Cl atom initiated oxidation of 1-alkenes under atmospheric conditions

Atmospheric Environment 67 (2013) 93e100

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Cl atom initiated oxidation of 1-alkenes under atmospheric conditions M. Walavalkar, A. Sharma, H.D. Alwe, K.K. Pushpa, S. Dhanya*, P.D. Naik, P.N. Bajaj Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

h i g h l i g h t s < Reports the rate coefficients of Cl reactions with1-alkenes (C6eC9) at 298 K. < Cl contributes as much as OH towards the degradation of 1-alkenes in MBL. < Increased role of H abstraction in higher alkenes. < No increase in dissociation and generation of small aldehydes (C < 4). < Major gas phase products in NOx free air are 1-chloro-2-ones/ols; enones; enols.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 July 2012 Received in revised form 22 October 2012 Accepted 26 October 2012

In view of the importance of the oxidation pathways of alkenes in the troposphere, and the significance of Cl atom as an oxidant in marine boundary layer (MBL) and polluted industrial atmosphere, the reactions of four 1-alkenes (C6eC9) with Cl atoms are investigated. The rate coefficients at 298 K are measured to be (4.0
0.5), (4.4
0.7), (5.5
0.9) and (5.9
1.7)  1010 cm3 molecule1 s1 for 1hexene, 1-heptene, 1-octene and 1-nonene, respectively. The quoted errors include the experimental 2s, along with the error in the reference rate coefficients. From the systematic increase in the rate coefficients with the number of carbon atoms, an approximate value for the average rate coefficient for hydrogen abstraction per CH2 group in alkenes is estimated to be (4.9
0.3)  1011 cm3 molecule1 s1. Based on these rate coefficients, the contribution of Cl atom reactions towards the degradation of these molecules is found to be comparable to that of OH radical reactions, under MBL conditions. The products identified in gas phase indicate that Cl atom addition occurs mainly at the terminal carbon, leading to the formation of 1-chloro-2-ketones and 1-chloro-2-ols. The major gas phase products from the alkenyl radicals (formed by H atom abstraction) are different positional isomers of long chain enols and enones. A preference for dissociation leading to an allyl radical, resulting in aldehydes, lower by three carbon atoms, is indicated. The observed relative yields suggest that in general, the increased contribution of the reactions of Cl atoms towards degradation of 1-alkenes in NOx free air does not result in an increase in the generation of small aldehydes (carbon number < 4), including chloroethanal, as compared to that in the reaction of 1-butene. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: 1-Alkenes Cl atom reactions Rate coefficients Tropospheric lifetimes Aldehyde formation

1. Introduction Alkenes, an important class of non-methane volatile organic compounds, contribute significantly towards generation of aldehydes in the troposphere (Luecken et al., 2012). These include alkenes from biogenic sources as well as from automobile fuels. Terminal alkenes (1-alkenes) are important precursors of formaldehyde in the polluted boundary layer (Wert et al., 2003). The major degradation pathways of alkenes are reactions with

* Corresponding author. E-mail address: [email protected] (S. Dhanya). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.039

tropospheric oxidants, OH, O3 and NO3 (Atkinson, 1997). The relative importance and consequences of these various possible oxidation pathways of alkenes in the troposphere are decided by the rate coefficients and mechanism of their reactions with the respective oxidants. In addition to the lower alkenes, many other alkenes with carbon number >7, are used as important intermediates in industry and they are also released to the atmosphere from various sources such as gasoline, structural fires, combustion of polyethylene, automobile emission, etc., though the percentage is less than that of the biogenic alkenes such as isoprene. Although 1-alkenes with higher carbon number (10e18) have found application as offshore drilling fluids, due to their very low vapour pressure they are unlikely to be released to the troposphere

