Kinetics of the Reactions of Hydroxyl Radicals with Aldehydes Studied under Atmospheric Conditions J. Alistair KerP and David W. Sheppard Department of Chemistry, The University of Birmingham, Birmingham B15 2TT, England
of HONO with -0.1 ppm each of NO and NO2 in synthetic air were prepared by acidifying aqueous NaN02 as described by Cox (9).The NO, concentration, consisting of NO NO2 HONO, and the individual component concentrations were determined by chemiluminescent analysis (Thermo-electron Model 12A) as previously described (9). The required amounts of ethylene were added by sweeping measured volumes of the gas into the bag with a stream of nitrogen. The aldehydes were added by syringe injection into a fast-flowing stream of nitrogen passing into the bag. When the bag was filled with all components, it was shaken, and the contents were allowed to mix until the measured concentrations of NO, , ethylene, and aldehyde were constant and reproducible. The mixture in the bag was then photolyzed with 8,12, or 16 fluorescent lamps (Philips L20/05; wavelength range, 300-450 nm) for periods of up to 1 h. During the course of a run, the ethylene and aldehyde concentrations were monitored by analysis of successive samples withdrawn from the bag with a gas-sampling valve. Analysis was by gas chromatography with flame ionization detection (Pye Model 104) and with the columns and conditions listed in Table I.
Mixtures of nitrous acid vapor in synthetic air containing low concentrations of ethylene and an aldehyde have been photolyzed in a 220-L Tedlar plastic bag at 298 K and atmospheric pressure. By observing the rates of consumption of the ethylene and the aldehyde, using gas-chromatographic analysis, we have measured relative rate constants (kG/kg)for the reactions HO RCHO products (6) and HO CpH4 products (5). The following absolute rate constants have been cm3 molecule-l s-l at 298 derived by taking k 5 = 8.0 X K: (kG/lOll cm3 molecule-l s-l) CH3CH0, 1.2 f 0.4; CzHgCHO, 1.8 f 0.1; n-C3H7CHO, 2.4 f 0.1; i-CsHyCHO, 2.7 f 0.5; n-C4H&HO, 2.6 & 0.4; i-C4HgCHO, 2.7 f 0.1; t C4HgCH0, 2.1 f 0.6; CH2=CHCHO, 1.9 f 0.2; CH3CH= CHCHO, 3.3 f 0.6; CGH~CHO, 1.1f 0.2. The results are discussed in relation to literature data on the analogous reactions of O(3P)atoms and CH3 radicals, and the implications of the results in terms of the “smog reactivity” of aldehydes are briefly considered.
+
-
+
+
-
Aldehydes are recognized as playing an important role in the chemistry of the polluted troposphere. They are emitted as primary pollutants from the partial oxidation of hydrocarbon fuels in internal combustion engines, and they arise as secondary pollutants from the atmospheric oxidation of hydrocarbons. A recent conference ( I ) on “Chemical Kinetic Data Needs for Modeling the Lower Troposphere” devoted an entire session to the kinetics and photochemistry of aldehydes. The photooxidations of formaldehyde and acetaldehyde have been studied in smog-chamber experiments (Z),and the data have been computer-simulated on the basis of proposed kinetic mechanisms ( 3 ) .In the case of acetaldehyde, several important steps in the mechanism have been characterized from kinetic studies under atmospheric conditions ( 4 ) . Kinetic and mechanistic data involving higher aldehydes are lacking, although it is generally accepted that the initial step in the photooxidative degradation of organic molecules under atmospheric conditions largely involves attack by hydroxyl radicals. Thus, recent attempts to model smog-chamber data ( 5 )and to model the formation of photochemical air pollution (6) have included reactions involving some of the higher aldehydes, for which it has been assumed that their reactivity toward hydroxyl radicals is similar to that of acetaldehyde and propionaldehyde. As a first step toward understanding the mechanisms of the photooxidation of the higher aldehydes, we report here measurements of the room-temperature rate constants for the reactions of hydroxyl radicals with C4, Cg, and other aldehydes. The reactions have been studied under atmospheric conditions by a competitive technique involving ethylene as a reference compound.
