Major Atmospheric Sink for Phenol and the Cresols. Reaction with the Nitrate Radical William P. L. Carter,' Arthur M. Winer, and James N. Pitts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 92521
Evidence has been obtained from studies using two different environmental chambers that phenol and the cresols react rapidly with the nitrate radical (NO3).When a relative rate technique reliable to within at least a factor of 2 was used, the following rate constants for reaction with NO3 a t 300 f 1 K were derived (cm3 molecule-l s-'): phenol, 2 X 10-l2; o-cresol, 1.2 X lo-''; rn-cresol, 7 X p-cresol, 1.3 X lo-". Methoxybenzene, benzaldehyde, and toluene reacted with NO3 too slowly to measure by the techniques employed in this cm3 study, and only upper limits of 1 3 X 10-15-8 X molecule-' s-' could be determined. These rate constants mean that reaction with NO3 can be a major sink for phenolic compounds under conditions of photochemical air pollution, especially at night. Possible mechanisms accounting for these results are discussed. Aromatic hydrocarbons constitute a significant portion of the reactive organics emitted into polluted urban atmospheres (1-4), and substituted phenols are among the products of their NO,-air photooxidation (5-12). However, the subsequent fates of these products in atmospheric systems are not well understood. For example, cresols are formed in environmental chamber irradiations of toluene-NO,-air mixtures (5-71, but, after their initial buildup, their concentrations decrease a t a rate faster than can be attributed to their known reactions with hydroxyl radicals (8, 13, 14) or with ozone (9, 13).Although this rapid decrease in the cresol concentration generally occurs around the time that NO is consumed and 0 3 begins to build up, it cannot be attributed to reaction between the cresol and 0 3 since the rate constants for these reactions have been measured (9,13) and are too low for them to be of importance in atmospheric systems. This apparent excess rate of decay of the cresols could possibly be due to an experimental artifact in sampling for cresols in the presence of 0 3 , and this possibility is examined in this study. An alternative possibility is that there could be some unknown aspect of the aromatic photooxidation mechanism causing cresol production to decrease around the time 0 3 formation begins or that some other species formed in such experiments reacts rapidly with cresols. For example, nitrate radicals (NOS) are formed in photochemical smog systems from the reaction of O3 with NO2 0 3
+ No2
--*
NO3
0 2
(1)
and it may be reaction with this species which accounts for the excess rates of cresol consumption. On the basis of results of experiments reported here, we believe that this is the most probable explanation. Exploratory Runs. Five experiments were conducted in which 200-1400 ppb of 0 3 was injected into the SAPRC -6OOO-L,all-glass chamber (15-17) containing 10-100 ppb of cresols; o-cresol was used most frequently, but similar results were observed for all three isomers. The O3 injections in the absence of NO, resulted in immediate and highly variable decreases in measured cresol concentrations, ranging from 1 to 30 ppb, with the amount of reduction having no obvious dependence on the absolute amount of cresol initially present, but tending to be somewhat greater when larger amounts of 0 3 were injected. In two experiments, second and third O3 injections were done, and additional cresol reductions, similar 0013-936X/81/0915-0829$01.25/0
in magnitude to those found following the first injections, were observed. In no case was complete consumption of cresol observed when O3 alone was injected, and no further changes in O3 and cresol concentrations were observed once mixing was attained following O3 injection. The reasons for the incomplete and variable loss of cresols observed following the injection of excess 0 3 in the absence of injected NO, are uncertain. However, this effect could be caused by NzO5 and NO3 impurities in the O3 injected, possibly formed in the ozonizer discharge from low levels of N2 in the ozonizer input gas ( 0 2 ) since we have previously observed significant production of NO2 and N2O5 from our ozonizer when N:!is included in the input gas (18).On the other hand, this effect cannot be due to a gas-phase reaction of cresols with 03,since continued cresol consumption did not occur in the presence of the excess 03,and the rate constant for the reaction of O3 with