J. Phys. Chem. 1984, 88, 1210-1215
1210
Rate Constants for the Gas-Phase Reactions of Nitrate Radicals with a Series of Organics in Air at 298 f 1 K Roger Atkinson,* Christopher N. Plum, William P. L. Carter, Arthur M. Winer, and James N. Pitts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 92521 (Received: June 6, 1983; In Final Form: August 1, 1983)
The nitrate radical is an important intermediate species in NOX-0,-air systems, but relatively few data are available concerning its reactions with organics. In this work we have determined rate constants for the gas-phase reactions of NO3 radicals with a series of organics at 298 1 K using two experimental techniques, one by monitoring the enhanced decay rates of N2O5 in the presence of a reactive organic and the other employing a relative rate technique. Based upon the most recent literature evaluation concerning the N205(+M) F! NO2 + NO, (+M) reactions and equilibrium constant, the rate constants (in cm3 propene, (4.2 f 0.9) X 10-l5; 1-butene, (5.4 molecule-' s-l units) determined were as follows: ethene, (6.1 k 2.6) X f 1.2) X 10-l5; isobutene, (1.70 f 0.19) X lo-',; cis-2-butene, (1.89 f 0.22) X IO-',; trans-2-butene, (2.11 f 0.24) X 10-13; 2-methyl-2-butene; (5.1 f 1.6) X 10-l2; 2,3-dimethyl-2-butene, (3.1 f 1.0) X lo-"; benzene, 51.1 X toluene, (2.0 formaldehyde, (3.23 f 0.26) X lo-'$ and m-xylene, (7.6 f 3.5) X naphthalene, (6.4 f 2.5) X f 1.1) X These rate constants for the alkenes (apart from ethene) and acetaldehyde are in good acetaldehyde, (1.34 f 0.28) X agreement with the only previous set of literature values, while aromatic cm-, hydrocarbons were observed for the first time to react with the NO3 radical. Thus, the rate constant for m-xylene is comparable to that for ethene, and naphthalene reacts with the NO3 radical a factor of 1.5 faster than does propene. Acetaldehyde is significantlymore reactive than is formaldehyde toward NO3 radicals, analogous to their reactions with O(,P) atoms and OH radicals. The atmospheric implications of these data, with regard to the loss of NO3 and organics and the formation of nitric acid and hence acid deposition, are briefly discussed.
*
tionally, it has been proposed that reactions of the NO, radical can lead to nitric acid formation during nighttime hours via several processes, including the reaction sequence9,10,15,16,18
Introduction The nitrate (NO3) radical has long been recognized as an important intermediate species in laboratory and environmental chamber N0,-03air systems.I4 More recently the NO3 radical has been identified and measured by long-path spectroscopic techniques in nighttime atmospheres5-I0 at a variety of locations in the USA and Europe. However, to date only a few roomtemperature studies of the gas-phase reactions of the NO, radical with organics have been carried o ~ t , l ' - 'including ~ the determination of rate constants for a series of alkenes,"J2 acetaldehyde," selected aromatic hydrocarbon^,^^,^^ and methoxy- and hydroxysubstituted a r 0 m a t i ~ s . I ~ These kinetic data, together with the observed ambient atmospheric concentrations of the NO, radical,+l0 indicate that the reaction of the NO, radical with the more reactive alkenes and the hydroxy-substituted aromatics may be an important nighttime Addisink for both NO, radicals and these
I
+ NO3
N2O5
N2O5
+ H20
2HNO,
-
and H-atom abstraction from organics such as aldehydes" and/or hydroxy-substituted a r ~ r n a t i c s ' ~ - ' ~ NO, NO3
+ RCHO
+ C,H50H
--
HNO,
+ RCO
HNO,
+ C6HS0.
Clearly, further studies of the kinetics and mechanisms of these reactions are of fundamental interest as well as being highly relevant to the chemistry of nighttime atmospheres. In this work, as part of a comprehensive study of these NO, radical reactions, we have determined rate constants for the reaction of NO3 radicals with a series of aldehydes, alkenes, and aromatic hydrocarbons at 298 f 1 K.