Results Photolysis of nitrous acid vapor is described by the following mechanism:
Environmental Science & Technology
+ hv -,HO + NO HO + HONO H2O + NO2
(2)
HO+NO+M-+HONO+M
(3)
HONO
HO
-
+ NOp + M
+
HONO2
(1)
+M
(4)
This gives rise to a steady-state concentration of hydroxyl radicals which can be determined by measuring the rate of consumption of ethylene added to the mixture. The hydroxyl-ethylene reaction HO
+ C2H4
-
products
(5)
initiates a chain of reaction leading to the formation of two molecules of formaldehyde and regenerating hydroxyl (7,lO). Table 1. Gas-Chromatographic Columns and Conditions column
Experimental Section The experimental system has been employed previously to measure the rate constants for hydroxyl radical reactions with a variety of organic substrates (7) and sulfur compounds (8). The hydroxyl radicals are generated by the photolysis (300-450 nm) if nitrous acid vapor in synthetic air a t 1-atm pressure and ambient temperature (298 f 4 K) in a 220-L heat-sealed Tedlar plastic bag. Mixtures containing -1 ppm 960
+
length/m
2 % PEG 4000/Chromosorb W-HP
1
10 % PEG 200/Chromosorb W-HP
1
10% PEG 200/Chromosorb W-HP
Phasepak Q
1.5
1
temp/K
aldehyde
388
benzaldehyde
353
crotonaldehyde acrolein
313 298
n-butyraldehyde
298 298
isobutyraldehyde n-valeraldehyde
298 298
isovaleraldehyde pivaldehyde
298
acetaldehyde
298
propionaldehyde
298
pivaldehyde
363
acetaldehyde
0013-936X/81/0915-0960$01.25/0 @ 1981 American Chemical Society
The rate of decay of ethylene is given by the expression -d In [CzHJdt
= ks[HO]
from which the steady-state concentration of hydroxyl can be deduced from the known value of k b = 8.0 X cm3 molecule-' s-1 at room temperature (11).For our experiments with 16 lamps, the steady-state concentrations of hydroxyl radicals were of the order of 5 X lo7 molecule ~ m - ~ . When the reaction mixture contains an aldehyde as well as ethylene, the reaction scheme must include the reaction HO
+ RCHO
-
products
(6)
which will be the major route for removing the aldehyde. Thus, for reactions 5 and 6 the measured rates of consumption of the ethylene and aldehyde are given by the equations
Discussion
-d[RCHO]/dt = k6[HO][RCHO] which can be integrated and combined to give
where the subscripts refer to the concentrations at time = 0 and t . A plot of In ([RCHO]O/[RCHO]~) vs. In ([C2H4]01 [CzH41t)yields the ratio of rate constants, ke/k5. A typical plot is shown in Figure 1for the ethylene-n-butyraldehyde system. Good straight-line plots were obtained for all of the aldehydes studied, indicating that the removal of both substrates is dominated by reaction with hydroxyl radicals. In each case the concentrations of ethylene and aldehyde were independently varied, typical ranges being for ethylene 2-7 ppm and for aldehyde 2.5-6 ppm. The absence of any marked effect of these concentration changes upon the above
. . 0 [L
0,3c
The present rate constant for acetaldehyde is in good agreement with previous data obtained from absolute rate constant measurements (14,17) and from relative rate constant measurements ( 4 , 1 5 ) .For propionaldehyde and benzaldehyde our rate constants are in good agreement with those of Niki et al. (15) derived from a system similar to ours but with analysis via Fourier transform infrared spectroscopy rather than gas chromatography. This general level of agreement between previous data and the present study gives some confidence in the data for the C4, C5, and other aldehydes for which no measurements have been reported. Gaffney