Carbonyl Products of the Gas Phase Reaction of Ozone with C5−C7

Sara M. Aschmann, Ernesto C. Tuazon, Janet Arey, and Roger Atkinson ... Yonghui Shu, Eric S. C. Kwok, Ernesto C. Tuazon, Roger Atkinson, and Janet Are...
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Environ. Sci. Technol. 1996, 30, 1321-1327

Carbonyl Products of the Gas Phase Reaction of Ozone with C5-C7 Alkenes ERIC GROSJEAN AND DANIEL GROSJEAN* DGA, Inc., 4526 Telephone Road, Suite 205, Ventura, California 93003

The gas phase reaction of ozone with alkenes, which is of critical importance in atmospheric chemistry, is still poorly understood. Major uncertainties regarding the reaction mechanism include the nature and the formation yields of the carbonyl products, the formation yields of the biradicals R1R2COO, and the subsequent reactions of these biradicals. In this study, the gas phase reaction of ozone with 1-pentene, 1-hexene, 1-heptene, 2,3-dimethyl-1-butene, cyclopentene, and 1-methylcyclohexene has been studied with sufficient cyclohexane added to scavenge OH. Carbonyl products were identified as their DNPH derivatives by liquid chromatography and chemical ionization mass spectrometry. Primary carbonyl formation yields for the 1-alkenes and 2,3-dimethyl-1-butene were close to the value of 1.0 that is consistent with the mechanism O3 + R1R2CdCH2 f R(HCHO + R1R2COO) + (1 - R)(R1COR2 + H2COO), where HCHO and R1COR2 are the primary carbonyls. Data for the 1-alkenes, R ) ca. 0.50, were consistent with about equal formation yields for the biradicals H2COO and RCHOO. Data for 2,3-dimethyl-1-butene, R ) 0.64 ( 0.01, were consistent with modest preferential formation of the disubstituted biradical. Carbonyls other than the primary carbonyls were measured, e.g., propanal from 1-pentene, consistent with the following reaction sequence: biradical f hydroperoxide f hydroxycarbonyl f R-hydroxyalkyl radical f carbonyl + HO2. The R-dicarbonyls CH3(CH2)nCOCHO, n ) 1, 2, 3, e.g., 2-oxobutanal from 1-pentene, and/or the corresponding β-hydroxycarbonyls CH3(CH2)nCHOHCHO were also observed to form from the 1-alkenes, and this with low yields of 0.02-0.03. Only limited information was obtained regarding carbonyl products of the cyclic compounds cyclopentene (butanal, yield 0.12 ( 0.00; glyoxal, yield 0.15 ( 0.01) and 1-methylcyclohexene (formaldehyde, yield 0.04 ( 0.01; 2-hexanone, yield ca. 0.04 ( 0.01). Butanal and 2-hexanone form by loss of CO2 from the carbonyl-bearing biradicals.

* Corresponding author: phone, 1-805-644-0125; FAX, 1-805-6440142.

0013-936X/96/0930-1321$12.00/0

 1996 American Chemical Society

Introduction The reaction of ozone with alkenes plays a major role in atmospheric chemistry and is directly relevant to ozone formation and to aerosol production from unsaturated organic compounds that are emitted by anthropogenic and biogenic sources (1-4). While the gas phase reaction of ozone with alkenes has been studied for many years, the reaction mechanism is still poorly understood. Recent studies have shown that the hydroxyl radical forms as a product of the ozone-alkene reaction (5, 6). Since OH reacts rapidly with alkenes (4), leading to carbonyl products, and also reacts rapidly with the carbonyl products of the ozone-alkene reaction, a better understanding of the mechanism of the ozone-alkene reaction requires product studies to be carried out under conditions that minimize “interferences” from OH. Product studies of this type have only been carried out in recent years, and this for a limited number of alkenes (5-8). We have identified the carbonyl products of the reaction of ozone with the C5-C7 alkenes 1-pentene, 1-hexene, 1-heptene, 2,3-dimethyl-1-butene, cyclopentene, and 1-methylcyclohexene. These alkenes were selected for their relevance to urban atmospheric chemistry (1, 3). The two cyclic alkenes were also selected for their importance as precursors to secondary organic aerosols (9, 10) and as simple structural homologues of an important category of biogenic emissions, the terpenes (11-13). Few studies have been made of the gas phase products of the reaction of ozone with C5-C7 alkenes (6, 14) and with cyclic alkenes (6, 15-18) under conditions relevant to the atmosphere. In this work, experiments have been carried out with ozonealkene mixtures in purified air, with sufficient cyclohexane added to scavenge OH. Carbonyl formation yields have been measured, and the corresponding reaction mechanisms are outlined. Information on carbonyl products and their yields in the reaction of ozone with C5-C7 alkenes is important for a number of reasons. There is much uncertainty regarding three major aspects of the ozone-alkene reaction mechanism: the nature and yields of the primary carbonyls, the relative abundance of the two biradicals formed from the 1,2,3-trioxolane adduct, and the subsequent reactions of the biradicals. The mechanism outlined in reaction 1 (see Results and Discussion) implies primary carbonyl formation yields of 1.0, and therefore, experimental data are needed to test the general applicability of reaction 1 to alkenes including C5-C7 alkenes. In addition, the relative abundances of the primary carbonyls provide information on the relative formation yields of the two biradicals. Experimental data on carbonyl products other than the primary carbonyls also provide insight into the reactions of the biradicals.

