FTIR gas-phase kinetic study of the reactions of Cl atoms with (CH3)2CCHC(O)H and CH3CHCHC(O)OCH3

Chemical Physics Letters 488 (2010) 135–139

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FTIR gas-phase kinetic study of the reactions of Cl atoms with (CH3)2C@CHC(O)H and CH3CH@CHC(O)OCH3 María B. Blanco a, Ian Barnes b, Mariano A. Teruel a,* a

Instituto de Investigaciones en Fisicoquímicas de Córdoba (INFIQC), Dpto. de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina b Physikalische Chemie/FBC, Bergische Universitaet Wuppertal, 42119 Wuppertal, Germany

a r t i c l e

i n f o

Article history: Received 29 December 2009 In final form 9 February 2010 Available online 12 February 2010

a b s t r a c t Rate coefficients for the gas-phase reactions of Cl atoms with (CH3)2C@CHC(O)H and CH3CH@CHC(O)OCH3 were determined to be (in units of 1010 cm3 molecule1 s1): 2.48 ± 0.71 and 2.04 ± 0.56, respectively. The experiments were performed by the relative technique in an environmental chamber with FTIR detection of the reactants at 298 K and 760 Torr. Structure activity relationships were developed for the reactions of Cl with a series of unsaturated esters and aldehydes. In addition, a comparison between the experimentally determined kCl with kCl predicted from k vs. HOMO correlations is presented. The atmospheric persistence of these compounds was calculated taking into account the measured rate coefficients. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction b,b-Dimethylacrolein is mainly used as an intermediate in closed systems for chemical synthesis. This substance is used further processed to citral and vitamin A. An estimated volume of <10 t/y was used for the reconstitution of essential oils and other natural products. Releases into the environment may occur during production and processing of b,b-dimethylacrolein as intermediate as well as from direct use of the substance and from formulation and use of products containing it [1]. During production and processing at the company BASF AG, the effluent concentration of the sewage treatment plant was below 20 lg/l. Less than 25 kg/y were emitted into the air. The world production of b,b-dimethylacrolein in the year 2001 ranged from 6000 to 13,000 metric tons; in Europe (1st producer) and Asia it ranged from 5000 to 10,000 and from 1000 to 3000 metric t/y, respectively. In 2001, the substance was not industrially produced in the United States [2]. At BASF AG (sole production site in Europe), the production and processing of b,b-dimethylacrolein takes place in closed systems. At BASF AG, b,b-dimethylacrolein is almost exclusively (99%) processed to citral, which serves as a fragrance and flavor for lemon aromas and as starting material for the synthesis of e.g. vitamin A. The remaining amount of 1% is directly poured into an ISO container by using special filling equipment [3].

* Corresponding author. Fax: +54 351 4334180. E-mail address: [email protected] (M.A. Teruel). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.02.023

b,b-Dimethylacrolein is used in small quantities as food flavor, in baked goods, chewing gums and non-alcoholic beverages [4]. Further, it is employed in confection frosting, frozen dairy, fruit ice, gelatin pudding, hard and soft candy and jam jelly. Methyl crotonate is a fragrance ingredient included in many fragrance mixtures. It may be found in fragrances used in decorative cosmetics, shampoos, toilet soaps and other toiletries as well as in non-cosmetic products such as household cleaners and detergents. Its use worldwide is in the region of less than 0.1 metric t/y [5]. The release of these oxygenated volatile organic compounds into the atmosphere is likely to contribute to the formation of ozone, peroxy acetyl nitrate (PAN) and other components of photochemical smog formed in urban areas through their reactions mainly with OH radicals and other oxidants like NO3 radicals and O3 molecules [6]. However, in recent years the oxidation of volatile organic compounds (VOCs) by the highly reactive chlorine atom has gained much attention mostly in the marine troposphere where significant chlorine atom concentrations may be present [7]. In order to assess the impact of these species on air quality, kinetic and mechanistic information on their tropospheric degradation is therefore, needed [6]. In this work, we report room temperature relative kinetic determinations of the rate coefficients for the reactions of Cl atoms with b,b-dimethylacrolein and methyl crotonate performed in a photoreactor at atmospheric pressure of air:

