The Hartley Photolysis of Ozone as a Source of Singlet Oxygen in Polluted Atmospheres R. H. Kummler, M. H. Bortner, and Theodore Baurer General Electric Space Sciences Laboratory, P. 0. Box 8555, Philadelphia, Pa. 19101
w Using experimentally observed solar irradiances and extinction coefficients, the first-order rate constant for the photolysis of ozone to form singlet oxygen is calculated. The minimum concentrations of O,('A,) in polluted urban atmospheres are then estimated on the basis of available quenching data. It is shown that these concentrations are sufficient to make 02(lAg)a much more important oxidizing agent than atomic oxygen in polluted environments.
F
ollowing a brief earlier treatment by Leighton (1961), the possibility that metastable singlet-state molecular oxygen, 0 2 ( I A Q rl&), plays a significant role in air pollution photochemistry has been the subject of recent, considerably renewed interest (Khan, Pitts, et a/., 1967; Pitts, 1967; Wilson, 1968). Prior to 1967, only three mechanisms regulating the population of singlet-state oxygen in the polluted atmosphere were recognized, viz.: photoexcitation a
+ * 02(lAg, '2,)
0 2 ( X 3 2 ) hv
b
(1)
collisionally induced absorption a
+ M + hv * Oz('A,, '8,)+ M
O2(X32)
b
(2)
and collisional quenching 02(lA,,
'20)
+M
+
+M
02(X32)
(3)
Thus, in his early work, Leighton (1961) inferred that the importance to air pollution photochemistry of singlet-state oxygen would probably be limited a t one atmosphere pressure by the supposedly fast Reaction 2b, collisionally induced emission, which he expected would be the dominant loss mechanism. However, he neglected the corresponding absorption step, Reaction 2a, which, by the principle of microscopic reversibility (Fisher and McCarty, 1966), ought then to be the dominant production mechanism, and which thereby would tend to counteract the presumed tendency toward decreased lifetimes of the metastable states of oxygen at ground-level pressures. In all this, it was assumed that de-excitation would occur only by the radiative processes l b and 2b-i.e., Reaction 3 was not taken into account. Subsequently, Badger, Wright, et a / . (I 965) investigated the collisionally induced absorption of oxygen (Reaction 2a) to the 'Ao state. I t may be shown from their results that the rate 248
Environmental Science & Technolog1
of Reaction 2a, as compared with Reaction l a , is greater by only a factor of about two, for a given solar flux at atmospheric pressure and that the comparative rates of Reactions 2b and 1b are similarly related. On the other hand, the rate constant given by Winer and Bayes (1966) for the collisional quenching of 0 # A g ) by O 2 in Reaction 3 suggests that that process may be fast enough a t one atmosphere to constitute the dominant 02(lA,) depletion term. Subsequent studies of the quenching of 02('A,) by both O2 and N2 confirm this result (Clark and Wayne, 1968). Thus, Leighton's original conclusion as to the limited importance of singlet-state oxygen in air pollution would seem to have been qualitatively correct, although not for the reason he imagined. Actually, Leighton's argument as modified to include the role of Reaction 3 is more valid as regards the importance of 02('8,)than that of the 'A, state. For, although the former state is produced considerably faster than the latter by Reaction 2a, collisionally induced absorption, it is also collisionally quenched more rapidly, as may be seen from a comparison of the results of Arnold, Kubo, er al. (1968) and Izod and Wayne (1969) with those of Clark and Wayne (1968) and Winer and Bayes (1966). The ' 2 , state is, therefore, unimportant if direct light absorption--e.g., by Reactions la and 2a-is the populating mechanism, or if its source term is not considerably greater than that for the ]A, state (Kummler and Bortner, 1967). The significant event leading to the recent regeneration of interest, despite these rapid depletion mechanisms for singletstate oxygen, has been the discovery by Khan, Pitts, et al. (1967) of a source of OZ(lAg)production via energy transfer from a complex organic donor molecule in the triplet state, (3D). The latter is thereby transformed to the ground singlet state (lD): 3D