and Levine (19) predicted rate constants of 3.0 X 10-l' and 3.5 X cm3 molecule-l s-l for HO reactioils with n - and isobutyraldehydes from linear free energy relationships between the experimental rate constants of O(3P) atoms and HO radicals in H-abstraction reactions with organic molecules. These predictions are in reasonable accord with the absolute rate constants derived here for n- and isobutyraldehydes (Table 111). A discussion of the present results for hydroxyl attack on the higher aldehydes must take account of the different pathways available for the reaction. On the basis of the bond dissociation energies (20) of the aldehyde C-H bond and those of primary, secondary, and tertiary C-H bonds in alkanes, it would be expected that the bulk of the abstraction by hydroxyl radicals would occur at the aldehyde group. Thus D(H-COH) = 87 f 1,D(H-COCH3) = 86.0 f 0.8, and D(H-COCzH5) = 87.4 f 1.0, but there are no experimental data for the higher aldehydes, while D(C-H) primary = 98 f 1,D(C-H) secon-
Table 11. Relative Rate Constants for the Reactions of Hydroxyl Radicals with Aldehydes and Ethylene at . 298 K: HO 4- RCHO -+ products (6); HO 4- C2H4 products ( 5 )
-
0
I 0
L1z
relation indicates that within our experimental conditions the kinetics were not influenced by products of the reactions. A further check was made on the rates of photodissociation of the aldehydes. Blank experiments were performed in each case consisting of the photolysis of a few ppm of aldehyde in a synthetic air mixture without added nitrous acid. The photolyses of all of the aldehydes were extremely slow in relation to the time scale of the hydroxyl radical experiments. The rate constants derived in the above manner for the range of aldehydes studied are presented in Table 11. The relative rate constants of Table I1 have been converted to absolute rate constants by taking a consensus literature ( 1 1 ) value of h5 = 8.0 X cm3 molecule-l s-l a t 298 K for the hydroxyl-ethylene reaction. The absolute rate constants are compared with existing literature values in Table 111.
0.2.
I I
C
relative lo k6/k5
k ( H 0 4- CH3CHO) = 1.0
CH3CHO
1.50 f 0.50
1.o
C~HSCHO
f 0.17 2.96 f 0.07 3.40 f 0.66 3.24 f 0.49 3.39 f 0.10 2.63 f 0.73 2.38 f 0.28 4.12 f 0.80 1.39 f 0.27
1.5 2.0 2.3 2.2 2.3
RCHO
n-CsHjCHO i-CaH7CHO n-CdHgCHO
I 0.05
i 0.10
I n ( IC,H,
l,/
IC2HL It 1
I
0.15
I
CC4HgCH0 f-C4H&HO CHp=CHCHO
Figure 1. Typical plot of In ([RCHO]o/[RCHO]t) vs. In ( [ C ~ H ~ ] O / [ C ~ H ~ CH3CH=CHCHO ]~) for the decay of n-butyraldehydeand ethylene during the photolysis of C~HSCHO HONO/air mixtures at atmospheric pressure and 298 K.
2.28
1.8 1.6 2.8 0.9
Volume 15, Number 8, August 1981
961
Table 111. Absolute Rate Constants for Hydroxyl Radical Reactions with Aldehydes: HO RCHO -, products (1)
Table IV. Relative Rate Constants for the Attack of HO, O(3P), and CH3 on Aldehydic C-H Bonds a
+
HO (298 K )
’ molecule-’
10’ k l/(cm3 aldehyde
HCHO
templK
S-1)
20.66
300
1.4 f 0.4
298
n-C3H7CH0 i-C3H7CH0 ~-C~HQCHO i-C4HgCH0 X~HQCHO CH2=CHCHO CH&H=CHCHO CsH5CHO
technique a
C2HsCHO
DF-MS
f?-C3H7CHO
(298 K)
CHs
(400 K )
1.o
1.o
1.1
1.6
1.1
1.3
2.2
1.1
DF-MS
i-C3H7CH0
1.5
2.7
1.4
1.o
1.0
299
14
FP-RF
1.4 f 0.1
298
15
relative
i-C4HSCH0
1.4
0.8
FP-RF
t44HgCHO
1.3
0.7
DF-MS
a For all three attackingspecies the rate constants have been obtained from the overall rate constants for reactionwith the molecule corrected for abstraction of H atoms from the alkyl groups in the aldehyde. Reference 22. References 23 and 24.
