Monitoring OH-Initiated Oxidation Kinetics of Isoprene and Its Products

A novel technique has been developed to simultaneously monitor the kinetics .... Ketone from the OH-Initiated Oxidation of Isoprene under NOx-Free Con...
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Environ. Sci. Technol. 2005, 39, 1030-1036

Monitoring OH-Initiated Oxidation Kinetics of Isoprene and Its Products Using Online Mass Spectrometry WOOJIN LEE,† MUNKHBAYAR BAASANDORJ, PHILIP S. STEVENS, AND RONALD A. HITES* School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405

A novel technique has been developed to simultaneously monitor the kinetics of the OH radical-initiated oxidation of isoprene and formation and oxidation of its products (methyl vinyl ketone, methacrolein, 3-methylfuran, and formaldehyde) using online mass spectrometry. The kinetics of isoprene and its products were investigated at 323 K and at 1 atm total pressure. The responses of 30 representative ions for isoprene and its products were monitored during the reaction, and their concentration profiles were calculated by linear algebraic equations, which resolve the measured mass spectra of representative ions into the responses of individual target organics, and by calibrations, which converted the responses to individual target concentrations. Using this method, yields of methyl vinyl ketone, methacrolein, and 3-methylfuran at 323 K were measured to be 14.4 ( 0.1%, 19.0 ( 0.2%, and 2.9 ( 0.2%, respectively, in excellent agreement with previously reported yields under NOx-free conditions. The reaction kinetics of isoprene and its oxidation products measured by the experimental procedure developed in this study were compared with those estimated by a kinetic model of isoprene oxidation. The observed methacrolein concentrations as a function of time were reproduced reasonably well by this model, while the observed methyl vinyl ketone concentration could be reproduced by including secondary reactions of some of the hydroperoxide products of isoprene oxidation. The observed 3-methylfuran concentrations could be reproduced using secondary cyclization reactions of some of the 1,4-hydroxycarbonyl products of isoprene oxidation. These results suggest that under low NOx conditions reactions of some of the hydroperoxides and hydroxycarbonyls produced from the OH-initiated oxidation of isoprene may be a significant source of methyl vinyl ketone and 3-methylfuran in the atmosphere.

significantly contribute to the formation of ozone in the troposphere (3). To predict air quality and to properly establish ozone control strategies, one needs a complete understanding of the isoprene-OH reaction’s kinetics and mechanisms under realistic atmospheric conditions. It has been reported that isoprene is oxidized by OH with rate constants in the range of (8.56 ( 0.26) × 10-11 to (11.1 ( 0.23) × 10-11 cm3 molecule-1 s-1 at 298 K (4-9). A negative temperature dependence for this reaction has been reported over the temperature range of 298-422 K (4, 7). OH can add to one of four positions in isoprene, resulting in the formation of four possible hydroxyalkyl radicals (10, 11); see Figure 1. Addition of OH to the 1- and 4-positions of the butadiene chain leads to the production of allylic radicals, allowing O2 addition to the carbon centers either β or δ to the OH group to form hydroxyalkyl peroxy radicals. Addition of OH to the 2- and 3-positions produces alkyl radicals that only allow O2 to be added to the carbon center β to the OH group. Therefore, a total of six isomeric hydroxyalkyl peroxy radicals are formed. Under polluted conditions in the troposphere, these peroxy radicals (RO2) react with NO to form HO2, NO2, ozone, and a variety of organic species, such as formaldehyde (HCHO), methyl vinyl ketone (MVK), methacrolein (MAC), 3-methylfuran (3MF), and other carbonyl and organic nitrate (RONO2) compounds including 1,4-hydroxycarbonyls. The measured total carbon mass balance of these products is reported to be between 60 and 97% (11-16): O2

OH + isoprene 98 RO2 RO2 + NO f RO + NO2 f RONO2 O2

RO 98 MVK + HCHO + HO2 O2

98 MAC + HCHO + HO2 O2

98 3MF + HO2 In contrast to high NOx conditions, there have been fewer studies about the formation and distribution of isoprene oxidation products in a NOx-free atmosphere. Under these conditions, self- and cross-reactions of the isoprene-based hydroxyl alkyl peroxy radicals lead to the formation of three major products (MVK, MAC, and HCHO) accounting for about one-third of the total carbon mass balance (17). Others have reported on the formation of C5 unsaturated diols (ROH), which account for 7% of the total carbon mass balance during the OH-initiated oxidation of isoprene (18), while theoretical studies suggest that C5 hydroperoxides (ROOH) and carbonyls make up the remaining first-generation products of isoprene oxidation under low NOx conditions (19): O2

