Ozonation, on the other hand, did not result in the formation of any of these volatile halogenated compounds. The concentration of these compounds actually decreased during ozonation, possibly resulting from both stripping and oxidation of these compounds by the ozone. Therefore, with the ozonation process, organic compounds can ultimately be converted to harmless carbon dioxide by intensive ozonation of water. Cost, however, appears to be the major obstacle to the wide application of ozonation. Effective coagulation and sedimentation in the early stages of water treatment would tend t o decrease ozone demand; incorporating UV irradiation and catalysts would enhance organic removal. These approaches would lower the ozonation cost. Further cost reductions can be realized through the development of efficient ozone generators and contactors and by obtaining knowledge about optimal conditions for ozonation reactions including such factors as reactor design, pH adjustment, temperature control, and application of UV. Future studies should therefore be directed toward minimizing costs by optimizing ozonation processes and toward assessing the health hazards associated with ozonation. Conclusions
Upon ozonation, 2-propanol was converted to acetone that in turn was oxidized to acetic and oxalic acids. Trace amounts of formaldehyde and formic acid were also detected in the ozonated acetone solution. Further ozonation of acetic acid resulted in the formation of giyoxylic acid that was oxidized readily to oxalic acid. The latter was a common end product of most ozonation reactions with organics and oxidized directly to carbon dioxide at a faster rate under the acidic condition. The removal of organics was greatly improved under UV irradiation. Methylene chloride, chloroform, carbon tetrachloride, bromodichloromethane, chlorodibromomethane, and bromoform were observed in the reaction mixtures of chlorinated ultrafiltration (UF) and reverse osmosis (RO) retentates of secondary effluent that consisted mainly of humic and fulvic acids, respectively. On the other hand, ozonation of UF and RO retentates did not result in the formation of these volatile halogenated organics.
Literature Cited (1) Dowty, B. J., Carlisle, D. R., Laseter, J. L., Enuiron. Sci. Technol.,
9,762 (1975). (2) Symons, J. M., et al., J . Am. Water Works Assoc., 67, 634 (1975). (3) DeWalle, F. B., Chian, E.S.K., J. Enuiron. Eng. Diu., ASCE, 100 (EE5), Proc. Paper 10867 (Oct. 1974). (4) Rook, J. J., J. Am. Water Works Assoc., 5,168 (1976). (5) Fang, H.H.P., Chian, E.S.K., Enuiron. Sci. Technol., 10, 364 (1976). (6) Guisti, D. M., Conway, R. A., Lawson, C. T., J . Water Pollut. Control Fed., 46,947 (1974). ( 7 ) Gould, J. P., Weber, W., Jr., ibid., 48,47 (1976). (8) Dobinson, F., Chem. Ind., 853 (1959). (9) Gilbert, E., “Ozonolysis of Chlorophenols and Maleic Acid in Aqueous Solution”, Proceedings of the 2nd Int. Ozone Symp., p 253, Montreal, P.Q., Canada, May 1975. (10) Ahmed, M. D., Kinney, C. R., J . Am. Chem. Soc., 72, 559 (1950). (11) Mateles, R. I., Chian, E.S.K., Enuiron. Sci. Technol., 3, 569 (1969). (12) Gleason, M. N., Gosselin, R. E., Hodge, H. C., Smith, R. P., “Clinical Toxicity of Commercial Products”, 3rd ed., Williams and Wilkins, Baltimore, Md., 1969. (13) Schechter, H., Water Res., 7,729 (1973). (14) Bethge, P. O., Lindstrom, K., Analyst, 99,137 (1974). (15) Webb. R. G.. Garrison. A. W.. Keith. L. H.. McGuire. J. J.. U.S. EPA Rep. No. EPA-R2-73-277,1973. (16) Kramer. D. N.. Klein, N., Baselice, R. A,, Anal. Chem., 31,250 (1959). (17) Houle, M. J., Long, D. E., Smette, D., Anal. Lett., 3, 401 (1970). (18) Chian, E.S.K.,Cheng, S. S.,DeWalle,F. B., Kuo,P.P.K.,Prog. Water. Technol.. 9.761 (1977). (19) Kuo, P.P.K., Chian, E.S.K., DeWalle, F. B., Kim, J. H., Anal. Chem., 49,1023 (1977). (20) , , “Standard Methods for the Examination of Water and Wastewater”, 13th ed., American Public Health Assoc., Washington, D.C., 1971. (21) Fang, H.H.P., Chian, E.S.K., J . Appl. Polymer Sci., 19. (22) Bunn, W. W., Haas, B. B., Deane, E. R., Kleopfler, R. D., Enuiron. Lett., 10, 205 (1975).