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directly. 1-octene has been observed in the urban atmosphere in the concentration range of 40 ppte48 ppb (Paulson and Seinfeld, 1992), and smog chamber studies have shown that its reactions with atmospheric oxidants, such as OH and O3 lead to the formation of secondary organic aerosol (SOA) (Forstner et al., 1997). The significance of the reactions of alkenes in the troposphere has prompted many measurements of the rate coefficient their reactions with tropospheric oxidants and recently these studies have been extended to the reactions of higher alkenes, including a series of 1-alkenes up to 14 carbon atoms, with OH (Aschmann and Atkinson, 2008), O3 (Mason et al., 2009; McGillen et al., 2008) and NO3 (Mason et al., 2009; Aschmann and Atkinson, 2011; Zhao et al., 2011). Cl atom is also considered to be an important oxidising species, in the remote marine boundary layer and polluted coastal areas and even in polluted mid-continental regions (Singh et al., 1996; Spicer et al., 1998; Tanaka et al., 2000; Thornton et al., 2010). The detection of 1-alkenes up to 1-hexene in the marine boundary layer (Bonsang et al., 1988) and that of the higher alkenes up to 1-decene in the polluted urban air (Forstner et al., 1997) emphasises the possibility of their reactions with Cl atoms. In spite of the importance of these reactions, the measurements of the rate coefficients are limited to molecules with carbon atoms up to five (C ¼ 5). The trend in the reactivity of Cl atoms with higher 1-alkenes, as well as the nature of products, can be different from those in the reactions of OH, O3 and NO3. While O3 and NO3 primarily react with the terminal double bond of 1-alkenes, in the case of OH, along with addition to the double bond, a minor channel of H atom abstraction reaction from the hydrocarbon chain is also considered to occur, with a molar yield increasing with increasing number of carbon atoms (Matsunaga et al., 2009). Although the mechanism of reaction of Cl atoms with alkenes is similar to that of OH, both abstraction and addition reactions are very fast, and have comparable rate coefficients in Cl atom reactions, whereas the rate coefficient of abstraction of H atom is an order of magnitude lower than that of addition to double bond in OH reactions. Hence, the increase in the rate coefficients of 1-alkenes with number of carbon atoms is expected to be more in Cl atom reaction, than that in OH reactions. This can result in an increasing role of Cl atom reactions for higher alkenes, in the atmospheric conditions such as marine boundary layer and polluted urban atmosphere. Thus, the dominant tropospheric degradation pathways of long chain alkenes, and their influence on tropospheric chemistry may change due to generation of chloroalkyl radicals by Cl atom addition, different from the OH added radicals, and increased formation of alkenyl radical by H abstraction, as compared to that formed by OH reactions. For assessing the relative importance of Cl atom reactions and their influence on tropospheric chemistry of the long chain 1-alkenes, the measurements of rate coefficients of these reactions as well as the detailed reaction mechanism of the chloroalkyl radicals and the alkenyl radicals are necessary. Although alkenyl radicals are formed during the OH induced oxidation of 1-alkenes in atmospheric conditions, many modelling studies take into account only addition reaction of OH (Pinho et al., 2006). The tropospheric chemistry of the alkyl radicals, formed by H atom abstraction from long linear n-alkanes, is well studied. Isomerisation reactions have been established to be important, along with the formation of the corresponding alcohols, ketones, and hydroperoxides as well as dissociation of alkoxy radicals to products with lower number of carbon atoms (Atkinson et al., 2008). The shorter and more branched alkoxy radicals are more prone to dissociation than isomerization. As compared to these alkyl radicals, detailed studies on the reactions of long alkenyl radicals and chloroalkyl radicals, which may be different from the corresponding alkyl radicals, are limited. The alkenyl radicals

formed by OH reactions with higher alkenes are suggested to give vinyl ketones and other unsaturated ketones, which have lower photochemical ozone forming potential than the products from OH addition, but there is no experimental characterization of these products (McGillen et al., 2007). While quantifying the SOA (Secondary Organic Aerosol) generation from reactions of OH with higher alkenes in the presence of NOx, the H atom abstraction pathways were found to be complex, and 1,4-hydroxy nitrates were assumed to be the only products (Matsunaga et al., 2009). In the case of chlorinated alkyl radicals, reaction mechanisms have been discussed for isoprene and a number of C4 molecules (Orlando et al., 2003). There are no reports on the longer chloroalkyl radicals, which may have different dissociation and isomerisation probabilities. In the present work, the rate coefficients of the reactions of Cl atoms with a series of 1-alkenes (C6eC9) are measured at room temperature, using relative rate method. These values are used to assess the significance of the reaction of Cl atom, as compared to that of ozone and OH radical, in the oxidation of these 1-alkenes, under the conditions of marine boundary layer and polluted atmosphere. The major products, resulting from the chloroalkyl and alkenyl radicals in air, in the absence of NOx, are also identified by GCeMS, and the possibility of direct generation of lower aldehydes (ethanal, propanal and butanal) from these radicals is examined by measuring their relative yields for each alkene. 2. Experimental Rate coefficients at room temperature (298 K) were determined using relative rate method by comparing the rate of decay of 1alkenes (Alk) due to their reactions with Cl atoms to that of a reference molecule (R), with known rate coefficient.