OH + isoprene 98 RO2

Introduction Isoprene (2-methyl-1,3-butadiene) is the most abundant biogenic hydrocarbon emitted into the atmosphere, with a global emission rate between 250 and 500 Tg/yr, exceeding that of anthropogenic hydrocarbons (1, 2). Because of the high reactivity of isoprene with the OH radical, isoprene can * Corresponding author e-mail: [email protected]. † Present address: Environment & Process Technology Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea. 1030

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RO2 + RO2 f RO + RO + O2 ROH + carbonyl + O2 f ROH RO2 + HO2 f ROOH + O2 O2

RO 98 MVK + HCHO + HO2 O2

98 MAC + HCHO + HO2 O2

98 3MF + HO2 10.1021/es049438f CCC: $30.25

 2005 American Chemical Society Published on Web 01/06/2005

FIGURE 1. Schematic mechanism for the formation of peroxy radicals and subsequent products from the OH-initiated oxidation of isoprene in the absence of NOx (19). The difficulty in detecting these latter species may be the result of heterogeneous loss of these polar and less volatile species on the walls of the reaction chamber (13). In this research, we have developed an experimental procedure (using online mass spectrometry) to simultaneously monitor the degradation of isoprene by OH and the formation of its oxidation products. This novel experimental procedure has been validated by comparing the kinetic data for isoprene and its products to those obtained from a model of the OH-initiated oxidation of isoprene under low NOx conditions. The results described in this study confirm our approach of measuring gas-phase reaction kinetics of target compounds and their products in order to test current models of the oxidation of volatile organic compounds under atmospheric conditions.

Experimental Section The experimental system used to monitor the reaction of isoprene and its products has been described in detail elsewhere (7, 20, 21). Briefly, the system features a ∼190 cm3 quartz reaction chamber mounted in a gas chromatographic oven (HP 5890). The chamber is connected to an electronimpact mass spectrometer (HP 5989A MS Engine) by a deactivated, fused silica tube (75 cm × 100 µm, J&W Scientific Inc.). With this experimental arrangement, we can control the reaction temperature precisely ((0.1 K), and we can continuously and simultaneously monitor the relative concentrations of isoprene and its reaction products as a function of time. Helium (99.999%, Gas Tech Inc.) was used to flush the chamber and to dilute the concentration of isoprene. The chamber was operated under static conditions at atmospheric pressure. OH radicals were produced in situ by the photolysis of hydrogen peroxide with UV radiation centered at 254 nm; four 8-W germicidal lamps (G8-T5, General Electric) were attached to the outside of the door of the GC oven, which had been modified with a 19 × 30 cm quartz window (7). We maximized the radiation in the reaction chamber by lining the inside of the GC oven with reflective tape. The chemicals used, their stated purities, their Chemical Abstract Registry Nos., and their suppliers are as follows:

