Tropospheric Reaction of OH with Selected Linear Ketones - American

Environ. Sci. Technol. 2005, 39, 814-820

Tropospheric Reaction of OH with Selected Linear Ketones: Kinetic Studies between 228 and 405 K E L E N A J I M EÄ N E Z , B E R N A B EÄ B A L L E S T E R O S , E R N E S T O M A R T IÄ N E Z , A N D J O S EÄ A L B A L A D E J O * Departamento de Quı´mica Fı´sica, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Avenida Camilo Jose´ Cela 10, 13071 Ciudad Real, Spain

The absolute rate coefficients for the tropospheric reactions of hydroxyl radical (OH) with a series of linear aliphatic ketones (2-butanone (k1), 2-pentanone (k2), 2-hexanone (k3), and 2-heptanone (k4)) were measured as a function of temperature (228-405 K) and pressure (45600 Torr of He) by the pulsed laser photolysis/laser induced fluorescence technique. These studies are essential to model the atmospheric chemistry of these ketones and their impact in the air quality. No pressure dependence of the rate coefficients was observed in the range studied. Thus, ki(298 K) (×10-12 cm3 molecule-1 s-1) were averaged over the pressure range studied yielding the following: (1.04 ( 0.74), (3.14 ( 0.40), (6.37 ( 1.40), and (8.22 ( 1.10), for 2-butanone (k1), 2-pentanone (k2), 2-hexanone (k3), and 2-heptanone (k4), respectively. k1 exhibits a slightly positive temperature dependence over the temperature range studied. A conventional Arrhenius expression describes the observed behavior. In contrast, the temperature dependence of k2-k4 shows a distinct deviation from the Arrhenius behavior. The best fit to our data was found to be described by the three-parameter expression: k(T) ) A + B exp(-C/T) in cm3 molecule-1 s-1. This work constitutes the first determination of the temperature dependence of k2-k4. Our results are compared with previous studies, when possible, and are discussed in terms of the H-abstraction by OH radicals. The atmospheric implications of these reactions are also discussed.

Introduction Carbonyl compounds are emitted into the atmosphere from a variety of natural and anthropogenic sources and play an important role in the formation of tropospheric ozone in urban and regional areas. These compounds are also produced in the atmosphere during the oxidation of hydrocarbons such as alkanes and alkenes. Carbonyl compounds, such as 2-butanone, 2-pentanone, and 2-heptanone, are present in the environment from natural sources as a consequence of the emissions from grass, clover, and some plant species (Spartium junceaum) (1). These ketones are also formed as a waste product resulting from industrial activities (in producing paints and other coatings, glues, cleaning, and flavoring agents, wood pulp, etc.) (2). The reaction with atmospheric radicals, such as OH, NO3, and Cl atoms, is expected to be the main loss process for * Corresponding author phone: 34 926 29 53 27; e-mail: [email protected]. 814

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these ketones in the troposphere. The atmospheric reaction of OH radicals with ketones plays an important role and determines the degradation of chemical pollutants and the formation of tropospheric ozone. Thus, the knowledge of the rate coefficients of the OH-initiated reactions of ketones is needed to better determine their effect on the troposphere. Therefore, an experimental kinetic study of the reactions of OH radicals with 2-butanone, 2-pentanone, 2-hexanone, and 2-heptanone has been carried out in this work.

OH + CH3C(O)C2H5 f Products k1

(1)


(2)


(3)


(4)

The rate coefficient measurements (ki, i ) 1-4) have been performed as a function of total pressure (45-600 Torr of He) and temperature (228-405 K) by using the pulsed laser photolysis/laser induced fluorescence (PLP-LIF) technique. Kinetics on the reaction of OH with 2-butanone (reaction 1) has been previously studied at room temperature by different authors using both relative (3-5) and absolute methods (6, 7). Also, k1 has been determined as a function of temperature by Wallington and Kurylo (6) using flash photolysis/resonance fluorescence (FP/RF) and by Le Calve´ et al. (7) using PLP-LIF. These authors observed a very slight temperature dependence of k1. As far as we know, the rate coefficient for the reactions of OH radicals with 2-pentanone, 2-hexanone, and 2-heptanone has only been studied at room temperature (6, 8, 9). The values of k2(298 K) measured by Wallington and Kurylo (6) and Atkinson et al. (8, 9) are in good agreement; however, there are two previous discrepant determinations of the rate coefficient for the reaction of OH with 2-heptanone and 2-hexanone (reactions 3 and 4). Thus, further room-temperature studies need to be performed to elucidate this discrepancy. Also a temperature dependence study of k2-k4 is necessary due to the lack of data at the temperatures of the upper troposphere. Because these ketones contain primary and secondary H atoms, and the temperature dependence of the rate coefficient changes with the nature of the abstracted H atom, the Arrhenius plots are likely to be curved. However, the degree of curvature to be expected is unknown. Therefore, this work constitutes the first determination of the temperature dependence of the rate coefficients for the reaction of OH with 2-pentanone, 2-hexanone, and 2-heptanone. These studies are essential to model the atmospheric chemistry of these ketones and their impact in the air quality. In this paper, a detailed investigation performed using a PLP-LIF experimental setup is presented for the kinetics of the reaction of OH radicals with some linear ketones. Our results are compared, when possible, with previous studies. The temperature dependence of ki will be discussed in terms of the H-abstraction by OH radicals.

