The Gas Phase Reaction of Ozone with 1,3-Butadiene: Formation Yields of Some Toxic Products Franz Kramp!, Suzanne E

Atmospheric Environment 34 (2000) 35}43

The gas phase reaction of ozone with 1,3-butadiene: formation yields of some toxic products Franz Kramp!, Suzanne E. Paulson" *

!Forschungszentrum Ju( lich, Institut fu( r Chemie und Dynamik der Geospha( re (ICG-2), P.O. Box 1913, 52425 Ju( lich, Germany "Department of Atmospheric Sciences, University of California at Los Angeles, Los Angeles, CA 90095}1565, USA Received 23 April 1999; received in revised form 7 July 1999; accepted 9 July 1999

Abstract

The formation yields of acrolein, 1,2-epoxy-3-butene and OH radicals have been measured from reaction of ozone with 1,3-butadiene at room temperature and atmosphere pressure. 1,3,5-Trimethyl benzene was added to scavenge OH radicals in measurements of product yields. In separate experiments, small quantities of 1,3,5-trimethyl benzene were added as a tracer for OH. Formation yields of acrolein of (52$7)%, 1,2-epoxy-3-butene of (3.1$0.5)% and OH radicals of (13$3)% were observed. In addition, the rate coe$cient of the gas-phase reaction of ozone with 1,2-epoxy-3-butene was measured both directly and relative to propene, "nding an average of (1.6$0.4);10\ cm molecule\ s\, respectively, at 296$2 K. The results are brie#y discussed in terms of the e!ect of atmospheric processing on the toxicity of 1,3-butadiene. ( 1999 Elsevier Science Ltd. All rights reserved.

Keywords: 1,2-Epoxy-3-butene; Ozone rate constant

1. Introduction Air Act. The ozone reaction also produces 1,2-epoxy-3- butene (Atkinson et al., 1994). Even though this epoxide 1,3-Butadiene is a common atmospheric pollutant that is only formed in yields of a few percent, because of its has been found to be carcinogenic due to its epoxide mutagenic potential, the epoxide could contribute to the metabolites (Himmelstein et al., 1997). It was identi"ed risk due to butadiene inhalation for human health in both as a Hazardous Air Pollutant in the 1990 Clean Air smoggy cities. Act (US public law 101}549) and as a Toxic Air Con- Ozone reacts with alkenes by adding across the taminant under California's air toxics program (AB 1807) double bond to form a primary ozonide which rapidly in 1992. Butadiene is a very volatile and widely used decomposes to a carbonyl compound and a vibrationally industrial chemical as well as a product of the incomplete excited carbonyl oxide (see Scheme 1). combustion of gasoline and diesel fuels, and is widely Butadiene also generates some 1,2-epoxy-3-butane, observed in ambient air at concentrations in the low ppt probably via pathway (R1c), (Atkinson et al., 1994). In the to low ppb range, with an average around 0.2 ppb (subur- gas phase, carbonyl oxides can then be collisionally sta- ban air, (EPA, 1993; Schmitz et al., 1997). In-vehicle bilized or undergo decomposition and/or isomerization exposures are typically higher; around 1.3 ppb (EPA, (e.g., Herron and Huie, 1977; Su et al., 1980; Horie and 1993). It reacts in the atmosphere with OH, O and NO Moortgat, 1991). For the C carbonyl oxide: to produce a number of potentially toxic products. All S P(+) ** ++ # three reactions produce acrolein and formaldehyde, [HCOO] HCOO ( Thermalized ) M (R2a) which are also considered air toxics under the 1990 Clean HC(O)OH#M (R2b) P # 2H CO (R2c) # * Corresponding author. HO CO (R2d) # E-mail addresses: [email protected] (F. Kramp), paul- CO H (R2e) [email protected] (S.E. Paulson) OH#HCO. (R2f)

Scheme 1.

Table 1 Summary of product yields from the O reaction with 1,3-butadiene under atmospheric conditions

Compound Yield (%) Yield (%) Yield (%) Yield (%) Yield (%) (this work) (a)! (b) (c) (d)!