Experimental Methods Alkene-Ozone-Cyclohexane Experiments. Experiments involving alkene-ozone-cyclohexane mixtures were carried out at RH ) 3-7% (hereafter dry air) and and at RH ) 55 ( 10% (hereafter humid air) with sufficient cyclohexane (Aldrich, purity >99.9%) added to scavenge the hydroxyl radical (5-7). The alkenes were commercially available (Aldrich) and were used without further purification. Stated

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FIGURE 1. Scatterplots for carbonyl formed vs 2,3-dimethyl-1-butene reacted in ozone-2,3-dimethyl-1-butene-cyclohexane experiments in dry air: 3-methyl-2-butanone (squares) and cyclohexanone (triangles). Open and filled symbols are for data from two separate experiments.

purities were 97% for 2,3-dimethyl-1-butene and for 1-methylcyclohexene, 99% for 1-pentene and cyclopentene, and >99% for 1-hexene and 1-heptene. Carbonyls were measured by sampling the reaction mixture, after all or nearly all the ozone had been consumed, using C18 cartridges coated with 2,4-dinitrophenylhydrazine (DNPH). The sampling flow rate was ca. 0.7 L/min, and the sampling duration was 30 or 60 min. In the experiments carried out at RH ) 3-7%, sampling of carbonyls on DNPHcoated C18 cartridges involved two sets of co-located cartridges to estimate overall method precision, one set of two cartridges in series to verify absence of breakthrough, and one control sample collected prior to the addition of ozone. Cartridge blanks and cartridge field controls were included as appropriate. Results for these control and method performance experiments have been described in detail elsewhere (19-21). For replicate analyses of the same cartridge sample, relative standard deviations (RSD) averaged 3.0, 3.6, 4.2, 3.9, 1.3, 2.8, 3.7, 3.5, 3.1, and 4.0% for formaldehyde, acetaldehyde, propanal, butanal, pentanal, hexanal, acetone, 3-methyl-2-butanone, cyclohexanone, and glyoxal, respectively. The corresponding RSD for colocated samples averaged 5.8, 6.7, 8.7, 6.1, 7.4, 8.0, 11.4, 1.3, 10.4, and 9.8%, respectively. Experiments in dry air were carried out at U.C. Riverside by Atkinson and co-workers as described previously (5, 6). Carbonyl yields in these experiments were calculated from the carbonyl concentration measured using DNPH-coated cartridges and the amount of alkene reacted, with the alkene concentration measured by the U.C. Riverside group using flame ionization gas chromatography (5, 6). Examples of carbonyl formation yield plots are shown in Figure 1 for 3-methyl-2-butanone and for cyclohexanone in the 2,3dimethyl-1-butene experiments. Experiments in humid air (four experiments for each alkene) were carried out in our laboratory using 3.7 m3 FEP Teflon film chambers (7, 12). Initial concentrations were 1.0-4.0 ppm alkene, 71-324 ppb ozone, and 400 ppm cyclohexane. Initial alkene to ozone concentration ratios were g10. Ozone was measured continuously by ultraviolet

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photometry (7). The ozone analyzer was calibrated according to the protocol recommended by the U.S. Environmental Protection Agency. The precision of the ozone measurements was 1-2 ppb in the range of ozone concentrations relevant to this study. Control experiments, described elsewhere (7, 8, 12) included measurements of the loss of ozone alone in purified humid air and of the loss of ozone in ozone-cyclohexane mixtures. The loss of carbonyls to the chamber walls was also measured for ca. 15 carbonyls, including those relevant to this study. Carbonyl loss rates were comparable in magnitude to those for ozone and, when compared to the duration of the ozone-alkene-cyclohexane experiments, made a negligible contribution to the measured carbonyl concentrations. For the experiments carried out in humid air, carbonyl yields were calculated from the measured carbonyl concentration and the amount of ozone reacted, i.e., assuming a 1:1 stoichiometry for the ozone-alkene reaction. This assumption appears reasonable since, with the initial concentrations listed above, essentially all of the OH formed reacts with cyclohexane (5-7) and the OH-alkene and OHcarbonyl product (e.g., OH-aldehyde) reactions are of negligible importance. Carbonyl yields thus calculated are listed in Table 1. Carbonyl Analysis. Samples collected on DNPH-coated C18 cartridges were analyzed by liquid chromatography (7, 19). Several separation methods were employed. All samples (five to six samples for each alkene studied) were analyzed using isocratic elution (53:47 by volume acetonitrile-water eluent) and single-wavelength detection (360 nm, with analyses repeated at 430 nm for at least one to two samples for each alkene studied) as previously described (7, 19). For several of the alkenes studied, samples were reanalyzed using the same conditions but at a lower column temperature. This resulted in a better separation of closely eluting carbonyl-DNPH derivatives, i.e., those of glyoxal and cyclohexanone in the 1-pentene experiments and those of pentanal and cyclohexanone in the 1-hexene and 1-heptene experiments. The DNPH derivatives of cyclohexanone and of 3-methyl-2-butanone co-eluted using 53: 47 acetonitrile-water eluent and were separated using a weaker eluent, 54:36:10 by volume acetonitrile-watertetrahydrofuran (21). To complement the isocratic elution methods listed above, samples were analyzed using a gradient elution method with diode array detection (19). The eluent was 49:51 by volume acetonitrile-water for the first 26 min, was increased to 100% acetonitrile over the next 14 min, and was held at 100% acetonitrile afterward. The diode array detector was used to record 200-600 nm UV-visible absorption spectra of the carbonyl-DNPH derivatives (19, 21). Several samples (at least one for each alkene studied) were also analyzed by chemical ionization mass spectrometry (19, 21, 22). Confirmation of the carbonyl structure in the samples collected in the alkene-ozone-cyclohexane experiments involved the comparison of retention times, 430/360 nm absorbance ratios, UV-visible spectra, and chemical ionization mass spectra to those of data libraries that we constructed using carbonyl-DNPH standards (19, 21). Quantitative analysis involved the use of external standards synthesized in our laboratory. Calibration curves, i.e., plots of absorbance at a given detection wavelength vs concentration, were constructed as described previously. The slopes of these calibration curves, i.e., response factors,