Alk þ Cl/products

(1)

R þ Cl/products

(2)

Assuming that alkene and the reference compound react only with Cl atoms, the fractional loss of alkene and R, are related by the standard expression,

"

½Alkt0 ln ½Alkt

#

 ¼

#  " ½Rt0 kAlk ln kR ½Rt

(I)

where [Alk]t0, [Alk]t and [R]t0, [R]t are the concentrations of the 1alkene and the reference compound at time 0 and t, respectively and kAlk and kR are the rate coefficients for reactions (1) and (2), respectively. Thus, the plots of logarithms of the ratios of fractional changes in the concentrations of both alkene and R at specific times give straight line with zero intercept and slope of kAlk/kR and the rate coefficient kAlk is calculated using the known value of kR. All the experiments were performed at 298
2 K. The details of the experimental set-up are given elsewhere (Sharma et al., 2010, 2011). The reaction mixture, consisting of alkenes (200e 250 ppm), reference molecule (200e300 ppm), CCl3COCl, the source molecule for Cl atom (500e600 ppm) and buffer gas (N2/ air), was taken in a quartz reactor cell of volume of about 1 L, having a sealed port, for taking out samples for concentration measurement. The total pressure was maintained at 800
3 Torr. Chlorine atoms were generated by the in situ photolysis of CCl3COCl at 254 nm. The mixture was photolysed for a period of 4e5 min, in steps of 40e50 s and the depletion of the alkene and the reference molecule were followed after each step, by measuring their concentration using a gas chromatograph (Shimadzu GC-2014) coupled with a flame ionization detector. Either packed 5% SE-30

M. Walavalkar et al. / Atmospheric Environment 67 (2013) 93e100

3. Results 3.1. Rate coefficient measurements Prior to the determination of the rate coefficients, the stability of the reaction mixtures with respect to wall losses and dark reactions was examined and found to be satisfactory for about 7 h, which is more than the total duration of a relative rate measurement. Since photolysis of the mixture without CCl3COCl did not show any significant decrease in the concentration of the alkene, direct photolysis can be ruled out. The reactions of Cl atom with each alkene, as well as reference molecule were carried out individually to ensure that the products of the reactions do not interfere with the GC measurement of the concentration of the alkenes and the reference molecules. The typical logarithmic plots of the relative decrease in the concentration of 1-hexene against that of 1-butene and n-butane, due to their reactions with Cl atoms, are shown in Fig. 1, in N2 as well as air. All the plots show near-zero intercept, but are shown with an offset in the Y axis so that the individual plots can be seen clearly. The presence of oxygen does not have any effect on the relative ratio, as shown in the figure. Similar plots of 1-heptene, 1octene and 1-nonene are shown in Figs. 2e4, respectively. In the case of 1-hexene, n-hexane could not be used as a reference molecule because of interference during GC analysis. 1,4cyclohexadiene (1,4-CHD), having a rate coefficient of (4.06
0.55)  1010 cm3 molecule1 s1 (Sharma et al., 2011), was used as a reference for 1-nonene, instead of n-butane, because of comparable rate coefficients. The average slope values determined from 4 to 9 measurements with each reference molecule are given in Table 1, along with the rate coefficients determined at 298 K. The errors listed for the rate coefficients

ln([1-Hexene]t /[1-Hexene]t )

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

ln([Reference]t /[Reference]t) Fig. 1. Decays of 1-hexene due to reaction with Cl atom, relative to n-butane in 800 torr of N2 (-) and air (,); relative to 1-butene in 800 torr of N2 (C) and air (B). -, , and B are shifted in Y-axis by 0.2, 0.07 and 0.07 respectively, for clarity.

include the errors in the rate coefficients of the reference molecules, in addition to the errors in the experimentally determined slope values. The rate coefficients of 1-propene, 1-butene, and 1pentene are also included in the Table for comparison. Unlike cyclic alkenes (Sharma et al., 2010), the relative rate ratios in nitrogen and air are the same, within experimental errors, in all these 1-alkenes. In 1-octene and 1-nonene, the determination of the rate coefficients in air was difficult due to interference from the products. The rate coefficients determined using different reference molecules are fairly consistent in 1-hexene and 1heptene, whereas there is a variation in 1-octene and 1-nonene, (almost 28% in 1-nonene), which could be due to larger uncertainty in the measurements for these molecules. The rate coefficient measurement of the reaction of NO3 with 1-nonene has been found to be influenced by heterogeneous reactions at the wall surface (Zhao et al., 2011), which may have some influence in the present experiments also, leading to higher uncertainties. Based on these

1.6

ln([1- Heptene]t /[1- Heptene]t )