o-cresol is known to be too low for that reaction to be of significance (9,13). No significant reduction in cresol concentrations occurred when 50 ppb of NO or NO:! were added to the glass chamber containing 20 ppb of o-cresol. However, upon the addition of 800-1OOO ppb of 0 3 to the cresol-NO or cresol-NO2 mixtures, complete disappearance of the cresol was observed. In order to determine whether this apparent disappearance was an artifact due to 0 3 reacting with the cresol when it was trapped on the Tenax-filled cartridges for analysis, an experiment was done where sufficient NO was added to remove the O3 remaining in the 03-NO, mixture following the consumption of cresol. No reappearance of the cresol was observed, indicating that the disappearance of cresol was real and not a sampling artifact due to the presence of 0 3 . Our observations of rapid and complete cresol consumption in the presence of 0 3 and NO2 are consistent with results previously and independently obtained by O'Brien (19). The fact that both 0 3 and NO2 are required for complete cresol consumption suggests a rapid reaction with NO3 or N2O5, since these are formed in the reactions of O3 with NO:! ( 2 0 ) . Since NO3 is a radical species which is known to react rapidly with olefins and aldehydes (21,22),we presume that it is this species which is responsible for the observed cresol consumption. Relative Rate Experiments. On the basis of the above arguments, a series of experiments was conducted in order to obtain estimates of the rate constants for the presumed reactions of NO3 with cresols and other representative aromatic compounds. These experiments utilized the 5800-LSAPRC evacuable chamber ( 2 3 , 2 4 )held a t 300 f 1 K and consisted of injecting 100-1000 ppb of O3 into the chamber which contained -100 ppb of the aromatic whose rate constant was to be determined, -100 ppb of an olefin whose rate constant for reaction with NO3 was known ( 2 0 , 2 2 ) ,and -5 ppm of NO2 in pure air (17)a t -5% relative humidity. The aromatic and olefin levels both before and after the O3 injections were monitored by gas chromatography; O3 and NO2 were monitored by UV absorption and by chemiluminescence, respectively. These experiments fell into two classes; those involving phenols, the cresols, and the more reactive olefins in which stable concentrations of the reactants were obtained within 1 h of 0 3 injection, and those involving only the less reactive organics in which consumption of organics continued up to
@ 1981 American Chemical Society
Volume 15, Number 7, July 1981
829
the time that the runs were terminated, 90-135 min after O3 injection. In both cases, if it is assumed that the aromatics are consumed only by reaction with NO3 (since the reaction of aromatics with O3 are known to be slow) (13,25) and that the reference olefins are consumed only by reaction with NO3 and with 03,then the consumption of the aromatic (AR) and the reference olefin (OL) can be described by the following rate equations: d[AR]/dt = -k 1[NO3][AR]
(1)
d[OL]/dt = -kz[NO3][0L] - k3[03][0L]
(11)
Equations I and I1 can be combined and integrated to obtain
+
where k l , kz, and k 3 are the rate constants for the NO3 AR, NO3 OL (20, 22), and 0 3 OL (26-28) reactions; [ARIO, [OLIO, [ARIf, and [OLIf are the reactant concentrations at times t o and tf, respectively, and S[O3] dt is the ozone dose measured or calculated for the period in question. Reaction of the reference olefin with O3 in most cases accounted for less than -33% of the olefin consumed, and thus reaction with NO3 was the major loss process for all of the reactive organics in this system. The possibility of radical intermediates formed from the 03-olefin reactions contributing to consumption of the organic reactants was examined by using kinetic computer model calculations (which assume 25% fragmentation to radicals following the 03-olefin reaction) (5,29) and was found to be negligible under the conditions employed in these experiments. As a test of the overall validity of this technique, experiments were carried out to measure NO3 olefin rate-constant ratios for several pairs of representative olefins. The specific olefins studied were propene, isobutene, cis- and trans-2butene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene. The resulting rate-constant ratios were found to be within f30% of those derived from the rate-constant measurements of Japar and Niki (22),who monitored NzO5 decays in NzO5NOz-olefin-air systems.
