sizer (SMPS; Model 3934, TSI Inc., Shoreview, MN, USA). Table 1 summarizes the experimental conditions in each run. The ozonolysis experiments of a- and b-pinene (runs 1e5) were conducted by adding ozone (580e2770 ppb) to a pinene-air mixture (141e307 ppb) in the chamber. No hydroxyl radical scavenger was used. After the pinene concentration decreased below 20 ppb in each ozonolysis experiment, SOA particles formed from pinene ozonolysis were collected on a Teflon membrane filter (Fluoropore, diameter: 47 mm, pore size: 1 mm; Sumitomo Electric, Osaka, Japan). The flow rate for filter sampling was 16.7 L min1. The filter sampling duration was 12 min. Methyl nitrite (902e1024 ppb) was then added to the reaction mixture. Methyl nitrite was prepared by adding aqueous sulfuric acid to a mixture of sodium nitrite and methanol. The reaction mixture was irradiated using filtered light from xenon arc lamps (wavelength: >300 nm, total power: 19 kW). Hydroxyl radicals were produced by the following reaction sequence: CH3ONO þ hn / CH3O þ NO,
(R3)
CH3O þ O2 / CH2O þ HO2,
(R4)
HO2 þ NO / OH þ NO2.
(R5)
A second filter sampling was conducted 1 h after the start of irradiation. A photooxidation experiment of nopinone (run 6) was conducted by irradiating a mixture of nopinone (143 ppb) and methyl nitrite (1074 ppb). SOA particles formed by the photooxidation of nopinone were collected on a filter 1 h after the start of irradiation. 2.2. LCeTOF analysis A 20-mL aliquot of adipic acid-13C6 methanol solution (32 ng/mL) was added to each filter sample as an internal standard. The filter sample was then sonicated in 5-mL methanol for 30 min. The extract of the filter sample was concentrated to near dryness under a gentle stream of nitrogen. The concentrated extract was dissolved in 0.5-mL formic acid-water-methanol solution (v/v/v ¼ 0.05/ 99.95/100) and was used as an analytical sample. A 10-mL aliquot of each analytical sample was loop-injected into a LCeTOF instrument (Agilent Technologies, Palo Alto, CA, USA). An octadecyl column (Model ODS-3, inner diameter: 3.0 mm, length: 150 mm, particle size: 5 mm, GL Science Inc., Tokyo, Japan) was used for sample separation. The mobile phases used for liquid chromatography were 0.05% formic acid aqueous solution and methanol. The total flow rate of the mobile phases was 0.4 mL min1. The column temperature was maintained at 298 K. The methanol concentration was initially 5%, increased to 90% for 30 min, maintained at 90% for 15 min, and maintained at 5% for 15 min. The sample compounds were analyzed by a negative mode electrospray ionization time-of-flight mass spectrometer with a mass accuracy of <3 ppm. The extracted-ion chromatograms (EICs) of the twelve masses shown in Table 2 were investigated in this study. The peak intensity of each EIC was corrected using the internal standard intensity and was then converted into the pinonic
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K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
3
Table 1 Experimental conditions and concentrations of aerosol produced in this study. Run no.
Reaction
1 2 3 4 5 6
b b b a a
a b c
[Reactant]0/ppb
-Pinene/O3c -Pinene/O3c -Pinene/O3c -Pinene/O3c -Pinene/O3c
144 141 284 307 154 143
Nopinone/CH3ONO þ light
[O3]0/ppb
620 2620 2770 670 580 e
[SOA]/mg m3 Period 1a
Period 2b
153 177 486 571 210 e
290 224 572 594 246 219
Filter sampling period before OH exposure. Filter sampling period after OH exposure. Methyl nitrite was added and irradiated after most pinene reacted with O3.
acid-equivalent concentration using calibration curves measured for adipic acid-13C6 and pinonic acid. Blank filters were prepared by sampling purified air in the chamber through Teflon filters. No impurities were identified by the LCeTOF analysis of the blank filters.