+ 02(X38)
-+
'D
+
02('A,,
'8,)
(4)
Such a mechanism is independent of the direct absorption of light by ground-state oxygen. However, insufficient evidence exists a t the present time regarding a specific organic compound which might combine: a sufficiently high absorption coefficient, a high probability for energy transfer, and adequate concentrations in the urban atmosphere to permit the assumption that the donor mechanism represented by Reaction 4 can be applied realistically to the solution of problems in air pollution photochemistry. The Hartley Photolysis Mechanism
In the present communication, we call attention to another source of singlet-state oxygen-viz., the photolysis of ozone in the Hartley region (3200 to 2000 A.): 0 3
+ h v +02* + o*
(5)
While this mechanism may produce less 02(lAg) than the maximum amount generated by Reaction 4, it nevertheless rests a t present on a somewhat stronger scientific basis, as described below, and S O must be considered t o predict minimum OZ('Ag)concentrations. Because the Hartley wavelength region is not greatly attenuated above the atmospheric ozone peak a t 25-km. altitude, the ozone photolysis (Reaction 5 ) in the upper atmosphere is extremely rapid. Serious attenuation exists a t ground level, below the atmospheric ozone layer, but the photolysis is still fast because the ozone density is much greater in polluted urban environments than in the upper atmosphere. A number of studies have suggested that singlet molecular oxygen is involved in the ultraviolet photolysis of ozone (Norrish and Wayne, 1965). It was thought initially, on the basis of spin-conservation arguments, that a t X < 3100 A , , singlet molecular oxygen was formed together with singlet atomic oxygen in the primary photochemical step (see DeMore and Raper, 1966, for a summary of the evidence): 03(lA)
+ hv -,0 2 ( * A gor
I&)
+ O(l0)
(sa)
Recent experiments (Izod and Wayne, 1968) have, however, suggested that in the presence of ground state molecular oxygen, the singlet molecules are excited mainly by the energy transfer process O(l0)
-
+ 02(X3Z)
Oz('Ag or lZ,)
+ O(3P)
(6)
Young and Black (1967) had already observed that the vacuum ultraviolet photolysis of 02,where O(*D)is a product, leads to the formation of 0 2 ( l & ) and it has now been shown (Wayne, 1968), that O,('A,) is also produced. The efficiency of the excitation process is in some doubt: Young and Black (1967) believe that one O2('8,) molecule is produced for every O(lD)atom quenched, while Izod and Wayne (1969) find that one molecule is excited to the ' 2 , state for about 500 atoms quenched [and suggest that the formation of Oz('A,) could be considerably more efficient]. At any rate, some O2(lAg) and 02('Z,) may be excited in Reaction 6 from the O('D) product of ozone photolysis. It may be noted that the direct production of 02('8,)in Reaction 5a requires X < 2600 A . and may, therefore, be ignored in the present analysis since all wavelengths less than 2900 A. are attenuated by the upper atmospheric ozone layer. The formation of O2('&), whether directly in Reaction 5a or indirectly via Reaction 6, requires X < 3100 A. and must, therefore, be considered as potentially important, since solar energy is available from 2900 to 3100 A. No direct evidence exists to support or refute high 02('5,)yields a t ambient ground-level pressure, but three indirect indications may be cited. Airglow and rocket measurements of the intensity of the 1.27-p band (Evans, Hunten, et nl., 1968; Gattinger and Vallance-Jones, 1966; Valiance-Jones and Harrison, 1958) cannot be explained unless ozone is the absorbing species (Houghton, 1965). Izod and Wayne (1968) have established that O3 photolysis in the presence of O2 produces 02(lA,), with the quantum yields increasing with increasing O2 pressure. There remains a question here as to why O2 should be needed, but this requirement is indeed fulfilled in the atmosphere. DeMore and Raper (1966) have established the O('D) quantum yield from Reaction 5a to be 0.4 a t 3100 A. Spin conservation and energetic considerations demand that the O2(lAO)yield, 4, also be 0.4.
