Kinetic Study of Gas-Phase Reactions of OH and NO3 Radicals and

Article pubs.acs.org/JPCA

Kinetic Study of Gas-Phase Reactions of OH and NO3 Radicals and O3 with iso-Butyl and tert-Butyl Vinyl Ethers Shouming Zhou,*,†,‡,§ Ian Barnes,† Tong Zhu,‡ and Thorsten Benter† †

Bergische Universitaet Wuppertal, Physikalische Chemie/FBC, Gauss Strasse 20, D-42119 Wuppertal, Germany State Key Joint Laboratory for Environmental Simulation and Pollution Control, College of Environmental Science, Peking University, 100871 Beijing, China

‡

S Supporting Information *

ABSTRACT: Using a relative rate technique, kinetic studies on the gas-phase reactions of OH radicals, ozone, and NO3 radicals with iso-butyl vinyl ether (iBVE) and tert-butyl vinyl ether (tBVE) have been performed in a 405 L Duran glass chamber at (298 ± 3) K and atmospheric pressure (750 ± 10 Torr) in synthetic air using in situ FTIR spectroscopy to monitor the reactants. The following rate coefficients (in units of cm3 molecule−1 s−1) have been obtained: (1.08 ± 0.23) × 10−10 and (1.25 ± 0.32) × 10−10 for the reactions of OH with iBVE and tBVE, respectively; (2.85 ± 0.62) × 10−16 and (5.30 ± 1.07) × 10−16 for the ozonolysis of iBVE and tBVE, respectively; and (1.99 ± 0.56) × 10−12 and (4.81 ± 1.01) × 10−12 for the reactions of NO3 with iBVE and tBVE, respectively. The rate coefficients for the NO3 reactions are first-time determinations. The measured rate coefficients are compared with estimates using current structure activity relationship (SAR) methods and the effects of the alkoxy group on the gas-phase reactivity of the alkyl vinyl ethers toward the oxidants are compared and discussed. In addition, estimates of the tropospheric lifetimes of iBVE and tBVE with respect to their reactions with OH, ozone, and NO3 for typical OH radical, ozone, and NO3 radical concentrations are made, and their relevance for the environmental fate of compounds is considered.

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oxidants, i.e., the OH radical, ozone, and the NO3 radical,10−25 and, under certain circumstances, possibly also Cl atoms.26 To date, most kinetic and product studies have focused on the gasphase reactions of linear alkyl vinyl ethers with OH, NO3, and O3. Using a variety of different techniques, rate coefficients for the room temperature gas-phase reactions of OH and NO3 radicals and ozone with a series of linear alkyl vinyl ethers, i.e., methyl vinyl ether (MVE), 10,12,14,20 ethyl vinyl ether (EVE),13−16,20,22,23 propyl vinyl ether (PVE),14,16,19,23 and butyl vinyl ether (BVE),14,16,19,23 as well as ethyleneglycol vinyl ethers (EGVE)27 have been reported in the literature. Temperature dependent studies of reactions of OH with a number of alkyl vinyl ethers have also been reported.13,14,19 The room temperature rate coefficients measured by different research groups for the reactions of the oxidants with EVE, PVE, and BVE are in good agreement, suggesting that the kinetics for the gas-phase reaction of these linear alkyl vinyl ethers is fairly well established. In contrast, kinetic studies on the gas-phase reactions of branched alkyl vinyl ethers are very limited. Rate coefficients for the reactions of OH with iso-butyl vinyl ether (iBVE) and tert-butyl vinyl ether (tBVE) have been reported by Mellouki14 and Thiault and Mellouki,19 while for

INTRODUCTION There is evidence for a large global source of oxygenated volatile organic compounds (OVOCs) in the atmosphere.1,2 These compounds are emitted directly into the atmosphere from biogenic and anthropogenic sources and are also formed in situ in the atmosphere from the oxidation of all hydrocarbons present within the atmosphere.3,4 OVOCs represent the most abundant class of organic carbon in the atmosphere. They are heavily involved in key atmospheric processes and play a central role in the chemical processes that determine the oxidizing capacity of the atmosphere.1,2 It is thought that oxygenated organics also make a significant contribution to the organic fraction of atmospheric aerosols.5 The increasing use of organic oxygenates as solvents has led to the need for knowledge on their contribution to (i) the oxidizing capacity of the atmosphere, (ii) toxic contaminant formation, (iii) indoor pollution, and (iv) secondary organic aerosol (SOA) formation.3−5 Therefore, a large number of investigations on the gas-phase reactions of many oxygenated organic compounds have been performed over the past decade, which have been the subject of a number of reviews.4−6 Vinyl ethers (ROCHCH2) are widely used in industry as oxygenated solvents and additives in different types of coatings7−9 and are released to the atmosphere entirely from anthropogenic sources. Because of the presence of the alkene moiety in vinyl ethers, it is expected that they will show moderate to high reactivity toward the major atmospheric © 2012 American Chemical Society

