Production of OH Radicals from the Reactions of C4–C6 Internal Alkenes and Styrenes with Ozone in the Gas Phase Grazyna E

Atmospheric Environment 36 (2002) 571–581

Production of OH radicals from the reactions of C4–C6 internal alkenes and styrenes with ozone in the gas phase Grazyna E. Orzechowskaa, Suzanne E. Paulsonb,*

a Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90095-1565, USA b Department of Atmospheric Sciences, University of California at Los Angeles, Los Angeles, CA 90095-1565, USA

Received 17 March 2001; accepted 21 August 2001

Abstract

OH formation from the ozonolysis reactions of seven internal alkenes with 4–6 carbons, styrene, trans-b-methyl styrene, and a-methyl styrene was studied using complementary techniques. A small-ratio relative-rate technique in which small quantities of OH tracers are added to monitor OH formation yields provided the following results: trans-2- butene, 0.6470.12; cis-2-butene, 0.3370.05; trans-2-pentene, 0.4670.08; cis-2-pentene, 0.2970.06; trans-3-hexene, 0.5370.08; cis-3-hexene, 0.3670.07; and 2-methyl-2-butene, 0.9870.24. For styrene, trans-b-methyl styrene, and a- methyl styrene, OH yields of 0.0770.04, 0.2270.09, and 0.2370.12 were measured, respectively. A second method, which monitors product formation from the OH reaction with 2-butanol was used to derive OH formation yields from 2,3-dimethyl-2-butene, 2-methyl-2-butene and cis-2-pentene, and provided yields of 0.9170.14, 0.8070.12, and 0.2770.07, respectively. The results are brieﬂy discussed in terms of the relationship between structures of these alkenes and OH formation. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Urban air; Alkene ozonolysis; OH radical; Indoor air

1. Introduction ppb levels (Table 1). Finally, styrene is a hazardous air pollutant under the 1990 Clean Air Act, and it is Urban environments contain a complex mixture observed in ambient and indoor air in the mid-pptC to of non-methane hydrocarbons (NMHCs) with mixing ppbC range (Chan et al., 1990; Rothweiler et al., 1992; ratios of 100–2000 ppbC (Jeffries, 1995; Brasseur et al., Zielinska et al., 1996; Fraser et al., 1998). 1999). Of this, alkenes account for about 10% of In the boundary layer, HOx ( ¼ OH, HO2,RO2) the total (Jeffries, 1995; Seinfeld, 1995), with terminal production from ozone–alkene reactions is a significant, alkenes the most abundant. Because of their reactivity, sometimes dominant contribution to the total HOx internal alkenes are generally at mid-ppt C to low-ppb C production during both day and night (Paulson and levels in ambient air. Data sets from the Southern Orlando, 1996; Ariya et al., 2000). For the particular California air quality study in ambient air (SCAQS) conditions in Los Angeles in 1987 and 1993 studied by in 1987 (Lurmann and Main, 1992), Los Angeles during Paulson and Orlando (1996), the alkene–ozone reaction summer of 1997 (George et al., 1999), the EPA 29 City generated more HOx than did O3 photolysis for most of Average (Jeffries, 1995), Atlanta roadway in summer the day. Although the internal alkenes are at the low ppb 1990 (Conner et al., 1995), and the BERLIOZ-1998 levels, they are substantial sources of HOx radicals. The campaign in rural Germany (Konrad et al., 2000), internal alkenes tend to generate more OH and react revealed concentrations of internal alkenes of 0.006–3 more rapidly with ozone (by a factor of 10 or more) than do terminal alkenes (e.g., Atkinson, 1997). For the Los *Corresponding author. Angeles data set 77% of the HOx source from ozone– E-mail address: [email protected] (S.E. Paulson). alkene reactions was estimated to come from internal

Table 1 Internal alkenes in ambient air at several locations

Alkene concentration (ppb)

