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Atmospheric Environment 44 (2010) 5407–5414

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Relative kinetic measurements of rate coefficients for the gas-phase reactionsof Cl atoms and OH radicals with a series of methyl alkyl esters

Nicole Schutze, Xiaoyin Zhong, Stefan Kirschbaum, Iustinian Bejan 1, Ian Barnes*, Thorsten BenterUniversity of Wuppertal, Faculty C – Department of Physical Chemistry, Gauss Strasse 20, D-42119 Wuppertal, Germany

a r t i c l e i n f o

Article history:Received 29 June 2009Received in revised form17 August 2009Accepted 19 August 2009

Keywords:Relative kineticsMethyl alkyl estersGas phaseHydroxyl radicalCl-atom

* Corresponding author. Tel.: þ49 202 439 2510; faE-mail address: barnes@uni-wuppertal.de (I. Barn

1 Present address: Department of Chemistry, UnivRoad, Cork, Ireland.

1352-2310/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.atmosenv.2009.08.011

a b s t r a c t

Relative kinetic studies have been performed on the reactions of Cl atoms with a series of methyl alkylesters in a 405-liter borosilicate glass chamber at (298 � 3) K and one atmosphere of synthetic airusing in situ FTIR spectroscopy to monitor the reactants. Rate coefficients (in units of cm3 molecule�1 s�1)were determined for the following compounds: methyl acetate (2.48 � 0.58) � 10�12; methyl propanoate(1.68 � 0.36) � 10�11; methyl butanoate (4.77 � 0.87) � 10�11; methyl pentanoate (7.84 � 1.15) � 10�11;methyl hexanoate (1.09 � 0.31) � 10�10; methyl heptanoate (1.56 � 0.37) � 10�10; methyl cyclohexanecarboxylate (3.32 � 0.76) � 10�10; methyl-2-methyl butanoate (9.41 � 1.39) � 10�11.

In addition rate coefficients (in units of 10�11 cm3 molecule�1 s�1) have been obtained for the reactions ofOH radicals with the following compounds: methyl butanoate (3.55� 0.71), methyl pentanoate (5.41� 1.08),and methyl-2-methyl butanoate (4.08 � 0.82).

Using the kinetic rate data tropospheric lifetimes for the methyl alkyl esters with respect to theirreactions with OH, and Cl have been estimated for typical ambient air concentrations of these oxidants.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Oxygenated volatile organic compounds (OVOCs) are emitteddirectly into the troposphere from biogenic and anthropogenicsources and are also formed in situ in the troposphere as a result ofthe atmospheric oxidation of all hydrocarbons. A wide range ofdifferent VOCs have been found in the atmosphere (Singh et al.,2001; Guenther et al., 1996).

There has been increased interest lately in the atmosphericchemistry of organic oxygenates mainly with respect to the rolesthat they play in indoor pollution and secondary organic aerosolformation and also the potential toxicity of some of the oxygenatesand their products (Kanakidou et al., 2005; Mellouki et al., 2003).

Simple C2 to C7 methyl alkyl esters (RC(O)OCH3, R ¼ alkyl) are animportant class of oxygenated VOCs which have fruity odors. Methylacetate (CH3C(O)OCH3), methyl propanoate (methyl propyrate; CH3

CH2C(O)OCH3), methyl butanoate (methyl butyrate; CH3CH2CH2C(O)OCH3), methyl pentanoate (methyl valerate; CH3CH2CH2CH2C(O)OCH3) and methyl-2-methyl butanoate (methyl-2-methyl butyrate;CH3CH2CH2(CH3)C(O)OCH3) are used to varying degrees as solvents,insecticides and reagents during the manufacture of perfumes and

x: þ49 202 439 2505.es).ersity College Cork, College

All rights reserved.

food flavorings (common synonyms are shown in brackets). Methylacetate, for example, is used as a solvent in fast drying paints suchas lacquers. It is also a solvent for waste film in the production ofcellulosic adhesives, a perfume solvent and is a reaction solvent indye production. Methyl butanoate and methyl pentanoate are ofparticular interest since they are used as convenient surrogates, incombustion studies of biodiesel fuels, for the methyl esters of long-chain fatty acids contained in the fuels (Hayes and Burgess, 2009;Huynh and Violi, 2008). Methyl pentanoate is also marketed underthe trade name Validol as a sedative.

The esters are also emitted into the atmosphere from naturalsources such as vegetation and fruit; for example, 2-methyl buta-noate esters occur widely in the aroma volatiles of fruit where theycontribute both to aroma and flavor (Paillard, 1990; Rowan et al.,1996). The esters can also be formed directly in the atmospherefrom the photooxidation alkyl ethers.

In comparison to the expansive kinetic and mechanistic databaseavailable on the gas-phase reactions of alkanes, alkenes and aromatichydrocarbons, with the atmospheric oxidants OH and NO3 radicals, Clatoms and O3, information on the gas-phase reactions of oxygenatedorganic compounds, such as alkyl esters, is currently fairly limited(Mellouki et al., 2003; http://www.era-orleans.org/eradb/).

Presented here is a relative kinetic study of the reaction of Cl atomswith a series of methyl alkyl (C2 to C7) esters. Although the reaction of Clatoms with esters in the atmosphere will only be of limited importancein the atmosphere, reaction with Cl is often used as a surrogate for the

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–54145408

more important reaction with OH radicals in product studies of theatmospheric photooxidation of OVOCs. Both the Cl-atom and OHradical initiated oxidation of the esters occurs via H-atom abstractionfrom the various –CHx– sites (x ¼ 1–3) in the molecule. Although theCl atoms reactions are not as selective as the OH radical reactions,depending on the experimental setup used for the analysis, they areoften experimentally easier to manage and analyze.

