Reactivity of E-butenedial with the major atmospheric oxidants

Atmospheric Environment 70 (2013) 351e360

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Reactivity of E-butenedial with the major atmospheric oxidants Pilar Martín a, b, *, Beatriz Cabañas a, b, Inmaculada Colmenar a, María Sagrario Salgado a, b, Florentina Villanueva a, c, Araceli Tapia a a

Instituto de Tecnologías Química y Medioambiental (ITQUIMA), Laboratorio de Contaminación Atmosférica, Universidad de Castilla La Mancha, Avda Camilo José Cela s/n, 13071 Ciudad Real, Spain b Departamento de Química Física, Facultad de Ciencias Químicas, Universidad de Castilla La Mancha, Avda Camilo José Cela 10, 13071 Ciudad Real, Spain c Parque Científico y Tecnológico de Albacete, Paseo de la Innovación 1, 02006 Albacete, Spain

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

< The degradation reactions of Ebutenedial with OH and NO3 radicals and Cl atoms were investigated. < For the reaction of Cl and NO3, these are the first data for rate coefficients. < The results show that the chemical structure of the organic compound is the decisive factor for NO3 reactions.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 September 2012 Received in revised form 17 January 2013 Accepted 21 January 2013

The degradation reactions of E-butenedial with OH and NO3 radicals and Cl atoms were investigated using a relative rate method. The experiments were carried out at w298
1 K and an atmospheric pressure of N2 or synthetic air as the bath gas. Three different sampling/detection methods have been used for the study with Cl, OH and NO3: (1) Solid-Phase Microextraction and Gas Chromatography with Flame Ionization Detection (SPME/GC-FID), (2) ‘in situ’ with long-path Fourier Transform Infrared Spectroscopy (FTIR), and (3) Tenax solid adsorbent and Gas Chromatography with Mass Spectrometry (Tenax/GCeMS) as the detection system. The measured rate coefficients for E-butenedial (cm3 molecule1 s1) are as follows: (1.35
0.29)  1010 for the Cl atom, (3.45
0.34)  1011 for the OH radical and (1.70
0.83)  1015 for the NO3 radical. For the reaction of Cl and NO3 these are the first rate coefficient data to be reported and in the case of OH the literature value is confirmed. This study confirms that the chemical structure of the organic substances does not influence on the reactivity with Cl, has a significant effect for OH reactions and is very important for NO3 reactions. Calculated atmospheric lifetimes are in the order of days for Cl and NO3 reactions and hours for OH. In the case of Cl atoms, a lifetime of 20 h is estimated in the early morning hours in urban coastal air. These shorter lifetimes imply that the degradation reactions of E-butenedial are of great importance because their reaction products are secondary pollutants that are involved in processes such as the formation of photochemical smog or peroxyacyl nitrates (PANs). Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Butenedial Kinetic study Atmospheric oxidants Relative method Atmospheric implications

1. Introduction * Corresponding author. Universidad de Castilla La Mancha, Facultad de Ciencias y Tecnologías Químicas, Departamento de Química Física Edf. Marie Curie, Avda Camilo José Cela s/n, 13071 Ciudad Real, Spain. Tel.: þ34 926295300; fax: þ34 926295318. E-mail address: [email protected] (P. Martín). 1352-2310/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2013.01.041

In the troposphere, aromatic hydrocarbons react with hydroxyl radicals (OH) by addition to the aromatic ring or by H-atom abstraction from the CeH bonds of the alkyl substituent(s). Under atmospheric conditions, OH radical addition dominates for toluene,

352

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

xylenes and trimethylbenzenes and leads to the formation of phenols, 1,2-dicarbonyls and a series of unsaturated 1,4-dicarbonyls (Atkinson, 2000) such as butenedial. Butenedial (OHCCH]CHCHO) has been identified as a primary product from the reaction of furan with the main atmospheric oxidants; chlorine (Villanueva et al., 2007) with molar yields of 11
2% and 1.6
4% for the E- and Z-butenedial isomers, respectively, OH (Bierbach et al., 1995; Gómez-Álvarez et al., 2009) where butenedial isomers have been quantified as the major products with a total estimated molar yield of >70% independently of the presence of NOx, and NO3 with an averaged formation molar yield of 77% for Z-butenedial (Berndt et al., 1997). Butenedial has also been detected after the OH photo-oxidation of reactive aromatic compounds such as toluene and ortho-xylene with molar yields of 11e32% and 10e29%, respectively (Arey et al., 2009). Very few experimental studies have been carried out on the chemistry of butenedial. The OH-butenedial reaction rate coefficients were measured by Bierbach et al. (1994) using the relative rate technique, with values of (5.2
0.1)  1011 for Z-butenedial and (2.4
0.8)  1011 cm3 molecule1 s1, which represents a lower limit, for E-butenedial. In this study the photolysis rate coefficient was determined for the butenedial isomers and it was concluded that, although the OH reaction was an important sink for butenedial, photolysis would be an even a stronger sink. Another study concerned the photolysis of butenedial at different wavelengths. It was concluded that, although the absorption crosssections of butenedial are small in the actinic UV region, the observed products (acrolein and 3H-furan-2one) may play an important role in tropospheric chemistry (Tang and Zhu, 2005). Liu et al. (1999) studied O3-initiated photo-oxidation of butenedial in a smog chamber and reported an O3-butenedial reaction rate coefficient of (1.6
0.1)  1018 cm3 molecule1 s1. In the absence of literature rate coefficient data for NO3- and Clbutenedial reactions and only a limit value for the reaction between the OH radical and E-butenedial, we determined laboratory rate coefficients for the degradation processes of E-butenedial with OH, NO3 and Cl atoms in order to contribute to the development of gas phase atmospheric chemical models and to better understand air pollution in urban and regional environments. 2. Materials and method Rate coefficients were determined for the gas phase reaction of E-butenedial with the atmospheric oxidants Cl atoms and OH and NO3 radicals at (298
1) K and approximately 1 atm of total pressure in N2/synthetic air. The values were obtained using a relative rate method in which the disappearance rate of E-butenedial was determined relative to that of a reference compound, the rate coefficient of which is reliably known, thereby allowing the absolute rate coefficient of E-butenedial to be determined. It was assumed that E-butenedial and the reference compound were removed only by reaction with OH and NO3 radicals or Cl atoms according to 1 and 2:

X þ E  butenedial ðSÞ/products ðkS Þ

(1)

X þ Reference compound ðRÞ/products ðkR Þ

(2)

where X is Cl, OH or NO3. As described in detail elsewhere (Atkinson et al., 1982) the decay of E-butenedial from [S]0 at time t ¼ 0 to [S]t at time t and the simultaneous loss of the reference compound from an initial concentration of [R]0 to [R]t at time t is given by Eq. (I):

 

½S0 k ½R0 ¼ S ln ln ½St kR ½Rt

(I)

Thus, a plot of {ln ([S]0/[S]t)} versus {ln ([R]0/[R]t)} should be a straight line passing through the origin. The slope of this line gives the ratio of rate coefficients kS/kR. The absolute rate coefficient, kS, can therefore be obtained if the rate coefficient kR of the reference compound is known. The reactions studied were measured relative to the reaction of different reference standards. Control experiments and literature studies (Bierbach et al., 1994; Thüner et al., 2003) showed that Z/E-butenedial is also subject to a photolysis process:

    E  butenedial S þ hy/products kph

(3)

Thus, when VIS radiation is used in the kinetic experiments, the decay of E-butenedial is governed by Reactions 1 and 3, thus making it necessary to correct the observed second-order rate coefficient in order to obtain the real second-order rate coefficient (kS). In this case, the decay of E-butenedial from [S]0 at time t ¼ 0 to [S]t at time t and the simultaneous loss of the reference compound from an initial concentration of [R]0 to [R]t at time t is given by Eq. (II):



  ½S0 k ½R0 ln  kph t ¼ S ln ½St kR ½Rt

(II)

where kph is the photolytic rate coefficient of E-butenedial and t is the reaction time, which is coincident with the photolysis time. This kph is determined by measuring the decay of E-butenedial at different times and plotting the data according to the first order equation.


½S0 ¼ kph t ln ½St

(III)

With a known kph, a plot of {ln([E-butenedial]0/[E-butenedial]t) e kpht} versus {ln([Reference]0/[Reference]t)} gives kS/kR as the slope of the straight line according to Eq. (II). Eq. (II) was used to determine the rate coefficient (kS) where the photolytic process could be important. Different experimental systems were used depending on the reaction studied and the systems available in the laboratory. Experimental conditions used for kinetic experiments and the reference compounds are given in Table 1. A detailed description of the experimental systems can be found elsewhere (Cabañas et al., 2005, 2008). 2.1. Cl reactions Two analytical systems were used: Gas Chromatography with Flame Ionization Detection (GC-FID) and Fourier Transform Infrared Spectroscopy (FTIR). For GC-FID experiments a Teflon-coated bag (Adtech) with a volume of 150 L was used as a reaction chamber. The bag was placed inside a rectangular cage with six VIS lamps (lmax ¼ 360 nm) and four UV lamps (lmax ¼ 254 nm) mounted on the walls. The concentrations of E-butenedial and the reference compounds were monitored by Gas Chromatography with Flame Ionization Detection (GC-FID, Hewlett Packard 5890 series II) and using a capillary column (30 m  0.32 mm  1 mm, Tracsil TRB-1701, Teknokroma) of intermediate polarity. Solid Phase Micro Extraction (SPME) was used as the sampling method. This sampling technique has been used in kinetic studies in which the reactants have a low vapour pressure (Cabañas et al., 2008). The fibre, a polymer mixture of divinylbenzene, carboxen and poly(dimethylsiloxane) [(DVB/CAR/ PDMS), 50/30 mm SPME, Supelco], was exposed to the gas mixture for 30 min, which is the shortest time required for E-butenedial to

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

353

Table 1 Experimental conditions for the reaction of Cl, OH and NO3 with E-butenedial. Radical Cl

e

OHf NO3

Reference compound (R) a

2-Methylfuran Cyclohexanea 2-Methylfuranb Cyclohexaneb 2-Methylpropeneb E-2-buteneb Acroleinc Propanalc

[E-butenedial]d (ppm)

[R]d (ppm)

Conversion E-butenedial (%)

Conversion R (%)

Total photolysis time (s)

8e16 8e14 10e13 10e13 10e13 10e13 1e2 1e2

9e18 11e22 10e13 10e13 16e32 11e32 3 3

39e63 34e58 40e66 57e68 60e64 57e69 54e58 61e74

73e90 66e84 40e82 42e65 43e46 48e65 60e62 13e29

50e90 600e1080 840e1020 1320e1440 1500 1500 e e

Method: aSPME/GC-FID; bFTIR; cTenax/GCeMS. d A number of injections of the unreacted mixture were carried out in order to determine the precision associated with the sampling method and to use in the error analysis (Brauers and Finlayson-Pitts, 1997). The standard deviations (s) of these replicate injections were typically about 2e4% for E-butenedial, 3% for acrolein, propanal and cyclohexane and 10% for 2-methylfuran. e Concentration of Cl atom precursors; [SOCl2] ¼ 20 ppm and [Cl2] ¼ 10 ppm. f Concentration of OH radical precursor; [CH3ONO] ¼ 10 ppm [NO] ¼ 10 ppm.