(1 m  3.12 mm) or Porapak-Q (1.8 m  3.12 mm) stainless steel columns or HT 8 (25 m  0.22 mm  0.25 mm) fused silica capillary column were employed for the separation with suitable temperature and flow conditions. Before and after each photolysis, the prepared reaction mixture was allowed to reach uniform distribution, which was confirmed by reproducibility of the concentration measured during consecutive GC samples. 1-butene, n-hexane and n-butane were the different reference molecules used, based on the least interference during the Gas Chromatography analysis, either from the molecule or from the products of its reaction with Cl atoms. The IUPAC recommended value of (2.05
0.25)  1010 cm3 molecule1 s1 (Atkinson et al., 2004) was used as the rate coefficient for the reaction of n-butane with Cl atoms. This value was also used for obtaining the corrected rate coefficient for n-hexane (Atkinson and Aschmann, 1985) as (3.15
0.40)  1010 cm3 molecule1 s1, which in turn, was used for calculation of the rate coefficient of 1-butene as (3.21
0.41)  1010 cm3 molecule1 s1, using the reported relative ratio against n-hexane (Coquet and Ariya, 2000). The products studies were carried out in reaction cells of 1 L as well as 2.7 L volume. The products were separated and analysed by Gas Chromatograph coupled with a Mass Spectrometer and FID (GCMS-QP2010, Shimadzu). Three different columns, namely carbowax, Q-Plot and HT8 were used for separation. Nitrogen gas of purity >99.9% (INOX Air Products Ltd., India) and ultrapure air (Zero grade; Chemtron Science Laboratories, India) were used as buffer gases. 1-Hexene (synthesis grade) from Spectrochem, 1-Heptene (97%), 1-Octene (98%) and 1-Nonene (96%) from SigmaeAldrich, n-Butane and 1-Butene from Matheson, were used without further purification. Samples of the compounds were stored in evacuated glass vessels and subjected to freezeepumpe thaw cycles prior to use.

95

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

ln([Reference]t /[Reference]t ) Fig. 2. Decays of 1-heptene due to reaction with Cl atom, relative to n-butane in 800 torr N2 (,) and air (-); relative to n-hexane in 800 torr of N2 (C) and air (B). ,, B and C are shifted in Y-axis by 0.1, 0.05 and - 0.05 respectively, for clarity.

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M. Walavalkar et al. / Atmospheric Environment 67 (2013) 93e100

ln([1-Octene]t /[1-Octene]t )

1.0

Table 1 The relative rate ratios and the rate coefficients calculated at room temperature for 1-alkenes with different reference molecules.

0.8 0.6 0.4 0.2 0.0 -0.2

0.0

0.1

0.2

0.3

0.4

Reference Buffer kAlk/kR gas

1-Hexene

1-Butene N2 Air n-Butane N2 Air

1.2 1.2 2.1 1.9







0.1 0.1 0.1 0.1

3.9 3.9 4.2 3.9







0.5 0.5 0.5 0.5

4.0
0.4

1-Heptene n-Hexane N2 Air 1-Butene N2 Air n-Butane N2 Air

1.3 1.3 1.4 1.4 2.5 2.0









0.1 0.1 0.1 0.1 0.2 0.2

4.0 4.0 4.5 4.5 5.0 4.2









0.6 0.5 0.6 0.6 0.7 0.7

4.4
0.7

1-Octene

n-Hexane N2 Air 1-Butene N2 Air n-Butane N2

1.7 1.6 1.9 1.9 2.6








0.1 0.1 0.1 0.3 0.3

5.2 5.1 6.0 5.9 5.3








0.7 0.8 0.9 1.1 0.8

5.5
0.9

1-Nonene n-Hexane N2 Air 1-Butene N2 1,4-CHD N2

1.9 1.6 2.1 1.2







0.3 0.5 0.3 0.2

5.9 5.2 6.7 5.8







1.3 1.7 1.2 1.3

5.9
1.7

0.5

ln([Reference]t /[Reference] t ) Fig. 3. Decays of 1-octene due to reaction with Cl atom, relative to n-butane in 800 torr of N2 (-) and 1-butene (,); relative to n-hexane in 800 torr of N2 (B) and air (C). -, , and C are shifted in Y-axis by 0.2, 0.05 and 0.07 respectively, for clarity.

individual values, the average rate coefficients for 1-hexene, 1heptene, 1-octene and 1-nonene are determined to be (4.0
0.5), (4.4
0.7), (5.5
0.9) and (5.9
1.7)  1010 cm3 molecule1 s1. The results show a systematic increase in the Cl atom rate coefficients with increasing number of carbon atoms of the 1alkenes. When plotted (Fig. 5), the rate coefficients show a linear dependence on the number of carbon atoms, with a slope of (4.9
0.3)  1011 cm3 molecule1 s1 per added eCH2e group. 3.2. Products study The chromatograms of the products, formed by the Cl atom initiated oxidation of these 1-alkenes in the presence of air, are found to be the same in cells of 1 L and 2.7 L capacity. The typical chromatograms are shown in Fig. 6, along with that of 1-butene. Fig. 6b shows the products eluted before 10 min in an expanded X scale. Two peaks are found to be dominant in the total ion

kAvg.  1010 k  1010 (cm3 molec1 s1) (cm3 molec1 s1)

Molecule

Propene (Atkinson et al., 2004) 1-Butene (Coquet and Ariya, 2000) 1-Pentene (Ezell et al., 2002)