Acrolein 52$6 51 Observed Formaldehyde (HCHO) 65 Observed Ethylene (19 Acetylene (19 b-Propiolactone (1 2,5-Dihydrofuran (0.5 Trace 1,2-Epoxy-3-butene 3.1$0.5 2.3$0.4! Observed 1,2,3,4-Diepoxybutane (1 Trace OH radicals 13$38(#4!3) CO 47 CO 23 Formic anhydride 9

!Study did not use an OH scavenger. (a) Niki et al. (1983); (b) Atkinson et al. (1994); (c) Atkinson and Aschmann (1993), (d) Liu et al. (1999). Also observed (at low levels): glycolaldehyde, glycidaldehyde, hydroxy acetone, glyoxal, methylglyoxal, malonaldehyde, 2,3-butanedione, butenedial, and several C and C compounds with multiple carbonyl and/or hydroxy groups.

Several recent studies have investigated OH formation (probably minor) products that are not observable with from ozone}alkene reactions (e.g., Atkinson and Asch- gas chromatography or FTIR. mann, 1993; Paulson et al., 1997; Gutbrod et al., 1997; In this study we report formation yields for acrolein, Donahue et al., 1998). For many alkenes, OH yields are 1,2-epoxy-3-butene, the OH radical, and upper limits for large and as such, these reactions are a signi"cant source the yields of several minor products of the ozone}bu- of the chain-initiating radicals that drive oxidizing tadiene reaction. The rate constant for the reaction of photochemistry in urban and rural air (Paulson and 1,2-epoxy-3-butene with O was also measured using Orlando, 1996). both absolute and relative rate techniques. Su$cient The results of previous studies on the butadiene reac- concentrations of 1,3,5-trimethylbenzene (TMB) or di-n- tion with O are summarized in Table 1. Using Fourier butylether (DBE) were added to scavenge OH in the transform infrared spectroscopy (FTIR), Niki et al. (1983) product studies. The results are discussed in terms of the observed acrolein, formaldehyde, CO, and CO together reaction mechanism and the e!ect of atmospheric pro- with small quantities of ethene, acetylene, and formic cessing on the toxicological potency of 1,3-butadiene. anhydride from the 1,3-butadiene ozone reaction, but did not use an OH scavenger. Atkinson et al. (1994) measured an 1,2-epoxy-3-butene yield (using an OH scaven- 2. Experimental description ger), and Atkinson and Aschmann (1993) estimated the OH formation yield. Recently, Liu et al. (1999) used Experiments were carried out in 250 l Te#on chambers a semi-quantitative derivatization technique to investi- at atmospheric pressure (760$10 Torr) and 296$2K. gate formation of carbonyls, including multi-functionals. Liquid compounds were evaporated into a stream of In addition to the above carbonyls, they observed several puri"ed air (Thermo Environmental Model 111) as the F. Kramp, S.E. Paulson / Atmospheric Environment 34 (2000) 35}43 37 chamber was "lled; gasses were added directly using other loss process for TMB (apart from a small amount graduated micro syringes. The quality of the zero air was of wall loss, (1.5%) is unlikely. Rate constants show checked periodically and concentrations of hydrocar- that H atoms rapidly combine with O,O( D) and O( P) bons, nitrogen oxides and ozone were always below the rapidly react to form O, and while it is conceivable that detection limits (about 15, 3 and 5 ppb, respectively). the carbonyl oxides may react with TMB, the alkene is Once the gas mixture was prepared, the chamber was a far more likely target, as are water, organic acids and mixed by moving the walls and left to stand for at least aldehydes. Relative loss of tracer pairs including TMB, 1 h to allow the reactant concentrations to stabilize. di-n-butyl ether, m-xylene and several other tracers in Several