reach equilibrium. The fibre was then inserted into the GC injector for thermal desorption (10 min at 270  C) followed by chromatographic separation. Chlorine atoms were generated from two precursors, (i) by the photolysis of thionyl chloride (SOCl2) at a wavelength of 254 nm and (ii) by the photolysis of Cl2 at a wavelength of 360 nm, in order to verify the results obtained. In previous experiments it was found that furan derivatives react with Cl2 in the dark (Cabañas et al., 2005) and, as a result, SOCl2 and Cl2 were used in the presence of 2-methylfuran and cyclohexane as reference compounds, respectively. Photolysis was typically carried out in steps of 10 s using SOCl2 and 120 s using Cl2, after which the lamps were turned off and the reaction mixture was sampled. In each experiment between five and nine photolysis sampling steps were carried out. The ‘in situ’ FTIR experiments were carried out using a cylindrical 50 L pyrex glass reactor. The reactants were injected into the reaction chamber using a vacuum line. A pumping system consisting of a rotary pump (Varian DS 302) was used to evacuate the reactor after every experiment. The photolysis system consisted of eight actinic fluorescent lamps (lmax ¼ 360 nm) spaced evenly around the reaction chamber. The chamber was equipped with a White-type multiple-reflection mirror system (Saturn Series Multi-Pass cells) with a base length of 1.35 m for sensitive in situ long path absorption monitoring of reactants and products in the IR spectral range 4000e650 cm1. The White system can operate at 72 traverses, giving a total optical path length of 200 m. The IR spectra were recorded with a resolution of 1 cm1 using a Thermo Nicolet 5700 FT-IR spectrophotometer equipped with a KBr beam splitter and a liquid nitrogen-cooled mercuryecadmiumetelluride (MCT) detector. In these experiments, Cl atoms were generated by continuous photolysis of Cl2. The total photolysis time was 14e24 min. Typically, for each spectrum 60 scans were co-added over 1 min and approximately 30 such spectra were recorded per experiment. The reactants were monitored at the following absorption frequencies (in cml): 1724 for E-butenedial, 2861 for cyclohexane and 792 for 2-methylfuran. Dark reactions for E-butenedial or the reference compound were found to be negligible for both the SPME/GC-FID and FTIR experiments.

described elsewhere (Taylor et al., 1980). The reactants were monitored at the following absorption frequencies (in cml): 1724 for E-butenedial, 962 for trans-2-butene and 889 for 2methylpropene. 2.3. NO3 reactions For the study of the reaction of E-butenedial with NO3 radicals, a GCeMS system was used due to overlap of the absorption bands of E-butenedial and the reference compounds in the FTIR system. NO3 radicals were produced by the thermal decomposition of N2O5 (Atkinson and Aschmann, 1984):

N2 O5 þ M 4NO2 þ NO3 þ M

(4)

and N2O5 was synthesized according to the method described by Schott and Davidson (1958). In this case a Teflon bag with a volume of 500 L was used in order to minimize the dilution effect of the additions of N2O5. Experiments were performed with N2 as the carrier gas. Samples of 100 cm3 were collected from the reaction chamber onto Tenax TA solid adsorbent with a gas syringe and were later desorbed in two thermal desorption stages at 250  C (PerkineElmer Turbomatrix 100). During the second thermal desorption the sample was injected into the GCeMS (Shimazu 17A QP5050) system where the analytes were separated on a capillary column and detected by mass spectrometry. Sampling was carried out after the addition of different quantities of N2O5 (1e7 ppm) and at subsequent intervals of 17 min. A total of ten additions were typically made in each experiment, leading to a depletion of 54e74% for E-butenedial. 2.4. Photolysis of E-butenedial A set of experiments was conducted to establish the photolytic loss of E-butenedial. The experiments to measure the photolysis rate coefficient (kph) were carried out in the same experimental system as the reactions of the Cl and OH radicals and under the same experimental conditions in order to use the kph in Eq. (II) and to correct the decay of E-butenedial concentration in the kinetic experiments where the photolytic process was important.

2.2. OH reactions

2.5. Chemicals

The experiments were carried out using the ‘in situ’ FTIR system described above. Hydroxyl radicals were generated by continuous photolysis of CH3ONO (Atkinson et al., 1982) in the presence of NO in air at a wavelength of 360 nm using eight VIS lamps. Methyl nitrite (CH3ONO) was previously generated in the laboratory as

The chemicals used in this study and their sources and purities were as follows: E-Butenedial was synthesized by a literature procedure (Frederico et al., 2003; Liu, 1999). The butenedial synthesized seems to be mainly E-butenedial, as evidenced by the GC chromatogram and mass and FTIR spectra (Figs A1eA3 respectively

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

in supplementary information). N2 (99.999%, Praxair), synthetic air (99.999%, Praxair), Cl2 (99%, Praxair) and NO (99%, Air Liquid). Organics: cyclohexane (99%, Aldrich), thionyl chloride SOCl2 (99%, Aldrich), 2-methylfuran (99%, Aldrich), 2-methylpropene (99%, Aldrich), trans-2-butene (99%, Aldrich), acrolein (90%, Aldrich) and propanal (97%, Aldrich). 3. Results

0.5 0.45 0.4

ln([S]0/[S]t])-kpht

354

0.35 0.3 0.25 0.2 0.15 0.1

3.1. Photolysis of E-butenedial

0.05 0 0

0.2

0.4

0.6

0.8

1

ln([R]0/[R]t) Fig. 2. Relative rate plot according to Eq. II for reaction of E-butenedial with Cl atoms using the FTIR system with cyclohexane as a reference compound.