Rate coefficients have been reported in the literature for thereaction of Cl with methyl acetate (MA) by Notario et al. (1998),Christensen et al. (2000) and Cuevas et al. (2005) who used pulsedphotolysis–resonance fluorescence (PP-RF), relative rate (RR) andpulsed laser photolysis–resonance fluorescence (PLP-RF) techniques,respectively. Notario et al. (1998) have also reported rate coefficientsfor the reactions of Cl with methyl propanoate (MP3), methylbutanoate (MB) and methyl pentanoate (MP5) using the PLP-RFtechnique. Rate coefficients for the reactions of Cl atoms with methylhexanoate (MH6), methyl heptanoate (MH7), methyl cyclohexanecarboxylate (MCC) and methyl-2-methyl butanoate (M2MB) are notavailable in the literature.

Wallington et al. (2006) have reported that there appears tobe a systematic discrepancy of approximately 20–30% between theresults of several relative rate studies and absolute studies of thereactions of Cl with formates and esters, with those determined byabsolute techniques being higher. They suggested that the over-estimation of the rate coefficients is possibly due, at least partially,to an additional loss channel for the Cl atoms in the absolutetechniques employed such as reaction with alkyl peroxy radicals(RO2). Since the rate coefficients of Cl with MP3, MB and MP5 haveonly been previously measured once using the PLP-RF technique(Notario et al., 1998) we have determined rate coefficients for thereactions of Cl with the methyl alkyl esters discussed above ina large volume photoreactor at room temperature using the relativerate kinetic technique with FTIR in order to quantitatively followthe loss of the reactant ester.

In addition, rate coefficients have been determined for the reac-tions of OH with MB, MP5 and M2MB. A rate coefficient is notavailable for the reaction of OH with M2MB in the literature. Ratecoefficients for the reactions of OH with MB and MP5 have beenpreviously determined at room temperature using flash photolysis–resonance fluorescence (FP–RF) (Wallington et al., 1988) and pulsedlaser photolysis–laser induced fluorescence (PLP-LIF) (Le Calve et al.,1997) and the results from both determinations are in good agree-ment. They have been determined in this study for the first time usingthe relative rate technique. As discussed later, the good agreementobtained between the OH rate coefficients determined in this workand those already available in the literature have served to verify thatthe subtraction procedures used in the analyses of the FTIR spectra arecorrect and precise.

2. Experimental methods

The experiments were performed in either a 405 l or 480 lborosilicate glass chamber at 298 � 3 K and 760 � 10 Torr totalpressure of synthetic air. The basic characteristics of both chambersare very similar and a detailed description of the 405 l chamber canbe found in Barnes et al. (1982, 1983). The 405 l reactor is equippedwith 18 and the 480 l with 20 fluorescent lamps (Philips TLA 40W/05,300� l� 450 nm, lmax¼ 360 nm) which are used for the photolyticproduction of radicals. Each reactor is equipped with a White-typemirror system mounted internally within the chamber, and coupledto an FTIR spectrometer (Fourier Transform-Infrared Spectrometer;Nicolet Magna 550) equipped with a globar as IR source anda liquid nitrogen cooled MCT-detector (mercury–cadmium–telluridedetector). The set-ups enable in situ infrared monitoring of bothreactants and products. The White systems in both reactors were

operated with a total optical absorption path of 50.4 m and infraredspectra were recorded with a spectral resolution of 1 cm�1.

Chlorine atoms were produced by the photolysis of molecularchlorine.

Cl2 D hn / Cl D Cl

The photolysis of CH3ONO in the presence of NO was used for theproduction of OH radicals:

CH3ONO D hnðlmax [ 360 nmÞ / CH3O D NO

CH3O D O2 / CH2O D HO2

HO2 D NO / NO2 D HO

The esters and other liquid compounds were added to the reactor atreduced pressure by injection into a heated inlet port which wasflushed with dry synthetic air. Gaseous compounds were injecteddirectly into the chamber using gas-tight syringes and mixed byturning on the fans installed in the chamber.

Rate coefficients for the reactions of both Cl atoms and OHradicals with the methyl alkyl esters were determined using therelative rate method (Finlayson-Pitts and Pitts, 2000) in which therelative disappearance rates of the ester and reference compound,whose rate coefficient with the reactive species (either Cl or OH) isreliably known, are monitored in the presence of either Cl atoms orOH radicals:

ester D Clðor OHÞ / products; kester

reference D Clðor OHÞ / products; kref

Additionally, the esters and the reference hydrocarbon(s) couldbe lost by deposition to the reactor walls or photolysis. Test experi-ments showed that corrections for wall losses were necessary forsome of the esters. The rate coefficients for the esters were derivedfrom the following equation:

ln�½ester�t0

½ester�t

��kwall ðt�t0Þ¼

kester

kref��

ln�½reference�t0

½reference�t

��(I)

where [ester]t0 and [reference]t0 represent the concentrations ofthe ester and reference hydrocarbon at reaction time t0; [ester]t and[reference]t represent the concentrations of the ester and referencehydrocarbon at time t; kwall is the first order loss rate for wall loss ofthe ester; and kester and kref are the rate coefficients for the reactionof Cl/OH with the ester and reference, respectively.

Typically 64 interferograms were co-added per spectrum result-ing in a scanning time of around 70 s and 20 spectra were recordedfor each experiment over a period of about 25 min. The initial reac-tant concentrations of the esters and reference compounds were inthe ranges 6–12 ppmV for the esters and 10–15 ppmV for the refer-ence hydrocarbons (1 ppm ¼ 2.46 � 1013 molecule cm�3 at 298 K).Initial concentrations of CH3ONO and Cl2 were typically w18 ppmVand w20 ppmV, respectively.