r
7 400
/
/
2900
3100
3000
3200
3300
W A V E L E V G T H . A. I208 - 9 r 7
Figure 1. Extinction coefficient, EA, of Inn and Tanaka (1953) for ozone photolysis, as averaged by Leighton (1961), presented as function of wavelength along with solar flux, J A , at ground level. Measured values at Mt. Wilson and downtown Los Angeles averaged from data of Stair,Waters, e t al. (1966) for October 6 and 12, 1965, 12:30 P.hl., P.D.S.T. Calculated curves plotted from data of Leighton (1961) for (a) zenith angle of 40", and (b) zenith angle of 60"
The
02('A,)
Concentration
Ultraviolet flux data down to 3100 A. have been obtained by Stair (1965) and Nader (1966, 1967) for the Los Angeles smog season. Some of these data are plotted in Figure 1, for typical hazy days (Stair, 1965), along with calculated values from Leighton (1961). A linear extrapolation to 3050 A. seems appropriate, and in view of the toe in the calculated curves, conservative. The extinction coefficient, E, for ozone photolysis (Leighton, 1961) by Reaction 5 (defined by I l l o = IO--ccb, where c is the ozone concentration in moles per liter and b is the path length in centimeters) is also given in Figure 1. A first-order rate constant in the linear absorption approximation may be calculated from the numerical integration: k,
=
3.84
x
Jx +A dX set.-'
lo-?'
(7)
using the data of Figure 1. Results for typical conditions are presented in Table I. Using the rate constant for collisional quenching of 02(14,) by O2 determined by Winer and Bayes (1966), the effective lifetime, T , of O2(lAU)a t ground level is 0.57 second. Hence, a steady-state concentration of 02('A,) will be reached in seconds with:
[od'Au)l
=
ka[oJ~
( 8)
Table I. Predicted Contributions to 02(lAu) Concentrations in Lower .4trnosphere from Ozone Photolysis
Condition Haze, Oct. 6, 12, 1965 (Stair, Waters, e t nl., 1966), 12:30 P.M., P.D.S.T. Clear, Oct. 1965, 12:30 P.M.
ka,
Location Downtown L.A.
Hr.-l
02(lig)/03
0.01
1.6
x 10P
Mt. Wilson
0.02
3.2
x
Leighton (1961), calcd. for zenith angle of 40"
0.076
12
x
10P
Volume 3, Number 3, March 1969 249
Table 11. Relative Reactivities of Oxygen Allotropes with 0.5 P.P.M. Ethylene Concentration, Rate Constant, Rate, Species Mole/L. L./Mole-Sec. Mole/L.-Sec.
0 O,(~A,) 0 3
4 x 10-16 3.2 x 1044 2 x 10-8
1.7 x lo8 1.7 x 108 2 x 103
1.4 X 1.1 x 10-13 8X
than are the relative ethylene consumption rates. Even on the latter basis, however, the additional Oz('A,) produced by other mechanisms (Pitts, Khan, et al., 1969) could change the relative rates cited in Table 11. Therefore, it would be very valuable to measure the 02('A,) concentrations in situ as an aid in establishing the aggregate effect of all populating mechanisms. Acknowledgment
The authors acknowledge helpful conversations with, and the encouragement of, J. N. Pitts, Jr., and R . P. Wayne. From this relation, we predict minimum 02(lA,) concentrations (relative to that of 0 3 ) in urban environments as indicated in Table I on the assumption that Nz is not much more efficient than 0 2 as a quencher, and that the quantum yield for 02('A,) formation is equal t o that for O(lD)-i.e., 4 = 0.4. Discussion and Conchsions
Evidence exists indicating that the reactivity of Oz(lAu) approaches that of 0 atoms. In the respective reactions with ozone:
+ OdlAg) + 0
and
0 3
-t
0 s
0 3
+
0 2
+ + +0 0 2
(9)
0 2
(10)
the rate constants for Reactions 9 and 10 are comparable (DASA Reaction Rate Handbook, 1967). For the respective reactions with O 2 to produce a free electron, 02-
and 02-
+0
+ Od'A,)
-+0 3
-
0 2
+e
(11)
+ +e
(12)
0 2
