Radical Dependence of the Yields of Methacrolein and Methyl Vinyl

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Environ. Sci. Technol. 2011, 45, 923–929

Radical Dependence of the Yields of Methacrolein and Methyl Vinyl Ketone from the OH-Initiated Oxidation of Isoprene under NOx-Free Conditions MARIA A. NAVARRO,† SEBASTIEN DUSANTER,† R O N A L D A . H I T E S , †,‡ A N D P H I L I P S . S T E V E N S * ,†,‡ Center for Research in Environmental Science, School of Public and Environmental Affairs, and Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

Received September 15, 2010. Revised manuscript received November 23, 2010. Accepted November 30, 2010.

Formation yields of methacrolein (MAC), methyl vinyl ketone (MVK), and 3-methyl furan (3MF) from the hydroxyl radical (OH) initiated oxidation of isoprene were investigated under NOxfree conditions (NOx ) NO + NO2) at 50 °C and 1 atm in a quartz reaction chamber coupled to a mass spectrometer. Yields of the primary products were measured at various OH and hydroperoxy (HO2) radical concentrations and were found to decrease as the HO2-to-isoprene-based peroxy radical (ISORO2) concentration ratio increases. This is likely the result of a competition between ISORO2 self- and cross-reactions that lead to the formation of the primary products, with reactions between these peroxy radicals and HO2 which can lead to the formation of peroxides. Under conditions with HO2/ISORO2 ratios close to 0.1, yields of MVK (15.5% ( 1.4%) and MAC (13.0% ( 1.2%) were higher than the yields of MVK (8.9% ( 0.9%) and MAC (10.9% ( 1.1%) measured under conditions with HO2/ ISORO2 ratios close to 1. This radical dependence of the yields was reproduced reasonably well by an explicit model of isoprene oxidation, suggesting that the model is able to reproduce the observed products yields under a realistic range of atmospheric HO2/ISORO2 ratios.

Introduction The chemical mechanism for the oxidation of isoprene is a subject of considerable interest in atmospheric chemistry. Isoprene (2-methyl-1,3-butadiene) is one of the most abundant biogenic hydrocarbons in the atmosphere, and it can rapidly react with the hydroxyl radical (OH) leading to the formation of ozone and secondary organic aerosols (1-4). Methyl vinyl ketone (MVK), methacrolein (MAC), 3-methyl furan (3MF), formaldehyde (HCHO), and organic nitrates are reported to be some of the most important products of the OH-initiated oxidation of isoprene (5). Formation yields of the main products (MAC and MVK) and the subsequent reactions of these products with tropospheric oxidants have * Corresponding author phone: (812) 855-4953; fax: (812) 8557802; e-mail: [email protected]. † Center for Research in Environmental Science. ‡ Department of Chemistry. 10.1021/es103147w

 2011 American Chemical Society

Published on Web 12/22/2010

been well-established in the presence of significant concentrations of NOx (NO + NO2) (6-8). However, recent measurements of hydroxyl (OH) and hydroperoxy (HO2) radicals in forest environments show serious discrepancies with modeled concentrations of these radicals, bringing into question our understanding of the chemistry of isoprene and other reactive biogenic species under low NOx mixing ratios (NO < 50 ppt) (9-12). Under similar low NOx mixing ratios, a recent modeling study of MVK and MAC concentrations in the tropical rainforest substantially overestimated the measured concentrations of these isoprene oxidation products (12). The atmospheric oxidation of isoprene is primarily initiated by the addition of OH to a double bond of the butadiene chain, leading to the formation of four potential hydroxyalkyl radicals. The addition of OH to the terminal carbons produces two allylic radicals, whereas the addition of OH to the internal carbons produces two alkyl radicals. Addition of O2 to these radicals leads to a total of six hydroxyalkyl peroxy radicals (ISORO2) (Supporting Information Figure S1). For simplicity, the E and Z isomers of the resulting δ-hydroxyalkyl peroxy radicals are not distinguished from each other in this figure, but including these isomers would result in a total of eight different ISORO2 radicals. Under low concentrations of NOx, ISORO2 radicals undergo self-and cross-reactions leading to the formation of hydroxy alkoxy radicals (R1O to R6O), which can decompose, react with O2, or isomerize leading to the formation of MVK, MAC, HCHO, and 3MF although there is still uncertainty associated with the mechanism of 3MF formation (13, 14) (Supporting Information Figure S1). Other products of these peroxy radical reactions include C5 unsaturated diols and hydroxycarbonyls (15-17) (Supporting Information Figure S1). Reactions of ISORO2 radicals with HO2 can compete with the self-and cross-reaction of ISORO2 radicals, terminating the radical propagation and leading to the formation of organic peroxides (ROOH) (Supporting Information Figure S1). Several studies have reported the yields of MVK, MAC, and 3MF under NOx-free conditions (Table 1). Miyoshi et al. (15), Ruppert and Becker (17), and Lee et al. (18) reported yields of MVK and MAC in the range of 14.4-17% and 17.8-19.0%. On the other hand, the study of Benkelberg et al. (19) found higher yields for both MVK and MAC (37.6% ( 1.5% and 35.0% ( 1.0%, respectively), as well as a 2-fold higher yield for 3MF (5.2% ( 0.5%) compared to that measured by Lee et al. (18) (2.9% ( 0.2%). Under low concentrations of NOx, it is expected that the product yields would exhibit a dependence on the HO2/ ISORO2 ratio due to the competition between the ISORO2 + ISORO2 and ISORO2 + HO2 reactions. In addition, ISORO2 reactions involving other organic peroxy radicals may also compete with the self- and cross-reactions of ISORO2 radicals and impact the product yields. Similarly, recently proposed mechanisms of OH regeneration from the oxidation of

