Gas-Phase Ozone Oxidation of Monoterpenes: Gaseous and ...

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244 JIANZHEN YU ET AL. aldehyde (8%), 3-caronic acid (4%), hydroxy caronaldehydes (3%), nor-3-caronic acid and isomers (2%), C 5 (2%), 3-caric acid (2%), hydroxy-3-caronic acid (1%), and pinic acid (1%). Caronaldehyde has been identified as a major product in a previous study with a yield of ≤8% in the gas phase (Hakola et al., 1994). Here the gas-phase caronaldehyde yield is estimated to be 7.8%. All the products added together account for 61% of the organic aerosol mass formed. Major aerosol products that contribute to >10% of organic aerosol mass are 3-caronic acid and 3-caric acid. Glasius et al. (1998) reported three aerosol products, 3-caric acid, 3-caronic acid, and nor-3-caric acid with aerosol phase yields of 0.5–5%, 0.1–0.7%, and 0.08–0.1%, respectively. 5. Gas-Particle Partitioning Many products identified in this study are detected in both gas and aerosol phases. Gas-particle partitioning for semi-volatile organic compounds have been successfully described by the equilibrium relation: K om,i = F i , (2) A i M o where K om,i is the gas-particle partitioning coefficient of species i, A i is the gas phase concentration (µg m −3 ) of compound i, andF i is the concentration of compound i (µg m −3 ) in the aerosol phase (Odum et al., 1996). A i and F i can be determined directly from denuder and filter samples, and M o is obtained from aerosol size distribution measurements. Table XIII lists the gas-particle partitioning coefficients of aerosol components generated in the ozone oxidation of the four monoterpenes. Gas-particle partitioning coefficients of semi-volatile compounds depend on their vapor pressures (Pankow, 1987); K om,i = 760RT MW om 10 6 ζ i pL,i o , (3) where R is the ideal gas constant (8.206 × 10 −5 m 3 atm mol −1 K −1 ), T is temperature (K), MW om is the number-averaged molecular weight of the organic matter (om) phase (g mol −1 ), ζ i is the molar fraction scale activity coefficient of compound i in the om phase, and p o L,i is the sub-cooled liquid vapor pressure (mm Hg) of compound i. Sub-cooled liquid vapor pressures of the major products in the β-pinene/O 3 and α-pinene/O 3 systems have been estimated using a modification of the Clausius–Claperyron equation (Schwarzenbach et al., 1993) (Table XIV). Measured K om,i values versus the estimated p o L,i are shown in Figure 10. As Figure 10 illustrates, the lower the vapor pressure, the higher the corresponding K om,i . For example, hydroxy pinonic acid, as a result of the presence of three polar functional groups, has the lowest vapor pressure (∼10 −5 mm Hg) among all identified products, and consequently exhibits the highest K om,i value. On the
Table XIII. Gas-particle partitioning coefficients (m 3 µg −1 ) of individual products at 306–308 K α-Pinene/O 3 3 -Carene/O 3 β-Pinene/O 3 Sabinene/O 3 Compound K om,i Compound K om,i Compound K om,i Compound K om,i Pinic acid 0.028 ± 0.001 3-Caric acid 0.033 Pinic acid 0.035 ± 0.009 Sabinic acid 0.027 Hydroxy 0.019 ± 0.013 Hydroxy 0.0010 Hydroxy pina- 0.0091 ± 0.0054 Hydroxy sabina 0.0037 pinonaldehydes caronaldehydes ketones ketones Pinonic acid 0.030 ± 0.005 3-Caronic acid 0.0090 Norpinonic acid and 0.013 ± 0.005 Nor-3-caronic acid 0.0047 Norpinonic acid and 0.014 ± 0.001 Norsabinonic acid 0.018 its isomers its isomers and its isomers Pinonaldehyde 0.0012 ± 0.0003 Caronaldehyde 0.0014 3-Oxo-pine ketone 0.0010 ± 0.0005 3-Oxo-sabina ketone 0.0012 Hydroxy pinonic 0.040 ± 0.010 Hydroxy caronic 0.0231 Hydroxy pinonic 0.066 ± 0.026 Sabina ketone 0.00053 acid acid acid A 13 0.025 ± 0.003 Hydroxy norpinonic 0.016 ± 0.009 acid GAS-PHASE OZONE OXIDATION OF MONOTERPENES 245
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