Receiued for review March 31,1977. Accepted June 24,1977. Work supported by the U S . Army Medical Research and Development Command under Contract No. DAMD 17-75-C-5006.Part of this paper was presented at the International Ozone Institute Workshop, Cincinnati, Ohio, Now. 17-19, 1976.
Contamination Effects on Ozone Formation in Smog Chambers Joseph J. Bufalini”, Theodore A. Walter, and Marijon M. Bufalini US. Environmental Protection Agency. Office of Research and Development, Environmental Sciences Research Laboratory, Research Triangle Park, N.C. 2771 1
High ozone concentrations observed when “clean” air is irradiated in smog chambers are a result of chamber contamination. Modeling results show that in the ppb range, NO2 and HONO can produce significant 0 3 levels when irradiated in the presence of formaldehyde. Nitrogen dioxide is slightly more efficient in producing ozone than HONO, whereas both are much more so than HON02. A mechanism for NO, regeneration from HONO2 photolysis is suggested. All of these contaminants (NOz, HONO, HON02, and CH20) can explain the high 0 3 values observed in smog chambers.
Smog chambers have been used since the early 1960’s to simulate atmospheric conditions to better understand photochemical smog production. The use of smog chambers for
simulating the chemistry of smog production has certain advantages over the use of aerometric field studies to elucidate smog phenomena. The temperature, relative humidity, solar intensity, and hydrocarbon distibution can all be controlled. The dependence of chemical reactions on one or more of these parameters can therefore be better established. Also, many of the meteorological effects can be eliminated, thus greatly simplifying the photochemical smog system. However, smog chambers have their own problems. The thermal rate of oxidation of NO in the presence of ultraviolet light does not follow the theoretical predictions. Wall contamination effects have been used to explain this phenomenon (I).This contamination can arise from the wall material itself or from material that had been irradiated in the chamber from previous use. Recent results ( 2 ) obtained from modeling the Bureau of Mines smog chamber data show that the reaction Volume 11, Number 13, December 1977
1181
mechanism that explains the chamber results with hydrocarbon and NO, concentrations in the 2-10 ppm range did not explain the results when the hydrocarbon and NO, concentrations were less than 1 ppm. To explain these low concentration runs, a “wall emission factor” had to be introduced in the mechanism. This paper is concerned with the problem of chamber contamination. We considered smog chamber data obtained by various experimenters and deduce that wall emissions in the form of HzCO, HON02, HONO, and NO2 are extremely important. Modeling data are presented to substantiate this conclusion.