Experimental Section The experimental setup employed in this work was the same as in our previous works on OH (10, 11) and other radicals (12-14); therefore, only a brief description is given here. Apparatus. The reaction vessel consisted of a Pyrex jacketed cell with an internal volume of ca. 300 cm3. The reactor was heated or cooled by circulating a fluid from a 10.1021/es049333c CCC: $30.25

 2005 American Chemical Society Published on Web 12/31/2004

thermostated bath through its jacket. At low temperatures, the quartz windows were externally purged with a small flow of He to avoid condensation of the room water vapor. Orthogonal side ports on the reactor were used to introduce the laser beams (perpendicular photolysis and probe beams), mount the photomultiplier detector, and allow the gas sample flow through the reactor. OH radicals were produced by the excimer laser photolysis of H2O2 at 248 nm (KrF). The photolysis laser energy, which was measured with a power meter at the exit of the LIF cell, was varied over the 0.2-4 mJ pulse-1 range. OH radicals were detected by pulsed laser induced fluorescence with excitation at 282 nm (A2Σ+, ν′ ) 1 r X2Π, ν′′ ) 0) using the doubled output of a Nd:YAG-pumped dye-laser. The fluorescence signal was detected with a photomultiplier tube (PMT) oriented perpendicular to the excitation beam. A bandpass filter (309 nm peak transmission; fwhm band-pass of 10 nm) mounted in front of the PMT was used to isolate the OH fluorescence signal. The OH temporal profiles were recorded as a function of the delay time between the photolysis and the probe laser pulses. The signal was averaged for between 20 and 100 times to increase the signal-to-noise ratio. An upper limit for the H2O2 concentration ([H2O2] < (0.46.1) × 1014 molecule cm-3) was obtained from the analysis of the OH temporal profiles recorded in the absence of reactant using the method described in Albaladejo et al. (10). The calculated [OH]0 ranged from 2 × 1010 molecule cm-3 to 9 × 1011 molecule cm-3. It was usually kept below 9 × 1011 molecule cm-3 to minimize unwanted complications from secondary radical-radical reactions. Absorption cross sections of 2-butanone and 2-pentanone were determined from 200 to 350 nm by two different groups (15, 16). At 248 nm the absorption cross sections for 2-butanone (2.168 × 10-20 cm2 from Martinez et al. (15) and 2.136 × 10-20 cm2 from Yuing et al. (16)) and 2-pentanone (2 × 10-20 cm2 from Martinez et al.; 15) are small. Thus, photolysis of ketones is negligible at 248 nm. As far as we know, the UV spectrum of 2-hexanone and 2-heptanone has not been measured, but absorption cross sections are expected to be small at 248 nm (≈10-20 cm2). The measurement of the ketone concentration was made using the calibrated mass flow, the total pressure, and temperature at the reaction cell. These concentrations were varied from 1 × 1014 molecule cm-3 to 2.3 × 1015 molecule cm-3. Total gas flow rates were measured using calibrated mass flow transducers and ranged from 250 to 500 sccm (standard cubic centimeter per minute). The linear gas flow velocity in the kinetic reaction cell ranged between 1 and 10 cm s-1, which was adequate to avoid accumulation of photoproducts between photolysis laser shots, 5 or 10 Hz. Pressures were measured using 100- and 1000-Torr capacitance manometers. The temperature of the gas in the reaction zone was measured with a thermocouple inserted directly in the gas flow ((1 K). Procedure. Kinetic measurements of the rate coefficient for reactions (1-4) were made under pseudo-first-order conditions in the OH radical, over the temperature and pressure ranges 228 to 388 K and 45 to 600 Torr (He bath gas), respectively. In the presence of a linear aliphatic ketone, the OH loss is attributed to reactions (1-4), ki, reaction with the precursor, H2O2, (k5), and also diffusion out of the detection zone (k6):

OH + H2O2 f HO2 + H2O OH f Loss

k6

k5

(5) (6)

The loss of OH in the absence of reactant is only attributed to reactions 5 and 6. Thus, the OH temporal profile followed

a simple exponential rate law

SOHt ) SOH0 exp(-k′t)

(7)

where SOHt and SOH0 represents the LIF signals at a time t, and a t ) 0, respectively. The LIF signal is directly proportional to the concentration of OH at each time. k′ is the pseudofirst-order rate coefficient obtained in the presence of ketone.