Cl þ ðCH3 Þ2 C@CHCðOÞH ! Products;

ð1Þ

Cl þ CH3 CH@CHCðOÞOCH3 ! Products:

ð2Þ

M.B. Blanco et al. / Chemical Physics Letters 488 (2010) 135–139

The reaction of methyl crotonate with Cl atoms has only been studied previously by Martín et al. [8] in a collapsible Teflon chamber with gas chromatography detection of the organics at atmospheric pressure and room temperature. To the best of our knowledge, this work provides the first kinetic study for the reaction of Cl atoms with (CH3)2C@CHC(O)H. Therefore, this is the first kinetic study of reaction (1) under atmospheric conditions. In addition, a structure activity relationship (SAR) method developed by Atkinson [9] is first used for Cl addition reactions to unsaturated esters and aldehydes. It is shown that this predictive technique is useful for obtaining an estimation of the unknown rate coefficients for the reactions of Cl atoms with unsaturated and oxygenated VOCs for which the rate coefficients are unknown. On the other hand, the kinetic data obtained in this work is compared with the calculated values from our previous correlation between the reactivity of the unsaturated compounds toward Cl atoms with the HOMO (Highest Occupied Molecular Orbital) of the unsaturated compounds [10]. The atmospheric lifetimes of the VOCs studied, with respect to reaction with Cl, have been calculated with the rate coefficients obtained in this work and compared with the lifetimes of b,b-dimethylacrolein and methyl crotonate due to other homogeneous sinks in the troposphere.

Isobutene as reference 1,3-butadiene as reference 0.36

0.27

0.18

0.09

0.00 0.0

0.1

0.2

0.3

0.4

0.5

0.6

ln[reference]0/[reference]τ Fig. 1. Relative rate data for the reaction of Cl with b,b-dimethylacrolein using isobutene (h) and 1,3-butadiene (s) as reference compounds at 298 K and atmospheric pressure of air.

99.999%), b,b-dimethylacrolein (Aldrich, 99%), methyl crotonate (Aldrich, 99%), isobutene (Messer Griesheim, 99%), 1,3-butadiene (Aldrich, 99 + %) and Cl2 (Messer Griesheim, >99.8%).

2. Experimental All the experiments were performed in a 1080 dm3 quartz-glass reaction chamber at a total pressure of 760 Torr (760 Torr = 101.325 kPa) and 298 ± 3 K in synthetic air. A detailed description of the reactor can be found elsewhere [11] and only a brief description is provided here. A pumping system consisting of a turbomolecular pump backed by a double-stage rotary fore pump was used to evacuate the reactor to 103 Torr. Three magnetically coupled Teflon mixing fans are mounted inside the chamber to ensure homogeneous mixing of the reactants. The photolysis system consists of 32 superactinic fluorescent lamps (Philips TL05 40 W: 320–480 nm, kmax = 360 nm), spaced evenly around the reaction vessel. The lamps are wired in parallel and can be switched individually, allowing a variation of the light intensity, and thus the photolysis frequency/radical production rate, within the chamber. The chamber is equipped with a White type multiple-reflection mirror system with a base length of (5.91 ± 0.01) m for sensitive in situ long path absorption monitoring of reactants and products in the IR spectral range 4000–700 cm1. The White system was operated at 82 traverses, giving a total optical path length of (484.7 ± 0.8) m. The IR spectra were recorded with a spectral resolution of 1 cm1 using a Nicolet Nexus FT-IR spectrometer, equipped with a liquid nitrogen cooled mercury–cadmium–telluride (MCT) detector. Typically, for each spectrum, 64 interferograms were co-added over 1 min and 15 such spectra were recorded per experiment. Chlorine atoms were generated by the photolysis of Cl2 with the fluorescent lamps:

Cl2 þ hm ! Cl

0.45

ln[(CH3)2C=CHC(O)H]0/[(CH3)2C=CHC(O)H]τ

136

ð3Þ

The initial concentrations of reactants in ppmV (1 ppmV = 2.46  1013 molecule cm3 at 298 K) were: b,b-dimethylacrolein, (0.3– 0.6); methyl crotonate, (0.6–0.9); Cl2, 4.6; isobutene, (1.4–1.9) and 1,3-butadiene, (0.9–1.0). The reactants were monitored at the following infrared absorption frequencies (in cm1): b,b-dimethylacrolein at 1125.6; methyl crotonate at 971 and 1322.9; isobutene at 890 and 1,3-butadiene at 908.4. 3. Materials The following chemicals, with purities as stated by the supplier, were used without further purification: synthetic air (Air Liquide,

4. Results Rate coefficients for the reactions of Cl atoms with b,b-dimethylacrolein and methyl crotonate were determined by comparing their rate of decay with that of the corresponding decay of the reference compounds:

Cl þ OVOC ! Products; kOVOC

ð4Þ

Cl þ Reference ! Products; kreference

ð5Þ

Provided that the reference compound and the reactant are lost only by reactions (4) and (5), then it can be shown that:

 ln

½OVOC0 ½OVOCt

 ¼

  k4 ½Reference0 ; ln k5 ½Referencet

ð6Þ

where [OVOC]0, [Reference]0, [OVOC]t and [Reference]t are the concentrations of the compounds studied and reference compound at times t = 0 and t, respectively, and k4 and k5 are the rate coefficients of reactions (4) and (5), respectively. The relative rate technique relies on the assumption that both the unsaturated compound and reference organics are removed solely by reaction with Cl atoms. To verify this assumption, various tests were performed to assess the loss of the reactants via reaction with molecular chlorine, photolysis and wall deposition. These processes that could interfere with the kinetic determinations were found to be negligible for both the unsaturated compounds and the reference compounds. Mixtures of the unsaturated compounds and reference compounds with Cl2 were stable in the dark when left in the chamber for the typical time span of the kinetic experiments (10–20 min). Moreover, in the absence of Cl2, photolysis of the mixtures (unsaturated compounds and reference compounds in air) did not show any decrease in the reactant concentrations over the time span of the experiments. Aldehydes could be lost by direct photolysis or adsorption on the smog chamber surface. For this reason, b,b-dimethylacrolein was introduced alone in the reactor. Then the reactor was irradiated using all fluorescent tubes for twice the duration of kinetic experiments to evaluate both photolysis and wall loss. No measurable changes were observed. Figs. 1 and 2 show the kinetic data obtained from the experiments plotted according to Eq. (6) for the reactions of Cl with the

137

ln[CH3CH=CHC(O)OCH3]0/[CH3CH=CHC(O)OCH3]τ

M.B. Blanco et al. / Chemical Physics Letters 488 (2010) 135–139

0.4

1

s1 ;

1

s1 :

k1 ¼ ð2:48  0:71Þ  1010 cm3 molecule

0.5

k2 ¼ ð2:04  0:56Þ  1010 cm3 molecule

Isobutene as reference 1,3-butadiene as reference

The errors quoted are twice the standard deviation arising from the least-squares fit of the straight lines, to which a contribution has been added to cover uncertainties in the reference rate coefficients.

0.3

5. Discussion 0.2

0.1

0.0 0.00

0.15

0.30

0.45

0.60

0.75

ln[reference]0/[reference]τ Fig. 2. Relative rate data for the reaction of Cl with methyl crotonate using isobutene (h) and 1,3-butadiene (s) as reference compounds at 298 K and atmospheric pressure of air.

Table 1 Reference compound, measured rate coefficient ratios, kOVOC/kreference, and the rate coefficients obtained for the reactions of Cl atoms with b,b-dimethylacrolein and methyl crotonate at 298 K in 760 Torr of air. OVOC