TABLE 1

Summary of Carbonyl Products and Their Formation Yields in Alkene-Ozone-Cyclohexane Experiments carbonyl 1-pentene formaldehyde propanal butanal cyclohexanoneb 2-oxobutanal 1-hexene formaldehyde butanal cyclohexanoneb pentanal 2-oxopentanal 1-heptene formaldehyde cyclohexanoneb pentanal hexanal 2-oxohexanal

yielda 0.505 ( 0.003 0.117 ( 0.023 0.496 ( 0.016 0.049 ( 0.002 0.029 ( 0.002 0.501 ( 0.006 0.043 ( 0.004 0.048 ( 0.007 0.536 ( 0.023 0.024 ( 0.001 0.511 ( 0.014 0.040 ( 0.013 0.056 ( 0.012 0.512 ( 0.049 0.025 ( 0.001

carbonyl 2,3-dimethyl-1-butene formaldehyde acetone 3-methyl-2-butanonec cyclohexanoneb cyclopentene butanal cyclohexanoneb glyoxal 5-oxopentanoic acid 1-methylcyclohexene formaldehyde cyclohexanoneb 2-hexanoned 6-oxoheptanoic acid

yielda 0.663 ( 0.010 0.215 ( 0.009 0.369 ( 0.012 0.117 ( 0.004 0.120 ( 0.001 0.033 ( 0.001 0.150 ( 0.010 e 0.040 ( 0.010 0.130 ( 0.010 0.040 ( 0.010 e

a Carbonyl formation yields (( one SD) measured in dry air (RH ) 3-7%) for 1-methylcyclohexene and in humid air (RH ) 55 ( 10%) for the other alkenes. b Product of the OH-cyclohexane reaction. c Calculated using the response factor of cyclohexanone. d Calculated using the response factor of hexanal. e Tentatively identified by chemical ionization mass spectrometry; yield not measured.

were used to calculate carbonyl concentrations in the samples collected in the alkene-ozone-cyclohexane experiments. The response factor for the DNPH derivative of 3-methyl-2-butanone was assumed to be the same as that for the DNPH derivative of cyclohexanone since the two compounds coeluted using one method and eluted close to each other using the other method; see above. Response factors measured for the DNPH derivatives of four C5 carbonyls that are isomers of 3-methyl-2-butanone are within 10% of each other (19), and the response factor of cyclohexanone-DNPH is within 15% of that of pentanalDNPH (19). This assumption was also supported by mass spectrometry data which showed that the relative abundances of the MH peaks of 3-methyl-2-butanone and cyclohexanone (M ) molecular weight of the carbonylDNPH derivative) matched well the ratio of concentrations calculated by liquid chromatography analysis. To verify day-to-day consistency in retention times and response factors, calibration standards were analyzed along with each batch of samples. Calibration standards employed in this study included two mixtures of carbonylDNPH derivatives, one prepared in our laboratory and the other obtained from a commercial supplier. Response factors measured for the 13 carbonyl-DNPH derivatives present in the two mixtures have been shown in previous work to agree within (4% (21). A limitation of the liquid chromatography method employed in this study is that β-hydroxycarbonyls yield the DNPH derivatives of the corresponding R-dicarbonyls, e.g., hydroxyacetaldehyde yields glyoxal-DNPH and hydroxyacetone yields methylglyoxal-DNPH (19). Thus, β-hydroxycarbonyls and the corresponding R-dicarbonyls could not be resolved when both carbonyls were formed in the same experiment. This limitation of the method was not of critical importance for this study since dicarbonyls were observed to form only from the 1-alkenes, and this in low yields of ca. 2-3%. The R-dicarbonyls tentatively identified as reaction products were 2-oxobutanal, 2-oxopentanal, and 2-oxohexanal (CH3(CH2)nCOCHO, n ) 1, 2, 3). No reference standards are available for these dicarbonyls. Their DNPH