0.45 0.4

ln([S]0 /[S] t )-k ph t

The results obtained for the experiments carried out in a 150 L Teflon bag and using radiation of 254 nm show that under these experimental conditions there is no photolytic loss of E-butenedial, probably due to the low irradiation time (steps of 10 s as explained above). In the experiments with VIS radiation values for kph were obtained from the slope of the plot ln[S]0/[S]t versus time in accordance with Eq. (III) and these were (3.6
0.03)  104 s1 and (4.8
0.1)  104 s1 in the 50 L reactor using N2 and synthetic air, respectively. The different kph values obtained when N2 or synthetic air were used could be due to the secondary reactions initiated by OH radicals, which could be formed in the air system (Kaiser and Wallington, 1996). The value of (3.6
0.03)  104 s1 was used in Eq. (II) because this corresponds only to the photolytic process. Processes of photoisomerization are not observed in our reaction systems.

0.35 0.3 0.25 0.2 2-methylpropene

0.15

3.2. Cl, OH and NO3 reactions

trans-2-butene

0.1

The reaction of Cl atoms with E-butenedial was studied using cyclohexane and 2-methylfuran as reference compounds in two experimental systems. Plots of the data obtained for the reactions of E-butenedial with Cl atoms and 2-methylfuran and cyclohexane as references, in accordance with Eqs. (I) and (II) are shown in Figs. 1 and 2 for SPME/GC-FID and FTIR, respectively. The relative plots for the reactions between OH and E-butenedial using 2methylpropene and trans-2-butene as reference compounds are shown in Fig. 3. The Y data in Figs. 2 and 3 were corrected taking into account the kph of E-butenedial. In the case of the NO3 radical, the relative plots according to Eq. (I) for the reaction of NO3 and Ebutenedial with acrolein and propanal are shown in Fig. 4. In all cases a good straight-line is observed, although in Fig. 4 the associated error is somewhat larger than in the study with Cl atoms or OH radicals due to the difficulties involved in studying slow reactions (1015 cm3 molecule1 s1).

0.05 0 0

0.2

0.4

0.6

0.8

ln([R]0 /[R]t) Fig. 3. Relative rate plot according to Eq. II for reaction of E-butenedial with OH radicals using trans-2-butene and 2-methylpropene as reference compounds.

The ratios of rate coefficients kS/kR for all experiments obtained by least-squares analyses are given in Table 2. These rate coefficient ratios are expressed on an absolute basis using the rate coefficient kR for the reference compounds and these values are also included in Table 2. In the case of Cl reactions, good agreement can be seen between the data obtained with the two experimental systems. The error limit for the ratio of rate coefficients kS/kR obtained by leastsquares analyses includes only the precision of the fit to our

1.2

0.7

0.8

0.6 Acrolein

0.5 ln([S0]/[S]t)

ln([S]0/[S]t)

0.8

0.4

0.4

Propanal

0.3 0.2 0.1

0 0

0.4

0.8

1.2

1.6

2

2.4

2.8

ln([R]0/[R]t) Fig. 1. Relative rate plot according to Eq. I for reaction of E-butenedial with Cl atoms using the SPME/GC-FID system with 2-methylfuran as a reference compound.

0 0

0.5

1 1.5 ln([R0]/[Rt])

2

2.5

Fig. 4. Relative rate plot according to Eq. I for reaction of E-butenedial with NO3 radicals and acrolein and propanal as reference compounds.

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

experimental data (
2s). The kS error limit, skS, was calculated taking into account the error limit from the slopes obtained from the regression analysis and the quoted error in the value of the rate coefficient for the reference compound. A weighted mean of all rate coefficient data for E-butenedial with Cl atoms and OH and NO3 radicals has been calculated and the values are shown in Table 2. The rate coefficients for the reaction with Cl atoms and NO3 radicals are the first such values to be reported. 4. Discussion The results obtained for the reaction of E-butenedial with Cl atoms and OH and NO3 radicals show intercepts at zero, within the error limit, in the relative rate plots. This fact, together with the straight lines obtained, indicates that there is no interference from other competitive reactions in the systems (apart from the photolytic process that was included in the case of Cl atoms and OH radicals). The rate coefficients found for Cl, OH and NO3 are given in Table 3 together with results from previous studies for the sake of comparison. Previous data for the Cl and NO3 rate coefficients determined here were not found, and thus a comparison could not be made. In the case of OH, our results can be compared with the literature value for the lower limit rate coefficient and these results are in reasonable agreement. It can be seen from the results in Table 3 that the rate coefficients for E-butenedial with the different oxidants are in the order kCl > kOH > kNO3 . 4.1. Reactivity vs. structure It is well established that Cl, OH and NO3 react with organic compounds by two basic mechanisms: abstraction of a hydrogen atom or addition to a C]C double bond (Atkinson and Aschmann, 1984; Finlayson-Pitts and Pitts, 2000). However, E-butendial has different functional groups (a double bond and aldehydic groups) so the initial attack by the oxidant could be addition to the double bond and/or abstraction of a hydrogen, and in the case of the abstraction process this could be abstraction of an aldehydic hydrogen and/or an alkylic hydrogen.