TABLE 1. Average Percent Yields of MAC, MVK, and 3MF under Low NOx Conditions Reported by Previous Studies Lee et al.a MAC 19.0 ( 0.2 MVK 14.4 ( 0.1 3MF 2.9 ( 0.2 a

Ref 18.

b

Miyoshi et al.b

Ruppert and Beckerc

22 17

17.8 ( 1.4 15.3 ( 1.2

Ref 15. c Ref 17.

d

Benkelberg et al.d 35.0 ( 1.0 37.6 ( 1.5 5.2 ( 0.5

Ref 19.

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isoprene may also impact the primary product yields under low NOx conditions. These new mechanisms include OH radical formation from RO2 + HO2 reactions (11, 20, 21), the reformation of OH through the formation of dihydroxyepoxides from the oxidation of isoprene hydroxyhydroperoxides (22), and the formation of HOx radicals from the unimolecular reactions of ISORO2 radicals (23, 24). Understanding the parameters controlling the formation yields of MAC and MVK is essential to accurately describe the chemical processing of isoprene in the troposphere. The following study investigates the OH-initiated oxidation of isoprene under NOx-free conditions at 50 °C and 1 atm. Yields of MAC, MVK, and 3MF were derived from their temporal concentration profiles at different radical concentrations. Adjusting the radical concentration allows measurements of the formation yields of the major isoprene oxidation products under various HO2/ISORO2 ratios to investigate the dependence of the yields on the relative concentrations of peroxy radicals in the atmosphere.

Experimental Section The experimental system employed for this study has been used previously for both relative-rate kinetic studies (25-30) and product studies (18). Briefly, the system features an ∼160 cm3 quartz reaction chamber mounted in a gas chromatographic oven (HP 5890, Hewlett-Packard) that maintains the temperature of the reactor at 50 ( 1 °C to reduce potential loss of products on the wall of the reactor. The chamber is connected to an electron-impact mass spectrometer (HP 5989A, Hewlett-Packard) by a deactivated fused-silica tube (45 cm long and 0.10 mm i.d., J&W Scientific Inc.). This transfer line is shorter than that used by Lee et al. (18), reducing wall loss of the analytes during transport to the MS ion source. Additional information on the experimental procedure is provided in the Supporting Information. OH radicals were produced inside the reaction chamber by photolysis of hydrogen peroxide (H2O2) near 295 nm (reaction R1): H2O2 + hν f OH + OH

9

Yield(X) )

(R1)

