Parameter inputs and relevant uncertainties

Table 1 lists information on the input parameters used to calculate the contributions of the OH(v = 9) + O multiquantum relaxation to the O2 AB emission. The table shows the values and associated uncertainty estimates at 298 K and, where available, an expression that describes their temperature dependence. The latest Evaluation on Chemical Kinetics of the Jet Propulsion Laboratory (JPL) was the reference of choice for several processes (31). Additional comments are provided next.

For the rate constant of the H + O3 reaction, experimental studies by Liu et al. (42) have corroborated the most recent JPL recommendation and provided new measurements at low temperatures. The yield of OH(v = 9) from H + O3 used in this work follows the recommendation of Adler-Golden (12), which was based on the critical evaluation by Steinfeld et al. (43). The OH(v = 9) total removal rate constant for collisions with O, O2, and N2 as well as the rate constant for the OH(v = 9) + O multiquantum relaxation adopted the values reported by Kalogerakis et al. and Sharma et al. (21, 25, 26) and assumed a temperature dependence similar to that observed by Thiebaud et al. (44). For the collisional relaxation of O2(b, v = 0) by O2, N2, and O, the values of the current JPL evaluation were used (31). Because of the uncertainty in the rate constant for O2(b, v = 0) by O, measured by Slanger and Black (45) at 298 K, and the lack of studies at low temperatures, a common practice in the literature (17, 18, 32) has been to use a lower limit of zero and a high value of 8 × 10−14 cm3 s−1, i.e., the measured rate constant at room temperature. We adopted this approach and, in Fig. 2, show results for the new source of O2 AB from OH(v = 9) + O using both limiting values of kO2bO, the rate constant for removal of O2 (b, v = 0) by O atoms.

Equations M10 to M15 used to calculate the contributions of the new source contain several kinetics parameters and variables, many of which have significant uncertainties. We will now consider the kinetics inputs before discussing specific details pertinent to the model atmospheric composition used. To estimate the uncertainty for the results presented in Fig. 2, we considered the uncertainty of all kinetic parameters in Eqs. M10 to M15 at a nominal temperature of 200 K. We used the information listed in Table 1 and an uncertainty estimate of 10% for the nascent yield y9 based on the uncertainties reported for the experimental measurements of Klenerman and Smith (46). The largest uncertainty contributions arise from parameters yO2b, LT(OH9), and kOH9NM with uncertainties of 25, 23, and 22%, respectively. The combined uncertainty of all parameters propagated through the formulas for O2 AB generated from OH(v = 9) + O amounts to an SD of 57%.

As already mentioned, the [N2], [O2], [H], and temperature inputs are the profiles of the NRLMSISE-00 model for the time of the rocket launch of interest. The atomic hydrogen layer displays a steep “ledge” near 80 km where its number density increases by almost an order of magnitude within a few kilometers. Moreover, the layer and its peak near 85 km may move during the night in a manner not accurately captured by the NRLMSISE-00 model (30). Thus, the total intensity of the new source for O2 AB could be affected, especially the slope of the emission’s onset at altitudes below 85 km.

The intensity of the OH(v) + O source of O2 AB strongly depends on atomic oxygen. Accurate determination of the O-atom profile has been a long-standing challenge for upper atmospheric studies, and simultaneous measurements of O atoms are usually missing from nightglow observations. Such information is available for the ETON and NLTE sounding rocket campaigns. Nevertheless, a discussion of substantial issues with these measurements is warranted. For ETON, Greer et al. (40) summarized the difficulties with partial detector saturation and signal interference in the measurements and how these were handled. The atomic oxygen input for our calculations is from the ETON P232H payload, the first of two payloads that performed these measurements. For the ETON O2 AB dataset, we used the average of two measurements from payloads P230H and P227H, launched 21 and 22 min before and after P232H (O-atom profile), respectively. The atomic oxygen profile from payload P232H can be considered representative of the conditions during the two O2 AB measurements that preceded and followed it. Nevertheless, the second ETON O-atom profile (P234H), recorded approximately 126 min after the first (P232H), differed significantly at most altitudes and had a peak O-atom concentration that was larger than that of P232H by approximately 35%. Thus, while the choice of the datasets aims at mitigating any inconsistencies in the measurements, we cannot exclude the possibility of systematic errors.

The NLTE data have its own set of challenges, mainly related to the calibration of the resonance fluorescence measurements. The O-atom dataset from the descent of the NLTE-2 flight was calibrated according to the work of Hedin et al. (32). These authors proposed a method that uses nightglow intensity profiles (O2 Chamberlain and AB bands) and the available empirical equations that relate the observed intensity to the O-atom concentration. From a study of several measured O-atom profiles, Hedin et al. (32) found that calibrating them in this manner resulted in an improvement of more than one order of magnitude in the spread of the retrieved O-atom concentrations. Using this approach for the NLTE-2 descent, we find an O-atom concentration peak of 4.2 × 1011 atoms/cm3 (Fig. 2). For the NLTE-2 ascent, Hedin et al. (32) obtained values from 4.4 × 1011 to 4.7 × 1011 atoms/cm3. The ascent and descent flight trajectories are separated by several kilometers and are likely subjected to different aerodynamic conditions. Given the uncertainties in the measurements and their calibration, and the fact that we now know that multiple production mechanisms are active, significant systematic errors cannot be excluded. Several authors have also considered complications in similar nightglow measurements such as the possibility of transient rocket-induced glow, nonlinearities in the measurements, or other contaminating emissions (32, 47, 48). The availability of accurate information on the atomic oxygen layer remains a challenge for the analysis and interpretation of nightglow emissions. Nevertheless, the uncertainties in the ETON and NLTE-2 atomic oxygen profiles do not put in question the conclusion that OH(v = 9) + O multiquantum relaxation is a significant source of O2 AB emission.

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