2.7 3.1
0.4 4.0
0.4

Rate coefficients at 298 K used for the reactions of Cl with reference molecules 1butene, n-butane, n-hexane and 1,4-CHD were (3.21
0.41), (2.05
0.25), (3.15
0.40) and (4.06
0.55)  1010 cm3 molecule1 s1 respectively [see text].

chromatogram (TIC) in all the alkenes, with systematically increasing retention times from 1-butene to 1-nonene, indicating the similarity of the products. In the case of 1-butene, they are clearly identified as 1-chloro-2-butanone (15.5 min) and 1-chloro2-butanol (18.41 min), from the mass spectra analysis and by comparing with the library data. Although there is no matching library data available for the corresponding peaks in higher alkenes, based on the mass spectral analysis, these are also assigned to the respective 1-chloro-2-ketone (lower retention time) and 1-chloro2-alcohol (higher retention time), as in the case of 1-butene. In 1-butene, low yields of chloroethanal (9.83 min) and H atom abstraction products such as 3-butene-2-one (4.17 min) and 3-

0.8

10

-1

0.6

10 x k / cm molecule s

-1

0.5

0.3 0.2 0.1 0.0

0.1 0.2 0.3 0.4 ln([Reference]t /[Reference]t )

0.5

Fig. 4. Decays of 1-nonene due to reaction with Cl atom, relative to 1-butene (-), nhexane (,) and 1,4-cyclohexadiene (C) in 800 torr of N2. - is shifted in Y-axis by 0.03, for clarity.

6

3

0.4

0.0

8

4

10

ln([1-Nonene]t /[1-Nonene]t)

0.7

2

0

2

3

4

5

6

7

8

9

10

number of carbon atoms Fig. 5. Plot of the rate coefficients of reactions of Cl atoms at 298 K against the number of carbon atoms in 1-alkenes.

M. Walavalkar et al. / Atmospheric Environment 67 (2013) 93e100

5

b

A

4

5

B

Intensity / 103

C D E

5

2

intensity / 10

A

B

3

4

97

5

6

9

10

a

6

D

4 C

1

3

2 3

2

4 5

1 6.0

8.0

10.0 Retention time /min

12.0

Fig. 7. Gas chromatogram showing the relative yield of lower aldehydes upon Cl atom initiated oxidation of 1-alkenes: 1) butene, 2) hexene, 3) heptene, 4) octene, 5) nonene. Column used is QPLOT (30 m  0.32 mm  10 mm) under isothermal conditions at 180  C. Number of molecules of 1-alkenes that have reacted/cm3 w2.4e3.0  1014.

4

2

10

15

20

25

30

retention time (min) Fig. 6. Total ion chromatogram of products of reactions of 1-alkenes with Cl atoms: A) butene, B) hexene C) heptene, D) octene, E) nonene. Column: CP WAX (30 m  0.25 mm  0.25 mm), with temperature programming, maintained at 32  C for 5 min and then raised the temperature to 200  C at the rate of 5  C min1.

butene-2-ol (8.36 min) are also observed along with an unidentified product (20.4 min). Although all the product peaks obtained in 1-hexene, 1-heptene, 1-octene and 1-nonene could not be assigned based on their mass spectra, some products and patterns are consistently observed in all of them. Formic acid (22 min) and propenal (2.62 min) are observed as minor products in all the alkenes. Enones and enols are observed in all cases, e.g., 5-hexene2-one (10 min) in 1-hexene, 1-Heptene-3-one and 6-heptene-3one (12.3 and 12.4 min) and heptenols (17.13, 17.31 min) in 1heptene. More isomers of enones and enols are detected in 1octene (15.5e16.8 and 19.2e20.7 min) and 1-nonene (19.85 and 22.25e23.37 min). In all cases, aldehydes with one carbon less than the respective parent molecules were detected in small yields. Aldehydes of lower carbon number (4) are also detected, but the corresponding peaks are small and some are merged either with peaks of air or parent molecules. For a better comparison, these lower aldehydes are separated and detected using column Q-plot and FID, for all alkenes, under identical conditions. Fig. 7 shows the relative yields of ethanal (A), propanal (B) and butanal (C) from 1-hexene, 1heptene, 1-octene and 1-nonene along with that of 1-butene. Chloroethanal (D) is observed only in 1-butene. The peaks are characterised by comparing with the standard molecules, as well as from their mass spectra. The total number of reacted molecules is almost the same for all. Although the presence of formaldehyde was detected, it is not observed using FID, probably due to lower sensitivity as well as lower yield. As compared to 1-butene, the yield of lower aldehydes is found to be decreasing in higher alkenes. However, it is observed that the yield of propanal is more

in 1-hexene whereas butanal is prominently seen in 1-heptene. Similarly, the total ion chromatogram in Fig. 6b shows that pentanal (4.9 min) is formed in 1-octene but not in other alkenes except 1hexene. There is an indication of hexanal (8.2 min) in 1-nonene, though the interference from the background makes it difficult to be clearly observed. Methyl furanone is detected in 1-hexene at 24.45 min and corresponding alkyl substituted furanones, lower by one carbon number, were observed in all other alkenes as minor products.