isoprene (99%, 78-79-5, Aldrich), MVK (99%, 78-94-4, Aldrich), MAC (95%, 78-85-3, Aldrich), HCHO (35 wt % solution in water, 50-00-0, Aldrich), 2-methylfuran (99%, 534-22-5, Aldrich), carbon tetrachloride (CCl4; 99.9%, 56-23-5, Aldrich), and hydrogen peroxide (50 wt % solution in water, 772284-1, Aldrich). Because 3MF was not commercially available, 2-methylfuran (2MF) was used for the calibration standard for 3MF. A preliminary experiment to select representative ions for isoprene and its products during the oxidation was conducted. Before the start of the experiment, helium was flushed through the reaction chamber for 30 min at a constant chamber temperature of 323 K. The helium was then directed through the hydrogen peroxide solution and into the chamber; the mixture of He and H2O2 was introduced into the chamber for 10 min. The experiment was started by closing the values attached to the reaction chamber to isolate the gas flows and by turning on the mass spectrometer, which had been adjusted to scan all m/z values between 25 and 200, except for some m/z values (28, 29, 32, 40, and 44) resulting from a small air leak into the chamber. CCl4 was used as an internal standard; thus, it was injected with isoprene for the determination of representative ions. [CCl4 was selected because it is essentially nonreactive with OH radicals (22).] An exact amount of isoprene (0.2 µL) and CCl4 (0.5 µL) was injected into the chamber after 2-3 min; this gave gas-phase reactant concentrations of 6.3 × 1015 and 16 × 1015 molecules‚cm-3, respectively. After a waiting period of 10 min to ensure mixing of the two compounds and establish a good baseline, the UV lamps were turned on to initiate reactions with OH. The reaction was allowed to proceed for 90 min; with one full scan mass spectrum obtained every 280 ms. After the lamps were turned off, the signal stabilized at a lower level for 5 min, and the mass spectrometer was then turned off. The responses were corrected by subtracting the background signal from all data points. The variation of each ion during the reaction was investigated, and 30 ions showing high response and significant change were selected as representative ions for isoprene, its products, and the internal standard. These m/z values are listed in the first column of Table 1. VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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of two matrices:

TABLE 1. Relative Abundances of Representative Ions of Six Target Organic Compounds ions (m/z) 26 30 35 37 38 39 41 42 43 47 49 50 51 52 53 54 55 65 67 68 69 70 71 81 82 83 84 117 119 121

isoprene

MAC

MVK

51

42 7

100

57 116 510 234 125 5

83 134 676 999 157 28

20 30 50 20 130 580

31 107 133 70 627 31

12 22 13 10 21 18

20 30 20 10 20 10 999

78 828 41

380 20

123 999 642 33

HCHO

3MF

580

208 24 89 126 496 83 42 43 4 89 36 315 152 600 336 79 5 5 6 4 3 3 584 999 108 10

AX ) M

CCl4

140 44 17 4 11 10 30 234 79 1

3 1 1 9 5 242 7 155 999 981 311

Using these ions, we conducted experiments to measure the kinetics of the OH-initiated oxidation of isoprene and its products. The same experimental procedure used for the first step was used to measure responses of representative ions. Isoprene (0.2 µL) and CCl4 (0.5 µL) were injected into the reaction chamber, and the responses of the 30 representative ions, selected in the first step, were measured for 100 min (reaction time of 90 min) using the mass spectrometer’s selected ion monitoring (SIM) mode. The responses were corrected by subtracting the background signal from all data points. The experiments were all conducted at 323 K. After turning off the mass spectrometer, the reaction chamber was flushed with helium, and the temperature of the GC oven was increased to clean the chamber prior to the next experiment. Because there is no separation of the compounds between the chamber and the mass spectrometer, at any given time during the experiment, the measured responses of the 30 ions represent the mass spectrum of the mixture of the several compounds in the chamber at that time. In other words, at any given time, the measured mass spectrum of the chamber’s contents is a linear combination of the mass spectra of the individual components, with each of the mass spectra weighted according to the abundance of that component in the chamber at that time. Thus, to follow the abundance of isoprene, its products, and the internal standard during the reaction, we developed a set of linear equations to resolve the measured mass spectra of the mixture into the responses of the individual target compounds. Although many products of the OH-isoprene reaction have been identified, we used only the four most abundant and well-known products (MVK, MAC, HCHO, and 3MF) as target compounds for the calculations in this study. Therefore, the responses of these four compounds plus isoprene itself and CCl4 were obtained as a function of time. The linear equations can be expressed in matrix notation by the product 1032

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(1)

where A is the 30 × 6 matrix of the relative abundances of the 30 representative ions for the six target compounds, X is the 6 × 1 matrix of the relative abundances of the six compounds, and M is the 30 × 1 matrix of the responses of the 30 ions measured every 280 ms during the experiment. The components of A were obtained from the National Institute of Standards and Technology database of reference mass spectra, and this matrix is shown in Table 1. The solution to this system of equations is