Alkene/Location SCAQS EPA 29 city Los Angeles Atlanta Roadway BERLIOZ rural 1987a averageb 1997c 1990d Germany 1998e trans-2-butene 0.25 1.05 0.02 2.0 0.01 cis-2-butene 0.23 0.88 NDf 1.6 0.01 trans-2-pentene ND 0.84 ND 2.9 0.008 cis-2-pentene 0.18 1.7 ND 1.6 0.008 trans-3-hexene ND ND ND 0.8 ND cis-3-hexene ND ND 0.12 ND 0.006 2-methyl-2-butene ND 0.06 ND 3.6 0.008

a Lurmann and Main (1992). b Jeﬀries (1995), based on 1 ppmC total hydrocarbons. c George et al. (1999). d Conner et al. (1995). e Konrad et al. (2000). f ND=no data.

cis-2-butene trans-2-butene cis-2-pentene trans-2-pentene cis-3-hexene trans-3-hexene

2-methyl-2-butene 2,3-dimethyl-2-butene styrene α-methyl styrene trans-β-methyl styrene

Fig. 1. Structures of C4–C6 internal alkenes and styrenes included in this study.

alkenes, and 63% from C5 or larger internal alkenes, for Rathman et al., 1999; McGill et al., 1999; Rickard et al., which OH yield measurements had never been made. 1999). Low pressure OH yields have also been reported Paulson and Orlando (1996) assumed that the OH for trans-2-butene, trans-3-hexene, and 2,3-dimethylbu- radical yields were the same for all internal alkenes in a tene (Kroll et al., 2001a). The OH formation yields for homologous series. However, OH formation from both the six other compounds investigated here are reported terminal and cyclo-alkenes drops oﬀ dramatically as the for the ﬁrst time. alkyl chain lengths increase (Paulson et al., 1999a; The OH yields were obtained either by using a small- Fenske et al., 2000a). This suggests that the relationship ratio relative-rate technique (SRRRT) developed in this between OH formation and alkene structure cannot be laboratory (Paulson et al., 1999b) or the 2-butanal predicted solely based on the immediate substituents on method developed by Chew and Atkinson (1996). The the double bond, as has been suggested (Atkinson, 1992; SRRRT takes advantage of the behavior of kinetics when Rickard et al., 1999), and underscores the importance of small quantities of tracers are added. The majority of OH measuring OH yields, particularly for alkenes with the reacts primarily with the alkene rather than the tracer, highest impact. but a large fraction of tracer is consumed. The 2-butanol In the present work, we investigate OH radical technique (Chew and Atkinson, 1996), in contrast, is formation from a series of larger internal alkenes for based on the determination of a product of the reaction which yields have not been previously reported, as well of OH with the tracer (2-butanone). An excess of the as yields for a series of styrenes, shown in Fig. 1. tracer scavenges over 95% of the OH radicals. Previous measurements of OH formation yields at atmospheric pressure (Table 4) have been reported for trans-2-butene (0.24–0.64), cis-2-butene (0.17–0.41), 2- 2. Mechanism of alkene–ozone reactions methyl-2-butene (0.81–0.93), and 2,3-dimethyl-2-butene (0.36–1.00) (Niki et al., 1986; Atkinson and Aschmann, A mechanism pioneered by Criegee (1975) describing 1993; Chew and Atkinson, 1996; Gutbrod et al., 1997; alkene–ozone chemistry continues to be investigated in G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581 573