3. Results and discussion 3.1. Time series and OH exposure Fig. 1 shows the typical time series of the b-pinene, nopinone, and SOA particle concentrations (run 2). Ozone was added to the reaction mixture at 0 min. The b-pinene concentration decreased and the nopinone concentration increased by the reaction of bpinene with ozone. The FT-IR technique overestimates the nopinone concentration by a factor of 1.7e2.5 owing to interference from the absorptions of other products formed from b-pinene oxidation (Yu et al., 1999). The b-pinene concentration decreased to the detection limit (8 ppb) at 32 min. The first filter sampling was conducted between 56 and 68 min (period 1). Methyl nitrite was added and lamps were turned on at 74 min. The nopinone concentration decreased and the SOA particle concentration increased by the reactions of oxidation products with OH radicals. The second filter sampling was conducted between 134 and 146 min (period 2). Table 1 summarizes the SOA particle mass concentrations observed during the filter sampling periods. The SOA particle mass concentration was calculated from the SMPS volume concentration assuming a particle density of 1 g cm3. No wall loss correction was performed. The OH exposure in this study was estimated by using the methyl nitrite, formaldehyde, NO2, and nopinone concentrations
Fig. 1. Time series of gas and aerosol concentrations measured during run 2 with bpinene; periods 1 and 2 show filter sampling periods.
measured by FT-IR. First, the time series of the OH concentration was estimated assuming a pseudo steady-state approximation:
½OH ¼ k1 ½CH3 ONO=ðk2 ½HCHO þ k3 ½NO2 þ k4 ½nopinoneÞ; (1) where k1 is the rate constant for the photolysis of methyl nitrite; k2, the rate constant for the OH þ HCHO reaction; k3, the rate constant for the OH þ NO2 reaction; and k4, the rate constant for the OH þ nopinone reaction. As the methyl nitrite concentration (~1 ppm) was higher than the initial b-pinene concentration (141e284 ppbv), the reactions of OH radical with b-pinene oxidation products other than nopinone will be negligible. The rate constant for the photolysis of methyl nitrite was determined to be (6.9 ± 2.7) 104 s1 under the present conditions. Literature
Table 2 Products investigated in this study; references: 1Yasmeen et al. (2010), 2Ma and Marston (2008), 3Claeys et al. (2009), 4Yu et al. (1999), 5this work, 6Kahnt et al. (2014), and 7Szmigielski et al. (2007). m/z
Molecular formula
Suggested product(s)
157.050 169.086 171.065 183.101 185.080 199.104 201.076
C7H10O4 C9H13O3 C8H12O4 C10H16O3 C9H14O4 C10H16O4 C9H14O5
203.055 217.071
C8H12O6 C9H14O6
343.139 357.156 367.175
C16H24O8 C17H26O8 C19H28O7
Terebic acid1 Pinalic-3-acid,2 pinalic-4-acid2 Terpenylic acid3 Pinonic acid4 Pinic acid,4 hydroxypinalic-3-acid2 10-Hydroxypinonic acid4 C9-carbonyl-dicarboxylic acid,5,6 C9-hydroxy-dicarbonyl-carboxylic acid5 3-Methyl-1,2,3-butanetricarboxylic acid (MBTCA)7 C9-tricarboxylic acid,5,6 C9-hydroxy-carbonyl-dicarboxylic acid5 Ester from terpenylic acid and diaterpenylic acid1 Ester from pinic acid and diaterpenylic acid1 Unknown1
Molecular structures and estimated saturated concentrations of compounds listed are shown in Fig. S1.