Results and Discussion Figures 1-3 present smog chamber data obtained by the irradiation of presumably “clean” air. The air cleansing process and/or chamber history was different in each case. The data in Figure 1 were obtained by irradiating air that was cleaned by passing outside ambient air through activated charcoal and particle filters. This air was then irradiated for 350 rnin in a 335 ft3 aluminum smog chamber with Teflon windows. After irradiating for 350 min, the formaldehyde concentration was over 0.25 ppm. (The background air prior to irradiations usually contained between 0-0.07 ppm CH20). Theoretical computer calculations were made to ascertain if it were possible to obtain this much CHzO from oxidation of methane since the cleaning process via charcoal scrubbing does not remove methane. These calculations show that when starting with 1.9 ppm CH4 and a few pphm of NO, and using a 46-step reaction mechanism, the amount of CH4 remaining after 350 min reaction time was 1.893 ppm. Therefore, the highest amount of CHzO that can be expected from the oxidation of CHI is only 7 ppb. An obvious explanation for the high CHzO is that other residual hydrocarbons not removed by charcoal scrubbing are oxidized to produce the high CH20. However, measurements of the nonmethane hydrocarbons in the “clean” air prior to irradiation indicate that they are usually of the order of 0.1 ppmC with the majority of these being saturated hydrocarbons. Because of the low concentration levels and the low reactivity ( 3 ) of these saturated hydrocarbons, this alternate explanation for the high CH20 was discounted. Other researchers have noted the formation of CHzO in these systems. Dimitriades and Wesson ( 4 ) found that air mixtures completely free of CHzO could not be obtained in their chamber. In addition to the production of CH20 when the “clean air” was irradiated, there was a buildup of 0 3 beginning when NO2 reached its maximum and NO was at a very low level. A t 350 min the 0 3 had reached 0.22 ppm but had not reached its maximum level. Figure 2 shows the data obtained from irradiating air in a 400 f t 3 aluminum chamber with Teflon windows. The air irradiated in this chamber had been previously cleansed by a series of procedures to produce “ultraclean” air. Ambient air, cleaned by passage through activated charcoal and particle filters, was used to purge the chamber. This “precleaned” air was then recirculated through: a combustion catalyst at 600°, Purafil odoroxidant to remove any NO formed in the combustion process, activated charcoal, and a final particle filter until the total nonmethane hydrocarbon was less than 30 ppbC (mostly ethane and propane), and NO, was less than the sensitivity of the NO, chemiluminescence monitor (less than 3 ppb). NO, and 0 3 are observed even though the air was “ultraclean”. After 450 min 0 3 had reached over 40 ppb, and NO, had reached 25 ppb (measured with chemiluminescent monitors). The data shown in Figure 3 were obtained from a cylindrical 400-L glass/Teflon chamber completely surrounded by ultraviolet lights. In this particular run, “ultraclean” air was 1182
Environmental Science & Technology
0
50
200
150
100
250
350
300
TIME (rnin.)
Figure 1. Results from
irradiation of treated ambient air in 335 ft3 aluminum Teflon smog chamber ( k 3 , = 0.26 min-’)
0
60
120
180
240
300
360
420
TIME (min.1
Figure 2. Results from irradiation of “ultraclean” air in 400 ft3 aluminum Teflon chamber ( k 3 , = 0.51 min-’)
o’20 0.18 0.16
2
L 1
0.14
0.06 0.04 NO,
0.02
I 0
400
800
1200
1600
2000
2400
0-0, 2800
3200
3600
TIME Imm.1
Results from irradiating “ultraclean”air for long periods in glass Teflon chamber (k3, = 0.50 min-’) Figure 3.