kTr 0 atmospheric studies. Ozone adds across the double bond Tracer þ OH þ M - R O2þM: ðR6Þ to form aprimaryozonide, which decomposes to form a vibrationally excited carbonyl oxide plus a carbonyl An OH yield may be derived from an analytical compound. For cis-2-butene: expression obtained from solving the ordinary differ- ential equations describing the kinetics of R4–R6, O * but the most accurate way to calculate the OH yield is OO ðR1Þ +O3 by solving the complete system of equations, including reactions of products and wall losses. The analytical and numerical solutions generally fall within 20% of O* one another (Paulson et al., 1999b). The data in O O O , + O this study were analyzed numerically. The OH yields O O O ðR2Þ for the internal alkenes using SRRRT were determined SYN ANTI for each tracer/alkene combination (usually two to Carbonyl Oxides Acetaldehyde four experiments) by comparing the output of the numerical model, described briefly below, with the data The resulting carbonyl oxides can have either a syn or Pusing a least squares fitting procedure. For each alkene, 2 anti configuration. There is a relatively high barrier to ðye ymÞ was calculated for all experimental data; interconversion between syn and anti carbonyl oxides ye and ym are the percentage of tracer reacted in the (DH0B30 kcal/mol; Anglada et al., 1999; Cremer et al., experiment and model, respectively. This process 1998; Fenske et al., 2000a). The syn isomers can undergo was repeated for model runs with different OH yields arapid1,4-hydrogen shift ( DH0B15 kcal/mol) (R3), until this quantity had been minimized. Random error and the resulting vinyl hydroperoxide can easily cleave limits (2s) were derived by varying the assumed OH to produce OH plus alkoxy radicals yield until 95% of points lay either above or below (DH0 ¼ 10215 kcal/mol). Because of the low barrier, it the calculated curve. The styrene experiments were is likely that most syn carbonyl oxides formed from gas analyzed individually, as described previously (e.g., phase reactions will produce OH. Fenske et al., 2000a). The numerical model is built on one developed for O O * * O HO propene experiments, and includes detailed RO chem- O H O OH + O 2 istry, wall losses and tracer oxidation reactions (Paulson H SYN ðR3Þ et al., 1999b). Any uncertainty in the tracer-OH rate constant directly translates into uncertainty in the OH Anti carbonyl oxides have accessible pathways to form yield, but calculated yields are not particularly sensitive 0B dioxiranes (DH 20 kcal/mol, Cremer et al., 1998; to assumptions made about the products. In these Anglada et al., 1999; Fenske et al., 2000a). These may analyses we used OH and O3-hydrocarbon reaction rate undergo rearrangement to carboxylic acids or, at higher constants recommended by Atkinson (1986, 1992, 1997) pressure, bimolecular oxygen atom transfer reactions or Kramp and Paulson (1998), with the following upon collision (Adam et al., 1989; Cremer et al., 1998). exceptions: rate constants for cis and trans-3-hexene Thus, the anti species are expected to give OH have not been measured, and were assumed to equal the inefficiently, if at all. averages of their respective homologues of 2-pentene and 2-heptene (Atkinson, 1997); values of 6.7 and 6.8 1011 cm3 molecule1 s1, respectively were used. 3. Experimental approach Styrene rate constants for reactions with OH and O3 of 5.8 1011 and 1.7 1017 cm3 molecule1 s1 (Atkin- The SRRRT (Paulson et al., 1999b), uses a small son, 1989; Tuazon et al., 1993), respectively, were quantity of OH tracer (i.e., o10% of the alkene assumed. The rate constant for the OH reaction with concentration), such that most of the OH reacts with a-methyl styrene was measured by Bignozzi et al. (1988) the alkene. Under these conditions, up to 45% of the in the same study with styrene, and was reported to be tracer is consumed. Tracers that react rapidly with OH the same as OH reaction with styrene, thus we have used but slowly with O3 are preferred: 1,3,5-trimethylbenzene the rate constant for styrene measured by Atkinson and (TMB), m-xylene (XYL), di-n-butyl ether (DBE), and Aschmann (1988) for a-methyl styrene. di-n-propyl ether (PRE), were chosen as tracers. The 2-butanol relative-rate technique (Chew and Atkinson, 1996) was used as a complementary method kO3 Alkene þ O3 - OH þ RO2þother products; ðR4Þ to determine OH formation yields. In these experiments a large excess of 2-butanol is added to scavenge X98% of OH, and 2-butanone formation is used to monitor the -kA Alkene þ OH þ M RO2þM; ðR5Þ extent of OH reaction. Since 2-butanone is formed 574 G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581 directly from the a-hydroxybutyl radical intermediate 5. Results reacting with O2 (R8), this method avoids uncertain RO2 chemistry. 