Interactive comment on “ Resolving the mesospheric nighttime 4 . 3 μ m emission puzzle : New model calculations improve agreement with SABER observations

This study investigates the impact of the additional vibrational energy transfer channel OH*→ O(1D)→ N2(v=1)→ CO2(v3), as recently proposed by theroretical and laboratory studies, on CO2 4.3 μm nightime radiances by means of NLTE radiative transfer simulations in comparison with SABER Channel 7 measurements. The authors show that the inclusion of the proposed new mechanism improves noticably the agreement between simulated and observed radiances.


Introduction
The SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on board the NASA TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics) satellite (Russell III et al., 1999) measures the limb ra-diance of the atmosphere in ten broadband infrared (IR) channels over an altitude range that spans the mesosphere and lower thermosphere (MLT).These measurements are aimed at retrieving various MLT parameters such as kinetic temperature, pressure, and densities of O 3 , H 2 O, CO 2 , O, and other constituents.
Recently, daytime temperature/pressure and CO 2 densities have been obtained from the SABER 15 µm and 4.3 µm emission observations (Rezac et al., 2015) using a self-consistent two-channel retrieval approach which accounts for strong coupling between both emissions.Although CO 2 is one of the key trace constituents of the MLT, whose 15 µm emission is a main steady source of cooling in this region, up to now no observations of this constituent at nighttime are available.Additionally, CO 2 has a relatively long chemical lifetime, therefore, it can act as a tracer for dynamical transport processes, such as molecular and eddy diffusion, transport by atmospheric tides and also for determining the residual mean circulation.However, little is still known about its distribution and variability, particularly about its diurnal variation and its distribution in polar night.The extensive SABER nighttime 4.3 µm radiance observations, which are supposed to fill this knowledge gap, remain, however, still unprocessed due to a lack of understanding of physical mechanisms generating this emission.As a result, nighttime temperatures are currently retrieved independently from the SABER 15 µm channel radiances using day-night mean CO 2 densities from the WACCM (Whole Atmosphere Community Climate Model) model (Garcia et al., 2007).
A detailed study of nighttime 4.3 µm emissions was conducted in by López-Puertas et al. (2004) aimed at determining the dominant mechanisms of exciting CO 2 (ν 3 ), where ν 3 is the asymmetric stretch mode that emits 4.3 µm radiation.The nighttime measurements of SABER channels 7 (4.3 µm), 8 (2.0 µm), and 9 (1.6 µm) for geomagnetically quiet conditions were analyzed, where channels 8 and 9 are sensitive to the OH (ν ≤9) overtone radiation from levels ν = 8-9 and ν = 3-5, respectively.López-Puertas et al. (2004) showed a positive correlation between 4.3 µm and both OH channel radiances at a tangent height of 85 km.This correlation was associated with the transfer (Kumer et al., 1978) of energy of the vibrationally excited OH(ν) produced in the chemical reaction and then further to CO 2 (ν 3 ) vibrations (hereafter "direct" mechanism).However, using laboratory rate coefficients of corresponding reactions the authors were unable to reproduce the 4.3 µm radiance observed by SABER.Although accounting for energy transfer from OH(ν) did provide a substantial enhancement to 4.3 µm emission, a 40% difference between simulated and observed radiance remained (for the SABER scan 22, orbit 01264, 77 Confining our consideration to quiet (non-auroral) nighttime conditions to avoid accounting for interactions between charged particles and molecules, whose mechanisms still remain poorly understood, we studied in detail the impact of both the "direct" mechanism alone and the combined effect of the two mechanisms on simulated nighttime SABER 4.3 µm radiances.We compared simulated radiances with the SABER measured radiances for various latitudes and seasons and present here results of this analysis.