irradiated for 1400 min at which time n-butane was added into the chamber air. The irradiation was then continued for another 2400 min (a total of 3600 rnin). After the previous chamber run, the chamber air was recirculated for 16 h through activated charcoal, a heated 0.5% rhodium on l/B-in. alumina pellet catalyst, Purafil, and activated charcoal. This air was then ozonized (1ppm 0 3 ) and held in the chamber for 6 h in the dark. The air was then recirculated again through the cleansing apparatus for 16 h. The measured background contaminants after the above cleansing were: NO, less than 1 ppb, NO less than 1ppb, 0 3 less than 1ppb, and THC less than 50 ppb. During the air irradiation phase of the experiment, 0 3 again built up to a concentration of over 20 ppb after 1400 min of irradiation. When n-butane was added at 1400 min, the 0 3 level continually increased to 0.12 ppm by 2100 min and leveled off at 0.15 ppm at 3000 min. The concentration of NO, at 3000 rnin as measured with a chemiluminescence monitor was 0.01 ppm. The concentration of NO, as measured by the Saltzman method showed that less than 10%of the 0.01 ppm was NOn. Chemiluminescent NO, monitors have been shown
Table 1. Photochemical Model Reactlon
(1) (2)
%
Rate
NO -I-NO2 2HONO 2HONO d NO NO2 f H20
+
-%
HONO HO f NO HO NO -+D HONO (5) HONO OH --+ NO2 H20 (6) HONO t HN03 --+ 2N02 H20 (3) (4)
+
+
+
-
+ + +
+ +
02
+
(7) CH4 OH Me02 H20 (8) Me02 NO --+ M e 0 t NO2 (9) Me02 HOP-+ M e 0 OH
+ O2
(10) MeO-%H2CO+H02
+ OH 02,CO + HO2 + H20 H2CO + OH -% HC03 + H20
(1 1) H2CO
(12)
-% HC03 4- HO2
-
(13) H2CO
(6)
1.5 X 4.5
(7)
3X 1 . 2 104 ~ 1 x 104
(8)a
(7) (9)
0.2
(7)
15 9.1 x 102 1 x 102
( 9) (9) (9)
5.0 x 103
(9)
3.5
x
103
( 7 0 ~
1.8
x
104
(70y
1.7
x
10-3
(70y
02
hu
(14)
CO f 2H02
H2CO
3.3
x
10-4
(70p
5.2
x
10-3
(9)
9.1 x 102 2 x 102 6.4 x 10-4
(9) (9)
02
(15) H P C O - % H ~ + C O
+ +
(16) HC03 -I-NO 02,NO2 HO2 4- C02 (17) H02 NO NO2 OH (18) 0 3 -WALL
+
(19)
03
A O('D) -I-
(21) q 3 P ) (22) O('D)
02
02
O3
A O(3P) HKJ
(23) O('D) --+ 20H (24) OH t O3 Hop (25) O3 H02 -+D OH
+
+ + 202 0 2
Reaction
Ref
c
(26)
4- NO2
-%OH
"03
2
x
(74)
2.5
(27) No3
+
-
NO2 4- O3
(28) HN03 OH --+ NO3 (29) NO2 OH HN03 (30) NO3 NO --+ 2N02
+ H20
+ + (31) NO2 5 NO + 0 (32) 4-NO -+ NO2 + O2 (33) + NO2 NO3 + O2 (34) NO3 + NO2 N2O5
10-5
--+
0 3
-
(35) N205 --+ NO3 4- NO2 H20
(36) N2O5 --+ 2HN03 (37) 2H02 H202 (38) 2H02 20H f
+
02
+
+ OH + C02
-%
(42) 2H0 0 3 4- H20 (43) HO H02 H20
+
(9)
02
(73) (9) d
21 4.6X 6.8 X I O 3
(7) (7) (9)
15
(7) e
x
103
(9)
5.3
( 9)
2 x 10-3 39 6.5 x 104
(9) ( 9) (9)
x x
(9) (9)
3.4 3.0
+ (44) OH -I-CO -% HOP + C02 (45) HO2
( 73)
1.9 x 102 1.5 x 104 1.1 x 104
10 5.3
0 2
(39) H202 20H (40) HC03 H02 --+ HO2 (41) 2H0 +H202
1.5
0.30
-
03
103 105
2 . 5 X 10'
(7)
02
+ HC03
1.8 X
02
+ H02 -I-CO
3.5 X
( 7Qb
3.0
( 15)
H2CO +H202
x
10-3
( 77)
(46) HO2 -I- H2CO -+ H202
2.1
x
10-2
(77)
(47) H2CO
4.2
x
105
(7)
(48) H2CO 4- NO3 02,CO 4-
8.5 X
loio
( 77)
x x
109 102
(77) (72) ( 73)