5.1. C4–C6 internal alkenes - d CH3CHðOHÞCH2CH3 þ OH CH3ðC ÞðOHÞCH2CH3 Initial conditions and resulting OH yields for cis-and ðR7Þ trans-2-butene, 2-pentene, 3-hexene, and 2-methyl-2- butene experiments using the small-ratio relative-rate technique (SRRRT) and 2-butanol technique are CH ðCdÞðOHÞCH CH þ O -CH ðOÞCH CH þ HO : 3 2 3 2 3 2 3 2 summarized in Tables 2 and 3, respectively. The ðR8Þ calculated OH yields in these tables include only random error and must be combined with systematic errors of Chew and Atkinson (1996) determined the yield of 2- 710–25% to generate the final uncertainties (e.g., butanone from the OH reaction with 2-butanol, which Paulson et al., 1999b). The final OH formation yields maybe combined with a small correction for OH together with values from the literature are summarized reaction with 2-butanone. The resulting factor, in Table 4, as follows: trans-2-butene, 0.6470.12; 7 0.695 0.073, was used to convert the measured 2- cis-2-butene, 0.3370.05; trans-2-pentene, 0.4670.08; butanone concentrations to give the quantity of 2- cis-2-pentene, 0.2970.06; trans-3-hexene, 0.5370.08; butanol that has reacted with OH. Slopes of plots of cis-3-hexene, 0.3670.07; and 2-methyl-2-butene, 2-butanone vs. alkene reacted were obtained using a 0.9870.24 using SRRRT. Fig. 2 shows representative least-squares regression analysis, and were then SRRRT results; all data from experiments using this corrected with this factor to determine the OH yield. technique and TMB as the tracer are plotted for the 2-butenes and 2-pentenes in Fig. 2.1 The 2-butanol method (OH yields in parentheses) was used for cis-2- 4. Experimental description pentene (0.2770.07), 2-methyl-2-butene (0.8070.12), and 2,3-dimethyl-2-butene (0.9170.14); these data are The chamber experiments have been described in plotted in Fig. 3. detail elsewhere (Paulson et al., 1999b) and are only OH yields have been measured previously for cis and briefly described here. Experiments were carried out at trans-2-butene, 2-methyl-2-butene and 2,3-dimethyl-2- 29672 K in Teflon chambers in the dark. Hydrocarbons butene (Table 4). Our values are all in good agreement were evaporated (liquids) or injected (gases) into a with those of Atkinson and Aschmann (1993) and Chew stream of purified air (Thermo-Environmental Model and Atkinson (1996), who used either the cyclohexa- 111) as the chamber was filled. Ozone was generated in none/cyclohexanol method or the 2-butanol technique, aliquots by flowing pure O2 at 0.1 l/min for 12–60 s, and with the values from Marston and co-workers through a mercury lamp generator (JeLight PS-3000- (Rickard et al., 1999; McGill et al., 1999) using a 30). The initial hydrocarbon concentrations were scavenger technique similar to SRRRT. The Gutbrod determined 30–60 min after filling the chamber. Next, a et al. (1997) values, derived by using CO as a scavenger, series of O3 aliquots were added, each immediately are in most cases lower than measurements using other following injection of a sample into the GC to allow methods, possibly for the reasons discussed by Horie maximum time for mixing and reaction before the next and Moortgat (1998). Niki et al. (1986) postulated OH measurement. Experiments lasted 3–6 h and had average formation and estimated its yield from excess 2,3- O3 concentrations of 0.5 ppm or less. dimethyl-2-butene consumption compared to O3 con- Purchased hydrocarbons had a stated purity of 99% sumption, finding ayield of 0.7 70.1, also in reasonable or better and were used as received. Cis and trans-3- agreement with more recent values. hexene were purchased from ChemSampCo. (Trenton, Our results using the 2-butanol method are within NJ). All other hydrocarbons were obtained from about 20% or better of our SRRRT results, under- Aldrich. Hydrocarbon concentrations were monitored scoring the fact that perfect accuracy free of systematic throughout the experiments with a gas chromatograph/ errors in such acomplex system is difficult to achieve.It flame ionization detector (GC/FID) (Hewlett-Packard should also be noted that from the existing literature 5890), equipped with aDB-1 (0.32 mm ID, 1 mm film, (Table 4) there does not appear to be a bias between the 30 m) or DB-624 (0.32 mm ID, 1.8 mm film, 30 m) tracer (this work, McGill et al., 1999) and scavenger column (J&W Scientific). The GC was calibrated daily techniques (this work, Chew and Atkinson, 1996; with a4.9 70.1 ppm cyclohexane standard (Scott Speci- Atkinson and Aschmann, 1993). alty Gases); and the number of carbon atoms in each Kroll et al. (2001a) have made direct, semi-quantita- alkene was used to calculate the FID response normal- tive pressure dependent OH yield measurements from ized to the cyclohexane calibration (Scanlon and Willis, 1985). 1 Additional data plots are available from the authors. G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581 575