4. Discussion The systematic increase in the rate coefficients of Cl atom reaction with higher 1-alkenes indicates increased contribution of abstraction reaction due to increasing number of CH2 groups, along with a smaller contribution from an increase in the addition rate coefficient due to increased inductive effect. The average increase observed here per CH2 group is (4.9
0.3)  1011 cm3 molecule1 s1. This can be considered as an approximate value of the rate coefficient for hydrogen abstraction per CH2 group in alkenes, neglecting the possible increase in the addition rate coefficient. This is only marginally lower than the rate coefficient derived for H abstraction from the secondary CH2 group, 5.8  1011 cm3 molecule1 s1, using the structure activity relationship (SAR) developed by Tyndall et al. (1997) and Atkinson (1997), with 9.3/9.14  1011 cm3 molecule1 s1 as the group rate coefficient for CH2 and 0.79/0.80 as the neighbouring effect due to the adjacent CH2 groups. Using the experimental rate coefficients at 298 K, and considering the reactivity of allyl group separately, Ezell et al. (2002) have derived a SAR for alkenes. The rate coefficients measured in the present study are in agreement with the values calculated using this SAR, within experimental limits. Among other tropospheric oxidants, a similar increase with number of carbon atoms is observed for the OH reaction. The increase per CH2 group, 2.0  1012 cm3 molecule1 s1, considered as the rate coefficient of abstraction reaction per CH2 group, is about 10 times less than the rate coefficient of addition reaction (Aschmann and Atkinson, 2008). The trend in the reactivity of NO3, where the reaction is only with the double bond, shows an increase in the rate coefficient up to C ¼ 7, due to the inductive effect of the alkyl group, and reaches a plateau beyond C ¼ 7 (Mason et al., 2009; Aschmann and Atkinson, 2011; Zhao et al., 2011). The rate coefficients of ozone reaction at room temperature are almost not

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affected by the increasing chain length of 1-alkenes (McGillen et al., 2008; Mason et al., 2009). Thus, among the tropospheric oxidants, the increase in the rate coefficients with number of carbon atom is largest for the reactions of Cl atom. In the Cl initiated oxidation of 1-butene, Orlando et al. (2003) have identified and quantified some products, namely, chloroethanal (20%), HC(O)Cl (<5%), formaldehyde (about 8%) and CO2 (<5%), using FTIR spectroscopy, at conditions, very similar to the present one (partial pressure of oxygen ¼ 150 torr). The possibility of abstraction reaction was not considered in the proposed reaction scheme. The products of the suggested reaction scheme are ethanal, propanal, chloroethanal, HC(O)Cl and 2-chloro-1-butanal from Cl addition at the terminal carbon and 1-chloro-2-butanone, 4hydroxy-2-chloro-1-butanal (via isomerisation), and formaldehyde, acetaldehyde and CO2 (via dissociation) from internal (at second C atom) addition of Cl. However, a quantitative estimation of the relative importance of the two addition channels is not known from this study. Due to the difficulty in quantification and the multiple possibilities of isomerisation and dissociation of the radicals leading to a variety of products, the present study also is not able to give an exact contribution of different reaction channels. But the present results identify some prominent reactions of the chloroalkyl and alkenyl radicals in the gas phase, as discussed below. The major gas phase products identified in all alkenes, 1-chloro2-ketones and 1-chloro-2-ols indicate the preferential addition of Cl atom at the terminal carbon, probably due to the stability of the secondary radical formed as compared to that of the primary radical generated by Cl addition at the second carbon atom. A general reaction scheme, involving the peroxy and alkoxy radicals, is proposed for the Cl initiated oxidation of these molecules (Fig. 8). The addition pathway is represented by Scheme 1A and abstraction pathway by Scheme 1B. The formation of chloroketones indicates the reaction of chloroalkoxy radicals with oxygen. The low yields of aldehyde, lower by one carbon atom indicates the dissociation of RCH2CH(O)CH2Cl at the terminal CeC bond (Orlando et al., 2003) to be only a minor channel of reaction of these alkoxy radicals. The only exception is 1-butene, where propanal is seen prominently (Fig. 7). The absence of chloroethanal in higher alkenes other than 1-butene indicates that dissociation at C2 is also not a very significant reaction in long chloroalkoxy radicals, RCH2CH(O)CH2Cl. In the case of corresponding hydroxyalkoxy radicals, isomerisation via a six member transition state leading to dihydroxy ketones is known to be a significant reaction. Similar reactions are also possible for chloroalkoxy radicals, as shown in the Scheme, and may be responsible for the low yield of smaller aldehydes. However, the corresponding polyfunctional products, chlorohydroxyketones, could not be identified in the gas phase by GCeMS, probably due to the experimental limitations or low vapour pressure of these molecules. Although any specific product directly resulting from Cl addition at second carbon atom could not be identified here, their presence can’t be ruled out since there are some product peaks that could not be clearly assigned. The observed formic acid/formaldehyde may arise from the radical, RCH2CH(Cl)C(O)H2, by dissociation. Among the different enones and enols formed by H abstraction, the presence of vinyl ketones, and enols with double bonds at C1 and keto/OH groups at C3 shows the formation of an allyl centered radical in all alkenes. Abstraction from alkyl CH2 groups is also indicated in higher alkenes, since corresponding enones and enols are also present as products, e.g., 5-hexene-2-one, 6-heptene-3one, 7-octene-2-one, 8-nonene-2-one and the corresponding alcohols. Since there are more unidentified peaks bunched very near to the identified enols and enones, it is possible that more positional isomers or unsaturated hydroxyl ketones, possible