to respond to a number of NO,-containing compounds ( 5 ) . These include PAN, alkyl nitrates, and HON02. HONO would also be expected to give a positive response. Thus, if the Saltzman method is specific for NO2, then less than 10% of the chemiluminescent-measured NO, value could be NOz. These data indicate that NO, and some organic fragment must be liberated from the chamber walls and, furthermore, that the wall emissions are not a result of heating effects since irradiating with infrared lights did not cause the NO, products to be liberated. To test this hypothesis, we decided to model a chamber that would contain varying low concentrations of HONO, HN03, NO2, and H2CO in a standard air mixture of 1.4 ppm CH4, 0.1 ppm CO, and 50% relative humidity. The mechanism and rate constants employed are shown in Table I. All rate constants except the photodissociation Reactions 3,13-15,19,20,26,27, and 31 and Reaction 21 are in units of ppm-' min-'. The photodissociation reactions are in units of min-l. Oxygen concentration was included in the rate constant when it participated in product formation. We made no attempt to model the three different pieces of data obtained from the three chambers because the experimental results could be reproduced by simply varying the various heterogeneous rate constants and introducing different wall emission factors. Instead, we chose to model some typical levels of pollutants expected from chamber walls. The 13 ppm of HCHO is certainly atypical. However, this concentration was used in an earlier study (16) where this con-
Ref
hsr
3.4
7.7 1.1 2.3
Rate
+ NO3 2HC03 4-
"03
"03
4- HO2
lo-'
0.59
( 70)b
f
a Assumed as 0.1 of k3,. Combined with ref. 7. Assumed. This value is usually dependent upon type of reactor. Ref. II-asumed 1.4 X min-'. Experimentalvalue. a Assumed. This is a heterogeneous rate. 'Assumed and combined with ref. 70.
centration produced over 0.1 ppm of 0 3 in a dirty bag chamber. Therefore, this high concentration of HCHO was modeled with low concentrations of NO,. The results obtained from the use of this mechanism are shown in Table 11. The computer runs were terminated after 10 h of radiation since longer irradiation times would be unrealistic in terms of one solar day of irradiation. In most of the cases modeled, the 0 3 levels were still increasing at the time the runs were terminated. Smog chamber results also indicate that the 0 3 levels would continue to increase if the irradiation time were greater than 10 h. An example of this is shown in Figure 3 for the experimental results obtained during the air irradiation phase of the experiment. The 0 3 levels increased from 15 ppb at 600 min to 24 ppb at 1400 min. If an irradiation were interrupted after a solar day of irradiation, held in the dark to simulate nighttime, and then irradiation again for a second day, we would expect the O3 level to decrease during the nighttime period, predominantly due to wall decomposition of 0 3 (Reaction 18),and to reaction with the small amount of NO, remaining in the gas phase (Reactions 32 and 33). The 0 3 would then increase on the second day of irradiation, reaching a level greater than that observed on the first day. This accumulation of 0 3 on the second day is assumed since the rate of 0 3 formation is greater than the rate of 0:jdegradation. That is, the wall reactions of O3 are too slow, and the amount of NO, is insufficient to titrate all the 0 3 , and a buildup of 0 3 would result. Volume 11, Number 13, December 1977
1183
Table It. Results from Photochemical Model HONO?, ppb
HONO, PPb
NO?, ppb
CH20,
ppm
(03) max, ppb
10 10 25 25
... ...
...
...
... ...
... , , .
5 6 7
...
5 5
...
...
...
...
a
...
...
5 5
13 0.05 13 0.05 13 0.05 13 0.05 13 13 13 13 0.05 0.05 13 0.05
7 10 18 15 a7 25 96 26 90 96 160 162 30 24 42 21
Run
no.