(1) Johnston, H. S. J. Am. Chem. SOC.1951, 73, 4542. (2) Niki, H.; Daby, E. E.; Weinstock, B. Adu. Chem. Ser. 1972, No. 113, 16. (3) Demerjim, K. L.; Kerr, J. A,; Calvert, J. G. Adu. Enuiron. Sci. Technol. 1974, 4, 1. (4) Graham, R. A.; Johnston, H. S. J . Phys. Chem. 1978,82, 254. (5) Platt, U.; Perner, D.; Winer, A. M.; Harris, G. W.; Pitts, J. N., Jr. Geophys. Res. Lett. 1980, 7, 89. (6) Noxon, J. F.;Norton, R. B.; Marovich, E. Geophys. Res. Lett. 1980, 7, 125. (7) Platt, U.; Perner, D.; Schroder, J.; Kessler, C.; Toennissen, A. J . Geophys. Res. 1981, 86, 11965. (8) Platt, U.; Perner, D.; Kessler, C., presented at the 2nd Symposium, Composition of the Nonurban Troposphere, Williamsburg, VA, May 25-28, 1982. (9) Platt, U. F.;Winer, A. M.; Biermann, H. W.; Pitts, J. N., Jr. Enuiron. Sci. Technol., in press. (10) Winer, A. M.; Pitts, J. N., Jr.; Platt, U.; Biermann, H. W. presented at the 185th National Meeting of the American Chemical Society, Seattle, WA, March 1983. (11) Morris, E. D., Jr.; Niki, H. J. Phys. Chem. 1974, 78, 1337. (12) Japar, S. M.; Niki, H. J. Phys. Chem. 1975, 79, 1629. ( 1 3) Bandow, H.; Okuda, M.; Akimoto, H. J. Phys. Chem. 1980,84,3604. (14) Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. Emiron. Sci. Technol. 1981, 15, 829. (15) Pitts, J. N., Jr. Enuiron. Health Perspect. 1983, 47, 115. (16) Pitts, J. N., Jr.; Harris, G. W.; Winer, A. M., presented at the 15th International Symposium on Free Radicals, Halifax, Nova Scotia, 1981.
0022-3654 ,/84 ,/2088- 121OS0 1S O / O
NO2
Experimental Section Two experimental techniques for the determination of rate constants for the reaction of NO, radicals with organics were used. The first of these techniques was based upon observing the increased decay rate of N2O5 in the presence of a reactive organic, as previously utilized by Niki and co-workers11J2in their pioneering studies of the gas-phase reactions of NO, radicals with organics. In these N,05-N02-organic-air mixtures, the reactions occurring are1'J2 N205 NO,
-
NO,
+ NO2
N20s NO,
M
-
+ NOz
(1)
N205
(2)
M +
loss of N205
+ organic
-
products
(3) (4)
(17) Atkinson, R.; Lloyd, A. C. J. Phys. Chem. ReJ Data, in press. (18) Morris, E. D., Jr.; Niki, H. J . Phys. Chem. 1973, 77, 1929.
