The rate at which oxygen atoms are produced is maximized at maximum NOz concentration in sunlight. This occurs at or just prior to the complete disappearance of nitric oxide concentrations. Ozone does not accumulate (increase in concentration) until after NOz reacts or decreases. It accumulates only when NO is not available to scavenge it. Most of the more reactive hydrocarbons have disappeared in reactions prior to the time either O3 maximizes or O2(lAP)can be formed in quantity from 03;only the slow or relatively slow reactors such as paraffins, and ethylene because of its relatively high concentration, remain. Oxygen atom concentration should maximize after the time of maximum rate of production of oxygen atoms-Le., sometime after the NOz maximum concentration. This may be prior to the ozone maximum or at about the same time as the ozone maximum. 02( IAg) concentration should maximize after the O3 maximum concentration and must be much less in concentration if it depends upon the photolysis of ozone. Kummler, Bortner, et al. (p. 249) indicate this to be true
By the time O3maximizes, eye irritation has become strong, plant damaging agents arc already forming, and visibility reduction has occurred. The question then is: Where lies the importance of singlet oxygen from the dissociation of ozone in the smog or air pollution problem? Pitts’ suggestion that a complex organic donor molecule in the triplet state is transformed and the energy is transferred to oxygen molecules, seems to be more reasonable and has greater possibility of being important than the photodissociation of ozone in the photochemical smog problem, but it is so far unsubstantiated by any evidence that this occurs in Los Angeles air. The relative importance of oxygen atoms, ozone and singlet oxygen molecules from photodissociation of ozone is shown in the following schematic equations:
NO
+ + NO + Nz04 2N02 NO2 + h~ + NO + 0 0 2
A
(1) (2)
in our previous article on this subject (Kummler, Bortner, et al., 1969), we will confine our remarks to the Los Angeles area for which the availability of solar flux data (Nader. 1967) enables us to present our calculations on a quantitative basis. In an environment like the L.A. area, the ozone concentration at noon typically exceeds 20 p.p.hm. (e.g., Stern, 1968; State of California, 1966) or a number density of over 4.9 X 1016/liter.For comparison, Bortner and Kummler (1968) reviewed the composition of the minor constituents in the upper atmosphere. They concluded on the basis of experimental data (Johnson, Purcell, et al. 1952; Miller and Stewart, 1956; Horton and Burger, 1966) that a typical maximum ozone concentration at about 30 km. is 3 X 101jmolecules/liter. This unique situation applies only to Los Angeles, and only during late summer or early fall, the “smog season,” which is the season of concern to us as the pertinent solar flux data were taken from Oct. 6-20, 1965. In S
1148 Environmental Science & Technology
Important and increases fast in importance as the reactivity of HC is increased.
r
i
I 1
+
+ Oa
NO keeps O3 from (accumulating. Important and increases slowly in im(HC), + [HCOJ portance as the “reactivity” of HC is increased. Cannot become important until Oaincreases or maximizes which must occur after NO disappears.
NO + NO’
+ 02
If any importance can be attached to singlet oxygen from the photodissociation of ozone, it might be in one or both of the following ways : That it puts an upper limit on ozone concentration since the higher the concentration, the more light can be absorbed, which reduces the ozone concentration by forming singlet oxygen molecules. That it changes slightly the number of oxygen atoms in the organic products of photochemical oxidation of hydrocarbons; say, from 3 to 2. We must conclude, therefore, that the importance of singlet oxygen from O3 photodissociation cannot compete with that of atomic oxygen in the photochemical smog process, especially during the morning hours when the photochemical reactions are really just getting started, and neither ozone nor singlet oxygen are present in measurable quantities.
