Conclusions The principal conclusion from these results is that freezing by the film-freezing principle permits efficient dewatering of sludge in short times at low AT. The ice film conducts heat sufficiently rapidly that the external heat transfer mechanism controls the process. Chemical addition is necessary to dewatering by freezing. The effectiveness of the alum addition points to a coagulation phenomenon. There is evidence that the coagulation (or whatever) process releases bound water irreversibly when a certain threshold condition, presumably A1 concentration in the supernatant, is achieved.
Finally, it seems clear that sludge dewatering by freezing deserves renewed attention. Literature Cited Bruce, A., Clements, G. S., Stephenson, R. A., Surzeyor 112, 849 and 50 (1953). Clements, G. S., Stephenson, R. J., Regan, C . J., J. Znst. Sewage Purif., Part 4, 318-37 (1950). Katz, W. J., Mason, D. G., Water Sewage Works 117, 110-14 (19 70). Receiced for reciew January 16, 1970. Accepted August 17, 1970.
CORRESPONDENCE
Hartley Photolysis of Ozone as a Source of Singlet Oxygen in Polluted Atmospheres
T
he article by R. H. Kummler, M. H. Bortner and Theodore Baurer on the “Hartley Photolysis of Ozone as a Source of Singlet Oxygen in Polluted Atmospheres” [ENVIRON. SCI.TECHNOL. 3,248-50 (1969)], states: On p. 249, that “the ozone density is much greater in polluted urban environments than in the upper atmosphere”; and On p. 250, that the photodissociation of ozone produces singlet oxygen, 02( lAo), whose “reactivity approaches that of 0-atoms.” In conjunction with certain other data, the authors seem to regard this as evidence that photolysis of ozone at ground level is sufficient to produce significant quantities of singlet oxygen and that this is a more important oxidizing agent in photochemical smog formation than atomic oxygen. This appears to be unsupportable for two reasons: The fact that the ozone density in urban environments exceeds that in the upper atmosphere only on infrequent occasions and then in only one urban area-Southern California; and The fact that such concentrations occur in urban environments only in the afternoon after the principal photochemical reactions involving hydrocarbons and oxides of nitrogen have taken place. That is, the ozone occurs primarily as a product of these reactions, not an initiator, and so must the singlet oxygen. For example, to enlarge on the first reason, the ozone concentration in the upper atmosphere exceeds 3 to 5 p.p.m. At the level of ozone maximum (approx. 25 km.), the barometfic pressure is about 33 one-thousandths of the ground-level pressure, and the temperature is about minus 53°C. Under these conditions, one mole occupies about 550 liters. One liter at 3 p.p.m., therefore, contains 6 X l o z 3molecules X 3 X 10-6/550 = 3.3 X l O I 5 molecules of ozone. Similarly, at 5 p.p.m., the ozone density = 5.5 X 101j molecules/liter. At ground level, if the oLone concentration is 0.1 p.p.m. (a high value for most cities), then the ozone density is 2.45 X 101j molecules/liter. The conclusion should therefore be that the
air aloft is more dense in the number of ozone molecules than at ground level. The single exception to this conclusion occurs in Los Angeles, only after the photochemical smog-forming reactions have proceeded almost to completion and the NOs concentration begins to decline, and only on those days when the temperature inversion has remained low all day. This inversion allows the ozone to accumulate. When the ground-level concentration reaches 0.15 p.p.m. by volume, it is about the same as that of the upper air, and only when it reaches or exceeds 0.25 p.p.m. does the ozone density of the ground-level concentration exceed that of the upper atmosphere. Further, in support of the second reason cited above, elevated concentrations of ozone occur only in the afternoon. If singlet oxygen-or any less usual form of oxygen molecule, such as atomic oxygen or ozone-is to be important in the photochemical reactions, it must be there in quantity in the early morning. This is the time of initiation of such reactions involving oxygen atoms from the dissociation of NOs and hydrocarbons, especially those termed “reactive” such as olefins and such substituted aromatic compounds as xylenes. At this time of day, the ozone concentration is so low it is not measurable. Since the significant production of 02( la,) is dependent upon the presence of relatively high concentrations of ozone, the concentration of 02(’Ag) at that time of day must be so vanishingly low as to be nonexistent. The most commonly accepted interpretation of the sequence of photochemical smog-forming reactions involves, first, the conversion of NO to NOz followed by the photodissociation of NOz to produce atomic oxygen. In the competition for oxygen atoms then taking place among oxygen molecules, nitric oxide, and hydrocarbons, the last named appeared to be favored, This is illustrated by the fact that introduction of such compounds, especially the more reactive types, accelerates the conversion of NO to NOs to a rate which is fantastic compared with that of the usual three-body reaction of 2 N 0 O2-+ 2N02. How this occurs is not clearly understood, but it is known that:
+
It occurs at very low ozone concentrations. It occurs, therefore, at very, very low 02( lAo) concentrations. Volume 4, Number 12, December 1970 1147
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