derivatives had retention times (relative to that of formaldehyde-DNPH) of 9.25, 12.6, and 17.9, consistent with those estimated from relationships between retention time and carbonyl carbon number for other dicarbonyls (19, 21). The retention time of 2-oxobutanal-DNPH was close to that of the DNPH derivative of the C4 dicarbonyl biacetyl and that of 2-oxopentanal-DNPH was close to that of the DNPH derivative of the C5 dicarbonyl 2,3-pentanedione. Their 200-600 nm absorption spectra and 430/360 nm absorbance ratios were also consistent with those for DNPH derivatives of other dicarbonyls for which reference standards are available (19). In addition, the retention time, absorption spectrum, and 430/360 nm absorbance ratio of 2-oxobutanal-DNPH in the 1-pentene experiments matched well those of the DNPH derivative of 2-oxobutanal tentatively identified as a product of the reaction of ozone with 2-ethylacrolein and with ethyl vinyl ketone, i.e., O3 + CH2 ) C(C2H5)CHO and O3 + C2H5COCHdCH2 f CH3CH2COCHO + HCHO + other products (23). The same good match was obtained for 2-oxopentanal formed in the 1-hexene experiments and in similar experiments carried out with trans-5-decene (unpublished results from this laboratory, 1995). Response factors for the R-dicarbonyls were estimated from data for structural homologues (glyoxal, methylglyoxal, biacetyl) using response factorretention time relationships (19, 21). The response factor thus calculated for 2-oxobutanal was within 10% of that for its C4 isomer biacetyl. The uncertainty on the response factors thus estimated is ca. (20%. Comparison of Carbonyl Measurement Methods. In the experiments carried out at RH ) 3-7%, Atkinson et al. (6) measured the formation yields of butanal from 1-pentene and cyclopentene, pentanal from 1-hexene, hexanal from 1-heptene, and 3-methyl-2-butanone from 2,3-dimethyl1-butene. These carbonyls were measured by gas chromatography with flame ionization detection (GC-FID) following sampling on Tenax cartridges and thermal desorption. In separate experiments, the same authors measured the formaldehyde yield from the alkenes listed above using Fourier transform infrared spectroscopy (FTIR). As is shown in Table 2, which also includes data for

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TABLE 2

Comparison of Formation Yields for Primary Carbonyls Measured by DNPH-LC, GC-FID, and FTIR in Ozone-Alkene-Cyclohexane Experiments formaldehyde

other carbonyls

alkene

FTIR dry aira,b

DNPH-LC humid airc

carbonyl

GC-FID, dry aira,b

DNPH-LC humid airc

1-pentene 1-hexene 1-heptene 1-octene 2,3-dimethyl-1-butene cyclopentene

0.595 ( 0.055 0.575 ( 0.057 0.533 ( 0.049 0.519 ( 0.054 0.766 ( 0.071

0.505 ( 0.006 0.501 ( 0.012 0.511 ( 0.028 0.476 ( 0.058d 0.663 ( 0.020

butanal pentanal hexanal heptanal 3-methyl-2-butanone butanal

0.541 ( 0.065 0.518 ( 0.095 0.582 ( 0.078 0.527 ( 0.070 0.391 ( 0.050 0.195 ( 0.027

0.496 ( 0.032 0.536 ( 0.046 0.512 ( 0.098 0.473 ( 0.046d 0.369 ( 0.024 0.120 ( 0.002

a From ref 6. FTIR and GC-FID measurements of formaldehyde and other carbonyls, respectively. b Carbonyl yield, carbonyl formed/reacted alkene; indicated errors are two standard deviations combined with uncertainty in alkene, carbonyl, and formaldehyde calibration factors of (5, (10, and (5%, respectively (6). c This study unless otherwise indicated, carbonyl yield, carbonyl formed/reacted ozone; indicated errors are two standard deviations. d From ref 25.

TABLE 3

Summary of Data for Unidentified Carbonyls in Ozone-Alkene-Cyclohexane Experiments in Humid Air carbonyl-DNPH derivatives alkenea 1-pentene 1-heptene 2,3-dimethyl-1-butene cyclopentene

retention

timeb

peak

0.84 1.59 1.66 13.3 0.82 1.37 1.63 2.37 5.64 10.84

htb

0.020 0.046 0.141 0.024 0.212f 0.915f 0.121f 0.569f 1.16f 0.057f

430/360 nm absorbance ratioc 0.17 0.22 0.27 0.82 0.20 0.19 0.80 0.45 0.19 0.80

comments

d e d e e

a

Unidentified carbonyls for these alkenes are consistent with those previously observed to form in alkene-ozone-cyclohexane experiments in dry air (24). b Relative to that of formaldehyde-DNPH. c This ratio is used as a diagnostic test for DNPH derivatives of monofunctional carbonyls vs those of dicarbonyls (19). d Possibly DNPH derivative of hydroxycarbonyl based on retention time and absorbance ratio. e Possibly DNPH derivative of dicarbonyls based on 430/360 nm absorbance ratio. f Relative to that of butanal-DNPH.

formaldehyde and heptanal from 1-octene (6, 25), there is reasonable agreement between carbonyl yields measured at RH ) 55 ( 10% using the DNPH cartridge-liquid chromatography method (DNPH-LC) and those measured at RH ) 3-7% by FTIR (formaldehyde) and by GC-FID (other carbonyls) . Possibly contributing to differences, in addition to calibration and experimental uncertainties for all three methods, are the differences in reporting carbonyl yields, i.e., carbonyl formed/alkene reacted for the GC-FID and FTIR methods and carbonyl formed/ozone reacted for the DNPH-LC method.