3. Results and discussion

3.1. OH radical reactions

Figs. 1–3 show plots the kinetic data plotted according to equation(I) for the reactions of OH with methyl butanoate (MB), methyl pen-tanoate (MP5) and methyl-2-methyl butanoate (M2MB) measuredrelative to ethene in synthetic air at 298 K. Linear least-squares fits tothe data give the rate coefficient ratios kMB/kethene ¼ 0.417 � 0.013,

Fig. 1. Plot of the kinetic data according to Eq (I) for the reaction of OH radicals withmethyl butanoate (MB).

Fig. 3. Plot of the kinetic data according to Eq (I) for the reaction of OH radicals withmethyl-2-methyl butanoate (M2M).

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–5414 5409

kMP5/kethene ¼ 0.635 � 0.038 and kM2MB/kethene ¼ 0.479 � 0.018.Using k(OH þ ethene) ¼ 8.52 � 10�12 cm3 molecule�1 s�1 for 298 K(Atkinson and Arey, 2003) gives:

kðOH D MBÞ [ ð3:55 ± 0:66Þ 3 10L12 cm3 moleculeL1 sL1

kðOH D MP5Þ [ ð5:02 ± 1:00Þ 3 10L12 cm3 moleculeL1 sL1

kðOH D M2MBÞ [ ð3:78 ± 0:76Þ 3 10L12 cm3 moleculeL1 sL1

The quoted errors of the rate coefficients are a combination ofthe least-squares standard deviations 2s plus an additional 15% tocover the uncertainty in the value of the reference rate coefficient(Calvert et al., 2000).

The rate coefficients are compared to available literature values inTable 1 where values estimated using i) the structure activity rela-tionship (SAR) approach described in Kwok and Atkinson (1995) andii) the group rate coefficients (GRCs) given for CHx (x ¼ 1, 2 or 3)groups in esters by Mellouki et al. (2003) are also listed.

As discussed by Kwok and Atkinson (1995), in the SAR method,calculation of overall H-atom abstraction rate coefficients for C–His based on estimation of group rate coefficients for H-abstractionfrom –CH3, –CH2–, and pCH– groups. These group rate coefficientsdepend on the identity of the substituents around these groupswith k(CH3–X) ¼ kprimF(X), k(X–CH2–Y) ¼ ksecF(X)F(Y), k(X–CH–Y(–Z)) ¼ ktertF(X)F(Y)F(Z), where kprim, ksec, and ktert are the grouprate coefficients for H-atom abstraction from –CH3, –CH2–, and

Fig. 2. Plot of the kinetic data according to Eq (I) for the reaction of OH radicals withmethyl pentanoate (MP5).

pCH– groups for a standard substituent, X, Y, and Z are thesubstituent groups, and F(X), F(Y), and F(Z) are the substituentfactors. For the OH reactions, kprim ¼ 0.136 � 10�12, ksec ¼0.934 � 10�12, ktert ¼ 1.94 � 10�12 (in units of cm3 molecule�1 s�1),F(–CH3) ¼ 1.00, F(–CH2–) ¼ F(pCH–) ¼ F(pCo) ¼ 1.23, F(–C(]O)OR)¼ 0.31 and F(–OC(]O)R) ¼ 1.6 have been used in the SAR calcula-tions (Kwok and Atkinson, 1995). In the group rate coefficientapproach of Mellouki et al. (2003) the available rate constant data at298 K for the reactions of OH radicals with alcohols, ethers, ketones,and esters, has been used to derive group rate coefficients for eachCHx (x ¼ 1, 2, or 3) group in these compounds as a function of theirposition in the alkyl group chain relative to the functional group.The method assumes that the reactivities of the groups on eitherside of the ester, ether and carbonyl groups are reasonably inde-pendent and additive. The paper of Mellouki et al. (2003) should beconsulted for details of the group rate coefficients and the proce-dures adopted for their calculation.

The rate coefficients determined in this work for OH withmethyl butanoate and methyl pentanoate are in good agreementwith the values obtained by Wallington et al. (1988) and Le Calveet al. (1997) using absolute methods. This study represents thefirst relative kinetic determinations of these rate coefficients. Therehave been no previous determinations of the rate coefficient for thereaction of OH with methyl-2-methyl butanoate. The SAR rateestimation method considerably underestimates the reactivity ofOH toward the esters in all cases (35–40%), whereas the estimatesusing CHx group rate coefficients specific for the ester functionalityare in excellence agreement with the measurements for methylbutanoate and methyl pentanoate but overestimated for methyl-2-methyl butanoate by w35%.

The rate coefficient measured for the reaction of OH withmethyl-2-methyl butanoate falls between those measured for OHwith methyl butanoate and methyl pentanoate. A similar reactivitypattern toward OH is observed for ketones of similar structure,i.e. 2-pentanone, 2-hexanone and 3-methyl-2-pentanone, wherethe rate coefficients at 298 K (in units of 10�12 cm3 molecule�1 s�1)are 4.4, 9.1 and 6.9, respectively (Atkinson and Arey, 2003). It is nowwell established that oxygenated functional groups activate botha and b and possibly g sites in molecules (Mellouki et al., 2003) themuch better performance of the group rate coefficient approachcompared to the SAR in the cases of methyl butanoate and methylpentanoate is probably due to the better incorporation of theselong-range effects in the group rate coefficients compared to theSAR method where only activation by directly adjacent substituentsis considered. Why the activation effects in the case of methyl-2-methyl butanoate are being overestimated by the GRC method ispresently not known.

Table 1Comparison of rate coefficients for the reactions of OH radicals with methyl alkyl esters determined in this work at 298� 3 K with available literature values and estimated ratecoefficients using the SAR and group rate coefficient approaches.