X ) (ATA)-1ATM

(2)

where AT is the 6 × 30 transpose of matrix A and (ATA)-1 is the 6 × 6 inverse matrix of the product of matrices AT and A. The responses of six target organics (X) at each sampling time were extracted from the responses of the 30 representative ions (M) using eq 2 and were saved in a data file. A calibration experiment was conducted to give response factors that could be used to convert the responses of isoprene and its products (X) to their concentrations. This experiment was conducted at 323 K, and the same experimental procedure as described above was used. After closing the valves, isoprene, its four major products, and CCl4 were injected into the reaction chamber. Six calibration levels of isoprene and its products (0.1-0.8 µL) were used without dilution, and an exact amount of CCl4 (0.5 µL) was injected as the internal standard at each level. A waiting period of 10 min was allowed for complete mixing, and the responses of the 30 representative ions were measured for 5 min without UV radiation. The data were corrected by subtracting the background signal from all data points. The responses of the 30 ions at each calibration level were converted to the responses of the six target organics using eq 2. The response of each target compound was averaged over the 5 min data acquisition time period, and a calibration equation for each target compound was obtained. This calibration was based on the internal standard method using a linear regression between its known concentration and the averaged responses at each level. The relative abundances of isoprene and its products (X) at each sampling time were converted to their concentrations using these calibration equations. The calibration experiment was conducted immediately after the isoprene experiment. The kinetic data measured by the experimental procedure were compared to simulations based on the mechanism for isoprene oxidation recommended by Jenkin et al. (19). A schematic of the model appears in Table 2. The model contains over 100 reactions and treats the reaction of each individual peroxy radical formed in the OH-initiated oxidation of isoprene. We used the rate constants for the degradation of isoprene from Gill and Hites (7) and the recommended rate constants for the peroxy and alkoxy radical reactions as described in Jenkin et al. (19). In this model, OH radicals are assumed to preferentially add to the terminal carbons with a branching ratio of 15% for peroxy radical (a), 45% for (b), 5% for (c), 5% for (d), 23% for (e), and 8% for (f); see Figure 1 for this radical nomenclature. Self- and cross-reactions of these individual peroxy radicals lead to the formation of individual alkoxy radicals that either decompose or isomerize to form the major reaction products. The initial OH radical concentration, which was more or less constant during the experiment, was fixed at 1.4 × 107 molecules‚cm-3, a value that was consistent with the observed isoprene decay. The model included secondary OH radical loss reactions of MAC, MVK, 3MF, and HCHO but did not include secondary

TABLE 2. Reactions and Rate Constants Used in the Chemical Model at 323 K reaction

ka

OH + isoprene f RO2b RO2 + RO2 f RO + RO + O2c RO2 + RO2 f C5 alcohol + C5 carbonyl + O2 RO (decomposition/O2) f MVK + HCHO + HO2d RO (decomposition/O2) f MAC + HCHO + HO2d RO (isomerization/O2) f 3MF + HO2 + H2Od RO2 + HO2 f ROOH (C5 hydroperoxide) + O2 OH + MVK f productse OH + MAC f productse OH + 3MF f productse OH + HCHO (+O2)f HO2 + CO + H2Oe OH + ROOH f RO2 + H2Of C5 1,4 hydroxycarbonyl f 3MF + H2Of

9.0 × 10-11 2.5 × 10-12-6.9 × 10-12 2.5 × 10-12-6.9 × 10-12 1 × 106 s-1 1 × 106 s-1 5 × 105 s-1 1.6 × 10-11 1.7 × 10-11 3.2 × 10-11 9.4 × 10-11 9.4 × 10-12 1.5 × 10-11 1.5 × 10-4 s-1

a Except where noted, rate constants are in units of cm3 molecule-1 s-1. b The different RO , radicals were speciated in the simulations, but for 2 simplicity, are not speciated in this table. c Individual rate constants for each peroxy radical reaction in the range shown were based on the 298 d K recommendations of Jenkin et al. (19). The fate of the individual alkoxy radicals depends on the structure of the individual radical as described in Jenkin et al. (19). e Rate constants based on the recommendations of Atkinson et al. (26). f Assumed based on observations.

FIGURE 2. Concentrations of isoprene and of its four major products (MVK, MAC, HCHO, and 3MF) as a result of isoprene’s gas-phase OH radical-initiated oxidation at 323 K. The concentrations of the compounds shown here were converted from the responses of 30 representative ions measured continuously during the reaction. reactions of organic hydroperoxides (ROOH), alcohols, carbonyls, or other products.