f 0.1 1.5 f 0.4
1.6 f 0.2
299
16 17 4 14
1.5 f 0.2
298
15
relative
1.2 f 0.4
298
this work
relativeb
3.1
298
18
DF-MS
2.1 f 0.1
298
15
relativeb
1.8 f 0.1
298
this work
relative
f 0.1 2.7 f 0.5 2.6 f 0.4 2.7 f 0.1 2.1 f 0.6 1.9 f 0.2 3.3 f 0.6 1.3 f 0.1 1.1 f 0.2
298
this work
relativeb
298
this work
relativeb
298
this work
relative
298
this work
relativeb
298 298 295
2.4
relative FP-RF
298
this work
relative
298
this work
relative
298
this work
relative
298 298
15 this work
relative relative
a DF = discharge flow: MS = mass spectrometry: FP = flash photolysis: RF = resonance fluorescence. Measured relative to k = 8.0 X om3molecule-’ s-’ at 298 K for reaction HO C2H4 products. Measuredrelative to k = 6.6 X cm3 molecule-’ s-’ at 295 K for reaction HO t HONO
+
-
-
H20 t N02.
dary = 95 f 1,and D(C-H) tertiary = 92 f 1kcal molm1.Abstraction of hydrogen from the alkyl group of the aldehydes will become more significant as the C-H bonds become weaker in going from primary to secondary to tertiary and as the number of C-H bonds increase. Direct evidence for hydroxyl attack on both parts of the aldehyde molecule comes from a study of the products of the oxidation of isobutyraldehyde at 713 K (21). The contributions to the observed rate constants made by abstraction from the alkyl group can be estimated by assuming that the rate constants for abstraction (per C-H bond) from primary, secondary, and tertiary C-H bonds in alkanes are applicable to the alkyl groups in aldehydes. Thus from a reanalysis of the available room-temperature rate-constant data, Atkinson et al. ( 1 1 ) suggest the following expression to fit the hydroxyl rate constants for alkanes:
+ +
kl(cm3 molecule-l s-l) = (6.5 X 10-14)Np (5.8 x 1 0 - 1 3 ) ~ ~(2.1 x
IO-12)~~
in which N,, N,, and Nt are the numbers of primary, secondary, and tertiary C-H bonds, respectively. Application of the above expression to the present data for the CZ-C~aliphatic aldehydes yields the corrected relative rate constants listed in Table IV. For comparison Table IV also lists the corresponding data for O(3P)atoms and CH3 radicals, where again the rate constants have been corrected for attack on the alkyl groups. Comparison of the sequence of relative rate constants for the HO radical and the O(3P) atom indicates that the HO 962
O(3P)
0.94 f 0.1
12.0
C2HsCHO
12 13
1.o
n-C4HsCHO
0.99 CH3CHO
ref
CH3CHO
Environmental Science & Technology
radical is much less selective than the O(3P) atom, which is to be expected on the basis of their relative reactivities. It is also interesting to note from Table IV that the sequence of relative reactivities for CH3 attack on the aliphatic aldehydes does not follow the patterns for O(3P) atoms or for HO radicals. For the CH3 radical reactions the rate constants, with the possible exception of t-C4H&HO, are little affected by the change in the alkyl group in the aldehydes. This may suggest that the bond dissociation energies of the aldehydic C-H bonds are not the only factors influencing these reactions, which is perhaps not surprising since CH3 radicals are known to behave as nucleophiles while O(3P)atoms and HO radicals are electrophilic. For the O(3P) atom reactions the Arrhenius parameters were measured (22), and, since the A factors were approximately constant throughout the C2-C4 aldehydes studied, the increasing rate constants corresponded to slightly decreasing activation energies in going from C2 to C4 aldehydes. This trend was taken to indicate a slight weakening of the aldehydic C-H bond in going from C2 to C4 (22).For the hydroxyl radicals the temperature coefficients have been measured for formaldehyde (14, 16) and acetaldehyde (14), and the activation energies found to be close to zero. In view of the higher rate constants for the hydroxyl radical with C3-Cg aldehydes, it seems likely that these will also have zero activation energies. The present HO radical results appear to indicate that any differences in the aldehydic C-H bond dissociation energies in the C3-C5 aldehydes must be small. A comparison of the hydroxyl rate constants for acrolein and crotonaldehyde with those of the other aldehydes suggests that, for both of these aldehydes, there is a contribution to the rate constant from the addition reaction to the double bond, and in the case of crotonaldehyde there is a further contribution from H abstraction a t the weak allylic C-H bonds. It has previously been shown (18,25) that there is a good correlation between HO reactivity and the potential of organic compounds to contribute to the formation of ozone in photochemical smog, i.e., “smog reactivity”. This tends to suggest that the reaction of HO radicals with the organic molecules serves as the rate-controlling step for the oxidation of NO to NO2 and subsequent formation of 0 3 . Such a simplistic mechanistic interpretation assumes that the ensuing radicals in the oxidation of the organic molecules all have comparable potential for converting NO to Nos. The HO rate constants reported here for C4, Cg, and unsaturated aldehydes would place these molecules in the most reactive of the five classes of smog reactivity (26). We are proceeding with product analyses for the HO-initiated photooxidations of the higher aliphatic aldehydes in an attempt to shed further light on the mechanisms of these important tropospheric reactions.