Received: June 18, 2012 Revised: August 15, 2012 Published: August 16, 2012 8885

dx.doi.org/10.1021/jp305992a | J. Phys. Chem. A 2012, 116, 8885−8892

The Journal of Physical Chemistry A

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the ozone reactions, rate coefficients have been reported by Mellouki14 in a non-peer-reviewed publication. To the best of our knowledge, the kinetics of the reactions of NO3 radicals with iBVE and tBVE have not yet been studied. In order to put the kinetic base for branched vinyl ethers on a more secure footing and to better assess the potential environmental impacts of vinyl ethers, we report here kinetic studies on the OH and NO3 radical and ozone initiated oxidation of iBVE and tBVE.

compounds. The following equation has been used to evaluate the kinetic data:

EXPERIMENTAL SECTION The kinetic investigations were carried out in a 405 L evacuable borosilicate glass tubular chamber closed at both ends with aluminum flanges. The experiments were conducted at (298 ± 3) K and (750 ± 10) Torr in synthetic air. A detailed description of the reactor can be found in the literature.28,29 Two types of photolysis sources are available for the experiments: 18 fluorescent lamps (Philips TLA 40 W/05; 300 ≤ λ ≤ 450 nm, λmax = 360 nm) spaced evenly around the outside of the reactor and 3 low-pressure mercury vapor lamps (Philips TUV40W; λmax = 254 nm) placed inside a quartz glass tube mounted centrally within the main chamber. A White-type mirror system (base path length 1.4 m) mounted internally within the chamber and coupled to an FTIR spectrometer (Fourier Transform-Infrared Spectrometer; Nicolet Magna 550) equipped with a globar IR source and a liquid nitrogen cooled MCT-detector (mercury−cadmium-tellurium detector) enables in situ infrared monitoring of reactants and products. The White system was operated with a total optical absorption path of 50.4 m and infrared spectra were recorded with a spectral resolution of 1 cm−1. The relative kinetic technique was used to investigate the rate coefficients for the reactions of iBVE and tBVE with OH and NO3 radical and O3. In the relative rate method, the disappearance of reactants, i.e., iBVE and tBVE, due to reaction with the reactive species (OH, O3, or NO3), is measured relative to that of a reference compound, whose rate coefficient with the reactive species is reliably known, e.g., for the OH radical reaction

where [reactant]t0 and [reference]t0 are the concentrations of the vinyl ethers and reference hydrocarbons, respectively, at time t0; [reactant]t and [reference]t are the corresponding concentrations at time t; k1 and k2 are the rate coefficients for the reaction of vinyl ethers and reference hydrocarbons with OH radical, respectively; k3 is the dark loss rate of each vinyl ether. Plots of ln([reactant]t0/[reactant]t) −k3(t − t0) versus ln([reference]t0/[reference]t) should give straight lines with slopes k1/k2. The rate coefficient k1 can be derived from the known rate coefficient k2. Since the dark reactions of the vinyl ethers are first order reactions, the rate k3 can be derived from the slope of plots of ln([reactant]t0/[reactant]t) versus time before reactions 1 and 2 are initiated. Frequent checks were made to verify that the dark losses of both the reactant and reference compounds did not change during the course of the reactions by measuring their rates in the pre- and postreaction periods. Typically, dark loss accounted for around 10% of the overall decay of both iBVE and tBVE in the OH kinetic experiments. The rate coefficients for the reactions of ozone and NO3 radicals with iBVE and tBVE have been determined using the relative kinetic technique in a manner analogous to that described above for the OH reactions and described in Zhou et al.16,27 OH Radical Reactions. The photolysis of CH3ONO−NO synthetic air mixtures using the fluorescent lamps (λmax = 360 nm) was used for the production of OH radicals:

⎧ [reactant]t ⎫ ⎪ 0 ⎬ ln⎨ − k 3(t − t0) ⎪ ⎩ [reactant]t ⎭ ⎪

=

■

reactant + OH → products,

reference + OH → products,

k1

k2

k3

⎪

(I)