Ester k (298 K) � 1012 (cm3 molecule�1 s�1) Techniquea Reference

Methyl butanoate

O

O

O

O (3.04 � 0.33) FP-RF Wallington et al. (1988)(3.30 � 0.25) PLP-LIF Le Calve et al. (1997)(3.29 � 0.66) RR This work1.89 SAR Kwok and Atkinson (1995)3.30 GRC Mellouki et al. (2003)

Methyl pentanoate

O

O (4.81 � 0.55) PLP-LIF Le Calve et al. (1997)(5.02 � 1.00) RR This work3.30 SAR Kwok and Atkinson (1995)5.08 GRC Mellouki et al. (2003)

Methyl-2-methyl-butanoateO

O (3.78 � 0.82) RR This work2.44 SAR Kwok and Atkinson (1995)5.21 GRC Mellouki et al. (2003)

a RR – relative rate; FP–RF – flash photolysis–resonance fluorescence; PLP–RF – pulsed laser photolysis–resonance fluorescence; SAR – structure reactivity relationship(Kwok and Atkinson, 1995); GRC: group rate coefficients (Mellouki et al., 2003).

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–54145410

3.2. Cl-atom reactions

Figs. 4–7 show plots of the kinetic data according to equation (I)for the reactions of Cl atoms with i) methyl acetate, ii) methylpropanonate, methyl butanonate, methyl pentanonate, methyl hex-anonate, and methyl heptanonate, iii) methyl-2-methyl butanoateand iv) methyl cyclohexane carboxylate, respectively. In everycase good linear correlations are obtained for all of the referencecompounds employed. It was attempted to use reference compoundswith rate coefficients as similar as possible to the ester underinvestigation. However, because of the overlap of infrared bands thiswas not always possible, thus, for several esters two or more refer-ence compounds have been used to check for potential artifacts in thespectral subtraction analyses and the experimental procedure.

Table 2 lists the measured rate coefficient ratios kester/kreference

and the values for the rate coefficients of Cl with the esters at 298 Kput on an absolute basis using the following reference compounds(with kCl in cm3 molecule�1 s�1 units): ethane (kCl ¼ 5.9 � 10�11;Atkinson et al., 2006), chloroethane (kCl ¼ 8.3 � 10�11; Wine andSemmes, 1983), ethene (kCl ¼ 1.1�10�10; Atkinson et al., 2006) andtoluene (kCl ¼ 5.84 � 10�11). All of the rate coefficients reported inthe literature for the reaction of Cl with toluene at 298 K, using bothabsolute and relative techniques, are in good agreement. The ratecoefficient given is an average value calculated from all the repor-ted literature values which are summarized in Fantechi et al. (1998)

Fig. 4. Plot of the kinetic data according to Eq (I) for the reaction of Cl atoms withmethyl acetate (MA) measured relative to the reaction of Cl with ethane.

and Smith et al. (2002). The value of Wine and Semmes (1983) forCl with chloroethane is in good agreement with more recentmeasurements by Bryukov et al. (2003).

The errors of the rate coefficients kester listed in Table 2 are theleast squares 2s standard deviations from the plots of the data plusan additional contribution to cover errors in the rate coefficients ofthe reference compounds, which are generally w15%. As can beseen in Table 2, in cases were more one than one referencecompound was used, there is good agreement between the ratecoefficients obtained using the different reference compounds.Therefore, we prefer to quote rate coefficients for these reactions asaverages of all the determinations. Averaging the values of the ratecoefficients and taking errors which encompass the extremes of alldeterminations for each reaction gives the rate coefficients listed askClþester (final) in the last column of Table 2.

The rate coefficients for the reactions of Cl with the methyl alkylesters determined in this study are compared with available liter-ature values in Table 3. The available rate coefficients for thereaction of Cl with methyl acetate are in good agreement withinthe given errors limits, however, the two absolute determinations(Notario et al., 1998 and Cuevas et al., 2005) are w20% higher thanthe two relative determinations (Christensen et al., 2000 and thiswork). The two relative determinations of the rate coefficient for

Fig. 5. Plot of the kinetic data according to Eq (I) for the reactions of Cl atoms withmethyl propanoate (MP3), methyl butanoate (MB), methyl pentanoate (MP5), methylhexanoate (MH6) and methyl heptanonate (MH7) measured related to Cl with ethene.

Fig. 6. Plots of the kinetic data according to Eq (I) for the reaction of Cl with methyl-2-methyl butanoate (M2MB) measured relative to Cl with ethane, toluene and ethene.

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–5414 5411

the reaction of Cl with methyl propanoate are in good agreementbut again the only absolute study by Notario et al. (1998) is higherby 20–30%. For the remaining methyl alkyl esters this studyrepresents the first relative rate determination of the rate coeffi-cients for the reactions of the compounds with Cl atoms. Notarioet al. (1998) have reported rate coefficients for the reactions of Clwith methyl butanoate and methyl pentanoate determined usingan absolute method. Both of the reported values are around a factorof two higher than the rate coefficients measured in this work usingthe relative kinetic technique.

It is well established that there are potential difficulties inabsolute studies of the reactions of Cl atoms with organics as hasbeen discussed recently in kinetic studies of the reactions of Cl withformates (Wallington et al., 2006; Ide et al., 2008) and acetates(Xing et al., 2009). The rate coefficients can be underestimated dueto regeneration of chlorine atoms from the reaction of molecularchlorine with the alkyl radicals produced in the reaction system:

Cl D RH / HCl D R

R D Cl2 / RCl D Cl

To reduce the regeneration of Cl atoms Notario et al. (1998) andCuevas et al. (2005) added O2 to their reaction mixtures whichconverts the alkyl radicals into alkyl peroxy radicals (RO2). However,

Fig. 7. Plot of the kinetic data according to Eq (I) for the reaction of Cl with methylcyclohexane carboxylate (MCHC) measured relative to Cl with ethene.

as discussed in several papers (Wallington et al., 2006; Ide et al.,2008; Xing et al., 2009) this leads to new complications such as i)reaction of Cl with RO2 which would lead to an overestimation of therate coefficient and ii) possible formation of O(3P), O(1D) and elec-tronically excited O2 (B3Sg

�) which would give unwanted secondary

chemistry. Ide et al. (2008) and Xing et al. (2009) in their absolutekinetic studies (PLP/Vacuum Ultraviolet (VUV)-LIF) of the reactionsof Cl with formates and acetates, respectively, have successfullydecoupled the time scales for chlorine atom loss and regenerationand found rate coefficients that are in excellent agreement withthose reported in relative kinetic studies. They both conclude thatunwanted secondary chemistry has lead to errors in the otherabsolute kinetic studies leading to the observed discrepancies.