1 2 3 4
,
.,
t(o3) max, min
600a 600a 600a 600a 138 349 127 320 137 128 128 127 289 600a 600a 6OOa
5 ... 9 10 10 10 ... 5 11 ... 5 5 5 5 12 10 5 5 13 10 14 ... lb ... 15 100 ... ... 16 100 ... , , . a O3concentration was still increasing when the computer run was terminated at 600 min. A constant concentration of 1 ppb of HONO in the gas phase during the computer run.
These theoretical calculations substantiate the hypothesis that nitrogen-containing species and some organic fragment must be liberated from the chamber walls and have led us to conclude that these wall emissions are probably H:,CO, HON02, HONO, and NOz. Ozone was produced in every run. The concentration ranged from 7 ppb up to 162 ppb depending upon the initial concentration and type of contaminant present. Even with only 1ppb af HONO constantly present in the gas phase and 0.05 ppm CHzO present as contaminants, the 0 3 level reached 24 ppb at 600 min and was still increasing when the computer run was terminated.
E f f e c t of Formaldehyde C o n t a m i n a n t The amount of CH20 had a remarkable effect on the 0 3 when CH20 was present with low ppb levels of either NO2 or HONO or when present with all three NO,-containing compounds (NO:,, HONO, and HON02 a t low ppb levels). When present at the 50 ppb level, CHzO produced less than 30 ppb O3 in the presence of any of the NO,-containing compounds. However, when CHzO was present at the 13 ppm level with either NOz or HONO, approximately three times as much 0 3 was produced. When all three NO,- containing compounds were present with CH20 at 13 ppm, approximately five-times as much 0 3 was produced. This effect can be attributed to Reactions 11-17 with Reactions 21 and 31. At 50 ppb the concentration of CH20 is insufficient to produce a significant concentration of RO2 and ROs radicals to reoxidize NO to NO2 after NO2 is photodissociated to NO and 0 3 . Effect of NO,-Containing C o n t a m i n a n t s Whenever the concentration of CH20 is sufficient to produce a significant concentration of RO2 and ROs radicals, NO,- containing contaminants have a noticeable effect on the level of 03.The magnitude of this effect is dependent upon whether the NO,-containing contaminants-HONO, HON02, and NOZ-are present individually or in combination with each other. From Table 11, it would appear that HONO2 is not as important a contaminant as HONO or NO2 for the formation of 0 3 . However, there is a possible problem with the absorption coefficient of HON02 in those calculations with HONO:, as 1184
Environmental Science 8 Technology
the only NO,-containing contaminant present. This possible problem and its consequences are discussed in detail below. The effect of NO2 as a contaminant is slightly greater than the effect of HONO as a contaminant, but in either case the effect is not large. When CH2O was present with HON02 at 100 ppb, more O3 was produced with CH2O at 13 ppm than with CH20 at 0.05 ppm. However, when the concentration of HONO2 was 10 ppb, more 0 3 was produced with CH20 at 0.05 ppm than with CH20 at 13 ppm. At 25 ppb HON02, the amount of O3 produced was more nearly the same for both concentrations of CH20.
Odd Hydrogen Radicals The theoretical profiles for OH and HO:, concentrations in runs 7 and 8 are shown in Figure 4. The run with the higher formaldehyde concentration has higher values for both OH and HOPradicals. The principal reason for this is because of Reactions 39 and 40. With 13 ppm CH2O at 200 min, the rate of Reaction 39 is 170 times greater than in the 50 ppb CHzO system. Reaction 40 is over lo4 times greater. The presence of these high levels of free radicals increases the rate of NO, depletion. Thus, in run number 7 , the NO and NO:, remaining after 600 min is 0.88 X lop6and 0.39 X ppm. However, in run number 8, the NO and NO2 are 0.96 X and 0.23 X lo-" ppm. This means that even at 600 min, the lower aldehyde containing run (number 8) will continue to produce ozone. This is confirmed by the presence of 0.025 ppm in number 8 as compared to 0.024 ppm ozone in run number 7.