CH3ONO + hν → CH3O + NO

(4)

(1)

CH3O + O2 → CH 2O + HO2

(5)

(2)

HO2 + NO → NO2 + HO

(6)

In this study, multiple reference compounds have been used. We have reported in our previous publications on studies of vinyl ethers16,27 that the compounds apart from being consumed due to reaction with OH were also lost to the chamber walls and additionally underwent reaction at the walls resulting in formation of an aldehyde and alcohol. The observed enhanced dark wall loss of the vinyl ethers was attributed to acid catalyzed hydrolysis of the compounds at the walls. The dark loss was not affected by the presence of light showing that photolysis was not an additional loss process for the vinyl ethers. The same behavior has been observed for the vinyl ethers under study here. The combined wall deposition and ensuing wall chemical transformations of iBVE and tBVE were found to obey first order kinetics, as found in the studies on other vinyl ethers, and can be represented by reactant → products,

⎛ ⎪ [reference] ⎫⎞ k1 ⎜ ⎧ t0 ⎬⎟ × ⎜ln⎨ ⎪ [reference]t ⎭⎟⎠ k2 ⎝ ⎩

The initial concentrations of vinyl ethers and reference compounds, i.e., isobutene and isoprene, were 4.0−5.0 ppm (1 ppm = 2.46 × 1013 molecules cm−3 at 298 K), those of CH3ONO and NO were 1.5−4.0 ppm and 9.8−19.8 ppm, respectively. NO was added to the reaction mixture to suppress the formation of O3 and NO3 radicals. Ozone and NO3 Radical Reactions. Ozone was added stepwise to premixed mixtures containing the vinyl ether, reference compounds and cyclohexane. Cyclohexane was present in excess to scavenge more than 95% of OH radicals produced in the reaction system. Ozone was generated by an electrical discharge in a flow of pure O2 using a custom built ozone generator. NO3 radicals were produced by the thermal dissociation of N2O5 prepared in solid form according to a literature method:30 N2O5 + M → NO3 + NO2 + M

(3)

Tests showed that, for the reference hydrocarbons used in the investigations, both wall loss and photolysis were negligible, and thus, no additional loss correction was applied for these

(7)

As for the investigations on the ozone reactions, the reactions of NO3 with vinyl ethers were performed using multiple additions of N2O5 to a mixture of the vinyl ether and reference 8886



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coefficient values of the reference hydrocarbons. Good linear relationships were found in the plots of the kinetic data for iBVE and tBVE using both reference compounds (Figure 1), and the values of the rate coefficients obtained using two different reference compounds are also in satisfactory agreement with one another (Table S1, Supporting Information). Therefore, we prefer to quote final rate coefficients for the reactions that are averages of both determinations. Averaging the values of the rate coefficients and taking errors that encompass the extremes of both determinations for each reaction gives rate coefficients at 298 K (in cm3 molecule−1 s−1) of k1(OH + iBVE) = (1.08 ± 0.23) × 10−10 and k1(OH + tBVE) = (1.25 ± 0.32) × 10−10, respectively. These values are listed in Table 1 where they are compared with the available literature kinetic data on the reaction of OH radicals with other short-chain alkyl vinyl ethers.

compounds. N2O5 was added to the chamber by passing air over the surface of solid N2O5, which was placed in a solid CO2−ethanol cooling trap at −50 °C. The initial concentrations of the reactants for the ozone experiments were vinyl ethers 2.5−5.5 ppm; cyclohexene 3.8− 7.3 ppm; O3 1.0−1.8 ppm; and cyclohexane 290 ppm. For the NO3 experiments, they were approximately: vinyl ethers 5.0 ppm and reference hydrocarbons, i.e., isoprene and 2,3dimethyl-1,3-butadiene, 4.8−5.5 ppm.