Literature Cited (1) Herron. J. T.. Huie, R. E., Hodgeson, J. A., Eds, NBS Spec. Publ. (U.S.)1979,No. 557. (2) Dimitriades, B.; Wesson, T. C. J. Air Pollut. Control Assoc. 1977, 22,33. (3) Demerjian, K. L.; Kerr, J. A.; Calvert, J. G. Adu. Enuiron. Sci. Technol. 1974,4,1. (4) Cox, R. A.; Derwent, R. G.; Holt, P. M.; Kerr, J. A. J . Chem. Soc., Faraday Trans. I 1976,72,2061. (5) Carter, W. P. L.; Lloyd, A. C.; Sprung, J. L.; Pitts, J. N., Jr. Int. J . Chem. Kinet. 1979,1I, 45. (6) Derwent, R. G.; Hov, 0. “Computer Modelling Studies of Photochemical Air Pollution Formation in North West Europe”; U.K. Atomic Energy Authority, AERE-R9434,1979. (7) Cox, R. A.; Derwent, R. G.; Williams, M. R. Enuiron. Sci. Technol. 1980,14,57. (8) Cox. R. A.: Sheuuard. D. Nature (London)1980.284,330. (9) Cox, R. A.J. P h o c h e m . 1974,3,175. (10) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1978,82,135. (11) Atkinson, R.: Darnall, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Adu. Photochem. 1979,II; 375. (12) Herron, J. T.; Penzhorn, R. D. J . Phys. Chem. 1969,73,191. (13) Morris, E. D., Jr.; Niki, H. J . Chem. Phys. 1971,55,1991. (14) Atkinson, R.; Pitts, J. N., Jr. J . Chem. Phys. 1978,68,3581. (15) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem. 1978,82,132.
(16) Stief. L. J.: Nava. D. F.: Pavne. W. A.: Michael, J. V. J . Chem. Phys. 1980,73,2254. (17) Morris. E. D.. Jr.: Stedman. D. H.: Niki. H. J . Am Chem Soc. 1971,93,3570. (18) Morris, E. D., Jr.; Niki, H. J . Phys. Chem 1971, 75, 3640. (19) Gaffney, J. S.; Levine, S. Z. Int. J . Chem. Kinet. 1979, 11, 1197. (20) Kerr, J. A.; Trotman-Dickenson, A. F. In “Handbook of Chemistry and Physics”, 60th ed.; CRC Press: Boca Raton, FL, 1980;pp F231-2. (21) Baldwin, R. R.; Cleugh, C. J.; Plaistowe, J. C.; Walker, R. W. J . Chem. Soc., Faraday Trans. 1 1979,75,1433. (22) Singleton, D. L.; Irwin, R. S.; Cvetanovii., R. J. Can. J. Chem. 1977,55,3321. (23) Birrell, R. N.; Trotman-Dickenson, A. F. J . Chem. Soc. 1960, 2059. (24) Kerr, J. A,; Parsonage, M. J. “Evaluated Kinetic Data on Gas Phase Hydrogen Transfer Reactions of Methyl Radicals”; Butterworths: London, 1976. (25) Darnall, K. R.; Lloyd, A. C.; Winer,A. M.; Pitts, J. N., Jr. Enuiron. Sei. Technol. 1972,10,692. (26) Dimitriades, B. “Proceedings of the Solvent Reactivity Conference”; U.S. Environmental Protection Agency: Research Triangle Park, NC, Nov 1974;EPA-650/3-74-010, Received for review December 15,1980. Accepted March 11,1981.