355

In order to enable a qualitative discussion of how the reactive elements (i.e. aldehydic hydrogen atom with the associated carbonyl group, other hydrogen atoms and double bonds) affect the reactivity of the aldehydes, Ullerstam et al. (2001) considered the situation based on a comparison of their aldehydes studied with ‘model’ compounds that contain only one or two of the reactive elements. The same approach can be applied in our work on E-butenedial. In our case other ‘model’ compounds are included in addition to those discussed by Ullerstam et al. (2001). The ‘model’ compounds considered are listed in Table 3 together with E-butenedial. 4.1.1. Cl atoms Ullerstam et al. (2001) observed that the rate coefficient for unsaturated aldehydes in the reaction with the main oxidants is lower than that for the alkene. This behaviour is due to the deactivating effect of the carbonyl group, which reduces the reactivity of the double bond. Similar behaviour was observed in our work for the reactions of E-butenedial with OH and NO3 radicals, but in the case of Cl atoms is not clear that the reactivity is influenced by the presence of double bond or carbonyl group, because as shown in Table 3, the rate coefficient of E-butenedial is lower than that for butane and butanal. This observation, together with a rate coefficient close to the gas kinetic limited value from gas collision theory (k in the order of 1010 cm3 molecule1 s1) and the slight differences between the rate coefficients [k in the range of (1.2e 4)  1010 cm3 molecule1 s1] for organic compounds with different structures, confirms that in the reaction with Cl atoms the reaction probability is high regardless of collision site and that the chemical structure plays a limited role in determining the reaction rate coefficient (Blanco et al., 2010; Cabañas et al., 2005; Ullerstam et al., 2001; Wang et al., 2002). The rate coefficient can be estimated using the structuree reactivity relationships (SAR approach) proposed by Blanco et al. (2010) according to the equation:

kcal ¼ kðbasicÞ

i ¼Y 41

ðCxiÞ

(IV)

i¼1

Table 2 Rate coefficients for the reaction of E-butenedial with the main atmospheric oxidants at atmospheric pressure of N2 gas and room temperature. Radical

d

kS/kR(
2s)e

k298K(
2s)

k298K(
2s)f

k298K(
2s)f

a

g

0.41
0.04 0.43
0.04 0.42
0.01 0.31
0.01 0.65
0.01 0.63
0.01 0.63
0.01 0.55
0.01 0.55
0.01 0.54
0.01 1.10
0.09 1.22
0.05 0.98
0.06 0.23
0.02 0.21
0.02 0.36
0.01

1.70 1.32 1.3 1.27 3.33 3.24 3.25 3.49 3.52 3.46 2.76 3.07 2.46 1.40 1.23 2.17



















1.47
0.24

1.35
0.29

Cl

b

OH

Reference

2-Methylfuran g Cyclohexane h Cyclohexane h 2-Methylfuran h 2-Methylpropene

h

c

NO3

E-2-butene

i

Acrolein

i

Propanal

0.19 0.15 0.12 0.13 0.34 0.32 0.33 0.18 0.16 0.16 0.99 1.02 0.84 0.37 0.36 0.45

1.29
0.18 3.27
0.38

3.45
0.34

3.49
0.19

2.72
1.08

1.70
0.83

1.52
0.45

k is given in 1  1010, 1  1011 and 1015 cm3 molecule1 s1, respectively. Reference rate coefficients. For Cl atoms: 2-methylfuran (4.1
0.2)  1010 cm3 molecule1 s1 (Cabañas et al., 2005); cyclohexane (3.07
0.12)  1010 cm3 molecule1 s1 (Aschmann and Atkinson, 1995). For OH radicals: 2-methylpropene (5.14
0.25)  1011 cm3 molecule1 s1 (Atkinson and Arey, 2003); E-2-butene (6.39
0.13)  1011 cm3 molecule1 s1 (Atkinson, 1997). For NO3 radicals: acrolein (2.5
0.4)  1015 cm3 molecule1 s1 (Cabañas et al., 2001a); propanal (6.0
0.6)  1015 cm3 molecule1 s1 (Cabañas et al., 2001). e Indicated error as two least-squares standard deviations. f Weighted average. g SPME/GC-FID. h FTIR. i thermal desorption and GCeMS.

a,b,c d

356

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

Table 3 Rate coefficients (cm3 molecule1 s1) for the reaction of E-butenedial with Cl atoms, OH and NO3 radicals at room temperature and atmospheric pressure and photolytic rate coefficient kph(s1). Some structurally related compounds are also included. Organic compound

Structure

E-butenedial

HOC

COH

Butane

Trans-2-butene

COH

Butanal

COH

Propanal

COH

Crotonaldehyde

b,bDimethylacrolein

COH

COH

Trans-2-hexenal

kOH (1011)

1.35
0.29a

3.45
0.34a 2.41b

1.70
0.83a

2.14
0.15e 2.05
0.06j

0.23
0.01e

0.046
0.002j

3
0.6f 4
0.5e

6.39
0.13g

390
0.27g

1.5
0.3f 1.4
0.4i

2.4
0.10j

11
1.5j

1.2
0.2f

2.0
0.10j

6.4
0.2j

f

COH

Acrolein

kCl (1010)

E,E-2,4-hexadienedial b c d e f g h i j k l m n o p q r s

HOC

2.0
0.2

2.2
0.4f 3.2
0.9m

4
0.3f 3.3
0.59e

2.48
0.71n

6.21
0.18e

1.9
0.22e

4.68
0.5e

12.1
0.43e

18.6
4.8e

16,000
2990s

7.6
0.8q

5.34
0.12p

COH

kO3 ð1018 Þ

kph (104)