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RESULTS AND DISCUSSION OH Radical Reaction. Figure 1 shows examples of kinetic data for the reactions of OH with iBVE and tBVE plotted

Table 1. Comparison of the Rate Coefficients (in cm3 molecule−1 s−1) Measured in the Present Work at 298 K for the Reactions of OH with Selected Vinyl Ethers with Values Reported in the Literature at the Same Temperature k × 1011

technique

MVE, CH3OCHCH2

3.35 ± 0.34 4.5 ± 0.7 6.4

EVE, C2H5OCHCH2

6.8 ± 0.7

FP−RFa relative rate SRRb estimation PLP−LIFc

PVE, C3H7OCHCH2

7.3 ± 0.9 7.79 ± 1.71 10 ± 1

relative rate relative rate PLP−LIFc

BVE, n-C4H9OCHCH2

11 ± 1 9.73 ± 1.94 11.0 ± 0.4 10 ± 1

relative rate relative rate relative rate PLP−LIFc

iBVE, i-C4H9OCHCH2

11 ± 1 11.3 ± 3.1 11 ± 1

relative rate relative rate PLP−LIFc

tBVE, t-C4H9OCHCH2

11 ± 1 10.8 ± 2.3 11 ± 1

relative rate relative rate PLP−-LIFc

11 ± 1 12.5 ± 3.2

relative rate relative rate

vinyl ether

Figure 1. Plots of the kinetic data according to eq I for the gas-phase reactions of OH radicals with iBVE (top) and tBVE (bottom).

refs Perry et al.10 Mellouki14 Grosjean and Williams12 Thiault et al.;13 Mellouki14 Mellouki14 Zhou et al.16 Mellouki;14 Thiault and Mellouki19 Mellouki14 Zhou et al.16 Peirone et al.25 Mellouki;14 Thiault and Mellouki19 Mellouki14 Zhou et al.16 Mellouki;14 Thiault and Mellouki19 Mellouki14 this work Mellouki;14 Thiault and Mellouki19 Mellouki14 this work

Flash photolysis−resonance fluorescence (FP−RF); Arrhenius expressions for the temperature range 299−427 K are reported. b Structure−reactivity relationship (SRR). cPulsed laser photolysis laser induced fluorescence (PLP−LIF); Arrhenius expressions for the temperature range 230−373 K are reported. a

according to eq I. The reference compounds employed and the rate coefficient ratios k1/k2 obtained from such plots from a minimum of three experiments are listed in Table S1, Supporting Information. The rate coefficients, k1, listed in Table S1, Supporting Information, for the reactions of OH with iBVE and tBVE were placed on an absolute basis using k2(OH + isobutene, 298 K) = (5.14 ± 1.03) × 10−11 and k2(OH + isoprene, 298 K) = (1.01 ± 0.20) × 10−10 cm3 molecule−1 s−1 31 for the reference hydrocarbon reactions with OH. The quoted errors for the values of k1 are a combination of the least-squares 2σ standard deviations obtained from the kinetic plots plus an additional 20% to cover potential uncertainties in the rate

From Table 1, it can be seen that the rate coefficients for the reaction of OH with iBVE and tBVE measured in this study are in excellent agreement with the values reported by Mellouki14 and Thiault and Mellouki,19 which were obtained using pulsed laser photolysis−laser induced fluorescence (PLP−LIF) and relative kinetic techniques. The generally good agreement between the various determinations of the rate coefficients for the reactions of OH with vinyl ethers (Table 1) suggests that 8887



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developed by Pfrang et al.33−36 for the reactions of OH, and also NO3, O3, and O(3P) with alkenes, unsaturated alcohols, ketones, ethers, and esters, fail to completely capture the reactivity of unsaturated ethers toward OH radicals. The discussion above on the contribution of H-atom abstraction to the overall rate coefficient suggests that the increased OH reactivity of alkyl vinyl ethers over that of their analogous alkenes is mainly due to activation of the double bond toward OH radical electrophilic addition by the −OR group in the alkyl vinyl ethers to a much greater extent than that caused by alkyl groups, −R. The substituent F factor of 1.3 for the −OR (R = alkyl) group used in the SAR of Kwok and Atkinson32 is based on the −OCH3 group in methyl vinyl ether. On the basis of the present database for alkyl vinyl ethers, much better predictions of the rate coefficients for the reactions of OH with alkyl vinyl ethers using the SAR estimation method would be achieved using a substituent F factor of 3.7 for −OR groups for R = C2 or higher. O3 Reaction. Figure 2 shows examples of the kinetic data plotted according to eq I for the reactions of iBVE and tBVE