As can be seen in Table 3 the discrepancy between the ratecoefficients determined with the absolute and the relative methodbecomes larger as the rate coefficient for Cl with the methyl alkylester increases, suggesting that in the absolute studies, withincreasing rate coefficient for the reaction of Cl with the ester thereis an increase in some secondary removal process for Cl atoms. Sinceconsistent values of the rate coefficients have been obtained for thereactions of Cl with some of the methyl alkyl esters studied in thiswork using different reference compounds and rate coefficientsobtained for reaction of OH with the esters are in good agreementwith literature values showing that the spectral analysis proceduresare in order we also conclude that the differences observed herebetween our relative values and the reported absolute values canprobably be attributed to secondary chemistry complications in theabsolute studies.

3.3. SAR for Cl þ esters

The structure activity relationship (SAR) method which wasinitially developed for estimating OH radical rate coefficients withorganics (Kwok and Atkinson, 1995) has also been applied to thereactions of Cl atoms with alkanes. In the SAR method the overallrate coefficient for H-atom abstraction is calculated by summingestimates of the group rate coefficients for the –CH3, –CH2– andpCH– entities in the molecule. The group rate coefficients depend onthe identity of the substituents adjacent to these groups andconsequently k(CH3–X) ¼ kprimF(X), k(X–CH2–Y) ¼ ksecF(X)F(Y) andk(X–CH2–Y(–Z)) ¼ ktertF(X)F(Y)F(Z). Aschmann and Atkinson (1995)have reported the following values for the parameters for Cl reac-tions with alkanes: kprim ¼ 3.32 � 10�11, ksec ¼ 8.34 � 10�11,ktert ¼ 6.09 � 10�11 (all in units of cm3 molecule�1 s�1) withF(CH3–) ¼ 1.00 and F(–CH2) ¼ F(pCH–) ¼ F(pCo) ¼ 0.79.

In their work on the reactions of Cl with acetates (CH3C(O)OR,R ¼ alkyl group) Notario et al. (1998), Cuevas et al. (2005) and Xinget al. (2009) have all noted that the CH3C(O)O– group has a deac-tivating effect on the Cl rate coefficient which extends to the b-position in the alkyl group. From their work Cuevas et al. (2005)and Xing et al. (2009) have determined substituent factorsF(RC(O)O–) and F(RC(O)CH2–) which can be used in SAR estima-tions of rate coefficients for reactions of Cl with esters. Cuevas et al.(2005) have calculated a factor F(–CO) ¼ 0.04 based on the ratecoefficient k ¼ 2.93 � 10�10 cm3 molecule�1 s�1 for the reaction ofCl with acetone from Albaladejo et al. (2003). If this factor is used ina SAR calculation of the rate coefficient for the reaction of Cl withthe acetyl group CH3C(O)O– in methyl acetate it results in a grouprate coefficient of 1.33 � 10�12 cm3 molecule�1 s�1. However,Christensen et al. (2000) have reported a Cl rate coefficient forCH3C(O)O– of <1.3� 10�13 cm3 molecule�1 s�1 which is an order ofmagnitude lower. This result implies a value of approximately0.004 for the substituent factor F(–C(O)OR) in esters and shows that

Table 2Measured rate coefficient ratios (kester/kreference) and rate coefficients for the reactions of Cl atoms with a series of methyl alkyl esters obtained in the present work at 298 Kusing the relative kinetic method.

Ester Reference hydrocarbon kester/krefer kClþester (cm3 molecule�1 s�1) kClþester(final) (cm3 molecule�1 s�1)

Methyl acetate Ethane 0.0421 (2.48 � 0.58) � 10�12 (2.48 � 0.58) � 10�12

Methyl propanoate Ethane 0.265 (1.56 � 0.36) � 10�11 (1.68 � 0.36) � 10�11

Ethene 0.163 (1.79 � 0.24) � 10�11

Methyl butanoate Chloroethane 5.355 (4.44 � 0.61) � 10�11 (4.77 � 0.87) � 10�11

Ethene 0.463 (5.09 � 0.87) � 10�11

Methyl pentanoate Ethene 0.713 (7.84 � 1.15) � 10�11 (7.84 � 1.15) � 10�11

Methyl hexanoate Ethene 0.992 (1.09 � 0.31) � 10�10 (1.09 � 0.31) � 10�10

Methyl heptanoate Ethene 1.422 (1.56 � 0.37) � 10�10 (1.56 � 0.37) � 10�10

Methyl-2-methyl butanoate Ethane 1.599 (9.43 � 1.39) � 10�11 (9.41 � 1.39) � 10�11

Ethene 0.908 (9.99 � 1.25) � 10�11

Toluene 1.510 (8.82 � 1.16) � 10�11

Methyl cyclohexane carboxylate Ethene 3.023 (3.32 � 0.76) � 10�10 (3.32 � 0.76) � 10�10

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–54145412

the whole ester entity –C(O)O– needs to be considered in deducingthe substituent factors for Cl-atom H-abstraction from the –CHx–groups around such groups and suggests that the effects on eachside of the ester may not be independent.