0 1984 American Chemical Societv -
The Journal of Physical Chemistry, Vol. 88, No. 6,1984 1211
Gas-Phase Reactions of NO, Radicals with Organics where reaction 3 may be a combination of a background wall reaction and a homogeneous gas-phase reaction with H20.19 Thus d[NzOsl/dt = -ki[N2Osl
+ k2[N021[N031 - h[Nz051
d[N03] /dZ = ki[NzO~]- k2[NO21[No,]
(1)
- k4[N031 [organic] (11)
Combining these two equations leads to the expression d[N20s]/dt
+ d[NO,]/dt
= -k,[NZOs]
- k4[N0,] [organic] (111)
and, under conditions where the equilibrium between N20s, NO,, and NO2 is attained (as was always the case in this work), then -d In [N2OS] dt
- =
(
k3
+
k,lorganicI)( K[N02]
K[N021 1 + K[N02]
)
D,= In ([pressure chamber],/[pressure chamber],,) = In ([tracer],,/ [tracer],) (VII) where (pressure chamber),, and [tracer],, are the initial pressure of the chamber and the initial concentration of the tracer, respectively, at time to, and (pressure chamber), and [tracer], are the corresponding quantities at time t . During these experiments, the dilution factor D,was typically 0.006 (Le., -0.6%) per NzO5 addition. Thus In ([organic],,/ [organic],) = k 4 j t [ N 0 3 ]dt
+ D,
(VIII)
fa
In ([reference organic],,/ [reference organic],) =
(Iv)
k s l ' [ N 0 3 ] dt
+ D, (IX)
10
Eliminating the integrated NO3 radical concentration from eq VI11 and IX leads to
where the equilibrium constant K is given by K = k2/kl = [N2051/mO2I[NO,I~ Since under the experimental conditions employed in this work K [ N 0 2 ] >> I , eq IV can be simplified to -d In [N20s]/dt = k3 + k4[organic]/K[N02]
In ([organic],,/[organic],) - D, = (k4/ks)[ln ([reference organic],,/ [reference organic],1 - D,] (XI
(V)
where [organic],, and [reference organic],, are the concentrations of the organic and reference organic, respectively, at time to, [organic], and [reference organic], are the corresponding con-K(d In [N2OS]/dt k,) = k4[organic]/[N02] (VI) centrations at time t , and k4 and ks are the rate constants for reactions 4 and 5 , respectively. Thus, under conditions such that the [organic]/[N02] ratio reHence, plots of In ([organic],,/[organic],) - D, against In mains constant during a given experiment, then from a series of ([reference organic],,/ [reference organic],) - D,should yield plots such experiments a plot of -K(d In [N205]/dt + k,) against of slope k4/ks, with a zero intercept. This analysis is totally [organic]/[N02] should yield a straight line with a slope of k4 analogous to that for the OH radical relative rate constant and zero intercept. It should be noted that the above analysis only technique recently developed in these laboratories utilizing the holds when the equilibrium between N2Os and NO2 NO3 can dark NZH4-03 reaction as a source of O H radicals.22 For this be attained (i.e., the N 2 0 s thermal decay rate k , must be much technique, the initial concentrations of the organic reactants were faster than the observed N 2 0 5 decay rates). This analytical -(2-3) X 1013 molecules cm-), and up to seven incremental expression was checked by computer model calculations using the accepted rate constants for the N2OS-NO2-NO3 s y ~ t e m 'and ~ * ~ ~ amounts of N2Os [-(0.5-5) X lo1, molecules cm-, of N,O, per addition] were added to the chamber throughout the experiments. the measured values of k4 and was found to introduce a negligible For the studies involving the more reactive alkenes, (2-20) X 1014 error for the conditions employed here. This analysis eliminates molecules cm-3 of NO2 were also included in the reaction mixtures any effects of small temperature changes from experiment to in order to extend the reaction times. experiment (a variation of 1 K changes the equilibrium constant All experiments were conducted in the SAPRC 5800-L therK by Analogous to the study of Japar and Niki,12 the mostated, Teflon-coated evacuable chamber. This evacuable concentrations of NOz in the N20S-N02-organic-air mixtures chamber facility has been described in detail