k′ ) ki[Ketone] + k0

(8)

k0 () k5[H2O2] + k6) is the first-order rate coefficient for the loss of OH in the absence of reactant. Both k′ and k0 were obtained from the nonlinear square fitting to the experimental OH profiles as a function of the reaction time. Values of k0 ranged from 54 to 874 s-1, depending on H2O2 concentration and temperature. k′ was measured at various ketone concentrations mantaining constant temperature and total pressure. The procedure was repeated for each pressure and temperature (see Tables 1-4). ki(T) was, then, obtained from the slope of the plot of k′-k0 versus [Ketone]. A null intercept was observed for all cases, indicating that there was no secondary chemistry interfering. Plots of eq 8 yielded intercepts in excellent agreement with the values of k0 measured in the absence of reactant. Figure 1a shows several examples of these plots for all the linear ketones studied at room temperature. In Figure 1b, a plot of k′2-k0 versus [2-pentanone] is shown for two temperatures, 253 and 388 K. Tables 1-4 list the absolute OH-rate coefficients obtained in this work as a function of temperature and total pressure for each ketone. No significant pressure dependence of the rate coefficients ki was found at any temperature and over the pressure range studied. Reagents. He (99.999%) used as supplied without further purification. Concentrated hydrogen peroxide (>90%) was prepared by bubbling He through a H2O2 sample initially at 50% concentration for several days prior to use. The H2O2 purity was determined by titration with a standard solution of KMnO4. During the kinetic measurements, the liquid solution of H2O2 was constantly bubbled in order to introduce the gas-phase H2O2 into the reaction cell. The ketones used in this investigation (2-butanone (99%), 2-pentanone (99.5%), 2-hexanone (98%), and 2-heptanone (99%) from Aldrich) were degassed by repeated freeze-pump-thaw cycles.

Results and Discussion This section is divided into two subsections for ease of presentation. The first part shows the results obtained at room temperature, and the second subsection is dedicated to the temperature dependence studies of the reactions of OH with C4-C7 linear ketones. Room-Temperature Measurements. A summary of the rate coefficients ki obtained in our laboratory from an absolute method is presented in Table 5. As can be seen, the effect of the length of the aliphatic chain on the rate coefficient ki is visible at 298 K and other temperatures (see also Tables 1-4). The OH reactivity increases with the number of C atoms in the aliphatic chain, indicating that the reaction is dominated by the attack of OH to the methylene (-CH2-) site. This trend is in excellent agreement with the structureand-reactivity, SAR, estimations of the rate coefficients ki. The SAR rate coefficients were calculated at room temperature using the parameters kprim, ksec, F(-CH3), F(-CH2-), F(-CH2C(O)R), and F(CO) given by Kwok and Atkinson (17). The calculated results, followed by the measured experimental values in parentheses, were 1.33 (1.04), 4.77 (3.14), 6.7 (6.37), and 8.2 (8.22) × 10-12 cm3 molecule-1 s-1 for 2-butanone, 2-pentanone, 2-hexanone, and 2-heptanone, respectively. VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Summary of the Measured Absolute Rate Coefficientsa for 2-Butanone as a Function of Temperature number [2-butanone] of runs (1014 molecule cm-3)

T (K) 228 238 248 253 273 298 313 318 343 363 388

30 10 10 20 50 60 30 10 30 10 20

0.45-6.40 0.44-4.74 0.42-4.55 0.36-5.77 0.37-5.33 0.44-9.70 0.33-4.63 0.39-4.40 0.37-4.22 0.35-3.82 0.33-3.72

range of k1′ (s-1)

k1 × 1012 (cm3 molecule-1 s-1)

115-730 105-445 159-560 88-652 119-538 134-1152 122-517 102-550 138-551 146-545 155-500

0.99 ( 0.04 0.80 ( 0.09 1.00 ( 0.06 0.91 ( 0.09 0.98 ( 0.04 1.04 ( 0.07b 0.93 ( 0.11 1.10 ( 0.08 0.98 ( 0.11 1.20 ( 0.12 1.09 ( 0.40

a Uncertainties are (2σ of the precision of the fit. Total pressure 100 Torr of He. b Average of k1 between 100 and 600 Torr.

TABLE 2. Summary of the Measured Absolute Rate Coefficientsa for the Reaction of OH with 2-Pentanone as a Function of Temperature range of p T (K) (Torr) 248 253 257 263 268 273 283 288 298 308 323 334 343 354 373 388 a

100 100-280 140 130-140 45-130 100-140 45-100 280 45-400 140-280 140 165 310 100-280 45-140 100-400

number [2-pentanone] of (1014 molecule runs cm-3) 10 70 40 20 40 40 40 20 120 30 20 20 20 40 40 50

0.34-1.60 0.88-23.0 0.24-3.40 0.64-4.10 0.97-6.40 0.28-4.00 0.43-6.00 1.10-6.00 0.28-7.80 1.20-6.10 0.90-4.50 1.00-4.50 3.10-10.0 1.20-9.10 0.70-8.50 0.44-17.0

range of k′2 (s-1)


430-1200 1087-5890 438-1974 721-2220 734-3125 680-2011 788-2864 795-2670 526-3367 457-2054 381-1500 484-1440 948-2880 508-2267 489-2294 425-4660

