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O + O + M association and the new source for O2 AB emission
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A previously unrecognized source of the O2 Atmospheric band emission in Earth’s nightglow

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Having established the important role of the source for O2 AB emission from OH(v = 9) + O multiquantum relaxation, it is informative to consider the three-body O + O + M association reaction and the implications of the new findings. The generally accepted mechanism for the formation of O2 AB emission from O-atom association was established in the 1980s (15, 17, 18). As mentioned, it involves excitation energy transfer from an initially formed electronically excited precursor to O2(b, v = 0). The exact nature of the excited precursor(s) and the detailed mechanistic steps are not well understood. This mechanism can be summarized by reactions M16 to M19$O+O+M→O2*+M(M=O2,N2,O)$(M16)$O2*+O2→O2(b,v=0)+O2$(M17)$O2(b,v=0)+M(M=O2,N2,O)→products$(M18)$O2(b,v=0)→O2(X)+hν$(M19)

Assuming steady-state conditions, the concentration of O2(b, v = 0) is given by (15)(M20)

The parameters kOOM, kpO2, yp, and ypO2 represent the rate constants and yields for processes M16 and M17, respectively. The term LT(O2*) describes the loss rate of the electronically excited precursor(M21)

Equations M20 and M11 have a rather similar dependence on [O], [O2], and [M]. This explains, in part, why the empirical equations for the O2 AB emission available in the literature since the 1980s were found to describe reasonably well the observed altitude dependence of the emission and have been used to assess this emission and retrieve atomic oxygen profiles (17, 18, 32). Nevertheless, it is important to reiterate the fact that the nature of the O2* precursor and the relevant mechanistic pathways originating from O + O + M are not well known. Moreover, the rate constants and yields kpi and ypi have not been quantified at mesospheric temperatures. In the literature, the parameters of the relevant empirical equations were simply adjusted to account for the observed O2 AB emission intensity.

From Fig. 2, it is evident that the contribution of the source of O2 AB emission from the OH(v = 9) + O process M5 peaks at a slightly lower altitude than the observed profile. The remainder emission originates from additional contributions by OH(v = 5 to 8) + O and termolecular association O + O + M. The former can be expected to have a rather similar altitude dependence to that of OH(v = 9) + O. Therefore, given the presence of the OH(v) + O new source of O2 AB emission, the reaction mechanisms M16 to M19 provide a less than adequate description of the O + O + M contributions to the O2 AB emission, which peak at a higher altitude than those from OH(v = 9) + O. This is a telltale sign that contributions to the O2 AB emission from O + O + M beyond the previously accepted basic mechanism most likely involve additional interactions with O atoms. The mechanism summarized in reactions M16 to M19 appears to be an oversimplification. Numerous relaxation pathways can lead from the multitude of electronically excited O2 states near the dissociation limit to the O2(b, v) vibrational level manifold and eventually to O2(b, v = 0) (16). Oxygen atoms undoubtedly play a significant role in these processes. As already mentioned, laboratory experiments determined that the rate constants for vibrational relaxation of O2(b, v = 1) by O and O2 are comparable at room temperature (19, 20). The faster the relaxation of O2(b, v > 1) levels by O atoms occurs compared with the relaxation of O2(b, v = 1) by O, the larger the influence O atoms will have in the population flow through the O2(b) vibrational level manifold toward O2(b, v = 0). Although not directly relevant to O2(b, v) + O, we note that the removal of OH(v = 9) by O atoms at 298 K is faster than that of OH(v = 1) by one order of magnitude (21, 51). It follows that we could include one more process in the mechanism for O + O + M generation of O2 AB emission$O2*+O→O2(b,v=0)+O$(M22)

The revised Eq. M20 for the steady-state concentration of O2(b, v = 0) when process M22 is included can be written as in reference (15)(M23)

Figure 3 presents the ETON and NLTE-2 data together with best-fit curves using Eq. M23 and the kinetics parameters of table S1. The room temperature value of kO2bO, 8 × 10−14 cm3 s−1, was used in the calculations. The figure shows two limiting cases for the new source of the O2 AB emission. First, we considered only OH(v = 9) + O as a lower limit for the source of O2 AB. Then, for an upper limit, we assumed that OH(v = 5 to 8) produced from the H + O3 reaction behave similarly to OH(v = 9) in collisions with O atoms. The corresponding yields for processes M17 and M23, (ypO2, ypO), in Fig. 3 (A to D) have the values (0.24, 0.11), (0.02, 0.16), (0.02, 0.41), and (0, 0.30), respectively. As expected, when a maximum contribution for OH(v) + O is considered (Fig. 3, B and D), ypO2 becomes less important than ypO. The fact that the optimized values for each limiting case differ significantly between ETON and NLTE-2 (Fig. 3, A versus C, and B versus D) appears to indicate some inconsistencies in the description of the mechanism. Nevertheless, invoking process M22 improves the agreement with the observed profile and may be a first step toward a more complete description of the O2 AB emission. Better characterization is still required of the mechanistic details of O + O + M association as well as the contributions from OH(v = 5 to 8) + O.

On the basis of the observed OH(v) vibrational population distributions reported in the literature (12, 14, 50), the steady-state population of OH(v = 9) is approximately one order of magnitude less than the cumulative population in levels v = 5 to 8 (Fig. 1C). This observation suggests that most of the observed O2 AB emission below 95 km originates from OH(v) + O. We eagerly envision that future studies will fully quantify the O2 AB source from OH(v ≥ 5) + O and the elusive mechanism of O + O + M association.

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