The light source was a 200 W Xe-Hg arc lamp (Hamamatsu Corporation) with a dichroic mirror system and a long-pass filter (WG-295 Andover Corporation) to eliminate wavelengths less than 295 nm. This light source replaced the lowpressure Hg lamps used previously by Lee et al. (18) decreasing the photolysis rate of H2O2 and allowing for the production of lower OH concentrations in these experiments (30). A mixture of approximately 0.5% O2 (zero grade, Indiana Oxygen) in He (99.999%, Gas Tech Inc.) was bubbled into a solution of hydrogen peroxide (H2O2, wt 50%, Sigma-Aldrich) and passed through the reaction chamber for 10 min before each experiment (see the Supporting Information). Several experiments were performed to select representative ions of isoprene, MAC, MVK, 3MF, HCHO, and CCl4. CCl4 was injected into the reaction chamber to track the dilution during each experiment since it does not react significantly with OH, and to track the sensitivity of the mass spectrometer using the procedure described by Lee et al. (18). Signals at m/z values of 67, 70, 55, 81, 30, and 117 were chosen to monitor isoprene, MAC, MVK, 3MF, HCHO, and CCl4, respectively. Their relative abundances, proportion of a specific m/z signal with respect to the most intense peak (100%), are given in Supporting Information Table S1. Note that for MAC and 3MF, the ion exhibiting the highest relative abundance was not selected due to significant contributions to this signal from other species. Because there is no separation of the compounds between the chamber and the mass spectrometer, the measured responses of each ion represent the mass spectrum of the mixture of the several 924

compounds in the chamber at that time. Supporting Information Table S1 indicates that several species contribute to the signal monitored at the m/z value selected for isoprene, MAC, MVK, 3MF, and CCl4. In order to determine the response of an individual species for a specific m/z value, the small contribution of the other detected species was accounted for by using a system of linear algebraic equations (see the Supporting Information). Calibration experiments were performed under similar conditions of temperature and pressure as during the photooxidation experiments to derive calibration factors to convert the individual signal measured for each species into concentrations (see the Supporting Information). For each photooxidation experiment, the reaction chamber was flushed with a mixture of He/O2/H2O2 for approximately 10 min and isolated from the experimental system. As described in the Supporting Information, different methods were used to introduce H2O2 into the chamber depending on the OH concentration required during the experiment. These approaches resulted in initial concentrations of H2O2 of approximately 1015-1017 molecule/cm3 which produced OH concentrations in the range of 106-107 molecule/cm3. For each experiment, 0.1 µL of both isoprene and CCl4 were injected into the reaction cell and the reactants were allowed to mix for 5 min before the UV lamp was turned on. Timeresolved signal profiles were recorded for 30-90 min, depending on the OH concentration generated. Concentration profiles were derived using calibration factors measured before or after each experiment. After each experiment, the reaction chamber was cleaned by increasing the oven temperature to 100 °C and flushing the reaction chamber with helium for 1 h. Yields of MAC, MVK, and 3MF were calculated after the reaction had been allowed to proceed for 5-10 min (corresponding to isoprene losses between 14% and 64%) as the ratio of the products and the loss of isoprene as shown in eqs 1 and 2:

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F)

([P]t - [P]0)F ([Iso]0 - [Iso]t)(e-kdilt)

(1)

kiso+OH - kp+OH (1 - e-kOH+iso[OH]t)e(kp+OH[OH]t+kdilt) kiso+OH 1 - e-(kiso+OH-kp+OH)[OH]t (2)

Here [ISO]0 and [P]0 are the concentrations of isoprene and an oxidation product (MAC, MVK, or 3MF) before illumination, whereas [ISO]t and [P]t are the concentrations after t seconds of illumination. The first-order loss of each species due to dilution (kdil) is derived from the exponential decay (e-kdilt) of the CCl4 signal. Note that eqs 1 and 2 also correct for losses due to reaction with OH using the recommended rate constants for kOH+MVK,, kOH+MAC, and kOH+3MF, respectively (31). Simulations of the production of MAC, MVK, 3MF, and HCHO were carried out using a model based on the explicit chemical mechanism described by Jenkin et al. (16) for the OH-initiated oxidation of isoprene (Supporting Information Table S2). This mechanism contains over 100 reactions and includes rate constants and branching ratios for the production of the six isomeric hydroxyalkyl peroxy radicals (ISORO2), their self-and cross-reactions, and the reaction of ISORO2 with other peroxy radicals such as CH3O2 and HO2. The original mechanism was modified to reflect recent advances in the understanding of the isoprene oxidation chemistry, including updated values for the rates of hydroxyalkoxy radical decomposition based on recent theoretical studies (14, 32-34), as well as additional production of MVK from

FIGURE 1. Observed concentration profiles of isoprene (black), MVK (blue), MAC (red), and 3MF (gold) at 50 °C and 1 atm. The OH concentration was 4 × 106 molecule/cm3 on average. Dashed line is the simulated profile for isoprene after accounting for dilution.