products of isomerisation reaction, are present. While deriving the SAR for the reaction of Cl atoms with alkenes (upto C ¼ 5), (Ezell et al., 2002) the rate coefficient of abstraction from the allyl group is found to be lower than that from secondary alkyl groups, though the bond dissociation energy of allyl CeH bond is less than that of secondary CeH bond. The results in isoprene and transbutene are also found to be inconclusive. In isoprene, with two double bonds and one primary allyl group, the yield of HCl indicates the abstraction reaction to account for about 15% of the total reaction (Suh and Zhang, 2000), whereas in trans-butene, with one double bond and two primary allyl groups, the contribution from abstraction is <10% (Orlando et al., 2003). The estimation of allyl hydrogen abstraction was difficult in the present study, due to probable dissociation reactions and difficulty in quantification, and thus more experimental results are desired in this direction. Grosjean et al. (1996) have studied the atmospheric chemistry of 1-octene and 1-decene and the major products from OH reaction are identified as aldehydes lower by one carbon atom and formaldehyde, formed by addition of OH at the terminal carbon atom. The alkenyl radicals, formed by abstraction reactions were considered to be important, and were suggested to give lower aldehydes, starting from those lower by 2 carbon atoms. This was not confirmed due to the interference from the secondary reactions of aldehydes generated by OH addition, which may also give lower aldehydes. In the present study, the products indicate the presence of different alkenyl radicals, e.g., 6-heptene-2-one, (CH] CH2CH2CH2CH2CH(]O)CH3) is a prominent product in 1-heptene, implying the formation of CH]CH2CH2CH2CH2CHCH3 radical. Even though lower aldehydes are observed, their yields appear to be lower than that of the undissociated ketones and alcohols. Among the lower aldehydes, those lower by three carbon atoms are more prominently seen. Thus, the preferred pathway of dissociation appears to be the one giving allyl radical, either from an alkoxy radical centered at allyl carbon or next carbon atom, as shown in scheme 1B, giving propenal and an aldehyde lower by three carbon atoms. The negligible presence of ethanal, propanal and butanal in octene and nonene indicates that these alkenyl radicals, chloroalkyl radicals or their corresponding alkoxy radicals do not undergo dissociation readily at any CeC bond, to give smaller aldehydes. The total absence of chloroethanal in all alkenes except 1-butene indicates that dissociation at C2 is not a preferred reaction of these long 1-chloro alkyl radicals. Alkyl substituted dihydrofuranones, lower by one carbon number are formed in all alkenes, the mechanism for their formation is not clear. One possibility for their formation is secondary reactions of aldehydes lower by one carbon atom (Forstner et al., 1997), one of the products formed, as discussed earlier. 4.1. Atmospheric implications Based on the rate coefficients determined in the present work, the relative importance of Cl atom reactions in the atmospheric degradation of 1-alkenes, as compared to the reactions with OH, O3 and NO3, are estimated by comparing the corresponding lifetimes as shown in Table 2. In the conditions such as marine boundary layer, where Cl atom concentration is about 105 molecule cm3 (Spicer et al., 1998), the reaction with Cl atom is competitive for all 1-alkenes and more dominant than the reaction with ozone. As the number of carbon atoms increase, from 1-butene to 1-nonene, the contribution of Cl atom increases from about one third to half of the total degradation reactions. However, the product analysis shows that the increased contribution of Cl atom initiated oxidation of higher alkenes does not result in an increased yield of lower aldehydes such as ethanal or propanal directly. The formation of

Scheme 1A: Addition Pathways R

+ Cl R

Cl

Cl

R

O2

CH2

O O

O2 HO2

Cl R

(OOH)