Table 2

Summary of initial conditions for C4–C6 experiments and calculated OH yields using the small-ratio relative-rate technique using tracers 1,3,5-trimethylbenzene (TMB); m-xylene (XYL); di-n-butyl ether (DBE); and di-n-propyl ether (PRE)

Alkene Initial concentration (ppm) Calculated OH yield72s random error

Alkene TMB XYL DBE PRE TMB XYL DBE PRE trans-2-butene 9.87 0.26 F 0.20 F 0.6070.02 0.7070.02 0.6370.01 F 9.63 0.27 F 0.22 F 10.4 0.29 F 0.23 F 9.50 0.27 F 0.20 F 10.7 0.29 0.28 FF 10.8 0.30 0.13 FF 9.04 0.25 0.29 FF cis-2-butene 13.1 0.35 F 0.27 F 0.3770.03 0.4070.04 0.3070.03 F 14.1 0.22 F 0.20 F 13.1 0.36 F 0.28 F 24.7 0.35 F 0.29 F 11.6 0.30 F 0.26 F 12.8 0.34 0.40 FF 17.7 0.29 0.31 FF 6.92 0.17 0.19 FF trans-2-pentene 8.64 0.08 FF0.22 0.4570.01 0.5070.01 F 0.4770.01 10.2 0.26 0.28 FF 7.10 0.25 0.27 FF cis-2-pentene 8.89 0.08 FF0.19 0.2570.03 0.3370.04 F 0.2870.04 9.51 0.24 FF0.28 10.1 0.21 FF0.21 10.7 0.25 0.29 FF 9.32 0.23 0.26 FF trans-3-hexene 8.42 0.21 FF0.26 0.5370.02 0.5370.02 F 0.5370.02 7.98 0.22 0.28 FF 8.11 0.22 0.29 FF cis-3-hexene 8.17 0.22 0.30 FF0.3570.03 0.3570.02 F 0.3770.06 8.84 0.25 FF0.30 8.57 0.24 FF0.30 2-methyl-2-butene 7.22 0.24 0.27 FF0.9570.05 1.0070.05 1.0070.06 F 8.31 0.27 F 0.22 F 8.31 0.25 F 0.19 F 7.44 0.25 F 0.18 F 8.71 0.18 F 0.20 F