6.14 ( 0.16 5.94 ( 0.56 4.95 ( 0.18 4.65 ( 0.36 4.64 ( 0.14 4.26 ( 0.32 3.99 ( 0.66 3.59 ( 0.10 3.14 ( 0.40 2.98 ( 0.38 2.90 ( 0.02 2.72 ( 0.01 2.71 ( 0.04 2.49 ( 0.20 2.56 ( 0.08 2.64 ( 0.06

TABLE 4. Temperature Dependence of the Absolute Rate Coefficients for the OH + 2-Heptanone Reaction between 260 and 405 K T (K)

p (Torr)

260 263 267 273 283 287 298 313 323 333 343 354 374 405

80 50 80 100 50 80-100 50-100 100-115 150 100-150 115 100-125 50-125 170

a

number [2-heptanone] of (1014 molecule cm-3) runs 8 8 9 8 7 17 27 16 9 18 9 18 22 9

0.42-2.10 0.11-1.00 0.20-2.00 0.17-1.70 0.15-1.00 0.13-2.00 0.10-2.80 0.80-4.40 0.54-2.70 0.43-2.10 0.84-4.10 0.43-2.10 0.51-3.60 0.48-2.30

range of k4′ (s-1)


925-2481 803-1700 600-2167 523-1737 776-1566 1569-1984 509-2768 1348-3800 860-2450 974-2144 1012-3260 595-1863 548-2880 702-1986

10.2 ( 0.60 9.46 ( 0.36 9.25 ( 0.28 8.53 ( 0.76 8.62 ( 0.46 8.20 ( 0.08 8.22 ( 1.10 7.54 ( 0.42 7.55 ( 0.40 7.55 ( 0.26 7.05 ( 0.12 6.97 ( 0.50 7.19 ( 0.44 7.17 ( 0.02

Uncertainties are (2σ of the precision of the fit.


TABLE 3. Summary of the Absolute Rate Coefficientsa for the OH + 2-Hexanone Reaction as a Function of Temperature range of p T (K) (Torr) 263 273 279 284 286 288 298 313 316 328 349 353 370 380 390 405 a

100-200 100-200 80 80-142 100-200 100 50-600 100 100 100 80-200 100-200 165 80-200 100 100-200

number [2-hexanone] of (1014 molecule runs cm-3) 17 26 10 27 16 10 27 7 9 18 17 14 4 18 9 18

range of k′3 (s-1)

0.40-5.40 1067-5200 0.23-2.90 597-2656 0.35-3.40 573-2816 0.20-5.50 747-3651 0.43-2.10 482-1323 0.30-3.00 200-2000 0.50-4.70 630-3500 0.74-5.70 486-3535 0.12-6.20 1067-3852 0.85-5.30 880-3600 0.30-8.50 444-4200 0.65-6.13 406-2982 11.0-18.0 7300-9160 0.68-7.80 550-4120 0.60-5.50 880-3810 0.66-5.70 477-2800

k3 × 1012 (cm3 molecule-1 s-1) 10.2 ( 1.20 7.84 ( 0.84 7.13 ( 0.87 7.34 ( 0.94 6.41 ( 0.20 6.30 ( 0.42 6.37 ( 1.40 5.97 ( 0.16 5.63 ( 0.10 5.32 ( 0.76 4.62 ( 0.46 4.84 ( 0.52 4.70 ( 0.20 4.61 ( 1.40 5.05 ( 0.26 4.50 ( 0.64


We have calculated the contribution of the reactivity of the aliphatic chain (R) to the global rate coefficient ki(298 K). The reactivity of the hydrocarbon chain in nonsymmetric ketones (CH3(CO)R) was calculated by subtracting the contribution of the R-CH3 adjacent to the carbonyl group. 816

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FIGURE 1. (a) Pseudo-first-order plots at room temperature for each ketone. (b) Plot of k′2-k0 versus [2-pentanone] at 253 K (full symbols) and 388 K (empty symbols). The rate coefficients obtained from the slopes are (2.66 ( 0.03) × 10-12 and (5.97 ( 0.08) × 10-12 cm3 molecule-1 s-1, respectively. The reactivity of the R-CH3 group is assumed to be one-half of the rate coefficient of the OH + acetone reaction. The more recently reported value of k(OH + acetone) by Gierzcak et al. (18) (1.77 × 10-13 cm3 molecule-1 s-1 at 298 K) was used to calculate the reactivity of the R-CH3 group (8.85 × 10-14 cm3 molecule-1 s-1). The calculated contributions were 91%

TABLE 5. Comparison of Room Temperature Rate Coefficients Obtained in This Work for the Reaction of OH Radicals with Linear Aliphatic Ketones with Literature Valuesa ketone

pT (Torr)

ki × 1012 (cm3 molecule-1 s-1)

techniqueb

reference

2-butanone

100-600 100-300 25-50 760 760 760 45-400 25-50 740 760 735 50-600 25-50 760 735 50-170 740 25-50