+
+
+
Discussion
+
The NO3 aromatic rate constants derived from this study are summarized in Table I, with the corresponding OH aromatic rate constants also shown for comparison. I t can be seen that, in contrast with OH, NO3 reacts with phenolic compounds at least 250 times more rapidly than it does with the other aromatics studied. This could be due either to a rapid hydrogen abstraction by NO3 from the weak phenolic 0-H bond or to reversible addition of NO3 to the aromatic ring followed by a six-center rearrangement to give the same products as hydrogen abstraction:
+
Table 1. Rate Constants for Reactions of Selected Aromatic Compounds with NO3 and OH 10l2k,cm3 molecule-’ s-l compd
toluene benzaldehyde methoxybenzene phenol o-cresol mcresol pcresol
NO3
S0.003 S0.008b S0.004b 2.0 f 0ACzd
12 f 2d.e 7 f Id*a 13.f 2 d,e
OH a
6
13 20
34 4a
38
cm3 a From Atkinson et al. (8). Based on /(NO3 4- propene) = 7.9 X cm3 molecule-’ s-‘(20,221. Based on /(NO3 cis2-butene)= 2.7 X molecule-’ s-‘ (20, 22). dTheuncertainty given reflects only random uncertainty in experimental measurementsof the relative rates, and not uncertainties associated with this technique in general or in the absolute NO3 + cis2-butene or 2-methyl-2-butene rate constants used. e Based on /(NO3 4- 2-methyl-2om3molecule-’ s-‘ (20, 22). butene) = 8.1 X
+
If rapid addition of NO3 to the aromatic ring (reaction 3) occurs for the cresols, it is reasonable to expect an analogous addition to be rapid for methoxy benzene and perhaps the other aromatics studied. However, a reaction analogous to the six-center rearrangement shown in reaction 3 is not possible for nonphenolic compounds, and thus decomposition of the adduct back to the original species may dominate. This may account for the observation of much slower overall rates of reaction of NO3 with those species. If the rate constants given in Table I are correct, and as long as at least moderate levels of NO, are present (220 ppb) and [Os] >> [NO], then the consumption of phenols and cresols by reaction with NO3 would dominate over consumption by OH, which has previously been believed to be their major atmospheric sink (13).These conditions are common in moderately polluted atmospheres. Under conditions of high 0 3 and NO2 and low light intensity (NO3photolysis being relatively rapid (19)), high NO3 concentrations are expected. Indeed, researchers a t this laboratory recently observed up to 355 ppt of NO3 -1 h after sunset during an air pollution episode in the Los Angeles basin (301, and under these conditions the halflife for phenolic compounds would be less than 1min. Further studies of this system, including more direct determination of rate constants for the reactions of nitrate radicals with aromatics, are presently underway in our laboratories. Acknowledgment We gratefully acknowledge many valuable discussions with Dr. R. Atkinson during the preparation of this manuscript. We thank Mr. G. Vogelaar, Ms. S. Aschmann, and Mr. F. Burleson for providing gas-chromatographic analyses and Mr. W. Long for valuable assistance in conducting the evacuable chamber experiments. Literature Cited (1) Lonneman. W. A,: Kooczvnski. S. L.: Darlev. “ , P. E.: Sutterfield. F.E. Enuiro; Sci. Tecinoi,, 1974,8, 229. (2) Kondo, J.; Akimoto, H. Adu. Enuiron. Sci. Technol. 