Wall Contamination The results of the experimental smog chamber runs, shown in Figures 2 and 3, indicated that NO,-containing contaminants and some organic fragments must be liberated from the chamber walls. Theoretical calculations suggest that these wall emissions are probably H2C0, HON02, HONO, and NOz. If NO,-containing compounds are absorbed on the walls on the chamber from previous irradiations, as is indicated by the experimental results shown in Figures 2 and 3, it would appear unlikely that the identity of the NO,-containing compound could be either HONO or NO2 since both of these compounds have a high rate of dissociation in the presence of ultraviolet light. Another explanation for the NO,-containing contamination in these chambers after the start of the irradiation is that HONO2-produced in an earlier experiment and adsorbed upon the chamber walls-undergoes photodissociation
A
t
i 20
-1
10
N
0
=
5
2
I
I 0 0
100
1 I
I
I
I
200
300
400
500
0 so0
I R R A D I A T I O N TIME lminl
Figure 4. Free radical concentration profiles for CH20 at 13 ppm (solid lines) and 0.05 ppm (dashed lines) with NO2 at 0.005 ppm
a t a higher rate than that used in the theoretical calculations. A photodissociation constant of 2.5 X min-’ was used for the dissociation of HONOz into OH and NO2. This value was obtained by extrapolating the absorption coefficient obtained by Johnston et al. (14). If HONO2 undergoes a bathochromic shift in the absorption region due to its adsorbed state, then the rate constant would be higher than that used in these calculations. This would then make HONOp a more important contaminant than is suggested by these calculations. Another possibility is that the nitric acid adsorbed on the wall surface would exist in an ionic state. This would enable HON02 to undergo nitrate-type photodissociation at longer wavelengths since nitrates will photodissociate at wavelengths greater than 300 mfi (17). Applicability of Smog Chamber Data t o Atmosphere
These findings raise some questions regarding the application of smog chamber data to derive reactivity scales for hydrocarbons and the utility of these scales for formulating control policies. When the principal concern is only one solar day of irradiation, the relative reactivities of hydrocarbons will remain the same as previously established. Olefinic hydrocarbons will produce the greatest amount of 0 3 , followed by the substituted aromatics and then the so-called unreactive paraffins. However, when longer irradiation times (comparable to more than one solar day) are considered, the short irradiation time reactivity scale will not be applicable. Experimental and theoretical results indicate that as long as NO, is available, all hydrocarbons will generate 03.The extent of the O3 accumulation depends upon the relative rates of 0 3 formation and O3 degradation. Surface (wall) decomposition of 0 3 and the reactions of 03 with NO, control the O3 degradation rate whereas the formation of O3 is controlled by the concentration of RO, (RO2, R03) radicals which react with NO to form NO2 which can photodissociate to form 03.For example, in the photooxidation of methane, O3 will not accumulate to any great extent since the 0 3 degradation rate is greater than the O3 formation rate. That is, the availability of R02 and ROBradicals to react with NO, is limited, because only one ROp radical can be formed from each oxidized CH4 molecule and the slow rate of oxidation. However, as long as some NO, is available to react with the limited number of ROp and R03 radicals, some O3 will be formed, although in limited amounts. Thus, when viewed in this manner all hydrocarbons will produce ozone, and the amount of ozone that will be found in a particular geographical area a t any one time will depend upon the hydrocarbons loading in the atmosphere, the ability of these hydrocarbons to produce RO, radicals, the amount of NO, that can be recycled by reaction, and the lifetime of 0 3 in the atmosphere. However, since these 03 precursors can be transported great distances, the slower reacting hydrocarbons can potentially form 0 3 in an area that has relatively low hydrocarbon emissions. Thus, it may be necessary to have near 100% control of hydrocarbons and NO, a t individual sources in highly populated and industrialized areas to maintain the total hydrocarbon loading of the atmosphere at a sufficiently low level to meet the O3 standard.