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values were used for the rate constants of k2, k3, and k4 (DeMore et al., 1997; Atkinson and Aschmann, 1993). The OH exposure was calculated by integrating the OH concentration over time. The OH exposure until the mean time of the second filter sampling was estimated to be (1.5 ± 0.6) 1011 molecule cm3 s; this corresponds to 1.7-days OH exposure at an OH concentration of 1 106 molecule cm3. If we reduce the nopinone concentration by a factor of 2, the OH exposure was estimated to be (1.6 ± 0.6) 1011 molecule cm3 s. The OH exposure was also estimated from the time series of the nopinone concentration and the rate constant for the reaction of nopinone with OH radical (Atkinson and Aschmann, 1993); the OH exposure was found to be (3.6 ± 0.4) 1011 molecule cm3 s using this method. The contribution from the nopinone SOA particle mass to D [SOA] was estimated; here, D[SOA] is the difference between the particle mass concentrations of periods 2 and 1. Nopinone produced by b-pinene ozonolysis in run 1 was evaluated to be 136 mg m3 using the FT-IR result of the initial b-pinene concentration and a literature value of the nopinone yield (17%; Yu et al., 1999). The SOA particle yield from nopinone in run 6 was calculated to be 0.28. As the SOA mass concentration was observed in a similar range in runs 1 and 6, the SOA particle yield value obtained from run 6 was applied to run 1, and the concentration of SOA particles formed from nopinone oxidation in run 1 was determined to be 27% of D[SOA]. A similar estimation was also performed using the data obtained from run 2; the concentration of SOA particles formed from nopinone photooxidation was estimated to be 78% of D[SOA]. The mass concentration of SOA particles formed form the photooxidation of nopinone will contribute to D[SOA] under the present conditions. 3.2. LCeTOF analysis of b- and a-pinene SOA particles Fig. 2a shows the pinonic acid-equivalent concentrations determined for the b-pinene SOA particles collected in run 1; particle wall loss corrections were not performed for the calculations of these concentrations. Two or more chromatographic peaks were observed in EICs with m/z 169.09, 185.08, 201.08, and 217.07. A single major chromatographic peak was observed in EICs with other masses. The total concentration of products was calculated for all twelve masses. Table 2 lists the products suggested for the twelve masses that were investigated (Yasmeen et al., 2010; Ma and Marston, 2008; Claeys et al., 2009; Yu et al., 1999; Szmigielski et al., 2007). As shown in Fig. S1, the retention times observed in this study for the peaks of terebic acid, terpenylic acid, pinonic acid, pinic acid, MBTCA, and the dimers of m/z 343.14, m/z 357.16, and m/ z 367.18 showed linear relationships with those observed with reverse-phase chromatography by Kristensen et al. (2014) and Yasmeen et al. (2010); these results support the present identifications for the eight products. The major products present in the sample collected before OH exposure were pinic acid, a product with m/z 357.16, and terpenylic acid (comprising 10.9%, 5.1%, and 4.2% of the total SOA particle mass, respectively). Fig. 2b shows the average period 2 to period 1 ratios determined using the results of runs 1e3 with b-pinene. The concentrations of terpenylic acid, C9H14O5, C9H14O6, MBTCA, and terebic acid increased after OH exposure. Terpenylic acid and C9H14O5 comprised 4.0% and 1.7% of the total aged particle mass, respectively. C9H14O6, MBTCA, and terebic acid each comprised less than 1% of the total aged particle mass. MBTCA is suggested to be formed from the OH-initiated oxidation of pinonic acid (Müller et al., 2012). Pinonic acid was also formed from b-pinene ozonolysis, although it was a minor product. The ratio of MBTCA to pinonic acid measured for aged bpinene SOA particles (1.60 ± 0.69) was similar to that measured for aged a-pinene SOA particles (1.30 ± 0.26). The OH-initiated
Fig. 2. Results of LCeTOF analysis for b-pinene SOA particles: (a) pinonic acidequivalent concentrations (run 1) and (b) period 2 to period 1 ratios (average of runs 1e3); dotted line is [period 1]/[period 2] ¼ 1.