1975,5,1. (3) Heuss, J. M.; Nebel, G. J.; DAlleva, B. A. Enuiron. Sci. Technol. 1974,8,641. (4) Mayrsohn, H.; Kuramato, M.; Crabtree, J. N.; Sothern, R. D.; Mano, S. H. “Atmospheric Hydrocarbon Concentrations JuneSeptember 1974”;California Air Resources Board, April 1975. (5) Atkinson, R.; Carter, W. P. L.; Darnall, K. R.; Winer, A.M.; Pitts, J. N., Jr. Znt. J . Chem. Kinet. 1980,12,779. (6) Pitts, J. N., Jr.; Darnall, K. R.; Carter, W. P. L.; Winer, A. M.; Atkinson, R. “Mechanisms of Photochemical Reactions in Urban Air”; Final Report, EPA-600/3-79-110, Nov 1979. (7) O’Brien, R. J.; Green, P. J.; Doty, R. A.; Vanderzanden, J. W.; Easton, R. R.; Irwin, R. P., presented at the 175th National Meeting ~
830
Environmental Science & Technology
of the American Chemical Society Meeting, Anaheim, CA, March 12-17,1978; “Nitrogenous Air Pollutants”; Grosjean, D., Ed.; Ann Arbor Press: Ann Arbor, MI, 1979; p 189. (8) Atkinson, R.; Darnall, K. R.; Lloyd, A. C.; Winer, A. M.; Pitts, J. N., Jr. Adu. Photochem. 1979,11,375. (9) Hendry, D. G. “Chemical Kinetic Data Needs for Modeling the Lower Trooosuhere”: Reston. VA, May 15-17, 1978; NBS Spec. Publ. (V.S:) 1979, NO. 557,89. (10) Hendry, D. G.; Baldwin, A. C.; Barker, J. R.; Golden, D. M. “Comouter Modeline of Simulated Photochemical Smog”: EPA600/3-’78-059, June G78. (11) Kenlev. R. A.: DavenDort. . . J. E.: Hendrv, - . D. G. J . Phys. Chem. I
.
1978,82,-io95. (12) Hoshino, M.; Akimoto, H.; Okuda, M. Bull. Chem. Soc. Jpn. 1978,51,718. (13) Atkinson, R.; Darnall, K. R.; Pitts, J. N., Jr. J. Phys. Chem. 1978, 82,2759. (14) Perry, R. A.; Atkinson, R.; Pitts, J. N., Jr. J . Phys. Chem. 1977, 81, 1607. (15) Doyle, G. J.; Lloyd, A. C.; Darnall, K. R.; Winer, A. M.; Pitts, J. N., Jr. Enuiron. Sci. Technol. 1975,9,237. (16) Llovd. A. C.: Darnall. K. R.: Winer. A. M.: Pitts. J. N.. Jr. J. Phvs. ‘ Chem: 1976,80,789. ’ (17) Dovle. G. J.: Bekowies. P. J.: Winer. A. M.: Pitts, J. N., Jr. Enuiron.-Sci. Technol. 1977; 11,45. (18) Pitts, J. N., Jr.; Carter, W. P. L.; Harris, G. W.; Winer, A. M.; ,
1
Graham, R. A. “Studies of Trace Gases from Corona Discharge Ozonizers”; Final Report to Chemical Manufacturer Association (Agreement No. 79/80), June 1980.
(19) O’Brien, R. J., Portland State University, private communication, 1980. (20) Graham, R. A.; Johnston, H. S. J. Phys. Chem. 1978,82,254. (21) Morris, E. D., Jr.; Niki, H. J . Phys. Chem. 1974, 78, 1337. (22) Japar, S. M.; Niki, H. J. Phys. Chem. 1975,79,1629. (23) Pitts, J. N., Jr.; Darnall, K. R.; Winer, A. M.; McAfee, J. M. “Mechanisms of Photochemical Reactions in Urban Air, 11. Smog Chamber Studies”; EPA-600/3-77-0146b, Feb 1977. (24) 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. (25) Pate, C. T.;Atkinson, R.; Pitts, J. N., Jr. J. Enuiron. Sci. Health, Part A 1976,11,1. (26) Herron, J. T.;Huie, R. E. J . Phys. Chem. 1974,78,2085. (27) Huie, R. E.; Herron, J. T. Int. J . Chem. Kinet., Symp. 1975,1, 165. (28) Japar, S. M.; Wu, C. H.; Niki, H. J . Phys. Chem. 1974, 78, 2318. (29) Carter, W. P. L.; Lloyd, A. C.; Sprung,J. L.; Pitts, J. N., Jr. Znt. J. Chem. Kinet. 1979,11,45. (30) Platt, U.; Perner, D.; Winer, A. M.; Harris, G. W.; Pitts, J. N., Jr., Geophys. Res. Lett. 1980, 7,89.