In the present work we have applied the SAR method to calculate Clrate coefficients for the methyl alky lesters (RC(O)OCH3) studied in thiswork. For the calculations we have used the values for kprim, ksec, ktert,F(CH3–), F(–CH2), and F(pCH–) given in Aschmann and Atkinson(1995), F(RC(O)O–)¼0.66 from Xing et al. (2009), F(–C(O)OCH3)¼0.04as deduced above (from Christensen et al., 2000) and a Cl rate coeffi-cient for CH3C(O)O– of o1.3 � 10�13 cm3 molecule�1 s�1. For thepurposes of the calculations it has been assumed that the effect ofthe –CO(O)CH3 only extends to the a-position on the acyl side of themolecule. The rate coefficients thus calculated are shown in Table 3. Itshould be noted that with the exception of methyl acetate the valuescalculated using F(–C(O)OCH3) ¼ 0.04 instead of 0.004 are onlymarginally higher.

As can be seen in Table 3 the SAR rate prediction method isconsiderably over-predicting the reactivity of the –CH2– groups evenup to the –CH2– group in the e-position. The SAR seems to consistentlyover-predict the reactivity by about a factor of 2 for C3 to C5 methylalkyl esters. The maximum discrepancy occurs for methyl butanoatethereafter the discrepancy starts to decrease. For the branched chainester methyl-2-methyl butanoate there is a þ30% difference betweenthe SAR prediction and the measured rate coefficient and in the case ofthe ring-containing ester methyl cyclohexane carboxylate there is�20% difference between the SAR prediction and the measured ratecoefficient. Taking into account the potential errors in the compari-sons both of these predictions are fairly reasonable.

In the straight chain methyl alkyl esters CH3(CH2)nC(O)OCH3

studied in this work the addition of a –CH2– group in the a, b, g, d and3-positions causes incremental increases in the overall rate coeffi-cients of 1.4, 3.09, 3.07, 3.46 and 4.3 � 10�11 cm3 molecule�1 s�1,respectively. Therefore, it is only from the 3-position onwards thatthe reactivity of the –CH2– is starting to increase to any appreciableextent. However, even in methyl heptanoate the terminal –CH2– andCH3–groups have still not reached a reactivity comparable to that inan alkane.

According to SAR predictions for Clþ alkanes reactions (Aschmannand Atkinson, 1995) the reactivity of a –CH2–- group surrounded bytwo other CH2 groups will be 5.20 � 10�11 cm3 molecule�1 s�1 andthat for a –CH2– group surrounded by a CH3 and CH2 group will be6.59 � 10�11 cm3 molecule�1 s�1.

Cl atoms appear to be behaving differently to OH radicals in theirreactions with methyl alkyl esters. According to Mellouki et al.

(2003) a –CH2– group in the b-position in the acyl side of the esterwill have a reactivity toward OH which is higher than that of the–CH2– groups in the g and d-positions, etc. with the –CH2– finallyreaching a reactivity comparable to that in an unsubstituted alkane.The activation of the –CH2– in the b-position is attributed to complexformation involving a six-membered cyclic transition state.

In the reactions of the compounds studied here with Cl the –CH2–group in the b-position, while obviously being much more reactivethan that in the a-position is just as reactive as the –CH2– groups inthe g� and d-positions and less reactive than the following –CH2–groups. The Cl-atom in combination with the –C(O)OCH3 groupappears to be somehow inhibiting H-atom abstraction in the alkylchain even up to C7. Temperature and pressure studies of the reac-tions in combination with quantum chemical calculations may helpto give an insight into the reasons for this apparent long-rangeinhibitive rate effect.

4. Atmospheric implications

The reactions of the esters with NO3 radicals and O3 are slow andbased on a comparison of reported rate coefficients for analogousalkanes (Atkinson and Arey, 2003) are expected to be of the order of�1 � 10�16 < 1 � 10�23 cm3 molecule�1 s�1, respectively, and thusof negligible importance for the atmospheric degradation of themethyl alkyl esters. Based on an average tropospheric concentra-tion of OH of ca. 1 �106 molecule cm�3 (24-h yearly global average,Prinn et al., 1995) reaction of the methyl alkyl esters with OHradicals will be the dominant removal process for the esters in theatmosphere with atmospheric lifetimes s of w70 h (s¼ 1/kOH[OH]).

Although the typical tropospheric background Cl-atom concen-tration is estimated to be <103 atom cm�3, which would result inatmospheric lifetimes ranging from 1.3 yr to 35 d for the estersstudied, in coastal areas and in the marine boundary layer, peakconcentrations of Cl atoms of 1�104 atom cm�3 or higher can occurduring sunrise (Spicer et al., 1998; Wingenter et al., 2005). In addi-tion, sources of inorganic Cl are now known to exist in some urbanareas with large industrial chemical manufacturing bases andpetroleum refining operations such as the Houston area in the US(Chang and Allen, 2006). Under such circumstances some of the fastreacting esters can of lifetimes of with respect to Cl-atom initiatedoxidation of w90 h and thus Cl-atom initiated can compare with theOH radical initiated decay.

Esters do not absorb UV light in the environmentally significantrange, (>290 nm), therefore, loss via photolysis will also be a negli-gible loss process for this class of organic compound in the

Table 3Comparison of rate coefficients (in cm3 molecule�1 s�1) for the reactions of Cl atoms with alkyl methyl esters at 298 � 3 K.