Results and Discussion Carbonyl Products. Carbonyls identified as reaction products in the alkene-ozone-cyclohexane experiments are listed in Table 1. These carbonyls were observed as reaction products in dry air and in humid air. They were positively identified by comparison with authentic standards using up to three methods, i.e., liquid chromatography with isocratic elution and single-wavelength detection, liquid chromatography with gradient elution and diode array detection, and chemical ionization mass spectrometry. Carbonyl products included formaldehyde, butanal, and propanal from 1-pentene; formaldehyde, pentanal, and butanal from 1-hexene; formaldehyde, hexanal, and pentanal from 1-heptene; formaldehyde, 3-methyl-2-butanone, and acetone from 2,3-dimethyl-1-butene; butanal, glyoxal, and, tentatively, 5-oxopentanoic acid from cyclopentene; and formaldehyde, 2-hexanone, and, tentatively, 6-oxoheptanoic acid from 1-methylcyclohexene. Cyclohexanone,

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a product of the OH-cyclohexane reaction (5-7), was observed in all experiments, thus providing evidence for the formation of the hydroxyl radical as a product of the reaction of ozone with the alkenes studied. Carbonyl formation yields were measured in dry air and in humid air. As discussed in detail elsewhere (24), the sampling performance of the DNPH-coated C18 cartridge was about the same in dry air and in humid air for the several ketones tested but was lower in dry air for several aldehydes, especially for formaldehyde. Thus, the formation yields listed in Table 1 are those measured at RH ) 55 ( 10%. Yields measured at RH 3-7% were reported previously (24). The one exception is 1-methylcyclohexene, which was studied at RH ) 3-7% and for which the carbonyl yields listed in Table 1 may be lower limits for actual yields, especially for formaldehyde (24). In addition to the carbonyls listed in Table 1, unidentified carbonyls were present in the samples collected in the ozone-alkene-cyclohexane experiments. These unidentified carbonyls are listed in Table 3, which includes retention and absorption parameters that may be useful for identification in future work (19, 21). The unidentified carbonyls listed in Table 3 are those recorded in experiments carried out in humid air. Those observed in the experiments with 1-pentene, 1-heptene, and 2,3-dimethyl-1-butene were minor products. Those from cyclopentene included three minor products and three products whose abundance was of the same magnitude as that of butanal; however, only minor unidentified products were observed for cyclopentene in dry air. For each of the 1-alkenes, one unknown

carbonyl was observed in dry air but not in humid air. For 1-methylcyclohexene, which was studied only at RH ) 3-7%, the chromatograms contained 12 unidentified peaks (not listed in Table 3) for which no structural assignment could be made from retention times, absorption spectra, and chemical ionization mass spectra. Formation Yields of Primary Carbonyls. The presently believed mechanism for the reaction of ozone with alkenes in the gas phase involves electrophilic addition and subsequent decomposition of the 1,2,3-trioxolane adduct into two carbonyls and two biradicals (refs 2-8, and references cited therein): O

R1R2C

CH2 + O3

O

O

C

C

R1 R2

H H

α(HCHO + R1R2COO) + (1 – α)(R1COR2 + H2COO)

(1)

where R1 ) H and R2 ) n-propyl, n-butyl, and n-pentyl for 1-pentene, 1-hexene, and 1-heptene, respectively, and R1 ) CH3 and R2 ) (CH3)2CH for 2,3-dimethyl-1-butene. The two primary carbonyls, i.e., those expected to form in reaction 1, were observed as major reaction products: butanal from 1-pentene, pentanal from 1-hexene, hexanal from 1-heptene, and formaldehyde from the three 1-alkenes. The sums of the formation yields of the two primary carbonyls were 1.00 ( 0.02 for 1-pentene, 1.04 ( 0.02 for 1-hexene, and 1.02 ( 0.05 for 1-heptene. These yields are consistent with those reported for other 1-alkenes, i.e., 1.03 ( 0.07 for formaldehyde from ethylene (8), 1.12 ( 0.05 for formaldehyde + acetaldehyde from propene (8), 0.98 ( 0.04 for formaldehyde + propanal from 1-butene (8), 0.95 ( 0.04 for formaldehyde + heptanal from 1-octene (25), and 1.02 ( 0.02 for formaldehyde + nonanal from 1-decene (25). As is shown in Figure 2, primary carbonyl formation yields for 1-alkenes are close to the value of 1.0, which is consistent with the mechanism summarized by reaction 1. For the 1,1-disubstituted alkene 2,3-dimethyl-1-butene, the sum of the yields of the two primary carbonyl products, formaldehyde and 3-methyl-2-butanone, was 1.03 ( 0.02. This yield is comparable to those for 1-alkenes and is also consistent with the mechanism summarized by reaction 1. Relative Formation Yields of Primary Carbonyls. The formation yields of the two primary carbonyls are also the formation yields of the corresponding biradicals; see reaction 1. From the yields of primary carbonyls listed in Table 1, the coefficient R in reaction 1 was calculated to be 0.50 ( 0.01 for 1-pentene, 0.48 ( 0.01 for 1-hexene, 0.50 ( 0.03 for 1-heptene (which compares to R ) 0.51 for the ozone-1-heptene reaction studied without scavenging OH; see ref 14), and 0.64 ( 0.01 for 2,3-dimethyl-1-butene. These values calculated from experimental data compare to R ) 0.50 for a 50:50 split of the 1,2,3-trioxolane adduct into the two biradicals. Thus, results for 1-pentene, 1-hexene, and 1-heptene do not indicate preferential formation of the more substituted biradical RCHOO as compared to H2COO, consistent with results for other 1-alkenes including propene, R ) 0.54 ( 0.02 (estimated in ref 8), 1-octene, R ) 0.50 ( 0.04 (25), and 1-decene, R ) 0.52 ( 0.02 (25), but not 1-butene, R ) 0.64 ( 0.04 (8). For 2,3-dimethyl-1butene, the value of R, 0.64 ( 0.01 suggests a modest preferential formation of the disubstituted biradical (CH3)2