If the lifetime of 0 3 in the real atmosphere is much lower than that observed in many chambers, then the results, i.e., production of high 0 3 concentrations when there is little initial NO, present, can only be applied to the atmosphere with caution. Ripperton et al. (18)suggest that in a rural area near Wilmington, Ohio, the O3 dark half-life a t ground level is approximately 10 h, and a t 600 m above ground level it is 20 h. One would expect that the daytime half-life would be lower since O3 will photodissociate with sunlight (11). The 0 3 regeneration steps are important; nonetheless 0 3 still degrades at a high rate. In smog chambers the O3 half-life, when irradiated, is generally of the order of 5-10 h, but in the dark, 0 3 half-life is as long as 36 h when the chamber surfaces are of inert material and have been properly acclimated. Although the surface to volume is much lower in polluted atmospheres than in smog chambers, there is apparently sufficient turbulence in the atmosphere to enable O3 to be destroyed on surfaces. Until the lifetime and fate of 0 3 and NO, in the atmosphere are firmly established, extrapolations from smog chamber conditions to real atmospheric conditions are questionable. Literature Cited (1) Bufalini, J. J., Kopczynski, S. L., Dodge, M. C., Enuiron. Lett., 3 (2), 101 (1972). (2) Dodge, M. C., “Combined Use of Modeling Techniques and Smog
Chamber Data to Derive Ozone-Precursor Relationships”, presented a t Proc. of Int. Conf. on Photochemical Oxidant and Its Control, Raleigh, N.C., Sept. 1976. (3) Dimitriades, B., Proc. of Solvent Reactivity Conf., EPA-650/374-010, NOV.1974. (4) Dimitriades, B., Wesson, T. C., J . Air Pollut Control Assoc., 22, 33 (1972). ( 5 ) Winer, A. M., Peters, J. W., Smith, J. P., Pitts, J. N., Jr., Enuiron. Sci. Technol., 8 , 1118 (1974). (6) Connell, P. S., Johnson, H. S., “Smog Chamber Conference”, Environmental Protection Agency, Research Triangle Park, N.C., Oct. 16-17. 1974. (7) Hecht, T.’A.,Seinfeld, J. H., Dodge, M. C., Enuiron. Sci. Technol., 8, 327 (1974). (8) Johnston, H. S., Pitts, J. N., Lewis, J., Zafonte, L., Moltershead, T., Atmospheric Chemistry and Physics”, Project Clean Air, Task Force Assessments, Vol4, Univ. of California, 1970. (9) Demerjian, K. L., Kerr, J . A,, Calvert, J. G., Adu. Enuiron. Sci. Technol., 4, l(1974). (10) Heicklen, J., Grant Report 800874. (11) Dodge, M. C., Hecht, T. A., Enuiron. Lett., 10, 257 (1975). (12) Davis, D. D., Can. J . Chem., 52,1405 (1974). (13) Garvin, D., Hampson, R. F., Eds., “Chemical Kinetics and Photochemical Data for Modelling Atmospheric Chemistry”, NBS Tech. Note 866, June 1975. (14) Johnston, H. S.,Chany, S.G., Whitten, G., J . Phys. Chem., 78, l(1974). (15) Morris, E. D., Jr., Niki, H., ibid., p 1337. (16) Bufalini, J . J., Gay, B. W., Jr., Brubaker, K. L., Enuiron. Sci. Technol., 6,816 (1972). (17) Daniels, M., Meyers, R. V., Belardo, E. V., J . Phys. Chem., 72, 389 (1968). (18) Ripperton, L. A., Eaton, W. C., Sickles, J . E., “A Study of the Oxidant-Precursor Relations Under Pollutant Transport Conditions”, Final Report, Contract No. 68-02-1296, Jan. 1976.
Received for reuiew October 21, 1976. Accepted June 27, 1977
Volume 11, Number 13, December 1977
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