ozone–alkene reactions using laser-induced fluorescence stabilization is necessary to facilitate OH production on ms timescales, for three of the alkenes studied here: from syn carbonyl oxides. trans-2-butene, trans-3-hexene and 2,3-dimethyl-2-butene. The highest pressures at which measurements were 5.2. Styrene, a-methyl styrene and trans-b-methyl styrene made were 50, 60 and 400 Torr, respectively. OH formation from each of these alkenes exhibits a large Initial conditions and resulting OH yields for the pressure dependence at pressures up to 60 Torr; OH styrenes are summarized in Table 5; data are plotted for from 2,3-dimethyl-2-butene flattens out above 100 Torr. trans-b-methyl styrene in Fig. 4. While experiments for If the results are normalized to 1 at low pressure, as C4–C6 alkenes were performed with nearly identical suggested by Kroll et al. (2001a), then the data suggest tracer/alkene ratios, and could thus be analyzed OH yields that are much lower than other measurements together, experiments for styrene were conducted with to date (Table 4). The source of this difference is a variable initial tracer to alkene ratios and were analyzed matter of some debate (e.g., Kroll et al., 2001b; Fenske individually. The uncertainties of OH yields for styrenes et al., 2000 b), however, it may arise from the difference shown in Table 5 reflect the random variability between in timescales; Kroll et al. (2001b) suggest that some experiments and need to be combined with a systematic 576 G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581 uncertainty of 730%. This data set has relatively high in the same range as the tracers, making the chromato- random errors because (1) in the case of styrene, the OH gram very congested and difficult to integrate accurately. yield is low but styrene reacts rapidly with OH, so that For styrene, the systematic uncertainty is also high very little of the tracer reacts. Data from the more slowly because the OH yield from O3 styrene is small, thus the reacting tracers DBE and XYL were discarded because importance of minor sources of OH and other tracer 4% or less of these tracers was consumed. (2) The losses is increased. For the other two compounds, the reactants and their products elute from the GC column largest uncertainty arises the OH–alkene reaction rate constant, which has not been measured. The average calculated OH yields and 2s experimental uncertainties Table 3 combined with systematic uncertainties for O3 reactions Summary of initial conditions and calculated OH yields for with styrene, trans-b-methyl styrene, and a-methyl experiments using the 2-butanol technique styrene are 0.0770.04, 0.2270.09 and 0.2370.12, Alkene Initial concentration OH yield respectively (Table 4). No previous measurements of (ppm) average72s OH formation from the O3 reaction with styrene or substituted styrenes have been made. Alkene 2-Butanol cis-2-pentene 2.63 1219 0.2770.07 2.68 1356 6. Discussion 3.00 1458 2-methyl-2-butene 2.41 1025 0.8070.12 As was pointed out by Atkinson and Aschmann 1.79 899 (1993) and others (Rathman et al., 1999; Rickard et al., 2.79 1466 1999; Paulson et al., 1999a), the broad features of OH 2.97 1684 formation from many alkenes may be explained by the 2,3-dimethyl-2-butene 2.13 888 0.9170.14 2.68 1456 availability of abstractable hydrogen atoms on the alkyl 0.644 786 groups adjacent to the carbonyl oxide moiety (R2). Presuming that OH formation is facile from syn

Table 4

Summary of OH yields for the reaction of a target alkene with O3 at room temperature and atmospheric pressure Alkene OH yield Reference

This work Others trans-2-butene 0.6470.12 0.64+0.32, 0.21 Atkinson and Aschmann (1993) 0.2470.02 Gutbrod et al. (1997) 0.5470.11 McGill et al. (1999) cis-2-butene 0.3370.05 0.41 +0.21, 0.14 Atkinson and Aschmann (1993) 0.1770.02 Gutbrod et al. (1997) 0.3370.07 McGill et al. (1999) trans-2-pentene 0.4670.08 FF cis-2-pentene 0.2970.06 FF 0.2770.07a trans-3-hexene 0.5370.08 FF cis-3-hexene 0.3670.07 FF 2-methyl-2-butene 0.9870.24 0.89+0.44, 0.30 Atkinson and Aschmann (1993) 0.8070.12a 0.9370.14a Chew and Atkinson (1996) 0.8170.16 McGill et al. (1999) 2,3-dimethyl-2-butene 0.9170.14a 0.8070.12a Chew and Atkinson (1996) 1.0 +0.5, 0.33 Atkinson and Aschmann (1993) 0.770.1 Niki et al. (1986) 0.3670.04 Gutbrod et al. (1997) 0.8970.24 Rickard et al. (1999) Styrene 0.0770.04 FF trans-b-Methyl styrene 0.2270.09 FF a-Methyl styrene 0.2370.12 FF

a Measured using 2-butanol as the OH scavenger. G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581 577

55 2.0 2, 3-dimethyl-2-butene 50

40 Trans 1.5 2-methyl-2-butene 35 Cis YOH = 0.60 + 0.02 30

25 1.0 YOH= 0.37 + 0.03 20

TMB reacted [%] 15 2-Butanone formed [ppm] formed 2-Butanone 0.5 10 cis-2-pentene