oxidation of pinonic acid will contribute to the formation of MBTCA present in aged b-pinene SOA particles. Terebic acid is suggested to be formed from the OH-initiated oxidation of terpenylic acid (Yasmeen et al., 2010). The total concentration of m/z 185.08 increased slightly after OH exposure. Among these products, pinic acid decreased by 16%e27% after OH exposure, whereas another minor product (probably hydroxypinalic-3-acid) increased after OH exposure. Fig. 3a shows the pinonic acid-equivalent concentrations determined for the a-pinene SOA particles collected in run 5. The major products present in the sample collected before OH exposure were pinic acid, 10-hydroxypinonic acid, and terpenylic acid (comprising 8.7%, 4.6%, and 3.6% of the total SOA particle mass, respectively). Fig. 3b shows the average period 2 to period 1 ratios determined using the results of runs 4 and 5. The concentrations of MBTCA, terebic acid, C9H14O5, and C9H14O6 increased after OH exposure. MBTCA comprised 1.0% of the total aged particle mass. Terebic acid, C9H14O5, and C9H14O6 each comprised only <1% of the total aged particle mass. As discussed previously, MBTCA and terebic acid will be formed from the OH-initiated oxidation of pinonic acid and terpenylic acid, respectively. Kahnt et al. (2014) suggested that C9H14O5 as well as C9H14O6 formed from the ozonolysis of campholenic aldehyde were C9-carbonyl-dicarboxylic acid and C9-tricarboxylic acid, respectively. As campholenic aldehyde is formed from the heterogeneous oxidation of a-pinene, C9carbonyl-dicarboxylic acid and C9-tricarboxylic acid will also be formed from the ozonolysis of a-pinene. 3.3. Comparisons with nopinone products Fig. 4 shows a comparison of the EICs observed for the b-pinene SOA particles with those observed for the a-pinene and nopinone SOA particles. Fig. 4a shows the EICs with m/z 171.07 observed for the b-pinene SOA particles. A peak appearing at 9.7 min and a
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K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
Fig. 3. Results of LCeTOF analysis for a-pinene SOA particles: (a) pinonic acidequivalent concentrations (run 5) and (b) period 2 to period 1 ratios (average of runs 4 and 5); dotted line is [period 1]/[period 2] ¼ 1.
minor peak appearing at 13.2 min are attributed to terpenylic acid and norpinic acid, respectively (Yasmeen et al., 2010). Terpenylic acid was present in the b-pinene SOA particles collected before OH exposure, suggesting that terpenylic acid is formed from b-pinene ozonolysis. The peak intensity of terpenylic acid increased after OH exposure. On the other hand, the peak intensity of terpenylic acid present in a-pinene SOA particles barely increased after OH exposure (Fig. 4b). Fig. 4c shows the EIC measured for nopinone SOA particles. Terpenylic acid was also formed from the photooxidation
5
of nopinone; this suggests that the OH-initiated oxidation of nopinone contributes to terpenylic acid formation during the aging of b-pinene SOA. Terpenylic acid formation from the OH-initiated nopinone oxidation is newly found in this study. Fig. 4d shows the EICs with m/z 201.08 observed for the bpinene SOA particles. Several chromatographic peaks were observed for the b-pinene SOA particles, suggesting that several isomers of C9H14O5 are present in these samples. The total signal of the C9H14O5 compounds increased after OH exposure, as discussed previously. The number of chromatographic peaks measured for the a-pinene SOA particles was smaller than that measured for the b-pinene SOA particles (Fig. 4e). The C9H14O5 compounds formed during the aging of b-pinene SOA contain not only the C9H14O5 compounds formed from a-pinene ozonolysis but also other C9H14O5 isomers. The C9H14O5 compounds formed from a-pinene ozonolysis include C9 carbonyl-dicarboxylic acids (Kahnt et al., 2014). The chromatographic peaks measured for nopinone photooxidation were similar to those measured for b-pinene oxidation (Fig. 4f). The OH-initiated oxidation of nopinone contributes to the formation of C9H14O5 products during the aging of b-pinene SOA. The EIC with m/z 217.07 measured for b-pinene SOA particles was also compared with those measured for a-pinene and nopinone SOA particles. The results of C9H14O6 compounds were very similar to those of C9H14O5 compounds. The C9H14O6 compounds formed during the aging of b-pinene SOA contain not only the C9H14O6 compounds formed from a-pinene ozonolysis but also other C9H14O6 isomers. The C9H14O6 compounds formed from apinene ozonolysis include a C9 tricarboxylic acid (Kahnt et al., 2014). The OH-initiated oxidation of nopinone contributes to the formation of C9H14O6 compounds during the aging of b-pinene SOA. The products with m/z 185.08 were also formed from nopinone photooxidation. However, pinic acid was barely formed from nopinone photooxidation. The product identified to be hydroxypinalic-3-acid was mainly formed from nopinone photooxidation. 3.4. Suggested formation mechanism for terpenylic acid Fig. 5 shows the suggested formation mechanism for terpenylic acid. Nopinone undergoes H abstraction from C-a carbon by OH
Fig. 4. Extracted-ion chromatograms (EICs) of m/z 171.07 measured for (a) run 1, (b) run 5, and (c) run 6, and EICs of m/z 201.08 measured for (d) run 1, (e) run 5, and (f) run 6.