measurements of the organic compounds in the the presence of ozone and alkenes have been studied dark chamber were made to establish the initial concen- by Paulson et al. (1997) and Marston et al. (1998); trations before addition of ozone. All organic compounds both studies are completely consistent with the tracers (DBE, TMB, 1,3-butadiene, acrolein, and 1,2-epoxy-3- reacting with OH. butene, Aldrich stated purity, 99, 98, 99, 96, and 98%, The rate coe$cient of the reaction between 1,2-epoxy- respectively), were used without further puri"cation. 3-butene and ozone was measured both directly and Ozone was generated by #owing pure oxygen relative to propene (Table 3). All experiments had added (150 ml/min) through a mercury lamp generator (JeLight, excess TMB to scavenge OH. For the relative measure- Model 600) and the ozone concentration inside the ments, ozone was added in three aliquots of about 2 ppm chamber was measured with UV absorption (Dasibi, each. In the direct measurements, O was added once Model 1003RS). and analyses of the hydrocarbon and ozone concentra- Experiments to investigate products of the 1,3-buta- tions were made every half hour during the 4}5 h experi- diene reaction with ozone are summarized in Fig. 3. ments. TMB or DBE were added in su$cient excess to trap Samples of organic compounds were drawn through more than 95% of the OH radicals formed. Ozone a heated Te#on line and analyzed with GC/#ame ioniz- was added every 30}60 min in aliquots, each resulting in ation detection (Hewlett-Packard 5890) equipped with l 0.5 to 2 ppm O inside the chamber. Analyses of a 30 m DB-1 column (J&W, ID 0.32 mm, 1 m "lm). The the composition of the reacting mixture were made every GC was calibrated daily using a 20.2$0.4 ppm n-hexane 30 min just before addition of the next ozone aliquot. standard (Scott Specialty Gases). Calibration standards Identi"cation of peaks was made both with authentic for TMB, DBE and propene (Scott Specialty Gases, all standards and with a gas chromatograph (GC)/mass certi"ed $2%) were run periodically and the per-car- spectrometer (Varian Saturn 2000 ion trap), with bon peak areas were compared to the n-hexane standard. electron impact ionization (EI) and acetonitrile chemical For TMB and propene, the agreement was better than ionization. $4%. Because of the oxygen atom in DBE, a peak area The small ratio relative-rate technique was used to equivalent to 7.3 carbons was observed. The concentra- measure OH formation (Paulson et al., 1999). Brie#y, tion of butadiene was calculated relative to the n-hexane small quantities of a tracer (TMB) that reacts with OH standard assuming a per-carbon peak area. This ap- but not O was added to monitor OH formation. Initial proach is very reliable for compounds containing tracer concentrations were less than 10% of the initial only carbon and hydrogen ($4%). In order to calcu- alkene concentrations (Table 2). Ozone was added in late concentrations for oxygenated products, the aliquots. To account for minor side-reactions such as loss FID peak areas were measured by two independent of the tracer to the wall and reactions of OH with methods. products, the data were analyzed numerically to derive First, in a set of four experiments, mixtures of acrolein an OH yield. It should be noted that it is evident from the and 1,2-epoxy-3-butene and n-hexane (as an internal rate constants that HO formed in these experiments will standard) in the 2}10 ppm range were generated in Tef- primarily react with either itself or an RO radical. Any lon bags by injection of the liquids into a known volume