Reference

kOVOC/ kreference

kOVOC (1010 cm3 molecule1 s1)

b,b-Dimethylacrolein (CH3)2C@CHC(O)H

Isobutene Isobutene 1,3-Butadiene 1,3-Butadiene 1,3-Butadiene

0.76 ± 0.02 0.77 ± 0.03 0.56 ± 0.02 0.59 ± 0.03 0.57 ± 0.02 Average

2.58 ± 0.28 2.62 ± 0.32 2.35 ± 0.31 2.48 ± 0.36 2.39 ± 0.31 2.48 ± 0.71

Methyl crotonate CH3CH@CHC(O)OCH3

Isobutene Isobutene 1,3-Butadiene 1,3-Butadiene 1,3-Butadiene

0.64 ± 0.03 0.63 ± 0.02 0.44 ± 0.01 0.43 ± 0.01 0.53 ± 0.02 Average

2.18 ± 0.28 2.14 ± 0.24 1.85 ± 0.22 1.81 ± 0.21 2.23 ± 0.30 2.04 ± 0.56

individual compounds studied measured relative to different reference compounds. Each plot represents a minimum of 2–3 experiments for each reference compound. Good linear relationships were obtained in all cases. The linearity of the plots with near-zero intercepts, combined with the fact that similar results were obtained for different initial concentrations of the unsaturated compound and reference organics, supports that complications due to secondary reactions in the experimental systems were negligible. The kOVOC/kreference ratios determined from the slopes of the straight-line plots in Figs. 1 and 2 are listed in Table 1 together with the absolute values of the rate coefficients, kOVOC, calculated from the kOVOC/kreference ratios. In order to place on an absolute basis the rate coefficients for the reactions of Cl with the unsaturated compound, the following values for the reactions of Cl with the reference compounds at 298 K were used: (3.40 ± 0.28)  1010 cm3 molecule1 s1 for Cl + isobutene [12] and (4.2 ± 0.4)  1010 cm3 molecule1 s1 for Cl + 1,3-butadiene [13]. The errors for the ratios kOVOC/kreference are only the 2r statistical errors. For both compounds, there is a good agreement between the values of kOVOC determined using two different reference compounds. Averaging the values of the rate coefficients and taking errors which encompass the extremes of both determinations for each reaction result in the following final values for the reaction rate coefficients at 298 K:

To the best of our knowledge, no kinetic data on the reaction of Cl atoms with b,b-dimethylacrolein have been reported. The present study, thus, is the first measurement of the rate constant of the reactions (1) and therefore, no direct comparison with the literature can be made. The value obtained in this study for the reaction of Cl atoms with methyl crotonate of k2 = (2.04 ± 0.56)  1010 cm3 molecule1 s1 is in good agreement, within the experimental error, with that recently reported by Martín et al. [8] of k2 = (2.21 ± 0.17)  1010 cm3 molecule1 s1 determined relative to the reactions of Cl with cyclohexane and n-pentane in (708 ± 8) Torr of N2 at (298 ± 2) by CG-FID analysis. It is interesting to compare the reactivity of the OVOCs towards Cl atoms with the corresponding alkenes since it has been postulated that the OVOCs react via similar addition mechanisms [6]. Table 2 lists the room-temperature rate coefficients of the reactions of Cl atoms with b,b-dimethylacrolein and methyl crotonate and the corresponding alkenes. A comparison between propene and 2-butene has been drawn for methyl crotonate. Although the corresponding alkene with an equivalent number of –CHx– groups is 2-butene, the methyl group in the –C(O)OCH3 entity is known to be highly unreactive toward Cl [16] and therefore, a comparison with the Cl + propene reaction is probably more realistic. The rate coefficients obtained for the reactions of b,b-dimethylacrolein and methyl crotonate with Cl are lower than those for the reactions of Cl with the corresponding alkenes (see Table 2). This behavior can be explained due to the negative inductive effect of the carbonyl-containing groups (–CHO or –C(O)OR) which will deactivate the double bond toward the main reaction pathway, i.e. the electrophilic Cl atom addition. The deactivation effect observed for the aldehyde (b,b-dimethylacrolein) is approximately the same as that found between the reaction of the ester (methyl crotonate) and propene with Cl atoms. This is somewhat surprising since the –CHO group is a reactive site (and therefore contributes to the global rate coefficient) and to some extent will compensate the electron withdrawing effect and thus the reactivity. However, as stated above, the reaction of Cl with the –C(O)OCH3 group is quite slow and in any respect will contribute to the global rate coefficient; the negative inductive effect will fully determine the reactivity. The nearly equivalent reduction in reactivity may result from the fact that a possible electron donating contribution of the – OCH3 entity in –C(O)OCH3 may counteract to some extent the deactivating effect of the carbonyl group and thus diminish the reducing effect on reactivity.