the kinetics for the gas-phase reaction of the alkyl vinyl ethers are fairly well established. However, the agreement between experimental rate data and values calculated using the structure activity reactivity (SAR) method of Kwok and Atkinson32 for reactions of OH with compounds containing oxygenated functional groups is in general rather poor. For example, the SAR method predicts rate coefficients of 4.83 × 10−11 and 3.56 × 10−11 cm3 molecule−1 s−1 for the reactions of OH with iBVE and tBVE, respectively, which are factors of approximately 2 and 3 lower than the experimental values determined in both this and other studies (Table 1). The available rate coefficient data for reactions of OH with saturated alcohols, ethers, ketones, and esters clearly indicate that the oxygenated functional groups have long-range activating effects with respect to H-atom abstraction at sites remote from the substituent group.4,14 The group reactivity approach applied by Mellouki et al.4 for estimating rate coefficients for the reactions of OH with oxygenated organics takes into account long-range activating effects of the oxygenated functional groups, giving more accurate estimates of the H-atom abstraction contribution to the overall rate coefficients for the reactions of OH oxygenated organics. This approach has been used to estimate the H-atom contribution to the overall rate coefficients measured for the reactions of OH with vinyl ethers up to C4 alkyl chains, and the values are given in Table S2, Supporting Information. The corresponding total H-atom abstraction rate coefficient contribution values estimated using the SAR method of Kwok and Atkinson32 for the vinyl ethers are also given. The SAR method of Kwok and Atkinson32 is known to fairly accurately predict the rate coefficients for the reactions of OH radicals with small alkenes, and this method has been used to estimate the H-atom contributions to the overall reactions for the reactions of OH, the alkenes analogous to those of the vinyl ethers, and these values are also listed in Table S2, Supporting Information, for comparison. As can be seen in Table S2, Supporting Information, the group reactivity approach of Mellouki et al.4 for estimating rate coefficients shows that long-range activating effects of the oxygenated functional ether group are contributing to an increase in the contribution from H-atom abstraction to the overall rate coefficients compared to that predicted by the SAR method of Kwok and Atkinson32 for the analogous alkenes. However, for C3 and above, the differences are only a few percent. The SAR method when applied to the alkyl vinyl ethers predicts H-atom abstraction contributions, which are only moderately less than those predicted by the group reactivity method.4 As discussed by Mellouki and co-workers13,14,19 and Zhou et 16 al. and illustrated by the percentages of the reactions proceeding by H-atom abstraction shown in Table S2, Supporting Information, the major reaction pathway for the reaction of OH radicals with alkyl vinyl ethers and also their analogous alkenes is the addition of OH to the double bond in the compounds. The dominance of the addition pathway is also supported by product studies,14,17 which to date have failed to detect products resulting from an H-atom abstraction channel. The rate coefficients for the reactions of OH radicals with alkyl vinyl ethers are a factor of 2 or more higher than those for the corresponding reactions with alkenes, and as discussed above, the SAR method of Kwok and Atkinson32 presently fails to capture the increased OH reactivity observed for the alkyl vinyl ethers. Similarly, the recent correlations and SARs

Figure 2. Plot of the kinetic data according to eq I for the gas-phase reactions of O3 with iBVE (top) and tBVE (bottom).

with ozone. The rate coefficient ratios, k1/k2, obtained from such plots and a minimum of three experiments for each reference compound used, are listed in Table S3, Supporting Information, and have been used in combination with k2(cyclohexene) = 8.1 × 10−17 and k2(cyclopentene) = 5.7 × 10−16 cm3 molecule−1 s−1 31 to put the rate coefficients for the 8888



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Table 2. Comparison of the Rate Coefficients (in cm3 molecule−1 s−1) Measured in the Present Work at 298 K for the Reactions of O3 with Selected Vinyl Ethers with Values Reported in the Literature at the Same Temperature vinyl ether EVE, C2H5OCHCH2

PVE, C3H7OCHCH2 BVE, n-C4H9OCHCH2 iBVE, i-C4H9OCHCH2 tBVE, t-C4H9OCHCH2

k × 1016

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refs

2.0 ± 0.2 1.54 ± 0.3 2.06 ± 0.42 1.50 ± 0.17 1.43 ± 0.08 2.4 ± 0.4 2.34 ± 0.48 2.9 ± 0.2 2.59 ± 0.52 3.1 ± 0.2 2.85 ± 0.62 5.0 ± 0.5 5.30 ± 1.07

concentration fit p-f-o kineticsa relative rate relative rate absolute S-CLb concentration fit relative rate concentration fit relative rate concentration fit relative rate concentration fit relative rate

Thiault et al.13 Grosjean and Grosjean21 Zhou et al.16 Al Mulla et al.24 Al Mulla et al.24 Mellouki14 Zhou et al.16 Mellouki14 Zhou et al.16 Mellouki14 this work Mellouki14 this work

a Pseudo-first-order kinetics. bAbsolute determination in a static system using chemiluminescence analysis to monitor the ozone decay in the presence of an excess of the ether.