1.04 ( 0.74 1.19 ( 0.18 1.15 ( 0.10 0.88 ( 0.09 2.61 3.3 ( 0.9 3.14 ( 0.40 4.00 ( 0.30 4.56 ( 0.30 4.98 ( 0.25 4.74 ( 0.14 6.37 ( 1.40 6.64 ( 0.56 8.92 ( 0.44 9.16 ( 0.61 8.22 ( 1.10 11.7 ( 1.10 8.67 ( 0.84

PLP-LIF PLP-LIF FP-RF RR-GC RR-GC RR-GC/FID PLP-LIF FP-RF RR-GC RR-GC RR-GC PLP-LIF FP-RF RR-GC RR-GC PLP-LIF RR-GC FP-RF

this work Le Calve´ et al. (7) Wallington and Kurylo (6) Cox et al. (5) Cox et al. (4) Winer et al. (3) this work Wallington and Kurylo (6) Atkinson et al. (9) Atkinson and Aschmann (8) Atkinson et al. (20) this work Wallington and Kurylo (6) Atkinson and Aschmann (8) Atkinson et al. (20) this work Atkinson et al. (9) Wallington and Kurylo (6)

2-pentanone

2-hexanone

2-heptanone

a Uncertainties stated by the authors; b LP-LIF, laser photolysis/laser induced fluorescence; RR-FTIR, relative rate/Fourier transform infrared spectroscopy; RR-GC/FID, relative rate/gas chromatography and flame ionization detection; RR-GC, relative rate/gas chromatography; FP-RF, flash photolysis/resonance fluorescence.

for the ethyl group in 2-butanone (k(C2H5) ) 9.5 × 10-13 cm3 molecule-1 s-1), 97% for the propyl group in 2-pentanone (k(C3H7) ) 3.05 × 10-12 cm3 molecule-1 s-1), 98% for C4H9 in 2-hexanone (k(C4H9) ) 6.3 × 10-12 cm3 molecule-1 s-1), and 99% for C5H11 in 2-heptanone (k(C5H11) ) 8.13 × 10-12 cm3 molecule-1 s-1). These results indicate that the H-atom abstraction is taking place mainly in the aliphatic chain (not in the CH3 group). The rate coefficients for the OH-reactivity of the aliphatic chain R is in excellent agreement with those calculated by Wallington and Kurylo (6) and Le Calve´ et al. (7). Comparison with Previous Studies. Table 5 also shows a comparison of the room-temperature rate coefficients obtained in this work with literature values obtained by relative methods at atmospheric pressure (3-5, 8, 9) and by absolute techniques at lower pressures (6, 7). The first value for the rate coefficient of the reaction of OH with 2-butanone, k1, was reported by Winer et al. (3) employing isobutene as a reference compound (k ) 5.06 × 10-12 cm3 molecule-1 s-1). As can be seen in Table 5, their k1 value is much greater than the rest. If the current recommended rate coefficient for the OH + isobutene reaction at 298 K (k ) 6.1 × 10-12 cm3 molecule-1 s-1)19 is used, the revised k1 is (2.73 ( 0.75) × 10-12 cm3 molecule-1 s-1. This value is, then, closer to our value, within the error limits given. Cox et al. (4, 5) reported two discrepant k1 values (see Table 5). Their revised rate coefficient was k1 ) (8.8 ( 0.09)× 10-13 cm3 molecule-1 s-1. Again, if the current recommendation of the rate coefficient for the reaction of OH with the reference compound employed by these authors (ethylene, k ) 8.52 × 10-12 cm3 molecule-1 s-1) (19) is used the obtained k1 ((9.37 ( 0.09) × 10-13 cm3 molecule-1 s-1) is in good agreement with our and previous absolute values (6, 7) but not with that reported by Winer et al. Cox et al. (5) stated that the difference between their value and that of Winer et al. could arise from a product interference in their GC analysis. However, they did not find any evidence of that product. On the other hand, our absolute value of k1 ((1.04 ( 0.74) × 10-12 cm3 molecule-1 s-1) is in excellent agreement with those obtained by Wallington and Kurylo (6) and Le Calve´ et al. (7) using photolysis techniques. The room-temperaturerate rate coefficients for the reaction of OH with 2-pentanone, k2(298 K), measured by relative methods are 45-58% higher than our absolute value. Also,