Table 2 Comparison of the rate coefficient values for the reactions of Cl atom with alkenes and unsaturated compounds studied at 298 K.

a b c

Alkene

k  1010 (cm3 molecule1 s1)

OVOC

k  1010 (cm3 molecule1 s1)

CH3CH@CH2 CH3CH@CH2CH3 (CH3)2C@CH2

2.7a 3.58b 3.38b

CH3CH@CHC(O)OCH3

2.04c

(CH3)2C@CHC(O)H

2.48c

From Ref. [14]. From Ref. [15]. This work.

138

M.B. Blanco et al. / Chemical Physics Letters 488 (2010) 135–139 Table 3 Comparison of the experimental (kexp) and calculated (kcalc) room-temperature rate coefficients for unsaturated esters and aldehydes.

3.6E-10 3.4E-10

Esters Aldehydes

kcalc  1010 (cm3 molecule1 s1)

kexp  1010 (cm3 molecule1 s1)

Unsaturated esters CH3CH@CHC(O)OCH3 CH2@C(CH3)C(O)O(CH2)3CH3 CH2@C(CH3)C(O)OCH3 CH2@CHC(O)OCH2CH3 CH2@CHC(O)O(CH2)3CH3 CH2@C(CH3)C(O)OCH2CH3 (CH3)2C@CHC(O)OCH3

2.69 2.96 2.52 2.43 2.51 2.86 2.92

2.04a 3.60b 2.82b 1.82c 2.94c 2.71c 3.58c

Unsaturated aldehydes (CH3)2C@CHC(O)H CH2@CHC(O)H CH2@C(CH3)C(O)H CH3CH@CHC(O)H

3.44 2.51 2.96 3.16

2.48a 2.20d 2.16d 2.49d

-1

-1

k calc (cm molecule s )

OVOC 3.2E-10 3E-10

3

2.8E-10

2.6E-10

2.4E-10

2.2E-10 1.5E-10

2E-10

2.5E-10 3

3E-10 -1

3.5E-10 4E-10 4.5E-10 -1

k exp (cm molecule s ) Fig. 3. Comparison of the calculated and experimental room-temperature Cl atom addition rate coefficients for unsaturated esters and unsaturated aldehydes at atmospheric pressure. The points in bold are the corresponding ester and aldehyde studied in this work.

The kinetic data obtained in this work together with that found in previous literature have been used to extend the structure activity relationship (SAR) approach developed by Atkinson to Cl atoms addition reactions [9], in particular, for different unsaturated esters and aldehydes. The method is based on the structure of the alkene, the number of unconjugated double bonds and the degree, identity, position and configuration of the substitution around the double bonds. The rate coefficients for the ‘basic structures’ are calculated from the average of the experimental rate coefficients for the following parent alkenes; basic structure CH2@CHR from propene with kbasic = 2.89  1010 cm3 molecule1 s1; CH2@CR2 from 2-methylpropene with kbasic = 3.40  1010 cm3 molecule1 s1; (E/Z)-RHC@CHR from (E/Z)-2-butene with kbasic = 3.63  1010 cm3 molecule1 s1; RHC@CR2 from 2-methyl-2-butene with kbasic = 3.95  1010 cm3 molecule1 s1 [17]. The reactivity factor for CH3 is set as unity. The group factors (Cx) are defined by the following equation:

kcalc ¼ kðbasicÞ 

i¼14 Y

ðCxiÞ:

i¼1

The values of Cx used are 0.74 for –C(O)OCH3, 0.84 for –C(O)OCH2CH3, 0.87 for –C(O)O(CH2)3CH3 and 0.87 for –C(O)H [17]. A correlation between the calculated and experimental roomtemperature rate coefficients for unsaturated esters and aldehydes with C@C bonds is provided in Fig. 3 and the corresponding values are listed in Table 3. A least-squares treatment of the data points in Fig. 3 yields the expressions:

For unsaturated esters; kcalc ¼ 0:21kexp  7:61; For unsaturated aldehydes; kcalc ¼ 1:34kexp þ 3:39: This correlation is such that estimations can be made of the rate coefficients for reactions which have not yet been investigated. The SAR estimation method provides rate coefficients for the reactions of Cl with b,b-dimethylacrolein and methyl crotonate which are close to the values reported in this work, showing that the method predicts the reactivity of a double bond adjacent to the carbonyl group of the ester or aldehyde group to within ± 25–30% of the measured values for methyl crotonate and b,bdimethylacrolein. The different trends observed in Fig. 3 for aldehydes and esters can be explained by considering the fact that different reaction mechanisms are operative for unsaturated esters and aldehydes.

a b c d

This work. From Ref. [10]. From Ref. [8]. From Ref. [15].

The reactions of OH radicals with formaldehyde, acetaldehyde and propanal and for the reactions of the NO3 radical with aldehydes proceed mainly by H-atom abstraction from the CHO group while significant contributions to the rate coefficients are expected from abstraction from other C–H bonds for higher aldehydes [18]. For the reaction of Cl atoms with unsaturated aldehydes, in addition to H-atom abstraction from the –CHO group, a significant fraction of the reaction will also proceed by Cl addition to the double bond. The contribution of H-atom abstraction is around 30% for the unsaturated aldehydes discussed here [19]. On the other hand, for the Cl-initiated oxidation of unsaturated esters, the major pathway is the addition of Cl to the double bond leadings to the formation of chloro-alkoxy radicals. From the rate coefficients in Table 3, it can be seen that the replacement of one H-atom by a methyl group in the olefinic carbon of the ester will increase the rate coefficient for the reaction of the compound with Cl atoms, i.e. for CH3CH@CHC(O)OCH3 and (CH3)2C@CHC(O)OCH3 by a factor of 1.75 (kCH3 CH@CHCðOÞOCH3 < kðCH3 Þ2 C@CHCðOÞOCH3 Þ . By contrast, the same replacement of a H atom in the aldehyde by a methyl group, e.g. CH3CH@CHC(O)H to give (CH3)2C@CHC(O)H, produces a negligible change in the Cl rate coefficient for the compounds. Furthermore, it should be noted that no significant differences are found between any of the experimental rate coefficients for the reactions of Cl atoms with the unsaturated aldehydes presented in Table 3 which shows that the reactivity toward Cl seems to be fairly independent of the substitution on the olefinic carbons for these unsaturated aldehydes. This tendency is difficult to explain since the contribution of the H-atom abstraction channel from the –CHO group to the overall rate coefficient is expected to be constant [19] and addition of methyl substituents would be expected to increase the reactivity, as observed for the unsaturated esters. The reactions are, however, fast and approaching collision controlled and it is possible that differences between the rate coefficients for changes in substitution may be minor and obscured by the experimental kinetic uncertainties. Additionally, the rate coefficients obtained can be compared with values calculated using the recently reported correlation between the reactivity of the unsaturated compounds toward Cl atoms and the HOMO of the unsaturated compounds [10]. The correlation obtained in our previous study for different groups of reactions is described as follows: 1 1

lnkCl ðcm3 molecule

s Þ ¼ ð0:1  0:1ÞEHOMO  ð21:2  2:9Þ:

ð7Þ

M.B. Blanco et al. / Chemical Physics Letters 488 (2010) 135–139 Table 4 Estimated tropospheric lifetimes of the unsaturated compounds studied in this work with Cl atoms, OH radicals, O3 molecules and NO3 radicals. OVOC

sCl

sOH

sO3

sNO3

b,b-Dimethylacrolein Methyl crotonate

5 daysa 6 daysa

2 hb –

9 daysc 4 daysd

– 13 dayse

Rate coefficients used in the calculation of the lifetimes were taken from: a This work. b kOH = 6.21  1011 cm3 molecule1 s1 [24]. c kO3 = 1.82  1018 cm3 molecule1 s1 [25]. d kO3 = 4.38  1018 cm3 molecule1 s1 [26]. e kNO3 = 1.85  1015 cm3 molecule1 s1 [27].