Received for review November 3,1980. Accepted March 9,1981. This work was supported primarily by the National Science Foundation, Directorate for Applied Science and Research Applications, Grant PFR7801004.
Effect of Peroxyacetyl Nitrate on the Initiation of Photochemical Smog William P. L. Carter,’ Arthur M. Winer, and James N. Pitts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 92521
Propene-NO, mixtures ( 4 . 5 ppm each) were irradiated in air in a 5800-L evacuable environmental chamber under simulated atmospheric conditions a t 300 K with 0 to 4 . 2 6 ppm added peroxyacetyl nitrate (PAN). A 5-7-min period elapsed between final reactant (NO and NOz) injection and the beginning of the irradiation, during which time -50% of the injected PAN was cdnsumed, and some oxidation of the initial NO and propene occurred. Upon irradiation, enhanced initial rates of 0 3 formation and hydrocarbon consumption were observed, the maximum 0 3 yield increased, and the time to reach ozone maximum decreased. These observations are attributed in part to the reactions of the radicals formed in the dark reaction of PAN with NO, and to a lesser extent to the production of NO, from the added PAN. Our results indicate that, if PAN is carried over from the previous air pollution episodes, it will enhance photochemical smog formation on subsequent days.
Introduction Peroxyacetyl nitrate (PAN), an important component of photochemical smog, is toxic to man and plants as well as being a strong lachrymator ( 1 , Z ) . The mechanism of its formation is reasonably well understood; it results from the NO,-air photooxidation of acetaldehyde (3-5) and other organics ( 5 , 6 )present in polluted atmospheres. Specifically it is formed from the reaction of NO2 with peroxyacetyl radicals formed in hydrocarbon-NO, photooxidations.
0
0
II
CH,C-00.
+ NO,
I1
CHIC-OONO,
--c
Sink processes for PAN in the atmosphere are less well understood. Smog-chamber studies (7,8)have shown that, once formed under simulated atmospheric conditions, PAN can be relatively stable, and, if NO levels remain low, it is not 0013-936X/81/0915-0831$01.25/0
consumed even upon continued irradiation. In the presence of NO, PAN is known to decompose via reaction 2 followed by reaction 3. 0 0 CH,C--OONO, I1 -t CH,C--OO* II + NO,
(2)
0
0 CH,COO. I1 + NO --.cCH,C--O* II +NO,
( 31
Although some loss of PAN, which can be attributed to reaction with NO emissions, has been observed during the night by kilometer pathlength FT-IR spectroscopy, the same study has shown that significant amounts of PAN can persist throughout the night (9). This suggests that in multiday photochemical air pollution episodes, or during stagnant atmospheric conditions, PAN formed on the previous day may persist during the night and possibly have an impact on photochemical smog formation on the following day. The possibility of ozone enhancement by residual PAN has been examined theoretically by Hendry and co-workers (10, 11). They predicted from model calculations that initially present PAN will significantly enhance the rate of photochemical smog formation due to radicals formed from its thermal decomposition (reactions 2 and 3). However, as far as we are aware, these predictions have not been experimentally tested. Consequently, we performed smog-chamber irradiations of half-ppm levels of propene and NO, in air with varying amounts of added PAN and report here the results of this study.
Experimental Section The experimental facilities and methods employed in the environmental-chamber experiments in this study have been discussed in detail elsewhere (7,12,13)and are only briefly described here. Irradiations were carried out in the 5800-L SAPRC evacuable, thermostated, Teflon-coated environ-
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