FIGURE 2. Observed concentration profiles of MVK (blue), MAC (red), and 3MF (gold) at 50 °C and 1 atm. Left: measurements using low OH concentration (approximately 4 × 106 molecule/cm3). Right: measurements using high OH concentration (approximately 2 × 107 molecule/cm3). Dashed lines are the simulated profiles after accounting for dilution. secondary reactions of some of the hydroperoxide products of isoprene (18). The rate constants in the model reflect currently recommended temperature dependences, and for those reactions without a recommended temperature dependence, the model used the recommended values at 298 K. Model simulations indicate that the yields of MVK and MAC are relatively insensitive to temperature, suggesting that the experimental results presented here at 323 K should be similar to those found at other temperatures. However, the temperature dependences for many of the reactions in the model have not been measured, which adds to the uncertainty of the model. In the simulations, t0 was set to be approximately 5 min after the beginning of the experiment to ensure that the species were well-mixed using measured and estimated initial concentrations of isoprene, H2O2, MVK, MAC, 3MF, and HCHO, as well as estimated photolysis frequencies for H2O2, HCHO, and other organic peroxide species (see the Supporting Information).

Results and Discussion Temporal Concentration Profiles. Figure 1 shows temporal concentration profiles recorded during a typical experiment. Small initial concentrations of MVK, MAC, 3MF, and HCHO

were usually observed prior to turning on the UV light and are likely due to the decomposition of isoprene in the stock solution. When the UV light is turned on, OH is produced by photolysis of H2O2 (reaction R1) inside the reaction chamber and the isoprene concentration decays due to its fast reaction with OH. In contrast to our previous study (18), the observed concentration of formaldehyde reaches a maximum between approximately 0 and 10 min and begins to decay as a result of losses due to photolysis and reaction with OH, similar to that predicted by the photochemical model (Supporting Information Figure S2). However, the measured concentration of formaldehyde has a high degree of uncertainty associated with the calibration factor as well as the contribution of nonphotolytic production to the observed signal (see the Supporting Information). Figure 2 shows the observed temporal concentration profiles of MVK, MAC, and 3MF obtained at both low (approximately 4 × 106 molecule/cm3) and at high OH concentrations (approximately 2 × 107 molecule/cm3). At low OH concentrations, the concentration of the isoprene oxidation products peaks approximately 20 min after the reaction is initiated. At high OH concentrations, the concentration of products peaks after approximately 10 min of reaction time. As can be seen from Figures 1 and 2, the model VOL. 45, NO. 3, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Formation of MAC, MVK, and 3MF as a function of isoprene loss under low OH concentration conditions, corrected for dilution and reaction with OH. The regression slopes (dashed lines) and correlation coefficients (r2) for this experiment are: MVK, 0.132 ( 0.001, 0994; MAC, 0.117 ( 0.010, 0.991; 3MF, 0.0437 ( 0.0001, 0990. is able to reproduce the observed concentration profiles of isoprene, MVK, and MAC reasonably well after accounting for dilution (kd ) 10-4-10-5 s-1), although the agreement is better at lower OH concentrations. For experiments conducted under higher OH concentrations, the model tends to underestimate the experimental concentrations of MVK and MAC by approximately 30-50%. In addition, it appears that the model tends to underestimate the overall loss of both MVK and MAC under both low and high OH concentrations based on the overall shape of the curves shown in Figure 2, especially at long reaction times. This may indicate that the model is underestimating the overall loss of these compounds, perhaps through heterogeneous or other processes. Including an additional loss process in the model equivalent to twice the loss by reaction with OH improves the agreement with the measured loss rates. However, this does not significantly affect the yields of MVK and MAC during the first 10 min of the simulation, as the calculated yields increase by less than 15% when the additional loss rate is included in the model. Given the uncertainty in the mechanism of formation of 3MF and the potential for heterogeneous formation of 3MF in atmospheric chambers (14), only the base model for 3MF formation as described previously was used to simulate the observed production of 3MF (18). Because of this uncertainty, the agreement between the measurements and the model shown in Figure 2 may be fortuitous. Additional measurements of the kinetics of 3MF formation and loss are needed to resolve this issue. Yields of MAC, MVK, and 3MF. Figure 3 shows typical formation curves of MAC, MVK, and 3MF corrected for dilution and reaction with OH as a function of isoprene loss for an experiment at low OH concentration (4 × 106 molecules/cm3), while Table 2 gives an average of the yields of MAC, MVK, and 3MF for average OH concentrations of 2.5 × 106 and 1.8 × 107 molecule/cm3 measured over several experiments. Yields measured during individual experiments are reported in Supporting Information Table S3. As can be seen from Table 2, the measured yields are significantly higher when the OH radical concentration is lower. In these experiments, the OH concentration generated inside the reaction chamber is adjusted by varying the concentration of H2O2. Increasing the concentration of H2O2 leads to higher OH concentrations, due to reaction R1, but also leads to an 926