Walter J. Hamming Chief Air Pollution Analyst Air Pollution Control District, LA. County Los Angeles, Calif. 90013
other cities and in clean air, an ozone maximum in vertical profile does not occur near ground level. At no location, however, may conclusions regarding the importance of particular mechanisms be made on the basis of the absolute ozone concentration. Rather, the relative ozone content of the air is the important parameter. In this discussion, the ratio of ozone to atomic oxygen densities should be the focal point. The reactions for the definition of this ratio are NO2 0f
NO2
+ h~
-P
NO
oz -k M +
+ 0%
+
+0
0 3
NOz
+
0 2
(1) (2) (3)
Reactions of 0 with hydrocarbons are several orders of magnitude slower than the ozone formation by Reaction 2 and play no significant role in determining the 0-atom concentration (Leighton, 1961). Similarly, the steady-state ozone
concentration is determined by Reactions 2 and 3. Reactions of the hydroxyl radical (Heicklen, 1969) cause slow changes in the cyclic relationship above, but at any given time the 0 and O3 steady-state concentrations should be excellent approximations to the actual case (4) (5)
Hence,
Thus, a mechanism dependent on ozone tends to be more important than an 0-atom mechanism in regions of low NO concentration--i.e., in “clean” air or in smaller and meteorologically favored cities, even though the absolute ozone concentration there is relatively lower than that in Los Angeles. Thus, the work of Kummler, Bortner, et al. (1969) applies more strongly out of Los Angeles than in all other parameters assumed constant. This was not previously noted. Since the rate constants k2 and k 3 are well established (DASA Reaction Rate Handbook, 1968), the time-dependent ratio of (03)as/(0)ss may be readily calculated from knowledge of the diurnal behavior of the NO concentration by substitutioninto Equation 6,
methylethylene (TME) (1.0) were obtained as: 2,3 dimethyl2-pentene, (0.7); 1-methylcyclopentene, (0.015); 1,2 dimethylcyclopentene, (0.4); 1,2 dimethylcyclohexene, (0.9). These rate constants were compared to the absolute TME value of ~ T X E= (1.0 i . 0.3) X lo9 ~ m mole-’ . ~ sec.-l, which places the O,(lA,) reactivity at less than ljl000 of the 0-atom reactivity. For this reason the Hartley photolysis will not generate enough singlet oxygen to make 02(lA,) generally important in the photochemistry of air pollution. However, the Hartley photolysis of ozone is not the only mechanism for On(lA,) production in urban atmospheres. Kummler and Bortner (1969) have verified the energy transfer mechanism originally suggested by Khan, Pitts, et al. (1967), as have Snelling (1968), Star, Sprung, et al. (1969), and others. This mechanism, if it proceeds with unit quantum yield, according to Pitts, Khan, et a!. (1969), would produce nearly 1000 times as much 02(lA,) as would the Hartley photolysis. Chemiexcitation, particularly via the O3reaction with organics noted by Murray (1970), might also produce 1000 times as much as ozone photolysis (Kummler and Bortner, 1970). Hence the criterion of Herron and Huie (1969), for O2(lAY) importance may still be met. Both 02*and 0 will produce the radicals-e.g., RCO or OH-necessary to convert NO to NOe(Altshuller and Bufalini, 1965), perhaps via the mechanism suggested by Heicklen (1969)
+ CO +COz + H H + Os + M + HOs + M HOZ + NO NO? + OH OH
(7)
+
where the NO concentration is in p.p.hm. This relationship is especially useful near the NO peak concentrations, when the ozone concentration, even though undetectable by presently employed instruments, is by no means unimportant. The efolding time constant for the achievement of a steady state in atomic oxygen is very nearly constant (14 psec.). Thus, for all practical purposes, the 0-atom peak coincides with the NOn peak. The ozone steady state is achieved more slowly, but even at a n N O concentration of 1 p.p.hm., the steady-state assumption should be valid after 1 min, based on Reaction 3. At the NO, peak, the NO concentration (Stern, 1968; State of California, 1966) is typically l p.p.hm., and in Los Angeles this occurs at about 8 to 9 a.m. At this time, the solar flux in the wavelength region of interest is about one-third that at noon. Therefore, the rate constant for the production of O,(lA,) by Hartley photolysis of ozone should be decreased by onethird (Kummler, Bortner, et al., 1969). The recent, more accurate O,(lA,) quenching data of Clark and Wayne (1969), indicate a lower effective lifetime of O?