(1.6
0.1)c

3.6
0.03a (3.9
0.3)b 10d

238
15.2h 190r

1.1
0.2f 2.5
0.4l

1.8
0.3 2.5
0.7e

E,E-2,4-hexadiene

a

j

kNO3 ð1015 Þ

<0.01k

6
0.8f 5.11
0.15e 16
1.91

0.12
0.02k

1.82
0.26e 1o

10p

9.56
0.25p

This work. (Bierbach et al., 1994). (Liu et al., 1999). (Thüner et al., 2003). (NIST, 2012). (Ullerstam et al., 2001). (Atkinson, 1997). (Wegener et al., 2007). (Cuevas et al., 2006). (Atkinson et al., 2006). (Magneron et al., 2002). (Cabañas et al., 2001a). (Wang et al., 2002). (Blanco et al., 2010). (O’Connor et al., 2006). (Klotz et al., 1995). (Klotz et al., 1999). (Ziemann and Atkinson, 2012). (Ellermann et al., 1992).

where k(basic) is the experimental rate coefficient for the ‘model’ alkene and Cxi is the group reactivity factor, which reflects the type of substituents, R, attached to the double bond. In the case of E-butenedial, k(basic) corresponds to the rate coefficient of (E/Z)-RHC ¼ CHR from (E/Z)-2-butene with kbasic ¼ 3.5  1010 cm3 molecule1 s1 (average value for data from Table 3). As E-butenedial has two eC(O)H groups, the value of Cxi is 0.87 for each eC(O)H (Blanco et al., 2010). This SAR estimation method provides a rate coefficient for the reaction of Cl with E-butenedial of 2.65  1010 cm3 molecule1 s1, which is around a factor of 2 higher than the value determined experimentally in this work (1.35  1010 cm3 molecule1 s1) and demonstrates that the method predicts the reactivity of a double

bond adjacent to the carbonyl group of the aldehyde group within
50% of the measured values for E-butenedial. Additionally, the rate coefficients obtained can be compared with values calculated using the recently reported correlation between the reactivity of unsaturated compounds towards Cl atoms and the HOMO of the unsaturated compounds (Blanco et al., 2010). The correlation obtained by Blanco et al. (2010) is represented as follows:

      lnkCl cm3 molecule1 s1 ¼ 0:1
0:1 EHOMO  21:2
2:9 (V)

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

The HOMO energy for E-butenedial was calculated using the GAUSSIAN 03 package (Frisch et al., 2001). The geometry optimizations and initial values of energies were obtained at the Hartreee Fock (HF) level, and an ab initio Hamiltonian with a 6-31 G (d,p) basis set was employed. The self-consistent field energies were subsequently calculated by MollerePlesset perturbation theory (MP2-SCF) with an ab initio Hamiltonian with a 6-311þþ G (d,p) basis set. EHOMO values of 11.401 eV (see Table A1 in supplementary information) were used to obtain the following rate coefficient: k ¼ 1.98  1010 cm3 molecule1 s1. This value is in good agreement with the reaction rate coefficient measured in this study (1.35  1010 cm3 molecule1 s1). Both methods are based on a mechanism in which the first step is radical addition to the double bond. 4.1.2. OH radicals When the rate coefficient values for E-butenedial are compared with those of the ‘model’ compounds in Table 3, it can be seen that the values fall between those of the alkene and saturated aldehyde compounds (trans-2-butene and butanal, respectively). This finding indicates a deactivating effect of the aldehydic groups to the addition process on the double bond. Such an effect is also observed if the rate coefficients for crotonaldehyde and b,b-dimethylacrolein are compared with that of E-butenedial (see Table 3), where the only difference between them is that crotonaldehyde and b,bdimethylacrolein have CH3 groups with a positive inductive effect (þI) versus the aldehydic groups in E-butenedial with a negative inductive effect (I). Moreover, the presence of two aldehydic groups in E-butenedial could also increase its rate coefficient due to the abstractions of the two aldehydic hydrogens, meaning that the I effect of the aldehydic groups is compensated and the rate coefficients for E-butenedial and crotonaldehyde must be similar. The effect of aldehydic H abstraction on the rate coefficient can be observed by comparing acrolein (with only one aldehydic hydrogen) with E-butenedial, where the presence of a second aldehydic group increases the rate coefficient. It can be seen from the data in Table 3 that the rate coefficient for E-butenedial obtained in this work is lower than the rate coefficient for OH reaction with E,E-2,4-hexandienedial. This finding is expected because the latter compound has one double bond more than E-butenedial. Calculations carried out using the SAR approach (Kwok and Atkinson, 1995) with the AOPWIM program (EPA, 2000) give a value of 4.1  1011 cm3 molecule1 s1 for the rate coefficient of E-butenedial. This value is consistent with that measured in this work and with the literature data (Bierbach et al., 1994). The rate coefficient obtained in this work for OH can also be compared with the value calculated using the reported correlation between the reactivity of unsaturated compounds towards OH atoms and the HOMO of the unsaturated compounds. The correlation obtained by Pfrang et al. (2006) is as follows:

  lnkOH cm3 molecule1 s1 ¼ 1:21EHOMO  12:34

(VI)

In this case, the calculated rate coefficient of 4.46  1012 cm3 molecule1 s1 is not in agreement with the rate coefficient of the reaction measured in this study (3.45  1011 cm3 molecule1 s1). This discrepancy could indicate that for the reaction of OH with E-butenedial, the addition of an OH radical to the double bond is not the sole pathway. The result obtained by Bierbach et al. (1994) in a study of the reaction products of E/Zbutenedial with OH, indicates that slightly less than 50% of the reaction probably proceeds by H atom abstraction from the aldehyde functional groups of butenedial. On the other hand, an SAR calculation carried out using the AOPWIM program gave a rate