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radicals. Lewis et al. (2005) conducted transition state theory calculations and found that the branching ratio for H abstraction from C-a carbon is 44%. The formed radical subsequently reacts with oxygen and NO to form the alkoxy radical a2. The branching ratios from the alkoxy radical a2 were determined using the rate constants for the decomposition of alkoxy radicals (Kroll and Seinfeld, 2008) and the H shift isomerization of alkoxy radicals (Vereecken and Peeters, 2010). The available pathways from a2 are the C(O)R dissociation (9.5 107 s1), CH2R dissociation (4.7 104 s1), 1,5-H shift (3.3 106 s1), and reaction with O2 (4.7 104 s1). 98% of the alkoxy radical a2 undergoes C(O)R dissociation, namely, the ringopening reaction, to form the acyl radical a3. The acyl radical a3 reacts with oxygen and NO to form the acyloxy radical a4. 4% of a4 decomposes to CO2 and the alkyl radical a5. The alkyl radical a5 reacts with oxygen and NO to form the alkoxy radical a6. 44% of a6 again undergoes ring-opening to form the tertiary alkyl radical a7. The formed tertiary alkyl radical a7 reacts with oxygen and NO to form the alkoxy radical a8. 98% of a8 undergoes the 1,5-H shift to form the acyl radical a9. The acyl radical a9 reacts with oxygen and NO to form the acyloxy radical a10. 15% of a10 undergoes the 1,6-H shift to form the acyl radical a11. The acyl radical a11 reacts with oxygen to form the acylperoxy radical a12. The acylperoxy radical a12 reacts with HO2 to form diaterpenylic acid (a13) and ozone (Moortgat et al., 1989). Diaterpenylic acid a13 undergoes selfdehydration to form terpenylic acid (a14). The saturation concentrations of a13 and a14 were evaluated using SPARC on-line calculator (Hilal et al., 2003) to be 6.8 and 45 mg m3, respectively (Table S1), suggesting that substantial fractions of a13 and a14 are present in the condensed phase. The dehydration of diaterpenylic acid likely occurs on the particle surface or in the liquid phase. Yasmeen et al. (2010) suggested that the dehydration from the hydroxyl group of diaterpenylic acid and carboxylic acid takes place in particle phase; this supports the self-dehydration mechanism suggested in this study for diaterpenylic acid. Of the reaction mechanism mentioned in this paragraph, the formation pathway for the alkoxy radical a8 from the oxidation of nopinone is the same as that suggested by Master Chemical Mechanism version 3.2 (MCM; Jenkin et al., 1997; Saunders et al., 2003; Jenkin, 2004). The formation mechanism for terpenylic acid from the reaction of the
alkoxy radical a8 is newly suggested in this study. The yield of diaterpenylic acid from nopinone photooxidation was estimated to be 0.11% at most; this maximum value was calculated by multiplying all the branching ratios referred and estimated here. The estimated maximum yield value was lower than the yield of terpenylic acid from nopinone photooxidation determined in run 6 (1.6%), suggesting that other formation pathways for terpenylic acid are available or the structure activity relationship rate constants for the alkoxy unimolecular reactions underestimate the yield of terpenylic acid. The present results showed that terpenylic acid is formed from the ozonolysis of both a- and b-pinene. Claeys et al. (2009) suggested that terpenylic acid is formed from a-pinene ozonolysis via the heterogeneous reaction of a-pinene oxide. However, no formation mechanism of terpenylic acid from b-pinene ozonolysis has been suggested. The acyloxy radical a4 is suggested to be produced during the ozonolysis of a- and b-pinene by the MCM model. The formation mechanism for terpenylic acid suggested in this study will also explain the formation of terpenylic acid from the ozonolysis of a- and b-pinene. The formation mechanisms for the acyloxy radical a4 from the ozonolysis of a- and b-pinene, as shown in Fig. 5, are the same as those proposed by the MCM model. The acyloxy radical a4 leads to terpenylic acid formation by a similar reaction pathway as that described for nopinone photooxidation. Diaterpenylic acid is a key monomer of ester dimer formation during the ozonolysis of a- and b-pinene (Yasmeen et al., 2010). The direct formation of diaterpenylic acid consistently explains the formation of ester dimers from a- and b-pinene ozonolysis under the present dry conditions. 3.5. Predicted formation pathways for C9 multifunctional compounds As the molecular structures of C9H14O5 and C9H14O6 were not suggested from the present experimental results, the probable formation pathways for C9H14O5 and C9H14O6 are predicted based on the rate constants for the unimolecular reactions of alkoxy radicals (Kroll and Seinfeld, 2008; Vereecken and Peeters, 2010). Fig. 6 shows the predicted formation pathways for C9
Fig. 5. Suggested formation mechanisms for terpenylic acid from the OH-initiated nopinone oxidation and the ozonolysis of a- and b-pinene.