0 020406080100 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (a) 2-Butene reacted [%] Alkene reacted [ppm]

45 Fig. 3. 2-butanone formation vs. 2,3-dimethyl-2-butene, 2- methyl-2-butene, and cis-2-pentene reacted in the presence of 40 suﬃcient 2-butanol to scavenge >98% of OH radicals. The Trans 35 data for 2,3-dimethyl-2-butene have been displaced vertically by Y = 0.45 + 0.01 OH 0.5 ppm for clarity. 30

25 Cis 20 Y = 0.25 + 0.03 has a gauche Me–Me interaction, while in the syn- 15 OH

TMB reacted [%] transition structure, the two methyl groups are nearly 10 anti, suggesting that trans-2-butene should generate

5 more syn-acetaldehyde oxide than cis-2-butene, and thus afford more OH (Fenske et al., 2000a). Recent MP2 0 0 20406080100 calculations by Rathman et al. (1999) predict the same (b) trend, although the MP2 method favors the syn- 2-Pentene reacted [%] transition structures more than the B3LYP method. Fig. 2. Data (symbols) and model calculations (lines) for trans- While these calculations provide a potential explanation and cis-isomers of 2-butene and 2-pentene for experiments of the difference in OH yield for cis- vs. trans-alkenes, using the small-ratio relative-rate technique and TMB as a they do not explain why trans-2-pentene and trans-3- tracer: (a) 2-butene, (b) 2-pentene. hexene have lower OH yields than the C4 homologue, while the C4–C6 series of cis-alkenes have about the same OH yields. carbonyl oxides, but not from anti, and formation of syn OH formation from linear terminal alkenes decreases vs. anti carbonyl oxides is largely under statistical monotonically with chain length (Paulson et al., 1999a). control, OH formation from unsubstituted internal This behavior can be qualitatively explained with the alkenes should be about 50%, 75% from a-methyl following arguments (Hasson et al., 2001). Ab initio internal alkenes and 100% from di-methyl substituted calculations suggest that the activation energies of the internal double bonds. Significant deviations from this four possible cycloreversion reactions of the primary simple model are clearly observed; trans alkenes produce ozonide (R2) are essentially independent of the carbon- more OH than cis isomers, and 2-methyl-2-butene chain length, and that pathway to formation of the anti- produces more than 75% OH. carbonyl oxide has the smallest barrier. As the size of the We have presented a theoretical explanation of the primary ozonide increases along the series, the internal higher OH yield from trans vs. cis-2-butene using energy of this species becomes distributed among a B3LYP/6-31G(d,p) calculations (Fenske et al., 2000a). greater number of degrees of freedom, and the lowest For cis-2-butene cycloreversion, the pathway leading to energy pathway becomes increasingly favored entropi- anti-acetaldehyde oxide is predicted to have an activa- cally. In this way the ratio of anti- to syn- carbonyl oxide tion barrier 3.0 kcal/mol lower than that leading to syn may increase along the 1-alkene series, causing the OH acetaldehyde oxide (Fenske et al., 2000a). For trans-2- yield to fall. butene, on the other hand, syn- and anti-transition states Ozonolysis of the internal alkenes may be complicated are predicted to have nearly the same energy. This is by the participation of diradical pathways (Fenske et al., because the otherwise favored anti-transition structure 2000b). In addition, OH may be formed from the 578 G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581

Table 5 Initial conditions and OH yields for styrene experiments

a Expt. no. Alkene Initial concentration (ppm) Calculated YOH Average YOH72sm Alkene XYL (X) or TMB DBE (D)