(‘A,) (0.088 sec.) at ground level than that previously employed. Thus the ratio of O?(lA,) to O s , based on the conditions of Table I in Kummler, Bortner, et al. (1969) will range from 8 X lo-* to 6 X lo-’ at the early morning NO2 peak. This implies a [ 0 2 ( 1A,)]8,/[O],, ratio of from 0.2 to 1.4 at the peak 0-atom concentration. At any later time, the O2(IA0) produced by Hartley photolysis alone will exceed, the 0 concentration. Then even at 8 :30 a.m. PDST, the presence of 02( la,) is as real as that of 0-atoms, although neither is present in measurable quantities. Hence, little insight into the importance of O2(lAU)is to be gained from a study of the diurnal variations of smog formation alone. Of’greater significance is the recent work of Herron and Huie (1969), in which the rate of constants for 0 2 ( l A g ) reactions with several hydrocarbons relative to that with tetra-
(8) (9) (10)
Before making a final decision on the importance of 02( ‘A,), its concentration in urban atmospheres must be measured to ascertain whether the ratio of 02( la,) to 0 will be greater than that required by the data of Herron and Huie (1969). Quite recently, Herron and Huie (1970) have reiterated the contention that, based on the relative concentrations of 0 and 0 2 ( l A e ) listed by us (Kummler, Bortner, et aI., 1969), the reactions of O?(’A,) with hydrocarbons of interest (Herron and Huie, 1969) are of relatively minor importance. However, we would again point out that the O,(lA,) concentration cited by Kummler, Bortner, et aI. (1969) was designated specifically as a minimum value, based only on the Hartley photolysis of ozone as a generating mechanism, and subject to significant enhancement by other mechanisms (Pitts, Khan, et a1.,1969). Literature Cited
Altshuller, A., Bufalini, J., Photochem. and Photobiol. 4, 97 (1965). Bortner, M., Kummler, R., NASA Report GE 9500-ECS-SR-1 on MSFC Contract No. NAS8-21320, Valley Forge, Pa., July 1968. “DASA Reaction Rate Handbook,” M. Bortner, Ed., DASA No. 1948, GE, Santa Barbara, Calif., 1968. Clark, I., Wayne, R., Proc. Roy. Soc. A 314,111 (1969). Heicklen, J., Symposium on Chemical Reactions in Urban Atmospheres, Warren, Mich., Oct. 1969. Herron, J., Huie, R., J. Chem. Phys. 51, 4164 (1969). Herron, J., Huie, R., ENVIRON. SCI. TECHNOL. 4, 685 (1970). Horton, B., Burger, F., J. Geophys. Res. 71, 4189 (1966). Johnson. F.. Purcell. J.. Tousev. R.. Watanabe,~.K.,J. Geophys. - . Res. 57, 157 (1952). ‘ Khan. A.. Pitts. J. Smith. E.. ENVIRON. SCI.TECHNOL. 1. 656 _
NOL.
3,248
I
I
1
Volume 4, Number 12, December 1970 1149
Kummler, R., Bortner, M., ENVIRON. SCI. TECHNOL. 3, 944 (1969). Kummler, R., Bortner, M., Ann. N . Y . Acud. Sci., to be published, 1970. Leighton, P., “Photochemistry of Air Pollution,” Academic Press, New York, 1961. Miller, D., Stewart, K., Proc. Roy. SOC.A 288, 540 (1965). Nader, J., PHS Pub. 999-AP-38, Cincinnati, Ohio, 1967. Murray, R., Ann. N . Y . Acad. Sci., to be published, 1970; Chem. Eng. News 48,34 (1970). Pitts, J., Khan, A., Smith, E., Wayne, R., ENVIRON. SCI. TECHNOL. 3,243 (1969). Snelling, D., Chem. Phys. Lett. 2, 346 (1968). Star, R., Sprung, J., Pitts, J., ENVIRON. SCI.TECHNOL. 3, 946 (1969).
State of California, Bureau of Air Sanitation, “Oxides of Nitrogen in Air Pollution,” Sacramento, Calif., 1966. Stern, A,, “Air Pollution,” Vol. I, Academic Press, New York, 1968.
R. H. Kummler Department of Chemical Engineering and Material Sciences Wayne State University Detroit, Mich. 48202
M. H. Bortner and Theodore Baurer Space Sciences Laboratory Space Division$ Philadelphia, Pa. 19101
CORRECTION
SPECTROPHOTOMETRIC DETERMINATION OF ATMOSPHERIC FLUORIDES In this article by P. W. West, G. R. Lyles, and J. L. Miller [ENVIRON. S’CI.TECHNOL. 4, 487 (1970)], paragraph 4, page 489, should read as follows: Lanthanum nitrate buffer (2 X 10-3M). Dissolve 0.866 g. La(N03)3.6H20and 102 g. NaOAc (anhy) in 250 ml. H20. The pH of this solution, when diluted 1 :lo, and containing acetone to the extent of 25 (v./v.) (see Sampling Technique), should be 4.1 to 4.2.
1150 Environmental Science & Technology
cO.