We calculated the HOMO energies for the unsaturated oxygenated compounds studied using the GAUSSIAN 03 package. The geometry optimizations and initial values of energies were obtained at the Hartree–Fock (HF) level, and ab initio Hamiltonian with a 6-31++ G (d,p) basis set was employed. The self-consistent field energies were then calculated by Moller–Plesset perturbation theory (MP4SCF) with an ab initio Hamiltonian with a 6-311++ G (d,p) basis set. The values of EHOMO of 9.66 eV for b,b-dimethylacrolein and 10.29 eV for methyl crotonate, were used to obtain the following and rate coefficients: k1 = 2.31  1010 cm3 molecule1 s1 k2 = 2.22  1010 cm3 molecule1 s1 through Eq. (7). These values are in close agreement with the rate coefficient of reactions (1) and (2) measured in this study of 2.48  1010 cm3 molecule1 s1 and 2.04  1010 cm3 molecule1 s1, respectively. Since the kinetic database for Cl addition reactions to unsaturated VOCs is quite limited, the methods developed in this work will represent particularly helpful resources for performing accurate estimations of rate coefficients for Cl atom addition reactions for which no experimental kinetic information is available. The rate coefficients summarised in Table 1 can be used to calculate the atmospheric lifetimes of the unsaturated compounds due to reaction with Cl atoms which can be compared to lifetimes due to their reactions with the other atmospheric oxidants. The atmospheric lifetimes can be calculated using the expression: sX = 1/kX [X] with X = OH, Cl, NO3 or O3, where kX is the rate coefficient for the reaction of the oxidant X with the ester, and [X] is the typical atmospheric concentration of the oxidant. For the calculations, the following oxidant concentrations have been used: [OH] = 2  106 radicals cm3 [20]; [Cl] = 1  104 atoms cm3 [21]; [NO3] = 5  108 radicals cm3 [22] and [O3] = 7  1011 molecules cm3 [23]. The estimated tropospheric lifetimes at room temperature of the unsaturated compounds with the typical oxidants (where data are available) are presented in Table 4. Photolytic loss of the esters will be negligible since they are photolytically stable in the actinic region of the electromagnetic spectrum [28]. However, aldehydes absorb actinic radiation in the range 270–340 nm and thus the estimates of the degradation under atmospheric conditions must include the photolysis contribution [29,30]. Yet, the lifetimes indicate that the unsaturated compounds are likely to be removed rapidly in the gas phase mainly by OH radicals for b,b-dimethylacrolein. Unfortunately, no data are available on the reactions of OH radicals with methyl crotonate, although, on the basis of structural similarities, it is probable that they will show a similar reactivity toward OH radicals such as b,b-dimethylacrolein, and thus have a similar lifetime with respect to reaction with this oxidant. Therefore, reactions with OH radical are also likely to be important removal processes for b,b-dimethylacrolein and methyl crotonate.