Table 2 Initial concentrations and TMB/1,3-butadiene ratio used in the OH-yield measurements

Experiment 1,3-Butadiene initial TMB initial Concentration ratio Calculated OH concentration (ppb) concentration (ppb) TMB/1,3-butadiene formation yield

9 2350 161 0.07 13 10 2700 242 0.09 15 11 3190 224 0.07 16 12 4260 225 0.05 13 38 F. Kramp, S.E. Paulson / Atmospheric Environment 34 (2000) 35}43

Table 3 Measurements of the rate coe$cient for the reaction between 1,2-epoxy-3-butene and ozone

Experiment 1,2-Epoxy-3-butene TMB initial Propene initial Ozone initial Observed bimolecular initial concentration concentration (ppb) concentration (ppb) concentration (ppb) reaction rate constant (ppb) (;10 cm molecule\ s\). Uncertainties are 2p

13! 1170 9030 800 Added in aliquots 1.3$0.03 14! 1120 8800 580 Added in aliquots 1.4$0.02 15" 1250 8750 * 2250 1.8$0.05 16" 575 6320 * 2050 1.7$0.02 17" 680 6970 * 2200 1.6$0.03

!Relative measurement. "Direct measurement.

of zero air. The reproducibility of these experiments was better than $3%. The resulting n-hexane was within 10% or better of the calculated value, but was lower in all cases. Using the internal standard to calculate the measured n-hexane concentration, concentrations of acrolein and 1,2-epoxy-3-butene resulted in FID peak areas equivalent to 1.9$0.2 and 2.9$0.3 carbons, respectively. Second, we measured the peak area/ppmv of acrolein directly using an instrument developed to measure total non-methane organic carbon (Paulson et al., 1996) by oxidizing acrolein samples to CO, reducing to CH, and comparing the FID peak area/ppmv to measurements of unaltered samples. The FID peak area for acrolein measured this way was equivalent to 2.1$0.2 carbons. To calculate acrolein yields in this study we have used 2.0 as the FID response. Wall losses are very minor for all compounds involved in this study (Kramp and Paulson, 1998), but were nevertheless in- cluded in each analysis used to derive the product yields. An upper limit for 1,3-butadiene wall loss rate of about 2;10\ s\ was measured. Fig. 1. Consumption of 1,2-epoxy-3-butene relative to propene from two experiments. The symbols indicate the data, and the line a linear least-squares regression. The rate constant is in 3. Results cm molecule\ s\.

3.1. Reaction of ozone with 1,2 epoxy-3-butene

The rate coe$cient for the ozone reaction with (1.7$0.3);10\ cm molecule\ s\. There are sev- 1,2-epoxy-3-butene was determined in direct and eral potential sources of the di!erence between the direct relative-rate experiments (Table 3). A linear least- and relative-rate measurements, including (probably squares regression (Fig. 1) of two experiments using small) errors from the propene}O reaction rate con- # " propene as the reference compound (k- propene stant, mixing (which tends to underestimate the more (1.01$0.25);10\ cm molecule\ s\, Atkinson, slowly reacting compound; R. Atkinson, pers. commun- $ ; \ 1994) resulted in a value of (1.4 0.4) 10 cm mol- ication), and errors in the initial concentrations of O or ecule\ s\ for the 1,2-epoxy-3-butene}ozone reaction. the hydrocarbons. In the calculations of 1,2-epoxy- Data from three direct experiments are shown in Fig. 2 3-butene yields below, we have used 1.6;10\ cm and lead to a slightly larger rate coe$cient of molecule\ s\. F. Kramp, S.E. Paulson / Atmospheric Environment 34 (2000) 35}43 39

3.2. Product yields for the reaction of butadiene with ozone ether and their oxidation products) except acrolein. # Chemical ionization with CHCN provided clean M 1 The major products of the reaction between ozone and spectra for 1,2-epoxy-3-butene, di-n-butylether, butanal 1,3-butadiene are acrolein, formaldehyde (HCHO) and and butyric acid}butylester but produced some frag- " the carbonyl oxides [HCOO] and [CH CH}CHOO] mentation or larger ions for acrolein and 1,3-buta- (R1, Scheme 2). In Fig. 3, acrolein, 1,2-epoxy-3-butene are diene. plotted versus 1,3-butadiene reacted. This data was ob- Both observed yields need to be corrected for their tained from eight experiments conducted with di!erent losses due to reaction with ozone. For this, the set of initial 1,3-butadiene concentrations and turnover rates. ordinary di!erential equations describing the reactions Linear least-squares regression of the entire data set in these experiments (including O and products) was results in yields of (50$2) and (2.4$0.2)% for acrolein solved numerically (Carter and Atkinson, 1988) to and 1,2-epoxy-3-butene, respectively. Identi"cation of account for secondary reactions in the derivation of the these compounds was con"rmed with authentic standards and GC/MS. With the ion trap, EI spectra agreed well with the National Institutes of Standards and Technology (1992) mass spectral libraries for all compounds (including 1,3-butadiene, di-n-butyl

