Dissociation of Metastable as a Potential Source of Atmospheric Odd Oxygen

An analysis of the possible dissociation of metastable oxygen molecules subject to constraints imposed by selection rules for molecular transitions, airglow observations, and atmospheric chemistry leads to the following conclusions. Dissociation of O2(bXI2g +) must produce a negligible number of oxygen atoms at all altitudes in the earth's atmosphere. However, if the dissociation cross section of O2(aXAg) has a maximum value in the range 10 -20 cm 2 to 10 -t9 cm 2, then the process O:(aXA(cid:127))+ hv--, O2(C3Au)--(cid:127) O(3p) q-O(3p) will constitute a significant, and potentially the major, source of odd oxygen in the uppermost stratosphere and mesosphere.


INTRODUCTION
In recent years there have been significant revisions in our quantitative knowledge of middle atmospheric odd oxygen sources. These include updated molecular parameters to describe O: absorption in the Schumann-Runge bands [Frederick and Hudson, 1980;Yoshino et al., 1983] and greatly reduced values of the O: cross section in the Herzberg continuum [Frederick and Mentall, 1982;Johnston et al., 1984]. A major consequence of these changes has been the wide realization that present photochemical models underestimate the observed ozone abundance in the upper stratosphere and lower mesosphere. The recent, thorough analysis of Froidevaux et al. [1985] clearly demonstrates that deficiencies still exist in our knowledge of the chemical mechanisms responsible for the observed high-altitude ozone abundance. In addition, the analysis of Frederick et al. [1984] concluded that the problem would best be removed by an increased source of odd oxygen as opposed to a reduced sink. Several potential mechanisms for increasing the atmospheric odd oxygen production rate exist. These involve reactions of excited states [Mitchell and Zemansky, 1971] and dissociation of molecular oxygen isotopes [Cicerone and McCrumb, 1980;Blake et al., 1984]. However, quantitative analyses of these processes generally show that the enhanced odd oxygen production at stratospheric and mesospheric altitudes is negligible in comparison to the background source from the Herzberg continuum and Schumann-Runge bands of 0:. This paper examines two potential sources of odd oxygen that have not received attention in previous atmospheric studies. These involve dissociation of the metastable a•Ag and b•Eg + states of molecular oxygen. The objectives of this work are (1) to assess the likelihood that these dissociations take place, given information from molecular physics, airglow observations, and atmospheric chemistry, and (2) to define the conditions under which these processes could be significant in comparison to dissociation of ground state O: in the Herzberg continuum and the Schumann-Runge bands. One must Copyright 1985 by the American Geophysical Union.
Paper number 5D0477. 0148-0227/85/005D-0477502.00 acknowledge the limitations of this analysis from the outset. In the absence of measurements that specify the dissociation

cross sections of O:(a•A•) and O:(b•Z•+), any conclusions
concerning the atmospheric importance of these processes must be considered speculative. However, by proposing and evaluating the potential of these new odd oxygen sources, this study seeks to provide the impetus for definitive theoretical and laboratory work. common occurrence [Garstang, 1962]. Furthermore, a dissociation that involves a forbidden transition can still have tremendous atmospheric significance. A prime example is the Herzberg continuum dissociation of O2, described by

(R3) O2(X3Zo -) d-h¾----• O2(A3Zu +)-• O(3p) + O(3p)
This process is strictly forbidden in Hund's coupling cases (a) and (b) by the change in parity from negative to positive [Herzberg, 1950]. The violation of this fairly rigorous selection rule leads to a small cross section whose peak value is slightly less than 1.0  [Hudson, 1971], exceeding that of the strictly forbidden process by 6 orders of magnitude.
Application of the selection rules presented by Herzberg [1950] shows that the most likely paths for dissociation of

Equation (1) omits reactions with atomic hydrogen and nitrogen since the rate coefficients of Schmidt and Schiff [1973] require the loss rate of O2(a•A,) in these processes to be negligibly small. In addition, the reaction of O2(a•A,) with 0 3 proceeds at a rate of approximately 2 x 10 -•8 cm 3 s -• at a
temperature of 270 K [Clark et al., 1970]. This process is therefore of no consequence relative to quenching by N2 and O2 with the rate coefficients of Table 1 [Bates, 1982], and its neglect in (1) is justified. Equation ( 8.5 x 10 -9 s-• tabulated by Bates [1982] and the assumption that absorption occurs at the discrete wavelength 757.85 nm where the ozone Chappuis bands provide a small optical depth as given in Table 2. The odd oxygen production rates corresponding to (R5) and (R6) are