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The short lifetimes for the unsaturated compounds, in the range of a few hours, imply that the unsaturated VOCs are likely to be removed rapidly in the gas phase close to their emission sources with removal by OH radicals. Nevertheless, in coastal areas and in the marine boundary layer, where peak concentrations of Cl atoms as high as 1  105 atom cm3 can occur [7,12], Cl-atom initiated degradation of b,b-dimethylacrolein and methyl crotonate can then be a significant, if not dominant, homogeneous loss process. Several authors have investigated the involvement of aldehydic compounds in the formation of oligomeric components in secondary organic aerosols [31,32] but further studies are required to evaluate the contribution of heterogeneous processes to the atmospheric degradation of aldehydes. Acknowledgments The authors wish to acknowledge RSC (UK), Deutsche Forschungsgemeinschaft (DFG), EU project EUROCHAMP, CONICET (Argentina), ANPCyT-FONCYT (Argentina), SECyT-UNC (Córdoba, Argentina), Fundación Antorchas (Argentina) and TWAS (Italy) for financial support. References [1] BASF AG, Technical Data Sheet, 3-METHYL-2-BUTENAL, 07/2002. [2] BUA, GDCh-Advisory Committee on Existing Chemicals of Environmental Relevance (BUA), 3-Methyl-2-butenal, BUA Report 194, As Per August 1996, English Reprint, 1998. [3] BASF AG, Product Safety, Data Assessment, SRC BCFWIN v2.14, 25 October 2002. [4] RIFM-FEMA, Database, 2002. [5] . [6] A. Mellouki, Le Bras, H. Sidebottom, Chem. Rev. 103 (2003) 5077. [7] C.W. Spicer, E.G. Chapman, B.J. Finlayson-Pitts, R.A. Plastridge, J.M. Hubbe, J.D. Fast, C.M. Berkowitz, Nature 394 (1998) 353. [8] M.P. Martín, M.P. Gallego-Iniesta, J.L. Espinosa, A. Tapia, B. Cabañas, M.S. Salgado, Environ. Sci. Pollut. Res., in press, doi:10.1007/s11356-009-0220-7. [9] R. Atkinson, Chem. Rev. 86 (1986) 69. [10] M.B. Blanco, I. Bejan, I. Barnes, P. Wiesen, M.A. Teruel, Atmos. Environ. 43 (2009) 5996. [11] I. Barnes, K.H. Becker, N.J. Mihalopoulos, Atmos. Chem. 18 (1994) 267. [12] M.J. Ezell, W. Wang, A.A. Ezell, G. Soskin, B.J. Finlayson-Pitts, Phys. Chem. Chem. Phys. 4 (2002) 5813. [13] M.L. Ragains, B.J. Finlayson-Pitts, J. Phys. Chem. A 101 (1997) 1509. [14] R. Atkinson, D.L. Baulch, R.A. Cox, J.N. Crowley, R.F. Hampson, R.G. Hynes, M.E. Jenkin, M.J. Rossi, J. Troe, Atmos. Chem. Phys. 6 (2006) 3625. [15] NIST-Chemical Kinetics Database on the Web, Standard Reference Database, 17, Version 7.0 (Web Version), Release 1.2., 2007, . [16] L.K. Christensen, J.C. Ball, T. Wallington, J. Phys. Chem. A 104 (2000) 345. [17] M.A. Teruel, M. Achad, M.B. Blanco, Chem. Phys. Lett. 479 (2009) 25. [18] R. Atkinson, J. Arey, J. Chem. Rev. 103 (2003) 4605. [19] W. Wang, M.J. Ezell, A.A. Ezell, G. Soskin, B.J. Finlayson-Pitts, Phys. Chem. Chem. Phys. 4 (2002) 1824. [20] R. Hein, P.J. Crutzen, M. Heimann, Global Biogeochem. Cycles 11 (1997) 43. [21] O.W. Wingenter, M.K. Kubo, N.J. Blake, T.W. Smith, D.R. Blake, F.S. Rowland, J. Geophys. Res. 101 (1996) 4331. [22] Y. Shu, R. Atkinson, J. Geophys. Res. 100 (1995) 7275. [23] J.A. Logan, J. Geophys. Res. 90 (1985) 463. [24] E.C. Tuazon, S.M. Aschmann, N. Nishino, J. Arey, R. Atkinson, Phys. Chem. Chem. Phys. 7 (2005) 2298. [25] K. Sato, B. Klotz, T. Taketsugu, T. Takayanagi, Phys. Chem. Chem. Phys. 6 (2004) 3969. [26] D. Grosjean, E. Grosjean, E.L. Williams, Int. J. Chem. Kinet. 25 (1993) 783. [27] C.E. Canosa-Mas, M.L. Flugge, M.D. King, R.P. Wayne, Phys. Chem. Chem. Phys. 7 (2005) 643. [28] M.A. Teruel, S.I. Lane, A. Mellouki, G. Solignac, G. Le Bras, Atmos. Environ. 40 (2006) 3764. [29] E. Martínez, A. Aranda, Y. Díaz-De-Mera, A. Rodríguez, D. Rodríguez, A. Notario, J. Atmos. Chem. 48 (2004) 283. [30] H. Plagens, Untersuchugen zum atmosphärenchemischen Abbau langkettiger Aldehyde, Ph.D. Thesis, Germany, 2001. [31] S. Gao et al., J. Phys. Chem. 108 (2004) 10147. [32] M. Kalberer et al., Science 303 (2004) 1659.