Fig. 3. Acrolein and 1,2-epoxy-3-butene concentrations vs. 1,3- Fig. 2. Absolute measurements of the rate coe$cient for the butadiene reacted (ppm) for all 7 experiments. The symbols 1,2-epoxy-3-butene}ozone reaction. The symbols indicate the indicate the data, and the lines linear least-squares regressions; data, and the lines linear least-squares regressions. Rate coe$- the uncertainties are 2p. The initial concentration of 1,3-bu- cients are given in ;10 cm molecule\ s\ and the uncer- tadiene was 2}16 ppmv and TMB or DBE were added in 25 or tainties are 2p. 50-fold excess, respectively, to scavenge OH.

Scheme 2. 40 F. Kramp, S.E. Paulson / Atmospheric Environment 34 (2000) 35}43 product yields. The reaction of ozone with acrolein uncertainties in the TMB rate constant and other minor ((2.9$1.0);10\ cm molecule\ s\; Atkinson, 1994) sources of systematic uncertainty results in an OH yield is about 20 times slower than its reaction with 1,3- of 13$3%. butadiene ((6.3$1.9);10\ cm molecule\ s\; Atkinson, 1997) and thus has a small e!ect; the resulting acrolein yield is 52%. The e!ect of the secondary con- 4. Discussion sumption of 1,2-epoxy-3-butene is more signi"cant; its corrected yield was 3.1%. Inclusion of uncertainties in A summary of product yields for the 1,3-butadiene FID peak area/ppm, rate coe$cients, and random errors reaction with ozone is shown in Table 1. The values for results in yields of (52$7)% and (3.1$0.5)% for ac- acrolein, 1,2-epoxy-3-butene and OH are all in agree- rolein and 1,2-epoxy-3-butadiene, respectively. ment with previous values within mutual uncertainties. Several small peaks appear on the chromatogram as In the case of acrolein, the agreement may be fortuitous. the butadiene is reacted away. E!orts to identify these In the study where OH scavengers were not employed products were unsuccessful. Possible minor products, (Niki et al., 1983) about 11% of the butadiene was pre- 2,5-dihydrofuran, b-propiolactone, ethylene and acety- sumably consumed by OH, but since the products of lene all remained below detection limits, indicating upper the butadiene reaction with OH in the absence of NOV limits for their formation yields are 1%. Since this study are expected to include acrolein, formaldehyde, and hy- was performed, Liu et al. (1999) have identi"ed or found droperoxides, not much e!ect on the observed yield of molecular weights for several possible additional prod- acrolein is expected. Our yield of the epoxide was identi- ucts (Table 1). cal to that of Atkinson et al. (1994) prior to correction for reaction with O, which they did not include. This cor- 3.3. OH formation yield rection produces a yield that is about 35% higher. More striking is the discrepancy between Niki et al.'s observa- The calculated OH yields for each tracer in four experi- tion of ethene and acetylene, which we could not corrob- ments, together with the initial conditions, are sum- orate. Our OH yield is about 60% higher than that marized in Table 2; data and calculations are shown in measured by Atkinson and Aschmann, 1993, although Fig. 4. The predicted behavior of TMB and butadiene both are again within the mutual uncertainties. This assuming OH yields of 13 and 16% are shown; 13% di!erence may arise from the unknown amount of HO provides the best "t for experiments 9 and 12, while that forms * the method used by Atkinson and As- experiments 10 and 11, where less than 4% of the TMB chmann, 1993 in that study is sensitive to HO forma- # was reacted, are best "t with slightly higher OH yields. tion. Formation of OH from HO O is likely very A weighted average of these data combined with the small with either method, but the cyclohexanol/cyclohexanone method measures formation of those products from the disproportionation reaction of the RO radical that results from the OH reaction with cyclohexane. RO}RO reactions compete with RO}HO reactions; the latter reactions are faster. The cyclohexanol/cyclohexanone method is based on a