Results and Discussion Figure 2 shows a plot of the OH-initiated oxidative degradation of isoprene and the formation of its four main degradation products (MVK, MAC, HCHO, and 3MF) at 323 K as a function of reaction time. Under these conditions, isoprene was completely degraded in 66 min, and the concentrations of MAC and MVK increased, reaching their maximum concentrations at 19 and 31 min, respectively, while the concentrations of HCHO and 3MF increased continuously throughout the experiment. The concentration of these four products accounts for approximately 30% of the total carbon consumed in the reaction, with the remaining products likely to be various peroxides, C5-diols, C5-carbonyl species, CO, and CO2 (19). The peroxides and C5-compounds were not detected in these experiments because these compounds were likely lost on the walls of the sampling inlet due to their high polarity. The production of MVK, MAC, and 3MF at 323 K increased linearly with the loss of isoprene during the first 20 min of the experiment as shown in Figure 3. In this figure, the solid lines are the observed production of each compound as a function of isoprene loss, while the dashed lines are the concentrations of each compound corrected for secondary

FIGURE 3. Concentrations of MAC, MVK, and 3MF as a function of isoprene loss during the first 20 min of the reaction. Solid lines are the observed concentrations; dashed lines and fits are the concentrations corrected for loss due to reaction with OH based on current rate constant recommendations (22, 25). The regression slopes and correlation coefficients (r2) are as follows: MAC, 0.190, 0.992; MVK, 0.144, 0.994; and 3MF, 0.029, 0.922. loss due to reaction with OH radicals (18). On the basis of the corrected data, the yields of MVK, MAC, and 3MF at 323 K were observed to be 14.4 ( 0.1%, 19.0 ( 0.2%, and 2.9 ( 0.2%, where the reported uncertainty is twice the standard error from the regression analysis. These yields of MVK and MAC are in excellent agreement with the yields under NOxfree conditions reported by Miyoshi et al. (17) of ∼17% and ∼22%, and by Ruppert and Becker (18) of 15.3 ( 1.2% and 17.8 ( 1.4%; both of these yields were measured at 295-303 K using an environmental chamber with FTIR detection of the products. The yield of 3MF reported here is also in agreement with the yield of 5.2 ( 0.5% as reported by Benkelberg et al. (23) using GC-MS techniques, although their reported yields of MVK and MAC (37.6 ( 1.5% and 35.0 ( 1.0%) are significantly higher than the values reported here. The agreement between the results reported here at 323 K with those obtained at room temperature suggests that these results are applicable to ambient temperatures in the atmosphere. Figures 4 and 5 compare the measured data at 323 K to those estimated by the above chemical model over the course of the experiment. In this model, MAC and HCHO are produced as a result of the decomposition of alkoxy radicals produced primarily from reactions of peroxy radical (e) and VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Comparison of observed concentration of isoprene and HCHO with those estimated by the chemical model.

FIGURE 5. Comparison of observed concentration of MAC, MVK, and 3MF with those estimated by the chemical model, including secondary reactions of isoprene products. to a lesser extent (d), while MVK and HCHO are produced as a result of the decomposition of alkoxy radicals produced from reactions of peroxy radical (b) and to a lesser extent (c):

In this mechanism, 3MF is produced as a result of isomerization reactions involving alkoxy radical products of the reactions of peroxy radicals (c) and (d). As can be seen from Figure 4, the observed increase in the concentration of HCHO during the course of the experiment is not reproduced by the chemical model. This may be due, in part, to the absence of secondary reactions 1034