the absolute k2(298 K) reported by Wallington and Kurylo (6) is 27% higher than that we report in this paper. No definite reason was found to explain that difference. Once again, the discrepancy between the rate coefficients obtained by absolute and relative methods is seen for the OH + 2-hexanone reaction (Table 5). The absolute rate coefficients measured in this work are in good agreement with those obtained by using the flash photolysis resonance fluorescence technique (6). Atkinson and Aschmann (8) tried to explain the discrepancy observed between their measured k2(298 K) and k3(298 K) and those measured by Wallington and Kurylo (6), suggesting as a cause a possible wall loss of the ketones in the relative system. These authors reexamined k2(298 K) and k3(298 K) using a bigger chamber and different reference compound (cyclohexane), and the obtained rate coefficients were in agreement with those previously reported (20). Therefore, Atkinson et al. (8) concluded that the difference could be attributed to systematic errors in one of the techniques (or both). Our measured rate coefficient for the OH + 2-heptanone, k4(298 K), is in excellent agreement with that previously measured by Wallington and Kurylo (6). However, k4(298 K) measured by Atkinson et al. (9), using a relative method, is more than 40% higher and is not within the experimental uncertainties of our measurements. Thus, it is clearly seen that there is a significant difference between the rate coefficients measured by relative and absolute methods. That difference decreases when the length of the aliphatic chain increases (from ca. 200% to ca. 40% to for reaction 1 and 4, respectively). The origin of the discrepancy observed between both methods is still unclear. Temperature Dependence of ki. The study of the temperature dependence of ki has been carried out by PLP-LIF. This work constitutes the first temperature dependence study of the OH kinetics with 2-pentanone, 2-hexanone, and 2-heptanone. The temperature range employed ranged between 228 and 405 K. The observed behavior of ki with temperature was different for 2-butanone and the higher ketones. The rate coefficients of OH + 2-butanone reaction exhibit a slightly positive temperature dependence (i.e., k1 increases when T increases), which is in agreement with the observations of Wallington and Kurylo (6) and Le Calve´ et al. (7) (see Figure 2a). The data were fitted to the Arrhenius equation (k(T) ) A exp(-Ea/RT)) yielding the following expression: VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) Temperature dependence of k1 between 228 and 388 K and comparison with previous studies. (b) Non-Arrhenius behavior of k2 between 248 and 388 K. (c) Non-Arrhenius behavior of k3 between 263 and 405 K. (d) Non-Arrhenius behavior of k4 between 260 and 405 K. Arrhenius expression (dashed line), k(T) ) AT 2 exp(-Ea/RT) (thin solid line) and k(T) ) A + B exp(-C/T) (thick solid line). Error bars represent ( 1σ.

TABLE 6. Temperature Dependence of the Rate Coefficients Obtained in This Work for 2-Butanone Together with Literature Valuesa T (K)

A × 1012 (cm3 molecule-1 s-1)

228-388 243-372c 240-440b

1.35 ( 0.35 1.51 ( 0.29 2.30 ( 1.10

Ea/R (K)

techniqueb

78 ( 52 PLP-LIF 60 ( 61 PLP-LIF 170 ( 120 FP-LIF

reference this work Le Calve´ et al. (7) Wallington and Kurylo (6)

a Uncertainties stated by the authors. For our data, the uncertainties are the combination of statistical and systematic errors; b LP-LIF, laser photolysis/laser induced fluorescence and FP-RF, flash photolysis/ resonance fluorescence.

k1(T) ) (1.35 ( 0.35) ×

10-12 exp(-(78 ( 52)/T) in cm3 molecule-1 s-1

Uncertainties are twice the standard deviation of combined statistical errors of the fit and estimated systematic errors. The estimated systematic uncertainties are (( 2σ): ( 2% in total pressure, ( 2% in flow rate, and less than 1% in temperature. The uncertainties in the ketone concentration in the stock mixture also contribute to the systematic errors. Thus, the overall uncertainty in the concentrations of the ketones in the reaction cell is estimated to be ( 5%. In Figure 2a, a representation of k1 (in logarithmic scale) versus 1/T is shown. In this figure, our results are compared 818

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with the only two kinetic studies on the OH + 2-butanone reaction reported (6, 7). In both studies a positive, but small, activation energy was measured. The magnitude of this activation energy differs between those studies. Wallington and Kurylo (6) reported an activation energy twice that of Le Calve´ et al. (7) (see Table 6). Like in the study of Le Calve´ et al. (7), the rate coefficients measured in this work were slightly dependent on temperature, and one can consider that the activation energy is, within the error limits, negligible. In contrast, a negative temperature dependence was observed for k2, k3, and k4, more pronounced below 298 K. The rate coefficients for reactions 2-4 also showed a nonArrhenius behavior, i.e. a curvature in the plot of ln k versus 1/T was observed. Panels b-d of Figure 2 show the temperature dependence observed for k2-k4 in the range studied. In these figures, different types of fits are included for comparison. Evidently, the Arrhenius expression (dashed lines) did not reproduce the experimental data over the temperature range studied. When the temperature dependence of k2-k4 was fitted to an expression of the type ki(T) ) AT 2 exp(-Ea/RT) (thin solid lines):

k2(T) ) 6.74 × 10-19 T 2 exp(1215/T) cm3 molecule-1 s-1 k3(T) ) 2.13 × 10-18 T 2 exp(1038/T) cm3 molecule-1 s-1 k4(T) ) 1.54 × 10-18 T 2 exp(1144/T) cm3 molecule-1 s-1 This fit is better that the Arrhenius one but not good enough

to represent the observed trend over the entire range. The best fit (represented by thick solid lines) to our data was found to be described by the expression k(T) ) A + B exp(-C/T) in cm3 molecule-1 s-1. The resulting A, B, and C parameters were obtained from the weighted least-squares fit to the 15-17 averaged rate coefficients which correspond to 40-70 individual measurements. Thus, recommended temperature dependence for the reaction of OH with these three linear ketones are (in cm3 molecule-1 s-1):