Non-LTE Model Applied
A non-LTE analysis was applied to CO 2 and OH using the non-LTE ALI-ARMS (Accelerated Lambda Iterations for Atmospheric Radiation and Molecular Spectra) code package (Kutepov et al. (1998), Gusev and Kutepov (2003), Feofilov and Kutepov (2012)), which is based on the Accelerated Lambda Iteration approach (Rybicki and Hummer, 1991).
Our CO 2 non-LTE model is described in detail by Feofilov and Kutepov (2012).We modified its nighttime version to account for the "direct" mechanism, reactions (R1-R3), in a way consistent with that of López-Puertas et al. ( 2004) and added the "indirect" mechanism of Sharma et al. (2015) and Kalogerakis et al. (2016) as described in detail below.Our OH non-LTE model resembles that of Xu et al. (2012).
2.1.1New Mechanism of CO 2 (ν 3 ) excitation at nighttime Sharma et al. (2015) suggested an additional mechanism that may contribute to the CO 2 (ν 3 ) excitation at nighttime, and discussed in detail its available experimental and theoretical evidence.According to this mechanism, highly vibrationally excited OH(ν), produced by reaction (R1), rapidly loses several quanta of vibrational excitation in collisions with O( 3 P) through a fast, spin-allowed, vibration-to-electronic energy transfer process that produces O( 1 D), Recently, Kalogerakis et al. (2016) have presented the first laboratory demonstration of this new OH(ν) + O( 3 P) relaxation pathway.
The production at nighttime of electronically excited O( 1 D) atoms in reaction (R4) has crucial importance.It triggers well known pumping mechanisms for daytime 4.3 µm emission (Nebel et al. (1994), Edwards et al. (1996)), where O( 1 D) atoms are first quenched by collisions with N 2 in a fast spin-forbidden energy transfer process then N 2 (ν) transfers its energy to ground state N 2 via a very fast single quantum VV process leaving N 2 molecules with an average of 2.2 vibrational quanta, which is followed by reaction (R3).

Collisional Rate Coefficients
We use, in our CO 2 non-LTE model, the same VT and VV collisional rate coefficients for the CO 2 lower vibrational levels as those of López-Puertas et al. ( 2004).However, a different scaling of these basic rates is applied for higher vibrational levels using the first-order perturbation theory as suggested by Shved et al. (1998).
The reaction rate coefficients applied in this study for modeling the transfer of OH(ν) vibrational energy to the CO 2 (ν 3 ) mode are displayed in Table 1.The total chemical production rate of OH(ν) in reaction (R1) was taken from Sander et al. (2011) and the associated branching ratios for ν were taken from Adler-Golden (1997).The rate coefficient for reaction (R2) was taken from Adler-Golden (1997), measured at room temperature, and multiplied by a low temperature factor of 1.4 (Lacoursière et al., 2003) for MLT regions.Following Sharma et al. (2015) and Kalogerakis et al. (2016) the rate coefficient of reaction (R4) for OH(ν=9) is (2.3±1)×10 −10 cm 3 s −1 for temperatures near 200 K.In this study, we applied this coefficient for each OH(ν ≥5).Additionally, OH(ν <5) collisions with O( 3 P) are considered completely inelastic and, therefore, we used for them the rate coefficient 3×10 −11 cm 3 s −1 from Caridade et al. (2013).The rate coefficient for the reaction O( 1 D) + N 2 (0) (reaction (R5) in Table 1) was taken from Sander et al. (2011) with accounting for the fact that 33% of the electronic energy is transferred to N 2 (Slanger and Black, 1974) producing, on average, 2.2 N 2 vibrational quanta.The rate coefficient for the reaction OH(ν ≤10) + O 2 (0) (reaction (R6) in Table 1) was taken from Adler-Golden (1997) and was scaled by a factor of 1.18 to account for MLT temperatures (Lacoursière et al. (2003), Thiebaud et al. (2010)).
3 Modeling Results

Vibrational Temperatures
The non-LTE population n ν of a molecular vibrational level ν is usually described by its vibrational temperature, T ν .From the Boltzmann formula, where T ν is defined by the degree of excitation of level ν against the ground level 0 and g ν and E ν are the statistical weight and the energy of level ν, respectively.If T ν =T kin then level ν is in LTE.km which is caused by an efficient VV exchange reaction (R3).from SABER measurements, OH for this and our calculations discussed below are taken from WACCM results (Garcia et al., 2007).Our calculations reproduce the result of (López-Puertas et al., 2004) very well between 70-95 km.There is a minor discrepancy around 87 km, where the OH peak resides, which is likely a result of OH density differences.Orange, red and green curves in Fig. 2 show results of our calculations with accounting for the combined effect of both "direct", reactions (R1-R3), and "indirect" reaction (R4) mechanisms.The three radiance profiles correspond to the range of rate coefficients reaction (R4) (see Table 1) within uncertainty limits estimated by Sharma et al. (2015).Accounting for the "indirect" mechanism "on top" of the "direct" one produces strong enhancement of 4.3 µm radiation for all runs in which the results display agreement to within (-23, +6.5)%, (-12, +10)%, and (-4, +20)% of SABER measurements, for rate coefficients 1.3, 2.3, and 3.3 × 10 −10 cm 3 sec −1 , respectively.