reactions of O3 with the iBVE and tBVE on an absolute basis. Corrections to the kinetic data for dark wall loss and dark reaction of the vinyl ethers in the O3 kinetic experiments were of the order of approximately 5% for both iBVE and tBVE. The values of the rate coefficients obtained for the reaction of O3 with iBVE using two different reference compounds are in excellent agreement, and we thus chose to quote a final rate coefficient for this reaction, which is an average of the two determinations. This results in final rate coefficients (in units of 10−16 cm3 molecule−1s−1) of (2.85 ± 0.62) and (5.30 ± 1.07) for the reactions of O3 with iBVE and tBVE, respectively. The quoted errors are again the combination of the least-squares standard 2σ deviations plus an additional 20% for uncertainties in the value of the rate coefficient for the reference(s). The rate coefficients determined in this work for the reactions of ozone with iBVE and tBVE are compared with available literature values in Table 2. The rate coefficients determined in the present work for the reactions of O3 with iBVE and tBVE are in excellent agreement with the only values determined by Mellouki14 from a best fit to concentration− time profiles measured in the EUPHORE chamber facility in Valencia, Spain (Table 2). The rate coefficient for the ozonolysis of tBVE is higher than BVE and iBVE by almost a factor of 2. As for the reactions of OH with alkyl vinyl ethers, the correlation and SAR methods of Pfrang et al.36 predict rate coefficients for the reactions of O3 with iBVE and tBVE that are much lower than those obtained experimentally. As Pfrang et al.36 point out, their SAR methodology assumes that the basic alkene structures possess −CH3 groups as substitutable R groups around the double bond. In the case of the unsaturated ethers studied here, an oxygen atom is directly attached to the double bond. Pfrang et al.36 argue that the lone pair at the oxygen atom is likely to affect the estimated highest occupied molecular orbital (HOMO) energies and thus place such compound outside the scope of their SAR methodology. As discussed by Zhou et al.,16 the rate coefficients for the ozonolysis of the alkyl vinyl ethers are much higher than those of the corresponding alkenes by factors of more than 20. This again reflects the strong electron donating effect of alkoxy groups, −OR, to the carbon−carbon double bond, which facilities the electrophilic addition of ozone to the double bond. One would intuitively think that the much higher rate coefficient for the reaction of tBVE with ozone compared to

the other vinyl ethers could be explained by the stronger electron donating effect of the (CH3)3CO− group compared to other alkyl groups. However, it is evident that this is not true for the corresponding alkenes where the rate coefficients for the ozonolysis drop off on proceeding from 1-hexene via 4-methyl1-pentene to 3,3-dimethyl-1-butene. This cannot be explained by simple electronic arguments since stabilization of radical intermediates will increase with increasing branching of the alkyl substituent groups. From a steric point of view, however, for the alkenes, an increase in the branching complexity of the alkyl substituents can hinder the addition of ozone to the reactive double bond of the alkene thus counteracting the inductive stabilization effect and resulting in a lowering of the rate coefficient. In vinyl ethers such as tBVE, the extra O atom between the double bond and the tertiary butyl group considerably lessens any steric hindrance, thus enabling effective inductive radical stabilization and a higher reactivity toward O3 addition at the double bond. NO3 Radical Reaction. Figures 3 shows examples of the kinetic data plotted according to eq I for the reactions of NO3 radicals with iBVE and tBVE using two reference compounds. Good linear relationships were obtained for both iBVE and tBVE with each reference compound employed. The rate coefficient ratios, k1/k2, obtained from these plots from a minimum of three experiments with each reference compound are listed in Table S4, Supporting Information. Using these ratios in combination with NO3 rate coefficient values at 298 K for the reference compounds of k2(isoprene) = 6.78 × 10−13 cm3 molecule−1 s−1, k2(2,3-dimethyl-1,3-butadiene) = 2.1 × 10−12 cm3 molecule−1 s−1, and k2(1,3-cycloheptadiene) = 6.5 × 10−12 cm3 molecule−1 s−1 37 leads to the rate coefficient k1 for the reactions of NO3 with iBVE and tBVE listed in Table S4, Supporting Information. The contribution of the combined dark wall and dark reaction losses of the vinyl ethers to the measured overall losses in the NO3 experiments were approximately 5%. The given errors are again standard 2σ deviations from the kinetic plots plus an additional 20% to cover uncertainties in the values of the rate coefficients for the reference compounds. As can be seen in Table S4, Supporting Information, the rate coefficients obtained for each vinyl ether using two different reference compounds are in reasonable agreement; thus, we prefer to quote final rate coefficients for the reactions that are averages of 8889