Ester k(Cl þ ester) Techniquea Reference

Methyl acetate

O

O (2.85 � 0.35) � 10�12 PP-RF Notario et al. (1998)(2.20 � 0.30) � 10�12 RR Christensen et al. (2000)(2.79 � 0.31) � 10�12 PLP-RF Cuevas et al. (2005)(2.48 � 0.58) � 10�12 RR This work2.32 � 10�12 SAR

Methyl propanoate

O

O (1.98 � 0.26) � 10�11 PP-RF Notario et al. (1998)(1.51 � 0.22) � 10�11 RR Cavalli et al. (2000)(1.68 � 0.36) � 10�11 RR This work3.17 � 10�11 SAR

Methyl butanoate

O

O(8.60 � 0.90) � 10�11 PP-RF Notario et al. (1998)(4.77 � 0.87) � 10�11 RR This work9.68 � 10�11 SAR

Methyl pentanoate

O

O(1.70 � 0.20) � 10�10 PP-RF Notario et al. (1998)(7.84 � 1.15) � 10�11 RR This work14.88 � 10�11 SAR

Methyl hexanoate

O

O(1.13 � 0.31) � 10�10 RR This work20.08 � 10�11 SAR

Methyl heptanoate

O

O(1.56 � 0.37) � 10�10 RR This work25.28 � 10�11 SAR

Methyl-2-methyl-butanoateO

O

(9.41 � 1.39) � 10�11 RR This work12.23 � 10�11 SAR

Methyl cyclohexane carboxlate

O

O

(3.32 � 0.76) � 10�10 RR This work26.37 � 10�11 SAR

a RR – relative rate; PP-RF – pulsed photolysis–resonance fluorescence; PLP-RF – pulsed paser photolysis–resonance fluorescence; SAR – structure activity relationship(Kwok and Atkinson, 1995).

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–5414 5413

troposphere. Although esters can be readily hydrolyzed they havelow Henry’s law coefficients (KH), for example KH is 8 M atm�1 formethyl acetate, and in general KH decreases with further increases inthe alkyl chain length (Sander, 1999). For species with Henry’s lawconstants smaller than 400 M atm�1, it can be calculated that lessthan 1% of their mass is dissolved in the aqueous phase inside a cloud.Therefore such species reside mainly in the gas phase in the atmo-sphere and this precludes wet deposition from being a significantatmospheric loss mechanism for the esters studied here.

Acknowledgements

Financial support of this work provided by the Deutsche For-schungsgemeinschaft (DFG) is gratefully acknowledged. The authorsthank one of the reviewers for their meticulous reading of themanuscript.

References

Albaladejo, J., Notario, A., Cuevas, C.A., Ballesteros, B., Martınez, E., 2003. Apulsed laser photolysis–resonance fluorescence kinetic study of the

atmospheric Cl atom-initiated oxidation of propene and a series of3-halopropenes at room temperature. Journal of Atmospheric Chemistry45, 35–50.

Aschmann, S.M., Atkinson, R., 1995. Rate constants for the gas-phase reactions ofalkanes with Cl atoms at 296 � 2 K. International Journal of Chemical Kinetics27, 613–622.

Atkinson, R., Arey, J., 2003. Atmospheric degradation of volatile organic compounds.Chemical Reviews 103, 4605–4638.

Atkinson, R., Baulch, D.L., Cox, R.A., Crowley, J.N., Hampson, R.F., Hynes, R.G.,Jenkin, M.E., Rossi, M.J., Troe, J., 2006. Evaluated kinetic and photochemical datafor atmospheric chemistry: volume II – gas phase reactions of organic species.Atmospheric Chemistry and Physics 6, 3625–4055.

Barnes, I., Bastian, V., Becker, K.H., Fink, E.H., Zabel, F., 1982. Reactivity studies oforganic substances towards hydroxyl radicals under atmospheric conditions.Atmospheric Environment 16, 545–550.

Barnes, I., Becker, K.H., Fink, E.H., Reimer, A., Zabel, F., Niki, H., 1983. Rate constantand products of the reaction CS2 þ OH in the presence of O2. InternationalJournal of Chemical Kinetics 15, 631–645.

Bryukov, M.G., Slagle, I.R., Knyazev, V.D., 2003. Kinetics of reactions of Cl atoms withethane, chloroethane, and 1,1-dichloroethane. Journal Physical Chemistry A 107(34), 6565–6573. doi:10.1021/jp0275138.

Cavalli, F., Barnes, I., Becker, K.H., Wallington, T.J., 2000. Atmospheric oxidationmechanism of methyl propionate. Journal of Physical Chemistry 104,11310–11317.

Chang, S., Allen, D.T., 2006. Atmospheric chlorine chemistry in Southeast Texas:impacts on ozone formation and control. Environmental Science and Tech-nology 40, 251–262.

N. Schutze et al. / Atmospheric Environment 44 (2010) 5407–54145414

Calvert, J.G., Atkinson, R., Kerr, J.A., Madronich, S., Moortgat, G.K., Wallington, T.J.,Yarwood, G., 2000. The Mechanisms of Atmospheric Oxidation of the Alkenes.Oxford University Press, Oxford, pp. 35–39.

Christensen, L.K., Ball, J.C., Wallington, T.J., 2000. Atmospheric oxidation mechanismof methyl acetate. Journal of Physical Chemistry 104, 345–351.

Cuevas, C.A., Notario, A., Martinez, E., Albaladejo, J., 2005. Influence of temperaturein the kinetics of the gas phase reactions of a series of acetates with Cl atoms.Atmospheric Environment 39, 5091–5099.

Fantechi, G., Jensen, N.R., Saastad, O., Hjorth, J., Peeters, J., 1998. Reactions of Clatoms with selected VOCs: kinetics, products and mechanisms. Journal ofAtmospheric Chemistry 31, 247–267.

Finlayson-Pitts, B.J., Pitts Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere.Theory, Experiments and Applications. Academic Press, San Diego, pp. 149–151.

Guenther, A., Zimmerman, P., Klinger, L., Greenberg, J., Eins, G., Davis, K., Pollock, W.,Westberg, H., Allwine, G., Geron, C., 1996. Estimates of regional natural volatileorganic compound fluxes from enclosure and ambient measurements. JournalGeophysics Research 101 (D1), 1345–1359.