Comparison of Measured and Simulated Radiances
We also modeled 4.3 µm emissions for two representative nights (solar zenith angle (SZA) greater than 100 • ) at solstice, 15 July 2010 (311 scans), and equinox, 10 October 2008 (524 scans), which are shown in Fig. 3.The residual 4.3 µm radiance (simulated-measured)/measured is displayed with accounting for the "direct" mechanism alone (Fig. 3a and 3c) and when both "direct" and "indirect" mechanisms are included (Fig. 3b and 3d).Figures 3a and 3b display nighttime scans taken on 15 July 2010.When only the "direct" mechanism is considered (Fig. 3a), SABER measurements are reproduced to within 20% for southern latitudes and 30% for northern latitudes up to 75 km.Above 75 km, SABER measurements are shown to be gradually under-predicted from 30-80% for all latitudes, where the larger differences occur at higher altitudes.When both "direct" and "indirect" mechanisms are included (Fig. 3b), the simulated radiation is in agreement with SABER measurements to within (-10, +20)% for the majority of mid-and tropical latitudes above 90 km.Below 90 km for mid-and tropical latitudes, simulations predict SABER measurement to within (-20, +10)%.The "indirect" mechanism enhances radiances from 20% at 80 km to 80% at 100 km.For higher latitudes between 60 • S and 80 • S, simulated emission show good agreement with measurements up to 95 km.However, above 95 km, the 4.3 µm emissions are still under-predicted between 20% and 60%.
This mismatched predictions may be hardly associated with any effects related to the geomagnetic activity since the kp index (<4) and the F10.7 index (=75) were low on this particular day.A more detailed investigation of this narrow altitude/latitude region is needed and will be performed in later studies.
Figures 3c and 3d display nighttime scans taken on 10 October 2008.Figure 3c shows agreement with SABER measurements to within 30% up to 75 km for all latitudes.Above 75 km, SABER measurements are shown to be gradually under-predicted from 40-70%, where percentages increase with higher altitudes.In the tropical regions, however, the disparity between simulated and SABER measurements is slightly greater at all altitudes compared to other regions.When both "direct" and "indirect" mechanisms are included (Fig. 3d), the simulated radiation is in agreement with SABER measurements to within (-20,+10)% for southern latitudes and (-10,+40)% for northern latitudes from 65-110 km.In both regions, radiance enhancements range from 20-30% below 80 km to up to 80-100% above 100 km.High atomic oxygen densities in some regions could be a result of the over-predictions for 4.3 µm emission modeling.In addition, unlike the solstice scans modeled in Fig. 3a and 3b, high latitude regions do not show any large under-predictions for equinox scenarios.Modeling emissions for alternative solstice and equinox nights, i.e.January and April, showed similar results as the nights modeled in Fig. 3.
Additionally, atomic oxygen densities retrieved by SABER have been reported to be at least 30% larger than other observations (Kaufmann et al., 2014).We found that lowering the atomic oxygen density by 50% reduces the 4.3 µm emission enhancement for all atmospheric scenarios, on average, by 5-20%, where the larger percentage differences occur at higher altitudes.