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Table 3. Comparison of the Rate Coefficients (in cm3 molecule−1 s−1) Measured in the Present Work at 298 K for the Reactions of NO3 with Selected Vinyl Ethers with Values Reported in the Literature at the Same Temperature vinyl ether MVE, CH3OCHCH2

k × 1012 0.47 0.72 ± 0.15

EVE, C2H5OCHCH2

1.40 ± 0.35 1.7 ± 1.3 1.31 ± 0.27

PVE, C3H7OCHCH2

1.85 ± 0.53 1.33 ± 0.30

BVE, n-C4H9OCHCH2

2.10 ± 0.54 1.70 ± 0.37

iBVE, i-C4H9OCHCH2

1.99 ± 0.56

tBVE, t-C4H9OCHCH2

4.81 ± 1.01

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refs

SRR and LFER relative rate relative rate relative rate relative rate relative rate relative rate relative rate relative rate relative rate relative rate

Grosjean and Williams12 Scarfogliero et al.30 Zhou et al.16 Pfrang et al.23 Scarfogliero et al.20 Zhou et al.16 Scarfogliero et al.20 Zhou et al.16 Scarfogliero et al.20 this work this work

ethers is similar to that observed for the OH and ozone reactions with the ethers. As was the case for ozone, the much larger NO3 rate coefficient for tBVE compared to those for BVE and iBVE is attributed to a large increase in the positive inductive contribution to the electron density at the double bond caused by the increase in the branching complexity of the tert-butyl group, which renders the bond more acceptive to electrophilic addition of the NO3 radical. The increase in rate coefficients for the reactions of the NO3 radical with the alkyl vinyl ethers with increasing electron donation to the double bond is fully in line with a mechanism involving mainly electrophilic addition of NO3 to the double bond. The rate coefficients for the reactions of NO3 with alkyl vinyl ethers are approximately 2 orders of magnitude higher than the corresponding rate coefficients for the reactions of NO3 with their analogous alkenes3 propene, 1-butene, 1-pentene, 1hexen,e and 3-methyl-1-butene, respectively, for the vinyl ethers listed in Table 3. However, whereas an increase in rate coefficient is observed for the reactions of NO3 with the vinyl ethers with increase in the electron donating power of the alkyl group, this does not seem to be the case for the alkenes (at least on the basis of the available data3), i.e., the rate coefficients for the reactions of NO3 with the linear 1-pentene and branched 3methyl-1-butene are very similar, 1.5 × 10−14 3 and 1.4 × 10−14,39 respectively. This might possibly reflect a higher degree of steric hindrance associated with NO3 addition to the alkenes compared with the vinyl ethers where an O atom separates the double bond from the alkyl group. Atmospheric Implications. The main atmospheric transformation processes for volatile organic compounds are reaction with OH radicals, ozone, NO3 radicals, and, under certain circumstances, possibly Cl atoms, photolysis, and wet/ dry deposition. Since the vinyl ethers do not absorb in the tropospheric actinic region,21 photolysis will be a negligible process. The compounds have very low Henry’s law constants; thus, similarly, wet deposition will also be a negligible loss process.

Figure 3. Plot of the kinetic data according to eq I for the gas-phase reactions of NO3 with iBVE (top) and tBVE (bottom).

the individual determinations with error limits that encompass the extremes of both determinations, which results in rate coefficients at 298 K (in 10−12 cm3 molecule−1 s−1 units) of 1.99 ± 0.56 and 4.81 ± 1.01 for the reactions of ozone with iBVE and tBVE, respectively. The rate coefficients for the reactions of NO3 with the alkyl vinyl ethers measured in this work are listed in Table 3 where they are compared with the available literature kinetic data on vinyl ethers. To the best of our knowledge, at the time of writing, no other reports of experimentally determined rate coefficients for reactions of NO3 with iBVE and tBVE have been published in the literature with which the values determined in this work can be compared. The SAR method developed by Kerdouci et al.38 for NO3 reactions predicts the rate coefficient for iBVE to within about 10%; however, the method developed for linear alkyl chains does not account for the increase in reactivity, which results in a degree of branching and consequently underpredicts the reactivity of NO3 toward tBVE by a factor of ∼2.4. The correlation and SAR approaches of Pfrang et al.33−36 underpredict the rate for NO3 with iBVE by a factor of ∼2.5 and that of NO3 with tBVE by only a factor of ∼1.3. As for the analogous OH radical and ozone reactions, the rate coefficients determined for the NO3 reactions show an increase with increasing carbon-chain length with k(MVE) < k(EVE) < k(PVE) < k(BVE) ≈ k(iBVE) < k(tBVE) (Table 3), indicating that the order of reactivity of NO3 toward alkyl vinyl 8890