FIGURE 2. Scatterplot of the sums of the concentrations of the two primary carbonyls vs reacted ozone in ozone-alkene-cyclohexane experiments carried out in humid air. The 1:1 line is shown for comparison with experimental data for formaldehyde + butanal from 1-pentene (solid squares), formaldehyde + pentanal from 1-hexene (open squares), formaldehyde + hexanal from 1-heptene (solid triangles), and formaldehyde + 3-methyl-2-butanone from 2,3dimethyl-1-butene (open diamonds). Also shown are data from ref 8 for formaldehyde + propanal from 1-butene (solid squares with open centers) and data from ref 25 for formaldehyde + heptanal from 1-octene (open triangles) and for formaldehyde + nonanal from 1-decene (solid diamonds).

CHC(CH3)OO as compared to H2COO, with the caveat that the measured value for R may be an upper limit for the actual value if formaldehyde also forms from the disubstituted biradical; see reactions 4a-c in the next section. Carbonyl Formation from Biradicals. Carbonyls other than the primary carbonyls were observed to form in the experiments with the 1-alkenes and with 2,3-dimethyl-1-butene: propanal from 1-pentene, butanal from 1-hexene, pentanal from 1-heptene, and acetone from 2,3dimethyl-1-butene. As discussed previously for other alkenes including structural homologues of those studied in this work (2-8, 26), these carbonyls may form via the biradical f unsaturated hydroperoxide f β-hydroxycarbonyl f carbon-carbon bond scission sequence shown below for the biradicals from 1-alkenes:

R1CH2CHOO f (R1CHdCHOOH)* f (R1CHOHCHO)* (2a) (R1CHOHCHO)* f R1CHOH + HCO

(2b)

Under the conditions of our study, the R-hydroxyalkyl radical formed in reaction 2b reacts with oxygen by H-atom abstraction from the OH-bearing carbon (3, 4):

R1CHOH + O2 f HO2 + R1CHO

(2c)

The formation yields listed in Table 1 indicate that the reaction sequence 2a-c accounts for ca. 3-9% of the total 1-alkene reaction. Dividing the yields of propanal, butanal, and pentanal in the 1-pentene, 1-hexene, and 1-heptene experiments by those of formaldehyde (taken to be also the yields of the RCHOO biradical) indicates that the

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reaction sequence 2a-c accounts for ca. 10-23% of the total biradical reactions, i.e., CH3(CH2)2CHOO f (0.23 ( 0.05)CH3CH2CHO in the 1-pentene experiments, CH3(CH2)3CHOO f (0.09 ( 0.01)CH3(CH2)2CHO in the 1-hexene experiments, and CH3(CH2)4CHOO f (0.11 ( 0.01)CH3(CH2)3CHO in the 1-heptene experiments (which compares to ca. 0.13 for the ozone-1-heptene reaction studied without scavenging OH; see ref 14). These values compare to CH3CH2CHOO f (0.20 ( 0.01)CH3CHO from 1-butene (8), CH3(CH2)5CHOO f 0.20 ( 0.04CH3(CH2)4CHO from 1-octene (25), and CH3(CH2)7CHOO f ca. 0.07CH3(CH2)6CHO from 1-decene (25). A reaction sequence analogous to reactions 2a-c is consistent with the observed formation of acetone from 2,3-dimethyl-1-butene:

to include high yields of HO2 under the conditions of this study]. Dicarbonyl Formation from Biradicals. The dicarbonyls CH3(CH2)nCOCHO with n ) 1, 2, 3 were observed to form in low yields (0.024-0.030; see Table 1) from the three 1-alkenes. Since β-hydroxycarbonyls and R-dicarbonyls lead to the same DNPH derivatives (those of R-dicarbonyls; see Experimental Section), the dicarbonyl yields given in Table 1 are actually those for R-dicarbonyls and/or the corresponding β-hydroxycarbonyls CH3(CH2)nCHOHCHO. These compounds may form from the corresponding biradicals in three pathways (2-8, 26), i.e. reaction 2a followed by

(RCHOHCHO)* f RCHOHCHO

(CH3)2CHC(CH3)OO f (CH3)2CdC(CH3)OOH f (CH3)2C(OH)COCH3 (3a) (CH3)2C(OH)COCH3 f CH3CO + (CH3)2COH (CH3)2COH + O2 f CH3COCH3 + HO2

(3b) (3c)

The yields of acetone and of formaldehyde in the 2,3dimethyl-1-butene experiments indicate that acetone accounts for 0.32 ( 0.01 of the total reactions of the biradical (CH3)2CHC(CH3)OO. The acetone yield is higher than those of 0.10-0.23 for aldehydes from the monosubstituted biradicals RCHOO. This may reflect the fact that formation of the unsaturated peroxide in reaction 3a involves H-atom abstraction from a weak tertiary C-H bond. Adding complexity to data interpretation in the case of 2,3dimethyl-1-butene is the possibility that formaldehyde, which forms as a primary carbonyl product in reaction 1, may also form from the biradical (CH3)2CHC(CH3)COO in a sequence analogous to reactions 3a-c but involving H-atom abstraction from the methyl group (instead of H-atom abstraction from the isopropyl group):