the rate constants are again comparable (Burt, Fehsenfeld, e t a/., 1968; DASA Reaction Rate Handbook, 1967). In addition, the work of Falick, Mahan, et al. (1965) suggested (Mahan, 1967) that in their reactions with ethylene, the rate constants for oxidation by 0 and Oz('A,) might be similar. In the latter case, comparative data for all the oxygen allotropes are available. Using the 02('A,) concentrations calculated in Table I and typical values of rate constants and concentrations for the 0 and O3oxidants taken from Leighton (1961), the relative rates of ethylene consumption are compared in Table 11. The comparative reactivities compiled in Table I1 show O,('A,) to be a significant potentia! oxidant despite recent laboratory studies which do not require the existence of a reactive intermediate such as Oz(lAO)*.g., Bufalini and Altshuller (1 967). The important difference between the laboratory and atmospheric cases lies in the cut-off of laboratory black-light at 3100 A. (Bufalini and Altshuller, 1967), whereas the major ozone photolysis occurs a t wavelengths below 3100 A. Thus, the rate of ozone photolysis in the laboratory is greatly attenuated compared to the atmospheric rates. Even if olefin consumption is unaffected by 02(1Ag), the concentrations and the still unknown subsequent role played by the products of each reaction are undoubtedly more meaningful indicators of the importance of each oxidant species listed
250 Environmental Science & Technolog)
Literature Cited Arnold, S., Kubo, M., Ogryzlo, E., Adv. Chem. Ser. 77, 133 (1968). Badger,'R., Wright, A., Whitlock, R., J . Cliem. Phys. 43, 4345 (1965). Bufalini, J. J., Altshuller, A. P., ENV~RON. S a . TECHKOL. 1, 133 (1967). Burt, J., Fehsenfeld, F., Albriten, D., Schiff, H., Ferguson, E., York University, Toronto, Canada, private communication, 1967. Clark, I. D., Wayne, R . P., Oxford University, Oxford, England, private communication, 1968. DASA Reaction Rate Handbook, M. Bortner, Ed., DASA No. 1948 (1967). DeMore, W. B., Raper, 0. F., J . Chem. Phys. 44,1780 (1966). Evans, W. F. J., Hunten, D. M., Llewellyn, E. J., VallanceJones, A., J . Geophys. Res. 73, 2885 (1968). Falick, A., Mahan, B., Myers, R., J . Chem. Phys. 42, 1387 (1965). Fisher, E., McCarty, M., Jr., J . Chem. Phys. 45, 781 (1966). Gattinger, R., Vallance-Jones, A., Planetary Space Sci. 14, l(1966). Houghton, J., Proc. Roy. SOC.A288, 545 (1965). Inn, E. C. Y . , Tanaka, Y . , J . Opt. SOC.Am. 43, 870 (1953). Izod, T. P. J., Wayne, R . P., Nuture 217, 947 (1968). Izod, T. P. J., Wayne, R. P., Proc. Roy. Soc. A, in press (1 969). Khan, A. U., Pitts, J. N., Jr., Smith, E. B., ENVIRON. SCI. TECHNOL. 1, 657 (1967). Kummler, R . H., Bortner, M. H., G E TIS Report R67SD20 (May 1967). Leighton, P. A., "Photochemistry of Air Pollution," pp. 447, Academic Press, New York, 1961. Mahan, B., University of California, Berkeley, Calif., private communication, 1967. Nader, J., Fourth International Biometeorological Congress, New Brunswick, N. J., August 1966. Nader, J., PHS publication 999-AP-38 (1967). Norrish, R. G. W., Wayne, R. P., Proc. Roy. SOC.A288, 200 (1965). Pitts, J. N., Jr. AAAS Divisional Symposium on Air Pollution, UCLA, June 19, 1967. Pitts, J. N., Jr., Khan, A. U., Smith, E. B., Wayne, R . P., SCI.TECHNOL. 3, 243 (1969). ENVIRON. Stair, R., Waters, W. R., and Jackson, J. K., NBS Report 9034 (1965). Vallance-Jones, A., Harrison, A. W., J . Atmospheric Terresr. P/ivs. 13. 45 (1958). Wayne, R,'P., Oxford University, Oxford, England, private communication, 1968. Wilson, W . , panel discussion, Third Middle Atlantic Regional Meeting, ACS, Philadelphia, Pa., February 1968. Winer, A. M., Bayes, K . D., J . Phys. Chem. 70, 302 (1966). Young, R. A., Black, G . , J . C/iem. Phys. 47, 2311 (1967). Received for review August 2 , 1968. Accepted December 9 , 1968.