H HC

Cl

Cl H O2

HC

O C H

O m i er

R

O Cl

R O

n, ti o

Cl

O2

Cl

+

Dissociation

OH

sa

HCOOH ,

Cl

R

Is

o Is

om

er

O

Dissociation

OH

Cl

R

R

isa tio n

R

C

RO2

Cl

R

RO2

O O

RO2

R

Cl

R

H

O2

Cl

O

H Chloro hydroxy ketones

C

R

O

Scheme 1B: Abstraction Pathways: + Cl

R

R

R

O2 O

O2

O R O

Unsaturated hydroxy ketones

R RO2 OH O

I

som

io sat e ri

n,

Iso

O2

me

ris

ati o

+

O2

n,O

2

R O

O

RO2

R

R

R

OH O

H R

R

C

+

O2 R

O O

+ O H2C

C H

Fig. 8. Schematics of the proposed reactions, leading to some of the major products observed in the present study, during the Cl atom initiated oxidation of 1-alkenes in NOx-free air.

Table 2 Tropospheric lifetimes (s) calculated for 1-alkenes with respect to reactions with different tropospheric oxidants. The concentrations used for Cl (Spicer et al., 1998), OH, NO3 and O3 (Atkinson, 2000) are 1.3  105, 2  106, 5  108 and 7  1011 molecules cm3, respectively. All the rate coefficients are in the unit of cm3 molecule1 s1. Sources of rate coefficients: kOH e Aschmann and Atkinson, 2008; (for propene e Atkinson et al., 2004). kO3 and kNO3 e Mason et al., 2009. kCl e present work (kCl for propene e Atkinson et al., 2004; for 1-butene e corrected value from Coquet and Ariya (2000), see text). Molecule

kOH  1011

sOH (h)

kO3  1017

sO3 (h)

kNO3  1014

sNO3 (h)

kCl 1010

sCl (h)

Propene 1-butene 1-pentene 1-hexene 1-heptene 1-octene 1-nonene 1-decene

2.9 3.2 3.2 3.7 3.9 4.1 4.3 4.6

4.8 4.3 4.3 3.8 3.5 3.6 3.2 3.0

0.97 0.93 1.07 1.01 1.05 1.25 e 1.11

40 43 37 39 38 32

0.9 1.3 1.5 1.8 2.0 2.4 e 2.6

59 43 37 31 28 23

2.7 3.2 4.0 4.0 4.4 5.5 5.9 e

8.2 6.9 5.6 5.6 5.1 4.0 3.3

36

21

100

M. Walavalkar et al. / Atmospheric Environment 67 (2013) 93e100

formaldehyde also appears to be small. The major products are long chain chloroketones, chloroalcohols, enols and enones. Some of these alcohols and ketones, with lower vapour pressure, are identified as products under smog chamber conditions and are known to form aerosols or undergo heterogeneous reactions at the aerosol surfaces, under atmospheric conditions. The experimental evidence for the reaction pathways of alkenyl radicals, obtained in this study are important in the ambient tropospheric conditions also, because the same radicals are generated by reactions of OH, the dominant pathway of degradation of these alkenes in ambient conditions. 5. Conclusion The rate coefficients of reactions of Cl atoms with 1-hexene, 1heptene, 1-octene and 1-nonene are measured at 298 K. There is a systematic increase in the rate coefficients with increasing number of carbon atoms in 1-alkenes. The results emphasise the importance of Cl atom reactions in the conditions of marine boundary layer and other similar situations with significant concentration of Cl atoms. The contribution of Cl atom reaction towards degradation is equal to that of OH reaction in the case of 1nonene. The consequences of these reactions are the formation of chloroalkyl radicals as well as the increase in the role of abstraction reaction, which lead to generation of alkenyl radicals centered at different carbon atoms. However, this does not lead to any increase in the yield of small aldehydes in the NOx free conditions employed in the present study, as compared to that in the OH reactions. The major products are chloroketones and chloroalcohols and a variety of enols and enones. The products support the formation of alkenyl radicals centered at the allyl position in all the alkenes. Among the different possibilities for dissociation of alkenyl or corresponding alkoxy radical, a preference for the one leading to an allyl radical is indicated. A complete understanding of the reaction mechanism requires identification of all the products and quantification. This may be more important with respect to reactions of alkenyl radicals, which are also formed by OH reactions, the major degradation pathway of alkenes under tropospheric conditions. Acknowledgements The authors are thankful to Dr. S.K. Sarkar, Head, Radiation and Photochemistry Division and Dr. T. Mukherjee, Director, Chemistry Group, Bhabha Atomic Research Centre, for their constant support and encouragement during this work. References Aschmann, S.M., Atkinson, R., 2011. Effect of Structure on the rate constants for reaction of NO3 radicals with a series of linear and branched C5eC7 1-alkenes at 296
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