(CH3)2CHC(CH3)COO f (CH3)2CHC(OOH)dCH2 f (CH3)2CHC(O)CH2OH (4a) (CH3)2CHC(O)CH2OH f (CH3)2CHCO + CH2OH (4b) CH2OH + O2 f HCHO + HO2

(4c)

The possible “competition” between reactions 3a-c and reactions 4a-c will be examined in future work. Yields of HO2 and CO associated with the biradical f hydroperoxide f carbon-carbon bond scission reaction sequence can be estimated from the corresponding measured carbonyl yields. For CO, reaction 2b followed by HCO + O2 f HO2 + CO (3, 4) results in yields of ca. 0.23 from CH3(CH2)2CHOO (1-pentene), ca. 0.09 from CH3(CH2)3CHOO (1-hexene), and ca. 0.11 from CH3(CH2)4CHOO (1heptene). For HO2, reaction 2b (followed by HCO + O2 f HO2 + CO) and reaction 2c result in yields of ca. 0.46 from CH3(CH2)2CHOO (1-pentene), ca. 0.18 from CH3(CH2)3CHOO (1-hexene), and ca. 0.22 from CH3(CH2)4CHOO (1heptene). Similarly, the estimated HO2 yield from 2,3dimethyl-1-butene, reaction 3a, is ca. 0.32 [the radical CH3CO formed in reaction 3b is expected to react with O2 to form CH3CO3 (3, 4) whose further reactions are not likely

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(5a)

f H2 + RCOCHO

(5b)

f OH + RCHCHO

(5c)

where the alkyl radical formed along with OH in reaction 5c is expected to react with O2, RCHCHO + O2 f RCH(O2)CHO, thus leading to products that possibly include the R-dicarbonyl RCOCHO. Cyclopentene and 1-Methylcyclohexene. Carbonyl products of the cyclopentene-ozone reaction in humid air included butanal and glyoxal, with combined yields of ca. 0.27 ( 0.01. Our yield of ca. 0.12 for butanal compares to literature values of 0.19 ( 0.03 (6) and 0.11 ( 0.01 (17) with and without scavenging OH, respectively. The formation of butanal from cyclopentene (and of pentanal from cyclohexene; see refs 18 and 25) is consistent with the following sequence, which involves loss of CO2 from the biradical (18): + O3 HC(O)(CH2)3CHOO

1,2,3-trioxolane

HC(O)(CH2)3CHOO

(6a)

HC(O)(CH2)3COOH (5-oxopentanoic acid)

(6b)

CO2 + butanal

(6c)

The oxo acid 5-oxopentanoic acid formed in reaction 6b by rearrangement of the biradical was tentatively identified by chemical ionization mass spectrometry. Vapor pressure considerations (9) indicate that a large fraction of this oxo acid may condense as aerosol under the conditions of our study. Substantial aerosol formation was observed in earlier work from mixtures of cyclopentene and ozone under similar experimental conditions (10). Pathways accounting for glyoxal, which was observed in this study but was not reported in earlier work carried out with (6) or without (17) scavenging OH, are not known at this time. Formaldehyde (yield 0.13 ( 0.03) was reported in an earlier study carried out without scavenging OH (17); none was detected in this study, corresponding to a yield of less than 0.002. Additional work is needed to examine these discrepancies and to obtain more information on the gas phase carbonyl products of the ozone-cyclopentene reaction. Carbonyl products of the ozone-1-methylcyclohexene reaction in dry air included formaldehyde (yield ) 4 ( 1%) and 2-hexanone (estimated yield 4 ( 1%). The oxo acid CH3C(O)(CH2)4COOH, 6-oxoheptanoic acid (or 5-acetylpentanoic acid), was tentatively identified by chemical ionization mass spectrometry. The formation of 2-hexanone and of 6-oxoheptanoic acid from 1-methylcyclohexene may involve reactions analogous to those leading

to butanal and to 5-oxopentanoic acid from cyclopentene, i.e. CH3

above, and/or products of the reactions of the alkenes with a small fraction of OH that was not scavenged by cyclohexane.

Acknowledgments + O3

1,2,3-trioxolane

CH3C(O)(CH2)4COOH

(7a)

HC(O)(CH2)4C(CH3)OO

(7b)

followed by, for the biradical formed in reaction 7a,

CH3C(O)(CH2)4CHOO f CH3C(O)(CH2)4COOH

(8a)

f CO2 + CH3C(O)(CH2)3CH3 (8b) Formaldehyde may form from the other biradical formed in reaction 7b via the biradical f peroxide reaction sequence discussed earlier for 1-alkenes; see reactions 2a-c:

HC(O)(CH2)4C(CH3)OO f HC(O)(CH2)4C(OOH)dCH2 f HCO(CH2)4C(O)CH2OH (9a) HC(O)(CH2)4C(O)CH2OH f CH2OH + RCO, where R ) HC(O)(CH2)4 (9b) CH2OH + O2 f HCHO + HO2