previou~ly,~~ and only were varied depending on the organic studied, with higher cona brief description will be given here. The chamber was fitted centrations being used for the more reactive organics in order to with an in situ multiple-reflection optical system which was inextend the reaction time. Typical initial reactant concentrations terfaced to a Fourier transform infrared (FT-IR) absorption were as follows (in molecules ~ m - ~ )N2O5, : (2-5) X lo1,; NO2, spectrometer. A 62.9-m path length was used in these experiments (2-280) X 1013;and organic, (6-340) X lo1,. resulting in detection limits of -7 X 10" molecules cm-3 for N20s, The second experimental method used was a relative rate 1 X 1 O l 2 molecules cm-, for HNO,, and -6 X 10" molecules technique and was based upon monitoring the relative decay rates for NO2. Water vapor concentrations were determined from of a series of organics, including at least one organic whose NO, wet bulb/dry bulb measurements of the purified matrix air used radical reaction rate constant was also determined from the ento fill the cham be^->^-^^ and were 1 4 X 10l6molecules cm-, (5% hanced N2Os decay rates in N20s--NO2-organic-air mixtures. relative humidity at 298 K). All rate constant determinations were Since the organics studied here react essentially negligibly with carried out at 298 1 K and -740-torr total pressure of air. N20s11,12 and N02,21then under the experimental conditions For the experiments in which N2O5 decay rates were used to employed the sole chemical loss process of these organics is due determine individual NO, radical reaction rate constants, N 2 0 S , to reaction with NO, radicals: NO2, and the organics were quantitatively monitored by FT-IR NO, + organic -* products (4) spectroscopy at a spectral resolution of 1 cm-'. For the relative rate constant studies, the reactant organics were analyzed prior NO, + reference organic products (5) to and during these reactions by gas chromatography with flame Additionally, small amounts of dilution occurred from the ionization detection (GC-FID), using the following columns: for trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene a incremental additions of N2OS to the reaction mixture. The dilution factor at time t was measured either by the pressure 20 ft X 0.125 in. stainless steel (SS) column packed with 5% change within the chamber (using an MKS Baratron capacitance DC703/C20M on 100/120 mesh AW, DMCS Chromosorb G, manometer) or by the change in concentration of a nonreactive operated at 333 K, for propene, 1-butene, isobutene, cis-2-butene, organic tracer. This factor, D,,is given by or
+
+
-
-
*
-
(19) Tuazon, E. C.; Atkinson, R.; Plum, C. N.; Winer, A. M.; Pitts, J. N., Jr. Geophys. Res. Lett. 1983, 10, 953. (20) Malko, M. W.; Troe, J. In?. J . Chem. Kinet. 1982, 14, 399. (21) Atkinson, R.; Aschmann, S. M.; Winer, A. M.; Pitts, J. N., Jr., Znt. J . Chem. Kinet., in press.
(22) Tuazon, E. C.; Carter, W. P. L.; Atkinson, R.; Pitts, J. N., Jr. Znt. J . Chem. Kinef. 1983, 15, 619. (23) Winer, A. M.; Graham, R. A.; Doyle, G. J.; Bekowies, P. J.; McAfee, J. M.; Pitts, J. N., Jr. Adu. Enuiron. Sci. Technol. 1980, 10, 461. (24) Doyle, G. J.; Bekowies, P. J.; Winer, A. M.; Pitts, J. N., Jr. Enuiron. Sci. Technol. 1977, 11, 45.
1212 The Journal of Physical Chemistry, VoZ.88, No. 6,1984
Atkinson et al.
r
3.5
n +
n
Lo
0 N
z
m
u
\
2.01
c n
Lo
0.2-
z
Y
\ \
0.08 0
20
40
60
80
100
120
TIME (min) Figure 1. Semilogarithmic plot of the N 2 0 5decay before and after the addition of 1.1 X loL4molecules of trans-2-butene at t = 52 min to an N2O,-NO2-air mixture.
\
PROPENE
LD
0 N
2
Y
C -
e
.b 0.5
U
X
9
Results Rate constants k4 for the reactions of NO3 radicals with a series of organics were obtained by using both of the techniques described above. For the technique which involved monitoring the enhanced rate of decay of N2O5in the presence of an organic, rate constants ( 2 5 ) Schott,
G.;Davidson, N. J . Am. Chem. SOC.1958, 80, 1841.