P(•A) = 2J(•A)[O2(a•A.)]
( where •,/•o, and •o are parameters peculiar to the transition being studied. A rigorous derivation of the constants • and ;t o from molecular physics considerations is not attempted here. Instead, the value • = 46.6, appropriate to the Herzberg continuum, is adopted. Study of the Schumann-Runge and Herzberg continuum cross sections shows that the peak values occur 40 _+ 10 nm shortward of the maximum wavelength at which dissociation is possible. Given this, the selections ;to = 255 nm for O2(alAa) and ;to = 311 nm for O2(bl•Ea+ ) are reasonable. Figure 2 presents the final models for a(•A, ;t) and a(•Z, ;t) in the case a0 = 5 x 10 -20 cm 2 for each. Note from (7) that a0 and ;to are not mathematically identical to the magnitude and wavelength of the cross-section maximum. However, these differences are insignificant, and the following discussion therefore refers to a0 as the peak cross section.

The dissociation rates J(•A) and J(•Z) and number densities [O2(alAa)] and [O2(b•Za+)] were then evaluated for a range
of a0 values to determine if and where the production rates of (3) and (4) can be significant in comparison to the odd oxygen production by dissociation of ground state 02. Clearly, one can always select a0 values that make processes (R5) and (R6) appear significant. However, important constraints must be met. These are as follows' (1) an acceptable a0 value must be much smaller than that expected for a quantum mechanically allowed transition, and (2) an acceptable a0 value should not be so great as to make dissociation a major loss process for

O2(a•Aa) and O2(b•a+). If a0 values can be found that simul-
taneously satisfy these constraints and still yield a significant production of odd oxygen, then the proposed source merits a detailed theoretical and laboratory analysis. x 10 -20 cm 2. Such peak cross sections are reasonable given the molecular physics arguments of the previous section. The proposed odd oxygen source becomes slightly more important on a percentage basis as one moves into the mesosphere.

RESULTS
Quenching of O2(aXAa) and attenuation of solar radiation makes the production rate P(xA) decrease rapidly below the stratopause. Figure 5 demonstrates that O2(aXAa) dissociation rates great enough to provide a significant source of odd oxygen still make minor contributions to the total loss rate. For a0-1 x 10-•9 cm 2, dissociation constitutes only 10.1% of the total loss rate at 70 km and decreases rapidly at lower altitudes. Indeed, Figure 6 shows that the O2(alAa) profiles computed for a0 = 0 and 1 x 10 -•9 cm 2 differ insignificantly. However, a value of a0 as great as 5 x 10 -•9 cm 2 would reduce the computed O2(alAa) number density at 70 km to 60-65% of that predicted in the absence of dissociation. We note that atmospheric O2(aXAa) abundances deduced from measurements of the 1.27-ttm airglow are in reasonable agreement with those shown in Figure 6. For example, the number densities reported by Evans et al. [1968] are approximately 1.3  x 10 xø, 2.6 x 10 lø, and 2.3 x 10 xø cm -3 at 65, 55, and 45 km, respectively. On the basis of this analysis the conclusion is as follows. If the peak cross section for O2(a•Aa) dissociation lies in the range 10 -20 to 10 -a9 cm 2, then this process provides a significant source of odd oxygen in the uppermost stratosphere and mesosphere. However, a peak cross section as large as 5 x 10-a9 cm 2 can probably be ruled out because of the consequent reduction in [O2(a •Aa) ].
Calculations based on (2) showed that a peak cross section of 1 x 10-•6 cm 2 would make the odd oxygen production rate in (R6) equal to that from the Herzberg continuum and Schumann-Runge bands at and above 60 km, with a rapid decrease in importance at altitudes below 50 km. Clearly, a cross section of this magnitude is unacceptable given the spin forbidden nature of the transition. Yet, smaller values of a0 would make process (R6) a negligible source of atmospheric odd oxygen. This negative result regarding the importance of O2(b•Ea+ ) dissociation might have been anticipated given the absence of observed airglow emissions corresponding to the reverse of (R6).

DISCUSSION
This analysis has shown that dissociation of O2(alAa) has the potential to provide a very substantial source of odd oxygen at and above the stratopause, provided the cross section exceeds 1 x 10 -20 cm 2 at its peak and has a wavelength dependence similar to that adopted here. Any contribution to odd oxygen production from dissociation of O2(b•o +) appears to be negligible.
The speculative nature of this work must again receive emphasis. Although nightglow observations clearly demonstrate that the 02(C3Au)--O2(alAa) transition occurs in the atmosphere, the major uncertainty in the present work is the assumed model for the O2(a 1Aa) dissociation cross section. If the shape shown in Figure 2 or the inferred peak magnitudes are grossly in error, then dissociation of O2(alAa) could in fact be an insignificant source of odd oxygen. However, in view of the discrepancies that exist between observed ozone profiles and those computed with current photochemical models, a more detailed theoretical and experimental examination of excited state chemistry would clearly be of value.