k2(T) ) (2.20 ( 0.24) × 10-12 + [(1.73 ( 0.35) × 10-15 exp((1917 ( 224)/T)] k3(T) ) (3.93 ( 0.60) × 10-12 + [(2.40 ( 0.52) × 10-14 exp((1351 ( 216)/T)] k4(T) ) (6.25 ( 0.76) × 10-12 + [(1.70 ( 0.90) × 10-14 exp((1378 ( 278)/T)] Uncertainties are twice the standard deviation of combined statistical errors of the fit and estimated systematic errors. The magnitude of the curvature (given by C values) is similar in the three ketones where a non-Arrhenius behavior is shown. Different reasons can lead to a curvature in the Arrhenius plots: (a) the presence of impurities that rapidly react with OH or (b) reactions 2-4 occur by several pathways with different temperature dependences. Some tests were done to verify if impurities were present in the ketone sample. The FTIR spectrum and the gas chromatography-mass spectometry/flame ionization detection (GC-MS/FID) analysis indicated that no significant amount of impurities was present. Thus, the curvature observed in the conventional Arrhenius plots cannot be attributed to the reactions of OH with impurities. Furthermore, the IR spectrum and the GC-MS/FID analysis were done before and after heating the liquid sample at 100 °C in order to check if possible thermal decomposition of the reactant could affect to the measured rate coefficients at high temperature. No difference was observed, indicating that no decomposition products were formed. In the series of linear ketones studied in this work there are different types of hydrogens (in positions R, β, etc.). If abstraction at each site followed the conventional Arrhenius expression, the overall temperature dependence of the rate coefficients ki would be curved due to the differences in the activation energies for the different channels. The presence of different types of hydrogen atoms (equivalent to different C-H bond strength) in this series of linear ketones leads to a deviation of the conventional Arrhenius expression. For example, the OH radical reactions with 2-pentanone can proceed by four pathways:

OH + CH3C(O)CH2CH2CH3 f CH2C(O)CH2CH2CH3 + H2O (2a) f CH3C(O)CHCH2CH3 + H2O

(2b)

f CH3C(O)CH2CHCH3 + H2O

(2c)

f CH3C(O)CH2CH2CH2 + H2O (2d) Aktinson et al. (9) determined that reaction pathways 2a-d account for 2%, 18%, 76%, and 3.5%, respectively, of the total reaction. The major reaction channel seems to be the H-abstraction from the CH2 group in β-position (reaction 2c). The complete product studies of these authors indicate that the mechanism of the reaction of OH with 2-pentanone is relatively straightforward and involves the reaction of the intermediate first-formed alkoxy radicals with O2 and de-

composition. These authors also reported an end-product study of the OH + 2-heptanone reaction (9). Atkinson et al. concluded that in the 2-heptanone system the first-formed intermediate alkoxy radical also isomerizes (the isomerization reaction potentially dominated in some cases). These authors also reported that the reaction channel predominant for 2-heptanone is H-abstraction from the β-CH2 group. No product studies on the OH reaction with 2-butanone and 2-hexanone have been reported. However, in light of the results of Atkinson et al., it is likely that H-atom abstraction occurs mainly in the methylene group adjacent to the keto group for 2-butanone (since it has no β-CH2 group) and in the β position (for 2-hexanone). Gierzcak et al. (18) reported a positive temperature dependence of the rate coefficients for the reaction of OH + acetone. This reaction also shows a deviation from Arrhenius behavior (curvature at high temperatures, in this case). However, the reaction of OH with 2-butanone has a negligible activation energy (obtained by fitting the data to an Arrhenius expression) and, again, a non-Arrhenius behavior is observed in this work for longer ketones (2pentanone, 2-hexanone, and 2-heptanone), being that the temperature dependence of k2-k4 is more pronounced at lower temperatures. Although we did not observe a pressure dependence of these rate coefficients, the H-abstraction reaction could proceed via an OH-addition complex. Some authors have postulated a six- or seven-membered ring complex which enhances the abstraction of a hydrogen atom from the β-CH2 group (6, 21). More recently, Gierzcak et al. (18) found that the acetonyl radical (CH3COCH2) was the major reaction product (independent of temperature and pressure) of the reaction of OH + acetone and deduced that this reaction channel proceeds via a hydrogen-bounded sixmembered ring complex. So, it seems reasonable that the same mechanism is operating for higher ketones. Tropospheric Implications. Volatile organic compounds are chemically removed from the troposphere, mainly, by its reaction with OH, NO3, O3, and halogen atoms. The reactions of ketones with O3 are extremely slow (an upper limit of 10-21 cm3 molecule-1 s-1 is accepted).19 Assuming an ozone concentration 24-h average of 7 × 1011 molecule cm-3 gives tropospheric lifetimes due to the O3-reaction (defined as 1/k[O3]) of more than 45 years. The kinetics of these ketones with NO3 radicals, the main tropospheric oxidant at nighttime, has not been reported. In comparison with the NO3 reactivity toward acetone, the rate coefficients for the ketones studied in this work would be less than 10-16 cm3 molecule-1 s-1 (19). So, their lifetimes due to the reaction with NO3 would be more than 9 years. The tropospheric lifetimes of the studied ketones due to the OH-reaction (τOH ) 1/ki[OH]) were calculated using an OH concentration (24-h averaged) of 106 radicals cm-3 (22). On the basis of the room-temperature rate coefficients obtained in this work, tropospheric lifetimes for 2-butanone, 2-pentanone, 2-hexanone, and 2-heptanone were estimated to be 11, 4, 2, and 1 days, respectively. In a recent kinetic study on Cl + 2-butanone, 2-pentanone, and 2-hexanone reactions (23), performed in our laboratory, the calculated tropospheric lifetimes at room temperature (considering a global concentration between 103 and 104 atoms cm-3) (24) were more than 35, 28, and 18 days, respectively. However, in marine regions, where a peak value of 105 atoms cm-3 has been observed (25), the Cl reaction can compete with the OH reaction (τCl would be between 2 and 4 days). Thus, the main tropospheric chemical removal of 2-butanone, 2-pentanone, and 2-hexanone seems to be the reaction of OH radicals. VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Acknowledgments The authors thank the Spanish Direccioń General de Ensen ˜ anza Superior e Investigacioń Cientı´fica and the Consejerıá de Ciencia y Tecnologıá (Junta de Comunidades de Castilla-La Mancha) for the supporting this research under projects BQU2001-1574, PAI-02-017, and GC-02-024.