NOTES
Allocation of Vehicular Emissions of Carbon Monoxide in El Paso, Texas, and Ciudad Juarez, Chihuahua Howard G. Applegate Department of Civil Engineering, University of Texas at El Paso, El Paso, Texas 79968
Some of the CO measured in the city of El Paso, Texas, has its origin either on federally owned land or in Mexico. Neither the city nor the state has jurisdiction over these two areas. Unless CO emissions from these areas are lowered, El Paso cannot come into compliance with federal standards.
Introduction The Environmental Protection Agency has declared El Paso, Texas, to be a nonattainment area in regard to carbon monoxide (CO). Local air pollution authorities believe some of the CO measured within El Paso is emitted from vehicles and in areas over which they have no control. This paper reports on the allocation of vehicular emissions of CO within the El Paso-Cd. Juarez (EPJAZ) airshed for 1977. For this study, the following broad categories were set up: local emissions-CO emitted by vehicles registered in and driving the streets of El Paso; federal-CO emitted by vehicles waiting to pass through customs on the international bridges and vehicles registered in and driving on Fort Bliss; foreign-CO emitted from vehicles registered in and driving the streets of Cd. Juarez. Methods
All CO measurements in El Paso were made by the Texas Air Control Board using EPA-approved methodology. All traffic data in El Paso were gathered by the Texas Department of Highways. Light-duty gasoline vehicles (LDGVs) are those passenger cars registered in El Paso by the Texas Department of Motor Vehicles as of March 1,1977; heavy-duty diesel vehicles (HDDVs) are commercial trucks, farm trucks, combination trucks, and buses registered on the same date. Total 0013-936X/81/0915-0963$01.25/0
vehicle miles traveled in El Paso were obtained from the Texas Department of Highways. The assumption was made that, since LDGVs constituted 81%of the total vehicles, they drove 81%of the total mileage. The number of vehicles crossing the international bridges between El Paso and Cd. Juarez was obtained from the U.S. Customs. They listed “freight carrier vehicles” and “other ground vehicles” in the report. For this study, the first were assumed to be HDDVs and the latter to be LDGVs. It was further assumed that the vehicles were warmed up and the average waiting time to pass through customs was 20 min. The total number of vehicles registered on Fort Bliss in 1977 was obtained from the post’s environmental office. It was assumed, in order to have a conservative estimate, that all were LDGVs. Officials from Fort Bliss estimated that the average vehicle traveled 5.49 mi day-l on the post in 1977. It proved impossible to obtain reliable figures on vehicle registration or miles traveled in Cd. Juarez. For this study, earlier published data were used to calculate that Cd. Juarez had 26 087 vehicles in 1977 (I,2). Further, it was assumed that the distribution between HDDVs and LDGVs was the same as in El Paso for the same year. This should give a conservative estimate of CO emissions since the percentage of HDDVs is probably much greater. A further conservative estimate was made that the mileage distribution between HDDVs and LDGVs was similar to that in El Paso. Buses, notorious polluters along the ent,ire border, are greater in number in Cd. Juarez and are driven more miles per day than in El Paso. Finally, it was assumed that the age distribution of vehicles in Cd. Juarez was the same as in El Paso. Since the tax on cars less than 5 yr old imported into Mexico is 100%of the purchase price, this estimate is very conservative. In addition, the quality of gasoline and diesel fuel in Cd. Juarez is lower than
@ 1981 American Chemical Society
Volume
15, Number 8, August 1981 963