Hayes, C.J., Burgess Jr., D.F., 2009. Exploring the oxidative decomposition of methylesters: methyl butanoate and methyl pentanoate as model compounds forbiodiesel. Proceedings of the Combustion Institute 32 (1), 263–270.

Huynh, L.K., Violi, A., 2008. Thermal decomposition of methyl butanoate: ab initiostudy of a biodiesel fuel surrogate. Journal Organic Chemistry 73 (1), 94–101.

Ide, T., Iwasaki, E., Matsumi, Y., Xing, J.–H., Takahashi, K., Wallington, T.J., 2008.Pulsed laser photolysis vacuum UV laser-induced fluorescence kinetic study ofthe reactions of Cl(2P3/2) atoms with ethyl formate, n-propyl formate, andn-butyl formate. Chemical Physics Letters 467, 70–73.

Kanakidou, M., Seinfeld, J.H., Pandis, S.N., Barnes, I., Dentener, F.J., Facchini, M.C., vanDingenen, R., Ervens, B., Nenes, A., Nielsen, C.J., Swietlicki, E., Putaud, J.P.,Balkanski, Y., Fuzzi, S., Horth, J., Moortgat, G.K., Winterhalter, R., Myhre, C.E.L.,Tsigaridis, K., Vignati, E., Stephanou, E.G., Wilson, J., 2005. Organic aerosol andglobal climate modelling: a review. Atmospheric Chemical Physics 5, 1053–1123.

Kwok, E.S.C., Atkinson, R., 1995. Estimation of hydroxyl radical reaction rateconstants for gas-phase organic compounds using a structure-reactivity rela-tionship: an update. Atmospheric Environment 29, 1685–1695.

Le Calve, S., Le Bras, G., Mellouki, A., 1997. Kinetic studies of OH reactions witha series of methyl esters. Journal of Physical Chemistry 101, 9137–9141.

Mellouki, A., Le Bras, G., Sidebottom, H., 2003. Kinetics and mechanisms of theoxidation of oxygenated organic compounds in the gas phase. ChemicalReviews 103, 5077–5096.

Notario, A., Le Bras, G., Mellouki, A., 1998. Absolute rate constants for reactions of Clatoms with a series of esters. The Journal of Physical Chemistry A 102, 3112–3117.

Paillard, N.M.M., 1990. The flavor of apples, pears and quinces. In: Marton, I.D.,MacLeod, A.J. (Eds.), Food Flavours Part C. The Flavour of Fruit. Elsevier,Amsterdam, pp. 1–41. 1990.

Prinn, R.G., Weiss, R.F., Miller, B.R., Huang, F.N., Alyea, J., Cunnold, D.M., Fraser, P.J.,Hartley, D.E., Simmonds, P.J., 1995. Atmospheric trends and lifetime of CH3CCI3and global OH concentrations. Science 269, 187–192.

Rowan, D.D., Lane, H.P., Allen, J.M., Fielder, S., Hunt, M.B., 1996. Biosynthesis of2-methylbutyl, 2-methyl-2-butenyl, and 2-methyl butanoate esters in RedDelicious and Granny Smith apples using deuterium-labeled substrates. JournalAgricultural Chemistry 44, 3276–3285.

Sander, R., 1999. Compilation of Henry’s Law Constants for Inorganic and OrganicSpecies of Potential Importance in Environmental Chemistry (Version 3).

Singh, H., Chen, Y., Standt, A., Jacob, D., Blake, V., Heikes, B., Snow, J., 2001. Evidencefrom the Pacific troposphere for large global sources of oxygenated organiccompounds. Nature 410, 1078–1081.

Smith, J.D., DeSain, J.D., Taatjes, C.A., 2002. Infrared laser absorption measurementsof HCl(v ¼ 1) production in reactions of Cl atoms with isobutene, methanol,acetaldehyde, and toluene at 295 K. Chemical Physics Letters 366, 417–425.

Spicer, C.W., Chapman, E.G., Finlayson-Pitts, B.J., Plastridge, R.A., Hubbe, J.M.,Fast, J.D., Berkowitz, C.M., 1998. Unexpectedly high concentrations of molecularchlorine in coastal air. Nature 394, 353–356.

Wallington, T.J., Dagaut, P., Liu, R., 1988. The gas phase reactions of hydroxyl radicalswith a series of esters over the temperature range 240–440. InternationalJournal of Chemical Kinetics 20, 177–186.

Wallington, T.J., Andino, J.M., Lorkovic, I.M., Kaiser, E.W., Marston, G., 1990. Pressuredependence of the reaction of chlorine atoms with ethene and acetylene in airat 295 K. Journal Physical Chemistry 94 (9), 3644–3648. doi:10.1021/j100372a052.

Wallington, T.J., Hurley, M.D., Haryanto, A., 2006. Kinetics of the gas phase reac-tions of chlorine atoms with a series of formates. Chemical Physics Letters 432,57–61.

Wine, P.H., Semmes, D.H., 1983. Kinetics of atomic chlorine (2PJ) reactions with thechloroethanes EtCl, MeCHCl2, ClCH2CH2Cl and ClCH2CHCl2. Journal PhysicalChemistry 87, 3572–3578.

Wingenter, O.W., Sive, B.C., Blake, N.J., Blake, D.R., Rowland, F.S., 2005. Atomic chlorineconcentrations derived from ethane and hydroxyl measurements over theequatorial Pacific Ocean: implication for dimethyl sulfide and bromine monoxide.Journal Geophysics Research 110, D20308. doi:10.1029/2005JD005875.

Xing, J.–H., Takahashi, K., Hurley, M.D., Wallington, T.J., 2009. Kinetics of the reac-tions of chlorine atoms with a series of acetates. Chemical Physics Letters 474,268–272.