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TABLE 2. Percent Yields of MAC, MVK, and 3MF at Different OH Concentrations and HO2/ISORO2 Ratios average [OH] molecules/cm3

HO2/ISORO2

MVKa

MACa

3MFa

2.5 × 106 1.8 × 107

∼0.1-0.2 ∼0.7-1

15.5 ( 1.4 8.9 ( 0.9

13.0 ( 1.2 10.9 ( 1.1

4.2 ( 0.4 1.9 ( 0.2

a Uncertainties represent 2 standard errors from multiple independent experiments (see the Supporting Information).

increase in the concentration of HO2 and ISORO2 radicals due to reactions R2 and R3: OH + H2O2 f HO2 + H2O

(R2)

OH + isoprene (+ O2) f ISORO2

(R3)

Because of the higher concentrations of H2O2 in the reaction chamber, the concentration of HO2 increases faster than the concentration of ISORO2, and as a result the HO2/ISORO2 ratio increases with increasing concentrations of H2O2. According to the reaction mechanism shown in Supporting Information Figure S1, a higher HO2/ISORO2 ratio could favor the production of hydroperoxides and other HO2 + ISORO2 reaction products over production of MAC, MVK, and 3MF from the ISORO2 + ISORO2 reactions, resulting in lower yields of the primary reaction products. Measured and modeled yields are shown as a function of the average concentration of OH and as a function of the HO2/ISORO2 ratio calculated from the photochemical model in Figure 4. As can be seen from this figure, the measured yields of MVK and MAC decrease when the HO2-to-ISORO2 ratio increases from approximately 0.1 to approximately 1. A similar trend was observed previously by Miyoshi et al., who found that the yields of MVK, MAC, and HCHO decreased as the isoprene/H2O2 ratio decreased due to the higher concentrations of HO2 in their system with higher concentrations of H2O2 (reaction R2) (15). This trend is also observed in the modeled results with the modeled MAC and MVK yields in reasonable agreement with the observed yields, especially at lower radical concentrations and a lower predicted HO2/ISORO2 ratio (Figure 4).

FIGURE 4. Percent yields of MVK (left) and MAC (right) as a function of the average OH concentrations estimated from the observed isoprene decay and as a function of calculated HO2/ISORO2 ratios estimated from model simulations. Error bars represent 2 standard errors from the calibration uncertainty. Values for each experiment are reported in Supporting Information Table S3. To confirm that the observed yields are dependent on the HO2/ISORO2 ratio and not on another process depending on the concentration of OH, an additional experiment was conducted where the lamp flux at 295 nm was attenuated with an additional band-pass filter (BG-40 Andover Corporation) to reduce the formation of OH radicals from H2O2 photolysis. The lower UV flux required higher concentrations of H2O2 to produce similar concentrations of OH radicals. In order to generate OH concentrations similar to those observed for the low OH concentration experiments (approximately 4 × 106 molecule/cm3) an additional aliquot of 25 µL of H2O2 was directly injected inside the reaction chamber. The lower intensity of the light, in conjunction with the higher concentration of H2O2, led to the production of approximately 5 × 106 molecule/cm3 of OH based on the observed loss of isoprene. However, the higher concentration of H2O2 in this experiment would lead to a higher HO2/ISORO2 ratio due to the increased rate of reaction R2 compared to R3 for this experiment. Measured formation yields of MAC (7.6% ( 1.4%) and MVK (6.6% ( 1.2%) in this experiment were lower than those measured at similar OH concentrations with lower concentrations of H2O2 and lower expected HO2/ ISORO2 ratios (Supporting Information Table S3). These measured yields are similar to the results at higher OH radical concentrations and higher HO2/ISORO2 ratios and suggest that the yields of MAC and MVK decrease with the HO2/ ISORO2 ratio independent of the OH concentration. These results are consistent with the reported formation yields of MVK and MAC from previous studies under low NOx conditions (Table 1) when differences in the expected HO2/ISORO2 ratios in each experimental system are taken into account. For example, the previous experiments carried out by Lee et al. on this system at the same temperature using an OH concentration of approximately 1.4 × 107 molecule/cm3 resulted in measured yields of MAC and MVK of 19.0% ( 0.2% and 14.4% ( 0.1%, respectively (18). These yields are higher than the yields reported here under similar OH radical concentrations. However, it is likely that the concentrations of H2O2 used during these previous experiments were significantly lower than the concentrations used in the present study. The germicidal lamps used by Lee et al. to produce radiation at 254 nm to initiate the oxidation likely required lower concentrations of H2O2 to produce similar concentrations of OH radicals due to the higher