357

coefficient of 33.75  1012 cm3 molecule1 s1 for the abstraction process and 7.39  1012 cm3 molecule1 s1 for the addition process. This result implies a contribution of 82% for the abstraction process to the overall rate coefficient. 4.1.3. NO3 radicals It can be observed (Table 3) that the rate coefficient for the reaction of NO3 radicals with E-butenedial is around four orders of magnitude smaller than the coefficients for OH. It can also be seen that the rate coefficient for E-butenedial is higher than that of its alkane (butane), in accordance with an increase in reactivity due to the abstraction of the aldehydic hydrogen versus the alkylic hydrogen or to the presence of double bond. On the other hand, the rate coefficient is lower than that of its alkene (trans-2-butene) and saturated aldehydes (butanal), a finding that indicates a deactivating effect of the aldehydic groups to the addition process on the double bond and a deactivating effect of the double bond on the aldehydic hydrogen abstraction process. All results indicate that the chemical structure of an organic compound has a significant influence during the attack by the NO3 radical, but more experiments about reaction products must be done in order to confirm it (Ullerstam et al., 2001). Predictions of rate coefficients for the NO3 radical are difficult and there are considerable inaccuracies associated with the estimation methods. Kerdouci et al. (2010) developed a new SAR for which the parametrization is exclusively based on experimental rate coefficients for the NO3 oxidation of VOCs. However, there is no literature information on aldehydic compounds. Another SAR, based on a correlation between the HOMO energy (EHOMO) and the rate coefficient, was proposed by Pfrang et al. (2006, 2007, 2008). The calculated values of EHOMO and this correlation were used to predict the rate coefficients for reactions of the NO3 radical with organic compounds selected to allow quantification of the effect of functional groups on reactivity. For the alkenes, 61% of the rate coefficients estimated by this SAR agree within a factor of two with experimental data, whereas rate coefficients for oxygenated VOCs are reproduced to within a factor of 3e5. In this work we estimated the rate coefficient for the reaction of NO3 and E-butenedial according to an SAR method developed by Pfrang et al. (2007), in which the rate coefficient, kSAR, can be estimated for substituted unsaturated compounds using Eq. IV, as explained above for the Cl reaction. In the case of the E-butenedial and NO3 reaction, the kbasic is 4.74  1013 cm3 molecule1 s1 (Pfrang et al., 2008). However, the f(eCHO) is not tabulated and this factor must therefore be calculated. For this purpose, two methyl groups (labelled R in Table A1, supplementary information) in the basic structures are substituted by aldehydic groups to generate 8 simple unsaturated dialdehydes. The resulting unsaturated dialdehyde species are listed in Table A1 in the supplementary information. The structures of these molecules were optimised for geometry and the HOMO energies were calculated using the GAUSSIAN 03 package (Frisch et al., 2001). With the HOMO energies and the correlation obtained by Canosa-Mas et al. (2005) for oxygenated compounds:

    ln k=cm3 molecule1 s1 ¼ 6:37
1:46 EHOMO   þ 32:70
14:94

(VII)

the values of the rate coefficients for the reaction of NO3 with the compounds are estimated. These estimated rate coefficients, kEst, were then compared with the values predicted for the basic structures, kbasic. A plot of kEst as a function of kbasic gave a line and the slope corresponds to f(eCHO)2 (Fig. A4, supplementary

358

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360 Table 4 Rate coefficients and atmospheric lifetimes determined for the reactions of Ebutenedial with the atmospheric oxidants OH and NO3 radicals, Cl atoms and ozone.

a

-12

log (kNO3/cm3 molecule-1s-1)

-12.5 -13

b

-13.5

2

-14 -14.5

1

3

Oxidant

k (cm3 molecule1 s1)

s

Cl OH NO3 O3

(1.35
0.29)  1010 (3.45
0.34)  1011 (1.70
0.83)  1015 (1.6
0.1)a  1018

9 days 8h 13 days 10 days

[Cl] ¼ 1  104 atoms cm3 (Wingenter et al., 1996); [OH] ¼ 1  106 radicals cm3 (Spivakovsky et al., 2000); [NO3] ¼ 5  108 radicals cm3 (Shu and Atkinson, 1995), [O3] ¼ 7.4  1011 molecules cm3 (Logan, 1985). a (Liu et al., 1999).

4

-15 -15.5

s¼

-16 -12

-11.5

-11

-10.5

-10

-9.5

-9

log (kOH/cm3 molecule-1 s-1) Fig. 5. Linear free energy plot of logðkNO3 Þ vs. log(kOH). a, Regression line for 16 addition reactions; b, regression line for 12 abstraction reactions. Closed squares represent, 1, acrolein, 2, crotonaldehyde, 3, E,E-2,4-hexadiendial and closed triangle 4, E-butenedial.