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involving the C5 hydroperoxides, alcohols, and carbonyls products of the oxidation mechanism, including possible photolysis reactions of these products. However, this discrepancy may also be due to measurement bias given that the concentration of HCHO was based on the measurement of a single ion at m/z 30 (see Table 1). Although the relative abundances of the ions at m/z 28 and 29 in the mass spectrum of HCHO are significant, they were not used here because of interferences from atmospheric N2. Future experiments conducted under nitrogen-free conditions would be needed to eliminate air leaks and to exclude the effect of atmospheric nitrogen on our measurements of HCHO. The production of MAC, MVK, and 3MF measured in our experiments agreed reasonably well with that predicted by the chemical model of Jenkin et al. (19) during the first 10 min of the experiment, suggesting that the production of these compounds is reasonably well-represented by the chemical mechanism (see Figure 5). The model also provides a reasonable fit to the observed MAC concentration over the course of the experiment, underestimating the peak MAC concentration by approximately 15%. These results suggest that the observed MAC concentration is primarily due to production by isoprene oxidation and loss by reaction with OH. However, without including other secondary reactions of the isoprene oxidation products, the base model of Jenkin et al. (19) underestimates the peak MVK concentration by approximately 30% and predicts that the peak MVK concentration should occur at approximately 25 min, while the observed MVK concentration continues to rise until peaking at after 30 min. This base model also predicts a decrease in the 3MF concentration after 10 min to due reaction with OH, while the observed concentration of 3MF continued to rise during the course of the experiment, as observed previously (18). These results suggest that secondary chemical reactions not included in the chemical model may contribute significantly to the production of MVK and 3MF over the course of the experiment. On the basis of the branching ratios for the OH addition to isoprene as suggested by Jenkin et al. (19), peroxy radical (b) has the highest concentration of all the peroxy radicals produced, resulting in a high concentration of the corresponding hydroperoxide after reaction with HO2. Reaction of this hydroperoxide with OH can also lead to formation of peroxy radical (b), which can react to form alkoxy radicals that can decompose to form MVK (24):

Including the OH + ROOH f ROO + H2O reaction for this hydroperoxide with a rate constant of 1.5 × 10-11 cm3 molecule-1 s-1 into the base model improves the agreement between the modeled MVK with the observed concentration (see Figure 5). This rate constant is similar to that recommended in the master chemical mechanism (MCM) for isoprene oxidation (24) and suggests that secondary reactions of isoprene oxidation products may be an important source of MVK under low NOx conditions.

Although the initial formation rate of 3MF is reproduced by the base chemical model of Jenkin et al. (19), the model predicts a rapid decrease in the concentration of 3MF due to reaction with the OH radical (k ) 9.4 × 10-11 cm3 molecule-1 s-1) (25). The observed increase in the concentration of 3MF during the entire course of the experiment may be due to secondary reactions of some of the products of the isoprene oxidation mechanism leading to the formation 3MF or a slower loss rate of 3MF due to reaction with OH. One possible secondary source of 3MF is the cyclization of some of the C5 1,4-hydroxycarbonyl products produced through reactions of peroxy radicals (a and f) and subsequent loss of water (15):

and 3-methylfuran under low NOx conditions. However, since these hydroperoxides and hydroxycarbonyls products are formed from peroxy radical reactions, the concentration of these products may be different under ambient atmospheric conditions, because the cross-reactions of isoprene-based peroxy radicals with other organic peroxy radicals may compete with the formation of these isoprene oxidation products. Clearly additional measurements of these product yields under a variety of atmospheric conditions are needed to confirm these results.

Acknowledgments This work is supported by the National Science Foundation Grant ATM-0106705.

Literature Cited

Including the formation of 3MF from the cyclization of these C5 1,4-hydroxycarbonyl products of isoprene oxidation in model (using a rate constant of 1.5 × 10-4 s-1) improves the agreement between the modeled and observed 3MF concentration (see Figure 5). The continuous rise in the observed 3MF formation relative to this model may suggest that other secondary reactions may also be contributing to the observed 3MF concentration, such as the reaction of OH with the C5 diol products of isoprene oxidation. These reactions may lead to the formation of the alkoxy radical precursors to 3MF formation:

Another possible 3MF source is reactions involving some of the hydroperoxides produced in the isoprene oxidation mechanism leading to the formation of 1,4-hydroxycarbonyls, perhaps catalyzed by reaction with OH (24), or photolysis reactions leading to precursors of 3MF. However, the influence of secondary heterogeneous reactions on the walls of the reactor leading to the formation of 3MF cannot be ruled out, and as a result, this subsequent production of 3MF may depend on the conditions of these experiments. Clearly additional measurements of the kinetics of 3MF formation and loss are needed to resolve this issue. These results suggest that secondary reactions of isoprene oxidation products, such as hydroperoxides and hydroxycarbonyls, may be a significant source of methylvinyl ketone

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Received for review April 14, 2004. Revised manuscript received October 4, 2004. Accepted November 10, 2004. ES049438F