Discussion and Conclusions
Kumer et al. (1978) first proposed the transfer of vibrational energy from chemically produced OH(ν) in the nighttime mesosphere to the CO 2 (ν 3 ) vibration, OH(ν) ⇒ N 2 (ν) ⇒ CO 2 (ν 3 ).The effect of this mechanism on the SABER nighttime 4.3 µm emission was studied in detail by López-Puertas et al. (2004), who showed that in order to match observations , an additional enhancement is needed that would be equivalent to the production of 2.8-3 N 2 (1) molecules for each quenching reaction OH(ν)+N 2 (0), instead of the currently accepted one N 2 (1) molecule.López-Puertas et al. (2004) concluded that the required 30% efficiency in the OH(ν)+N 2 (0) energy transfer ". . .is, in principle, possible, although the mechanism(s) whereby the energy is transferred is (are) not currently known".
Recently, Sharma et al. (2015) suggested a new efficient "indirect" channel of the OH(ν) energy transfer to the N 2 (ν) vibrations, OH(ν) ⇒ O( 1 D) ⇒ N 2 (ν) and showed that it may provide an additional enhancement of the MLT nighttime 4.3 µm emission.Kalogerakis et al. (2016) provided a definitive laboratory confirmation of this new OH(ν) + O vibrational relaxation pathway and measured its rate for OH(ν=9)+O.We added the new "indirect" OH(ν) + O energy transfer channel to the "direct" OH(ν) + N 2 (0) mechanism (using the currently accepted efficiency of 1).Our non-LTE model of the nighttime CO 2 emissions assumes for the "indirect" channel a rate coefficient that is independent of the OH vibrational level.We studied in detail the impact of the combined "direct" and "indirect" mechanisms with simulated SABER/TIMED nighttime 4.3 µm limb radiances and found that, while accounting for the "direct" mechanism alone leads to under-predicting the SABER measured radiances by up to 80%, inclusion of the new "indirect" channel in the model results in a significant reduction of these differences bringing them to (-20, +30)% for the majority of latitudes during equinox and solstice nights.This significant improvement suggests that the missing nighttime mechanism of CO 2 (ν 3 ) pumping has finally been identified.This confidence is based on the fact that the new mechanism accounts for most of the discrepancies for a large variety of atmospheric situations, leaving little room for other processes (that cannot be excluded, but are not expected to be significant   .06, 0.10, 0.17, 0.30, 0.52, 0.91, 1.6, 7, 4.8, 6) c fν (ν=1-10) = (1.9, 4, 7.7, 13, 25, 43, 102, 119, 309, 207) 13

Figure 1
Figure1shows the vibrational temperatures of the CO 2 levels of four isotopes, giving origin to 4.3 µm bands, which dominate the SABER nighttime signal(López-Puertas et al., 2004).These results were obtained for SABER scan 22, orbit 01264, 77 • N, 03 March 2002.The same scan was used for the detailed analysis presented in the work byLópez-Puertas et al. (2004).The kinetic temperature retrieved for this scan from the SABER 15 µm radiances and vibrational temperature of N 2 (1) are also shown.Dashed and solid lines in Fig.1represent simulations with accounting for "direct" mechanism (reactions (R1-R3) alone) and with additionally implemented "indirect" mechanism (reaction (R4)), respectively.Vibrational temperatures of CO 2 levels and N 2 (1) depart from LTE around 65 km.The additional accounting for reaction (R4) provides an increase of vibrational temperatures in the MLT.At 90 km, the T ν of 626(00011) increases by 22 K, that of N 2 (1) increases by 26 K, whereas the minor isotopes (636, 628, and 627) and 626(01111) show a smaller enhancement of 3-8 K.In both simulations, CO 2 (00011) of main isotope 626 and N 2 (1) have almost identical vibrational temperatures up to ∼87

Figure 2
Figure 2 displays the measured SABER channel 7 (4.3 µm) radiance (black) for the scan described in Sect.3.1.The violet curve in this figure represents the 4.3 µm simulated signal for this scan obtained by López-Puertas et al. (2004), long dash curve in Fig.10of this paper, with accounting for contribution in the channel 7 radiance emitted by OH(ν ≤10), and applying reactions (R1-R3) only ("direct" mechanism with currently accepted efficiency 1).Our calculations for this scan with accounting for "direct" mechanism alone are given by the blue curve.They also account for OH emission contribution and use inputs identical to those of(López-Puertas et al., 2004) except for OH densities.WhereasLópez-Puertas et al. (2004) retrieved OH densities
the 4.3 µm radiance (i.e. via O 2 and direct energy transfer from OH to CO 2 ) were tested but found to be • N, 03 Mar 2002, which was studied in detail) for altitudes above 70 km.In order to fit measurements, on average, the authors found that 2.8-3 N 2 (1) molecules (instead of the currently accepted value of 1) are needed to be produced after each quenching of OH(ν) molecule in reaction (R2).Alternative excitation mechanisms that were theorized to enhance ).Further improvements will require optimizing the set of rate coefficients used for OH(ν) relaxation by O( 3 P) and O 2 at mesospheric temperatures and, in particular, understanding the dependence of the indirect mechanism on the OH vibrational level.Relevant laboratory measurements and theoretical calculations are sorely needed to understand these relaxation rates and the quantitative details of the applicable mechanistic pathways.Nevertheless, the results presented here clearly demonstrate significant progress in understanding the mechanisms of the nighttime excitation of N 2 (1) and generating the nighttime CO 2 4.3 µm emission, and represent an important step towards developing the algorithm(s) suitable for retrieving CO 2 densities in the MLT from the SABER nighttime limb radiances.