typical yield of those compounds from reacted cyclohexane (a 55% yield, derived from experiments with a-pinene and *-carene (Atkinson et al., 1992), and is the source of the uncertainty that is attached by Atkinson and co-workers. Clearly, one explanation for the cyclohexane method to result in a lower OH yield is for the HO yield to be higher than typical. The initial O}butadiene reaction produces formaldehyde and a C carbonyl oxide (R1a), acrolein and aCcarbonyl oxide (R1b), or 1,2-epoxy-3-butene and O (R1c). A 52% yield of acrolein indicates that the primary ozonide decompose with more regard to sta- tistics than to thermochemistry; consistent with other unbranched alkenes (Grosjean and Grosjean, 1997 C515). The 65% formaldehyde yield reported by Niki et Fig. 4. OH-yield derived from TMB reacted vs. 1,3-butadiene al. (1983) is reasonably consistent with this split; addi- reacted. The symbols indicate the data, and the lines model tional formaldehyde can be formed via a number of calculations for di!erent assumed OH formation yields. secondary pathways. Epoxide formation (R1c) is a minor F. Kramp, S.E. Paulson / Atmospheric Environment 34 (2000) 35}43 41 channel that has also been observed from isoprene, which Formaldehyde is a ubiquitous pollutant that is both produces a similar yield of &5% (Paulson et al., 1992; directly emitted and a product of numerous photochemi- Atkinson et al., 1994). Several reaction pathways for the cal reactions, such that butadiene photooxidation con- C carbonyl oxides from the butadiene reaction with tributes only a tiny fraction of ambient formaldehyde. ozone are shown in Scheme 2. Syn- and anti-carbonyl Butadiene can be a major precursor to acrolein, however. oxides probably do not equilibrate e$ciently (Scheme 2a) Acrolein is considered an air toxic because of its acute due to a &28 kcal/mol activation barrier for isomeriz- toxicity; exposure to concentrations of 10 ppm can be ation (Kuwata et al., unpublished work). The possible fatal, and upper respiratory tract irritation may result formation channels for acetylene, ethene and b- from concentrations as low as 170 ppb (ATSDR, 1989; propiolactone (Scheme 2e}g), appear to be inoperative, Hazardous Substances Data Bank, 1993). Away from consistent with isoprene, which does not form an analog- strong sources, oxidation of 1,3-butadiene will clearly not ous lactone or methyl acetylene. Isoprene does, however, result in acrolein concentrations that are high enough to produce about 5% propene (Paulson et al., 1992; Carter produce acute toxicity e!ects. The question remains if the and Atkinson, 1996), presumably through the pathway 1,2-epoxy-3-butene and 1,2 : 3,4-diepoxybutane, both of analogous to Scheme 1. which are more carcinogenic than butadiene itself, for- Butadiene's OH formation yield is reasonable com- med in the atmospheric reaction of 1,3-butadiene with $ pared to ethene, which has a yield of 0.18 0.06 (Paulson O preserves or increases the human health risk posed by et al., 1998). Quantum chemical calculations have led to butadiene. In urban air, the ratio of O to OH generally the idea that ethene decomposes via a four-membered fall in the range from 1;10 to 2;10 (Seinfeld and ring to produce OH (Gutbrod et al., 1997; Donahue et al., Pandis, 1997). Given the rate constants for reaction ; \ 1998), although this mechanism (shown for the C car- of 1,3-butadiene with OH and O of 6.7 10 bonyl oxide, Scheme 1b) predicts OH yields that are and 7.5;10\ cm moleccule\ s\, respectively, the much lower than observed. Presumably, the same mecha- amount of 1,2-epoxy-3-butene produced as butadiene is nism operative for ethene is at work for 1,3-butadiene, oxidized in the atmosphere should be from 0.03 to 0.7%. through both the C carbonyl oxide and the anti-C We estimate a reaction rate for the OH reaction with ; \ \ \ carbonyl oxide. The C syn-carbonyl oxide cannot rear- 1,2-epoxy-3-butene of 3 10 cm molecule s range this way due to lack of an available H atom for based on structure}activity relationships (Kwok and abstraction. 