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K. Sato et al. / Atmospheric Environment xxx (2015) 1e9
multifunctional compounds from the OH-initiated oxidation of nopinone. The branching ratios for H abstraction from the C-a, Cb, C-c, C-d, and C-e carbons of nopinone were evaluated to be 44%, 13%, 10%, 3%, and 23%, respectively (Lewis et al., 2005). Only the most probable pathway for C9-hydroxy-dicarbonyl-carboxylic acid or C9-carbonyl-dicarboxylic acid from each H abstraction pathway was depicted, although we found two or more formation pathways from each H abstraction pathway. Note that the formation of other isomers that are not depicted here are also possible. (a) If we assume the H abstraction from C-a carbon, only very minor pathways were found for C9H14O5 products. The products formed by the H abstraction from C-a carbon mainly undergo dissociation pathways. (b) The H abstraction from C-b carbon leads to the formation of the alkyl radical b1. The formed radical reacts with oxygen and NO to form the alkoxy radical b2. 4% of b2 undergoes ring-opening to form alkyl radical b3. The formed alkyl radical b3 reacts with oxygen and NO to form the alkoxy radical b4. 80% of b4 undergoes ring-opening to form tertiary alkyl radical b5. The tertiary alkyl radical b5 reacts with oxygen and NO to form the alkoxy radical b6. 75% of b6 undergoes the 1,5-H shift to form the acyl radical b7. The acyl radical b7 reacts with oxygen to form the acylperoxy radical b8. This radical reacts with HO2 to form the C9-hydroxyldicarbonyl-carboxylic acid b9. The b9 product isomerizes to the cyclic hemiacetal b10 in the condensed phase (Lim and Ziemann, 2009). The yield of the b9 product from nopinone photooxidation was estimated to be 0.29% at most. (c) The H abstraction from C-c carbon leads to the formation of the tertiary alkyl radical c1. The formed radical reacts with oxygen and NO to form the alkoxy radical c2. 97% of c2
7
undergoes ring-opening to form the tertiary alkyl radical c3. This radical reacts with oxygen and NO to form the alkoxy radical c4. 77% of c4 undergoes 1,5-H shift to form the alkyl radical c5. The formed alkyl radical reacts with oxygen and NO to form the alkoxy radical c6. 97% of the resulting radical undergoes ring-opening to form the acyl radical c7. This acyl radical reacts with oxygen to form the acylperoxy radical c8. The acylperoxy radical c8 reacts with HO2 to form the C9hydroxyl-dicarbonyl-carboxylic acid c9. This product undergoes cyclization to form the cyclic hemiacetal c10 in the condensed phase. An aldehyde group of the cyclic hemiacetal c10 is oxidized in the gas phase or in the condensed phase to form the cyclic hemiacetal c11. A ring-opening product would also be formed from the decomposition of c11 in the condensed phase, although this product is not shown. The yield of the c9 product from nopinone photooxidation was estimated to be 7.2% at most. (d) The H abstraction from C-d carbon leads to the formation of the alkyl radical d1. The formed radical reacts with oxygen and NO to form the alkoxy radical d2. 10% of the alkoxy radical d2 undergoes ring-opening to form the alkyl radical d3. This radical again reacts with oxygen and NO to form the alkoxy radical d4. 94% of d4 undergoes ring-opening to form the acyl radical d5. The acyl radical d5 reacts with oxygen and NO to form the acyloxy radical d6. 5% of d6 undergoes the 1,6-H shift to form the acyl radical d7. The formed radical reacts with O2 to form the acyl peroxy radical d8. The acyl peroxy radical d8 reacts with HO2 to form the C9-carbonyldicarboxylic acid d9. The radical d9 oxidizes to form the C9tricarboxylic acid d10. The yield of the d9 product from nopinone photooxidation was estimated to be 0.014% at most.