information). The factor obtained is 2  104. This factor and the kbasic were used to estimate a rate coefficient using Eq. (IV). The value obtained was 9.57  1017 cm3 molecule1 s1 and this is almost two orders of magnitude lower than the experimental rate coefficient obtained in this work. Once again this discrepancy indicates that different mechanisms, other than the addition to the double bond, must occur in the reaction of NO3 with the aldehydic compound. 4.2. Linear correlations It is known that a series of organic compounds react in the same way with different atmospheric oxidants and several correlations have been proposed in order to predict rate coefficients (Atkinson, 1994; Wayne, 2000). These correlations can also be used to obtain information about the first stage of the mechanism by which the reaction proceeds. In order to obtain more information about the reactivity of E-butenedial with NO3 radicals, two linear correlations of logðkNO3 Þ and log(kOH) were plotted in Fig. 5. With this aim in mind, literature values for the rate coefficients for the reactions of OH and NO3 radicals with alkanes and unsaturated organic compound were used (Atkinson and Arey, 2003; Klotz et al., 1995, 1999; Magneron et al., 2002; Ullerstam et al., 2001). The relevant data are listed in Table A2 in the supplementary information. The data for E,E-2,4-hexadiendial and E-butenedial are also plotted in the figure (square number 3 and triangle number 4, respectively) together with those for other unsaturated aldehydes such as acrolein (square 1) and crotonaldehyde (square 2). The data for E,E-2,4hexadiendial, E-butenedial and acrolein fall near to the abstraction line (line b) but crotonaldehyde falls between the two lines. This fact indicates that an ordinary abstraction of the aldehydic hydrogen atom takes place in the reaction of NO3 radical and Ebutenedial to a significant extent and this is in agreement with the result obtained from the other correlations explained above. 4.3. Atmospheric implications The rate coefficients summarized in Table 3 can be used to calculate the atmospheric lifetimes of E-butenedial due to reactions with Cl, OH, NO3 and O3. The atmospheric lifetimes can be calculated using the expression:

1 kx ½X

(VIII)

with X ¼ OH, Cl, NO3 or O3, where kx is the rate coefficient for the reaction of the oxidant X with E-butenedial, and [X] is the typical atmospheric concentration of the oxidant. The estimated tropospheric lifetimes at room temperature of the unsaturated compounds with typical oxidants are presented in Table 4. The degradation reactions of E-butenedial with the main tropospheric oxidants are of the order of days e except for the OH radical, which has a lifetime of 8 h. Therefore, the main sink for Ebutenedial degradation initiated by atmospheric oxidants is the reaction with the OH radical. However, in coastal areas and in the marine boundary layer, where peak concentrations of Cl atoms as high as 1  105 atoms cm3 can occur (Spicer et al., 1998), lifetimes calculated with this concentration are ten times lower (20 h), indicating that Cl atom-initiated degradation of this compound can be an important homogeneous loss process. Photolytic loss of the E-butenedial may also be important since it absorbs actinic radiation in the range 270e340 nm (Tang and Zhu, 2005). A photolysis rate coefficient of 3.4  104 s1 or 1  103 s1 is measured when radiation at 254 nm (Bierbach et al., 1994) or solar radiation (Thüner et al., 2003) is used, respectively. The photolysis rate coefficients obtained for butenedial in all cases are very high, even more than for other aldehydes such as acrolein or trans-crotonaldehyde (Magneron et al., 2002; O’Connor et al., 2006) (see Table 3). If a photolysis rate coefficient of 1  103 s1 is taken to estimate the lifetime, a value of 0.3 h is obtained. This calculation indicates that the photolysis reaction will be the major atmospheric sink for E-butenedial. These lifetimes are in the range of hours, implying that E-butenedial is likely to be removed in the gas phase close to their sources. These shorter lifetimes imply that the degradation reactions of E-butenedial are of great importance because their reaction products are secondary pollutants that are involved in processes such as the formation of photochemical smog or peroxyacyl nitrates (PANs) (Wayne, 2000). 5. Conclusions The work reported here is the first kinetic study carried out to date on the reactions of E-butenedial with Cl and NO3 radicals. This study confirms that the chemical structure of the organic substances does not influence on the reactivity with Cl, has a significant effect for OH reactions and is crucial for NO3 reactions. The presence of aldehydic carbonyl groups in an unsaturated compound leads to a decrease in the reactivity of a conjugated double bond in the reaction with radicals, especially with OH and NO3 radicals. The reactions of unsaturated aldehydes with NO3 have rate coefficients that are lower than those of the corresponding aliphatic aldehyde, indicating that the reactivity of the aldehydic hydrogen atom is affected by the double bond. The latter effect is

P. Martín et al. / Atmospheric Environment 70 (2013) 351e360

not clear on comparing acrolein and E-butenedial. The plot of logðkNO3 Þ versus log kOH for E-butenedial, together with the result obtained by SAR methods in the case of OH radical, shows an important contribution to the mechanism of direct aldehydic hydrogen abstraction. Atmospheric lifetimes of 8 h for OH reactions and w10 days for the rest of the oxidants were determined and this lifetime is longer than the lifetime obtained if a direct photolysis process is considered. As a result, it can be concluded that the main sink for Ebutenedial is the photolysis process and this could have significant atmospheric implications on a regional scale. Acknowledgements Inmaculada Colmenar González thanks the Junta de Comunidades de Castilla La Mancha for a personal grant. Financial support was provided by the Consejería de Ciencia y Tecnología of Junta de Comunidades de Castilla-La Mancha and Ministerio de Ciencia e Innovación for projects PII1I09-0202-3992 and ENE200767529CO2-02, respectively. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2013.01.041. References Arey, J., Obermeyer, G., Aschamann, S.M., Chattopadhyay, S., Cusick, R.D., Atkinson, R., 2009. Dicarbonyl products of the OH radical-initiated reaction of a series of aromatic hydrocarbons. Environmental Science and Technology 43, 683e689. Aschmann, S.M., Atkinson, R., 1995. Rate coefficient for the gas-phase reaction of alkanes with Cl atoms at 296
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