absorption cross section of H2O2 at 254 nm (6.7 × 10-20 cm2) compared to that at 295 nm used in this study (0.90 × 10-20 cm2) (35). As a result, the HO2/ISORO2 ratio in the study of Lee et al. is likely lower for the higher OH concentrations used and could be similar to the ratio reported here under lower OH concentrations, leading to similar reported yields of MAC, MVK, and 3MF. Similar yields of MAC and MVK were reported by Miyoshi et al. at 303 ( 1 K using OH concentrations of approximately 5.2 × 107 molecule/cm3 but relatively low H2O2 concentrations (1.3 × 1015 molecule/cm3) and UV wavelengths shorter than 220 nm (15). On the other hand, Benkelberg et al. used lower concentrations of H2O2 (3 × 1014 to 1015 molecule/cm3) and higher concentrations of isoprene (2.5 × 1016 molecule/cm3) that likely reduced the production of HO2 in their experiments leading to a lower HO2/ISORO2 ratio. This may explain the higher formation yields measured for both MVK and MAC reported in their study (19); however, additional experiments under similar conditions of temperature and bath gas composition are needed to confirm the results reported here. Implications for Atmospheric Conditions. Midday HO2 mixing ratios measured during the PROPHET (Program for Research on Oxidants: Photochemistry, Emission and Transport) campaign during the summer of 1998 ranged from 10 to 25 pptv, with a median value of approximately 16 pptv, while median midday mixing ratios of RO2 (ISORO2 being the most prominent) were estimated to be approximately 32 pptv with median mixing ratios of NO of 66 ppt (9). This results in a midday HO2/RO2 ratio of approximately 0.5 for this forested environment where isoprene emissions dominate OH radical reactivity (9). Although modeling the formation of MVK and MAC at the PROPHET site would require including production from the ISORO2 + NO reaction, the explicit model used in the present study is in reasonable agreement with the experimentally observed yield under these radical ratios (Figure 4). This gives confidence in the ability of this model to accurately describe the peroxy radical self-and cross-reactions leading to the formation of MAC and MVK under typical atmospheric peroxy radical concentrations. The model does tend to underestimate the yields of both MVK and MAC at HO2/RO2 ratios greater than approximately 0.6. This may suggest that the model may be overestimating the rates of radical termination by the HO2 + RO2 reaction (36), either through an overestimation of the rate constant VOL. 45, NO. 3, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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for this reaction or due to an underestimation of the rates of radical cycling from this reaction. A preliminary analysis suggests that a reduction of the rate constant for the ISORO2 + HO2 reaction by a factor of 3 improves the agreement between the measured and modeled MAC and MVK concentration profiles shown in Figure 2, whereas including HOx radical production from proposed unimolecular reactions of ISORO2 radicals (23, 24) does not appear have a significant impact on the modeled yields. This latter result may reflect the high concentration of peroxy radicals in these experiments resulting in peroxy radical self-and cross-reactions dominating the fate of these radicals. A detailed sensitivity analysis of the model, as well an analysis of the impact of recently proposed radical cycling mechanisms (11, 12, 23) on the observed yields of MAC and MVK, will appear in a subsequent paper.

Acknowledgments We thank Dr. Daekyun Kim for experimental assistance. This work is supported by the National Science Foundation, Grant AGS-0622815.

Supporting Information Available Additional details on the experimental procedure, including the abundance (%) of representative ions for the MAC, MVK, 3MF, HCHO, and CCl4, and chemical reactions and rate constants used in the photochemical model. This material is available free of charge via the Internet at http:// pubs.acs.org.

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