(9c)

In the same ozone-1-methylcyclohexene-cyclohexane experiments in dry air, Atkinson et al. (6) did not report formaldehyde, 2-hexanone, and 6-oxoheptanoic acid as reaction products. They reported one product, the dicarbonyl 5-acetylpentanal, CH3C(O)(CH2)4CHO, with an estimated yield of ca. 0.10 ( 0.02. This carbonyl may form by loss of an O atom from the two biradicals formed in reactions 7a and b. Examination of our chromatograms and of the corresponding mass spectra gave no indication for the presence of 5-acetylpentanal; an estimated upper limit for its formation yield is ca. 0.06. More information is obviously needed regarding gas phase carbonyl products of the ozone-1-methylcyclohexene reaction. Other Minor Carbonyl Products. In addition to the unidentified carbonyls listed in Table 3, several carbonyls that are not listed in Table 1 were observed to form in low yields in the following experiments: 1-pentene, acetaldehyde (yield 0.05) and glyoxal (0.02); 1-hexene, propanal (0.02); 1-heptene, acetaldehyde (0.04), propanal (0.02), and butanal (0.03); 2,3-dimethyl-1-butene, acetaldehyde (0.02) and butanal (0.02); cyclopentene, propanal (0.04). These carbonyls may be carbonyl impurities, ozone-alkene reaction products of alkene impurities, actual ozone-alkene reaction products via pathways other than those discussed

This work has been sponsored by the Coordinating Research Council, Inc. (CRC), Atlanta, GA, Contract CRC-APRACAQ-1-2-94. Experiments in dry air were carried out by Dr. Roger Atkinson and co-workers at SAPRC, U.C. Riverside; see ref 6. Denise M. Velez prepared the draft and final versions of the manuscript.

Literature Cited (1) National Research Council. Rethinking the ozone problem in urban and regional air pollution; National Academy Press: Washington, DC, 1991. (2) Atkinson, R.; Carter, W. P. L. Chem. Rev. 1984, 84, 437. (3) Carter, W. P. L. Atmos. Environ. 1990, 24A, 481. (4) Atkinson, R. J. Phys. Chem. Ref. Data, Monogr. 2 1994. (5) Atkinson, R.; Aschmann, S. M. Environ. Sci. Technol. 1993, 27, 1357. (6) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Environ. Sci. Technol. 1995, 29, 1860. (7) Grosjean, D.; Grosjean, E.; Williams, E. L., II Environ. Sci. Technol. 1994, 28, 186. (8) Grosjean, E.; de Andrade, J. B.; Grosjean, D. Environ. Sci. Technol. 1996, 30, 975. (9) Grosjean, D.; Friedlander, S. K. In The Character and Origins of Smog Aerosols; Hidy, G. M., et al., Ed.; Wiley: New York, 1979; Chapter 19, pp 435-473. (10) Grosjean, D.; Williams, E. L., II; Grosjean, E.; Novakov, T. Aerosol Sci. Technol. 1994, 21, 306. (11) Atkinson, R.; Aschmann, S. M.; Arey, J.; Shorees, B. J. Geophys. Res. 1992, 97, 6065. (12) Grosjean, D.; Williams, E. L., II; Grosjean, E.; Andino, J. M.; Seinfeld, J. H. Environ. Sci. Technol. 1993, 27, 2754. (13) Hatakeyama, S.; Izumi, K.; Fukuyama, T.; Akimoto, H. J. Geophys. Res. 1989, 94, 13,013. (14) Grosjean, D. Sci. Total Environ. 1984, 37, 195. (15) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Environ. Sci. Technol. 1983, 17, 312A. (16) Hatakeyama, S.; Akimoto, H. Bull. Chem. Soc. Jpn. 1990, 63, 2701. (17) Hatakeyama, S.; Ohno, M.; Weng, J.; Tagaki, H.; Akimoto, H. Environ. Sci. Technol. 1987, 21, 52. (18) Hatakeyama, S.; Tanonaka, T.; Weng, J.; Bandow, H.; Takagi, H.; Akimoto, H. Environ. Sci. Technol. 1985, 19, 935. (19) Grosjean, E.; Grosjean, D. Int. J. Environ. Anal. Chem. 1995, 61, 47. (20) Grosjean, E.; Grosjean, D. Int. J. Environ. Anal. Chem. 1995, 61, 343. (21) Grosjean, E.; Grosjean, D.; Fraser, M. P.; Cass, G. R. An air quality model evaluation data set for organics. 2. C1-C14 carbonyls in Los Angeles air. Environ. Sci. Technol., submitted. (22) Grosjean, D. Anal. Chem. 1983, 55, 2436. (23) Grosjean, D.; Grosjean, E.; Williams, E. L., II Isr. J. Chem. 1994, 34, 365. (24) Grosjean, E.; Grosjean, D. Environ. Sci. Technol. 1996, 30, 859. (25) Grosjean, E.; Grosjean, D.; Seinfeld, J. H. Environ. Sci. Technol. 1996, 30, 1038. (26) Grosjean, D. Environ. Sci. Technol. 1990, 24, 1428.

Received for review July 17, 1995. Revised manuscript received December 6, 1995. Accepted December 8, 1995.X ES950529+ X

Abstract published in Advance ACS Abstracts, February 15, 1996.

VOL. 30, NO. 4, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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