,
I -
20 [ETHENE]
IO
OO I
I I
I
and trans-2-butene a 36 ft X 0.125 in. SS column packed with 10%2,4-dimethylsulfolane on 60/80 mesh firebrick, operated at 273 K; for ethene a 5 ft X 0.125 in. SS column packed with Porapak N (80/100 mesh), operated at 333 K; and for benzene, toluene, and m-xylene a 10 ft X 0.125 in. SS column of 10% Carbowax 600 on C-22 firebrick, operated at 348 K. Naphthalene was also monitored by GC-FID, using a 6 ft X 0.25 in. glass column packed with Super Pak 20M, temperature programmed from 323 to 373 K at 10 K m i d . For this reactant, gas samples of 100-cm3volume were withdrawn from the chamber into 0.25 in. X 3.25 in. glass traps packed with Tenax G C 60/80 mesh. These samples were then transferred by the carrier gas at -523 K from this trap to the column head which was at 323 K, followed by the temperature programming of the column as noted above. N 2 0 5was prepared by the method of Schott and D a v i d s ~ n . ~ ~ NOz was added to a stream of ozonized oxygen at atmospheric pressure, and the N z 0 5was collected at 196 K and purified by vacuum distillation. The NzOs, as prepared in this manner, always contained 215% NOz after injection into the chamber. For the experiments in which N2O5 decay rates were used to determine individual NO3 radical reaction rate constants, known pressures of the reactants, as monitored by an MKS Baratron capacitance manometer, were added to the chamber from calibrated Pyrex bulbs. For the relative rate constant studies the reactants, except for naphthalene (see below), were introduced into the chamber from all-glass, gas-tight syringes or from Pyrex bulbs by a stream of ultrahigh-purity Nz. Naphthalene was introduced into the chamber by passing ultrahigh-purity N z at a known flow rate through a Pyrex tube (0.25 in. 0.d.) filled with naphthalene. The organics, of stated purity levels >99%, were used as supplied, except for formaldehyde and acetaldehyde, which were prepared as described below. N o impurities in the alkenes or aromatics were observed by gas chromatography or by FT-IR absorption spectroscopy. H C H O was prepared by heating previously dried paraformaldehyde (>95% purity) at 373 K under vacuum. The effluent was passed through a trap at 179 K to remove HzO and possible disproportionation products, and formaldehyde was collected in a trap at 77 K. Acetaldehyde (reagent grade) was degassed under vacuum and distilled to a trap at 156 K under vacuum.
ETHENE
Y
0
30
40
/ [ NO21 I
1
2
3
I
[PROPENE]/[NO2] I
I
0
I 0.04
I
I
I
[trans-2-
I
1
I
0.12
0 08
0 16
BUTENE]/[NO*]
Figure 2. Plot of eq VI for ethene, propene, and trans-2-butene.
-I
u
s
8-
6rr)
E
/
4-
L" 0 I
0
v, 0
I 4
1
I
0
,
I
I
12
[ORGANIC] /[NO21 Figure 3. Plot of eq VI for formaldehyde and acetaldehyde.
were determined for ethene, propene, trans-2-butene, formaldehyde, acetaldehyde, and m-xylene. In all cases the NzO5 decays were observed to be exponential within the experimental error limits, as shown by the linearity of plots of In ([NZO5],,/ [N205],)against time ( t - to). For this technique, the background NzO5 decay rate k3 (Le., that in the absence of an added organic) was monitored by FT-IR spectroscopy for -45 min, and then a known concentration of the organic was added to the NzOSNOz-air mixture and the increased NZO, decay rate determined. Figure 1 shows a typical semilogarithmic plot of the N205concentration as a function of time before and after the addition of trans-2-butene to an N205-N02-air mixture. From these experiments, plots of eq VI such as those shown in Figures 2 and 3 were obtained. The rate constants k4 obtained from the slopes of these plots by least-squares analyses are given
Gas-Phase Reactions of NO3 Radicals with Organics
The Journal of Physical Chemistry, Vol. 88, No. 6, I984 1213 :.4r
L
40 2-METHYL-2-BUTENE
L
f zOl
n
,
c T
- t/ “L Y
W
z
W I3
l o -
PROPENE
5
IO
20
I n ([trons-E-BUTENE]
30 tQ/[trons-Z-BUTENE]
1 40
t)
I 50
- Dt
m
-I \
u
c c
Figure 4. Plot of eq X for propene, isobutene, cis-&butene, and 2methyl-2-butene,with trans-2-butene as the reference organic.