1,3-butadiene might be expected to produce Atkinson, 1995). As such, in urban air, the O reaction about 75% of the OH from ethene, exactly as observed. consumes at most 10% of the epoxide; the remainder is The rate constant for the O reaction with 1,2-epoxy- consumed by OH. If we then assume that the 1,2 : 3,4- 3-butene, (1.6$0.4);10\ cm molecule\ s\, is, diepoxide has a yield that is the same as that for as far as we know, the "rst measurement of the O reac- O reacting with butadiene, then the yield of 1,2 : 3,4- tion rate for an unsaturated epoxide. Relatively few rate diepoxide is not more than 0.005% of the reacted 1,3- constants for O reactions with oxygenates have been butadiene. measured. In contrast to OH}alkene reactions, the Pharmacokinetic modeling of 1,3-butadiene inhalation rate constants for ozone}alkene reactions are generally and metabolism indicates that roughly 20% of the 1,3- quite sensitive to substituents around the double bond. butadiene inhaled is absorbed and converted to 1,2- The rate constants for the similar reactions between epoxy-3-butene (Himmelstein et al., 1997). The inhalation O and 1-butene, methylvinylketone, and acrolein are 10, uptake e$ciency of 1,2-epoxy-3-butene and 1,2 : 3,4- 5 and 0.7;10\ cm molecule\ s\, respectively (At- diepoxybutane are at least 30 times greater than 1,3- kinson and Carter, 1984). The e!ect of an epoxide oxygen butadiene (Weast, 1986; Sweeney et al., 1997). Even so, appears to fall in the range suggested by acrolein and the toxicity of inhaled 1,2-epoxy-3-butene is not more methylvinylketone. than "ve times that of 1,3-butadiene. Likewise, 1,2 : 3,4- diepoxybutane has a maximum toxicity that is about 500 times greater than that of 1,3-butadiene. As 1,3-butadiene 5. Human exposure reacts, therefore, on balance its toxicity based on carcino- genicty is reduced; the production of carcinogenic epox- 1,3-Butadiene is considered an air toxic because of its ide products replaces from less than 1 up to about 5% of potential carcinogenic e!ects based primarily on rat and its initial toxicity. It should be emphasized that the prod- mice studies (Himmelstein et al., 1997). The active com- ucts atmospheric reactions of 1,3-butadiene with NO, pounds appear to be butadiene metabolites 1,2-epoxy-3- which are expected to include carbonyl nitrates (Barnes butene and 1,2 : 3,4-diepoxybutane; the latter is about et al., 1990), some of the unknown products of the ozone 100 times more potent (Kallioniemi et al., 1992; Coch- reaction with butadiene, as well as some of the products rane and Skopec, 1994; Himmelstein et al., 1997). of the OH radical chemistry (e.g., malonaldehyde re- As butadiene reacts in the atmosphere with OH and O, cently discovered by Liu et al., 1999) may also be quite it is converted primarily to acrolein and formaldehyde. toxic. 42 F. Kramp, S.E. Paulson / Atmospheric Environment 34 (2000) 35}43

Acknowledgements EPA, 1993. Motor vehicle-related air toxics study. Environ- mental Protection Agency, EPA Report No. 420-R-93-005. Grateful acknowledgement is made to the University Grosjean, E., Grosjean, D., 1997. Gas phase reaction of alkenes of California Toxic Substances Teaching and Research with ozone: formation yields of primary carbonyls and bi- Program and to the National Science Foundation (grant radicals. Env. Sci. Technol. 31, 2421}2427. Gutbrod, R., Kraka, E., Schindler, R.N., Cremer, D., 1997. CATM-9629577) for partial support of this research, Kinetic and theoretical investigation of the gas-phase and to Grazyna Orzechowska for assistance with the ozonolysis of isoprene: carbonyl oxides as an important GC/MS. Myeong Chung and Dr. Richard Meller mea- source for OH radicals in the atmosphere. Journal of Ameri- sured the FID responses for acrolein using the total can Chemical Society 119, 7330}7342. non-methane organic carbon analyzer. 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