(14)

(15)

Literature Cited (1) Kesselmeier, J.; Staudt, M. Biogenic volatile organic compounds (VOC): An overview on emission, physiology and ecology. J. Atmos. Chem. 1999, 33, 23-88. (2) Toxic Release Inventory Report; Environmental Protection Agency: 2001. (3) Winer, A. M.; Lloyd, A. C.; Darnall, K. R.; Pitts, J. N. Relative rate constants for the reaction of the hydroxyl radical with selected ketones, chloroethenes, and monoterpenes hydrocarbons. J. Phys. Chem. 1976, 80, 1635-1639. (4) Cox, R. A.; Derwent, R. G.; Williams, M. R. Atmospheric photooxidation reactions. Rates, reactivity, and mechanism for reaction of organic compounds with hydroxyl radicals. Environ. Sci. Technol. 1980, 14, 57. (5) Cox, R. A.; Patrick, K. F.; Chant, S. A. Mechanism of atmospheric photooxidation of organic compounds. reactions of alkoxy radicals in oxidation of n-butane and simple ketones. Environ. Sci. Technol. 1981, 15, 587-592. (6) Wallington, T. J.; Kurylo, M. J. Flash photolysis resonance fluorescence investigation of the gas-phase reactions of OH radicals with a series of aliphatic ketones over the temperature range 240-440 K. J. Phys. Chem. 1987, 91, 5050-5054. (7) Le Calve, S.; Hitier, D.; Le Bras, G.; Mellouki, A. Kinetic studies of OH reactions with a series of ketones. J. Phys. Chem. A 1998, 102, 4579-4584. (8) Atkinson, R.; Aschmann, S. M. Flash-photolysis resonance fluorescence investigation of the gas-phase reactions of OH radicals with a series of aliphatic ketones over the temperature range 240-440 K. J. Phys. Chem. 1988, 92, 4008-4008. (9) Atkinson, R.; Tuazon, E. C.; Aschmann, S. M. Atmospheric chemistry of 2-pentanone and 2-heptanone. Environ. Sci. Technol. 2000, 34, 623-631. (10) Albaladejo, J.; Ballesteros, B.; Jimenez, E.; Martin, P.; Martinez, E. A PLP-LIF kinetic study of the atmospheric reactivity of a series of C-4-C-7 saturated and unsaturated aliphatic aldehydes with OH. Atmos. Environ. 2002, 36, 3231-3239. (11) Albaladejo, J.; Ballesteros, B.; Jimenez, E.; Diaz de Mera, Y.; Martinez, E. Gas-phase OH radical-initiated oxidation of the 3-halopropenes studied by PLP-LIF in the temperature range 228-388 K. Atmos. Environ. 2003, 37, 2919-2926. (12) Martinez, E.; Albaladejo, J.; Notario, A.; Jimenez, E. A study of the atmospheric reaction of CH3S with O3 as a function of temperature. Atmos. Environ. 2000, 34, 5295-5302. (13) Martinez, E.; Albaladejo, J.; Jimenez, E.; Notario, A.; de Mera, Y. D. Temperature dependence of the limiting low- and high-

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Received for review April 30, 2004. Revised manuscript received July 22, 2004. Accepted November 10, 2004. ES049333C

Tropospheric Reaction of OH with Selected Linear Ketones - American

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