W
t
z
W I3
m
P
v
C
-zI
t J 8:
- 08u I
c
0
W W
o’8
1 c 1
+W
Y
\
0
W
0 02
00
--
04
W
E
0
6
08
z
I n ([PROPENE] t,/[PROPENE]t)-Dt
Figure 6. Plot of eq X for ethene and 1-butene, with propene as the
$1
reference organic.
0’
a z
0.12
W
II:
0
u
-
11
In( [2-METHYL-2-BUTENE]
+o/[2-METHYL-2-BUTENE]t)
- Dt
Figure 5. Plot of eq X for 2,3-dimethyl-2-buteneand 2-methyl-2-butene. The differing symbols refer to the separate experiments carried out.
in Table I. Since a variation of 1 K changes the equilibrium the uncertainties in the values of k4 given constant K by in Table I include the uncertainties in K due to the measured temperature changes during the experiment combined with two least-squares standard deviations of the slopes of these plots. The equilibrium constants K used to derive the rate constants k4 were those determined from the Arrhenius expression given by Malko and Troe20 in their recent evaluation [ K = (1.33 X 10-27)(T/ 300)0”2e1’080/T cm3 molecule-’, = 1.87 X lo-“ cm3 molecule-’ at 298 K]. The background N 2 0 5 decay rates k3 were typically observed to be 1.5 X lo4 s-’ (at a relative humidity of 55%). Using the relative rate constant technique, we studied the following sets of organics: ethene, propene, and 1-butene; propene, isobutene, cis-2-butene, trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene; trans-2-butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene; ethene, benzene, toluene, m-xylene, and naphthalene; and naphthalene and propene. The dilution factor D,was determined by pressure measurements before and after the addition of N205or, for the faster reacting alkenes, by using an alkane (cyclohexane, isobutane, or n-octane) tracer. Representative plots of eq X are shown in Figures 4-7, and the rate constant ratios k41k5obtained by least-squares analyses of these data are given in Table 11. The error limits given in Table I1 reflect the estimated uncertainties in the dilution factor D,
-
I
/ n
z a
TOLUENE
W
II:
2 0.04
v
C
BENZENE
--0
0
n -
n-1~
I
0.04
1
I 0.08
1
I 0.12
In ([ETHENE] to/[ETHENE]
I
I 0.16
+)-Dt
Figure 7. Plot of eq X for benzene, toluene, and m-xylene with ethene
as the reference organic.
combined with two least-squares standard deviations of the plots of eq X. In all cases the intercepts of these plots as derived by least-squares analyses were within two standard deviations of zero.
Discussion The data obtained by using both experimental techniques are, as seen from Figures 2-7, in good accord?with eq VI or X, respectively. The observation that the N205decay rate increases linearly with the [organic]/[N02] ratio and not with the concentration of the organic shows that the NO, radical, and not N205,is the reactive species, as previously concluded by Japar and Niki.12 The rate constant ratios k4/k5given in Table I1 may be placed on an “absolute” basis (but still linearly dependent on the value of the equilibrium constant used) using the rate constants for ethene, propene, trans-2-butene given in Table I determined from
1214 The Journal of Physical Chemistry, Vol. 88, No. 6,1984
Atkinson et al.
TABLE I: Rate Constants k , (in cm3 molecule-’ s-’) for the Reaction of NO, Radicals with a Series of Organicsa this work organic
from N,O decay ratesb
I-butene isobutene cis-2-butene trans- 2-bu tene 2-methyl-2-butene 2,3-dimethyl-2-butene benzene toluene m-xylene naphthalene formaldehyde acetaldehyde
(2.10
i
0.18) x 10-13
(1.15
i
0.20) x
(3.23 (1.34
i
t
lit. valuese
ref
(1.09 5 0.12) x 10-1’ (6.2 i 0.4) X lo-’’ (3.5 1 0.9) x IO-I’ (9.1 t 1.0) x 10-1’ (1.3 .t 0.12) x 10-13 (2.1 i 0.23) x 10-13 (1.6 i 0.12) x 1 0 3 3 (6.4 i 0.6) x 10.’’ (4.3 i 0.6) x IO-’’