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and NO3 (Table S4) with iBVE and tBVE and estimated atmospheric lifetime for iBVE and tBVE with respect to degradation by OH radicals, ozone, and NO3 radicals (Table S5). This material is available free of charge via the Internet at http://pubs.acs.org.

Because of the presence of the alkene moiety, vinyl ethers show high reactivity toward OH radicals, ozone, and NO3 radicals, and Mellouki et al.4 have concluded that reaction with these species represents the major degradation pathways of vinyl ethers in the troposphere. The reactions of OH, NO3, and O3 with straight-chained alkyl vinyl ethers are known to give alkyl formates and HCHO as major products, and in the case of the NO3 reactions, organic nitrates are also formed in considerable yield.4,17 Ongoing work on the branched alkyl vinyl ethers, iBVE and tBVE, indicates that this is also the case for these compounds and will be the subject of a future publication. The rate coefficients determined in this study for the reactions of OH radicals, O3, and NO3 radicals with iBVE and tBVE have been used to estimate the atmospheric lifetimes of the vinyl ethers studied in this work with respect to degradation by these oxidant species. The atmospheric lifetime τ with respect to loss by an oxidant species is defined as

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Corresponding Author

*Phone: +1 416 946 7359. Fax: +1 416 946 7359. E-mail: [email protected]. Present Address §

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work by the European Commission within the project MOST (contract EVK2-CT-2001-00114) and the EU project EUROCHAMP is gratefully acknowledged.

τ = 1/k[species]

where k is the rate coefficient for the reaction of the reactive species (OH, NO3, or O3) with the vinyl ethers determined in this study, and [species] is the typical concentration of the reactive species. The average tropospheric concentrations of OH radicals, ozone, and NO3 radicals used in calculations of the lifetimes were ca. 1.6 × 106 (12 h daytime average40), 7 × 1011 (24 h average concentration41), and 5.0 × 108 (12 h nighttime average42,43) molecules cm−3, respectively. These are the values most commonly used in publications presenting calculations of the lifetimes of organic compounds with respect to reactions with OH radicals, ozone, and NO3 radicals. However, it should be born in mind when comparing the lifetimes that the nighttime atmospheric concentrations of NO3 radicals can be highly variable and, depending on the availability of NOx, may range from nearly zero to 2.0 × 109 molecules cm−3 or higher.44 The atmospheric lifetimes of iBVE and tBVE with respect to reactions with OH, NO3, and ozone are shown in Table S5, Supporting Information. As with other alkyl vinyl ethers, the atmospheric lifetimes for the reactions of iBVE and tBVE with OH, NO3, and ozone are all less than two hours in all cases. Thus, all three loss processes can make significant contributions to the degradation of iBVE and tBVE. The short lifetimes of the vinyl ethers show that they will be quickly degraded when emitted to the atmosphere and will only be actively involved in tropospheric chemistry on local to regional scales. The losses of the vinyl ethers observed in the chamber experiments due to the catalyzed hydrolysis at the acidic chamber walls11 indicate the possibility of a contribution to the atmospheric removal of the vinyl ethers by catalyzed degradation on acidic particle and aerosol surfaces.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Measured rate coefficient ratios, k1/k2, and values of the rate coefficients k1 (in cm3 molecule−1 s−1) for the reactions of OH radical with iBVE and tBVE (Table S1), comparison of the overall rate coefficients for the reactions of OH radicals with vinyl ethers and their analogous alkenes at 298 K with the estimated rate coefficient contributions for H-atom abstraction from the alkyl groups in the compounds (Table S2), measured rate coefficient ratios, k1/k2, and values of the rate coefficients k1 (in cm3 molecule−1 s−1) for the reactions of O3 (Table S3) 8891



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Kinetic Study of Gas-Phase Reactions of OH and NO3 Radicals and

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