Fig. 6. Predicted formation pathways and structures of C9 multifunctional products based on the rate constants for unimolecular reactions of alkoxy radicals (Kroll and Seinfeld, 2008; Vereecken and Peeters, 2010).
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(e) The H abstraction from C-e carbon leads to the formation of the tertiary alkyl radical e1. The formed radical reacts with oxygen and NO to form the alkoxy radical e2. 94% of e2 undergoes ring-opening to form the acyl radical e3. The formed radical reacts with oxygen and NO to from the acyloxy radical e4. 5% of e4 undergoes the 1,6-H shift to form the alkyl radical e5. The formed radical reacts with oxygen and NO to form the alkoxy radical e6. The formed radical mainly undergoes ring-opening to form the acyl radical e7. This radical reacts with oxygen to form the acylperoxy radical e8. The resulting radical reacts with HO2 to form the C9-carbonyldicarboxylic acid e9. The oxidation of the e9 product also forms the C9-tricarboxylic acid d10. The yield of the e9 product from nopinone photooxidation was estimated to be 1.1% at most. Of the reaction mechanisms shown in Fig. 6, the formation pathways for the alkoxy radicals b6, c4, and d2 from the oxidation of nopinone are the same as those suggested by the MCM model. The oxidation mechanisms of the alkoxyl radicals b6, c4 and d2 and the oxidation mechanism of nopinone via the tertiary alkyl radical e1 are newly suggested in this study. Among the products with m/z 201.08, the b10, c10, d9, and e9 products had the saturated concentrations in a range 0.1e2.4 mg m3 as listed in Table S1, suggesting that these products exist mainly in the condensed phase. On the other hand, the b9 and c9 products had the saturated concentrations in a range 61e102 mg m3, suggesting that these products exist mainly in the gas phase under ambient conditions although these products exist mainly in the particle phase under present experimental conditions. The products with m/z 217.07, i.e., the d10 and c11 products, had the saturated concentrations lower than 102 mg m3, suggesting that these products are low-volatility organic compounds which exist basically in the condensed phase.
4. Conclusions In this study, the gases and particles formed from the ozonolysis of b- and a-pinene were exposed to OH radicals in a laboratory chamber. The total OH exposure in this study was estimated to be 1.5e3.6 1011 molecules cm3 s. As a result of OH exposure, the SOA particle concentration increased. SOA particle formation from the photooxidation of nopinone, formed from the ozonolysis of bpinene, contributes to the increase in the particle concentration during the OH exposure. Pinic acid and terpenylic acid are abundantly present in the b- and a-pinene ozonolysis particles. Terpenylic acid and C9H14O5 products present in b-pinene SOA particles increased upon exposing SOA to OH radicals, whereas MBTCA present in a-pinene SOA particles increased upon exposing SOA to OH radicals. Terpenylic acid and similar C9H14O5 products were also formed from the photooxidation of nopinone. The OH-initiated oxidation of nopinone contributes to terpenylic acid and C9H14O5 formation during the aging of b-pinene SOA. The formation mechanism for terpenylic acid from nopinone oxidation was newly suggested. Diaterpenylic acid formed from the oxidative doublering opening of nopinone undergoes self-dehydration to form terpenylic acid in the condensed phase. The formation of terpenylic acid from the ozonolysis of a- and b-pinene can also be explained by similar processes. The formation pathways for C9H14O5 from nopinone photooxidation and the molecular structures of C9H14O5 were predicted based on the rate constants for the unimolecular reactions of alkoxy radicals. The predicted structures of C9H14O5 were C9-carbonyl-dicarboxylic acid and the acyclic and cyclic forms of C9-hydroxyl-dicarbonyl-carboxylic acid.
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Please cite this article in press as: Sato, K., et al., Terpenylic acid and nine-carbon multifunctional compounds formed during the aging of bpinene ozonolysis secondary organic aerosol, Atmospheric Environment (2015), http://dx.doi.org/10.1016/j.atmosenv.2015.08.047