SBT1031 Styrene 7.60 0.408Db 0.861 0.07 0.0770.03 SXT717 10.3 0.539Xb 0.426 0.08 SXT718 9.44 0.44Xb 0.365 0.06 ST57 9.72 F 0.466 0.09 ST519 8.89 F 0.426 0.09 ST65 7.62 F 0.469 0.05 TMST79 trans-b-Methyl styrene 10.9 F 0.519 0.15 0.2270.04 TMST78 10.5 F 0.516 0.20 TBSXT722 10.8 0.433X 0.419 0.17 0.17 TBSXT825 14.3 0.519X 0.478 0.17 0.25 TBSXT305 9.62 0.499X 0.436 0.29 0.29 TBSXDT311 6.31 0.212D 0.274 0.24 0.24 AMSXT724 a-methyl styrene 10.4 0.484X 0.422 0.20 0.2370.06 0.19 AMSXT614 4.36 0.548X 0.380 0.32 0.22

a sm=standard deviation of the mean. b These results were discarded because less than 4% of the tracer reacted.

30 themselves be sources of OH (Gutbrod and Kraka, EXPT. TRACER YOH 1997; Fenske et al., 2000a). Laser-induced ﬂuorescence TBSXT-722 TMB 0.17 experiments by Gutman and Nelson (1983) and by 25 TBSXT-722 XYL 0.17 TBSXT-825 TMB 0.17 Lorenz et al. (1985) reveal OH to be one of the minor TBSXT-825 XYL 0.25 products of the reaction of the parent vinoxy radical 20 TBSDT-311 TMB 0.24 TBSDT-311 DBE 0.24 with O2. None of these complications, however, provides a clear explanation of the lower OH yields from trans-2- 15 pentene and trans-3-hexene relative to trans-2-butene. The OH yield from 2-methyl-2-butene, 0.8970.17 10 may be higher than 0.75 since addition of a methyl

Tracer Reacted [%] Reacted Tracer substituent to adouble bond perturbs the cycloreversion

5 transition states. Calculations by Fenske et al. (2000a) on 1-methylcyclohexene showed that the methyl group functions as an electron donor, stabilizing the incipient 0 0204060 partial positive charge on the carbon atom of the carbonyl oxide. This causes the methyl-substituted trans-b-Methylstyrene reacted [%] carbonyl oxide transition states to be lowest in energy. Fig. 4. Data (symbols) and model calculations (lines) for trans- Since methyl substituted carbonyl oxides always have b-methyl styrene. Tracers and corresponding OH yields abstractable hydrogen atoms, lower transition state assumed in the calculations are indicated in the legend. energies leading to their formation may result in higher OH yields. The OH yield for styrene (0.0770.04) is roughly 40% decomposition of vibrationally excited carboxylic acids, that of ethylene (yOH ¼ 0:1870:06; Paulson et al., which likely arise from the isomerization of dioxiranes 1999b), indicating that OH formation from styrene (Cremer et al., 1998). Finally, the vinoxy radicals co- may be due entirely to the C1 fragment; the vinyl generated with OH in the proposed mechanism (R3) can hydroperoxide channel (R3) is not available to the G.E. Orzechowska, S.E. Paulson / Atmospheric Environment 36 (2002) 571–581 579 phenyl substituted carbonyl oxide. Since the benzalde- Acknowledgements hyde formation yield is 41%, it is likely that its co- product, the C1 carbonyl oxide, should also be formed This research has been supported by a grant from the with a yield of about 40%, predicting an OH yield that is US Environmental Protection Agency National Center remarkably consistent with the observed OH formation. for Environmental Research’s Science to Achieve trans-b-methyl styrene and a-methyl styrene can be Results (STAR) program, through grant R826236-01- thought of as propenes with phenyl substituents on the 0. It has not been subjected to any EPA review and 1- and 2-carbon, respectively. This substitution appears therefore does not necessarily reflect the views of the to reduce the OH yield by about a third. Unfortunately, Agency, and no official endorsement should be inferred. aldehyde yields from these compounds have not been Assistance with styrene and 2,3-dimethyl-2-butene measured, so that the quantities of the different carbonyl experiments was provided by Myeong Y. Chung and oxides resulting from cycloreversion of the primary Andy W. Ho. The authors appreciate the helpful ozonide are not known. discussion and comments from Drs. Alam Hasson and Keith Kuwata.

7. Ambient and indoor air

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