Technical Note: On the possibly missing mechanism of 15 μm emission in the mesosphere–lower thermosphere (MLT)

R D Sharma

doi:10.5194/acp-15-1661-2015

Abstract

Abstract. Accurate knowledge of the rate as well as the mechanism of excitation of the bending mode of CO2 is necessary for reliable modeling of the mesosphere–lower thermosphere (MLT) region of the atmosphere. Assuming the excitation mechanism to be thermal collisions with atomic oxygen, the rate coefficient derived from the observed 15 μm emission by space-based experiments (kATM = 6.0 × 10−12 cm3s−1) differs from the laboratory measurements (kLAB =(1.5-2.5) × 10−12 cm3s−1) by a factor of 2–4. The general circulation models (GCMs) of Earth, Venus, and Mars have chosen to use a median value of kGCM = 3.0 × 10−12 cm3s−1 for this rate coefficient. As a first step to resolve the discrepancies between the three rate coefficients, we attempt to find the source of disagreement between the first two. It is pointed out that a large magnitude of the difference between these two rate coefficients (kx ≡ kATM - kLAB) requires that the unknown mechanism involve one or both major species: N2, O. Because of the rapidly decreasing volume mixing ratio (VMR) of CO2 with altitude, the exciting partner must be long lived and transfer energy efficiently. It is shown that thermal collisions with N2, mediated by a near-resonant rotation-to-vibration (RV) energy transfer process, while giving a reasonable rate coefficient kVR for de-excitation of the bending mode of CO2, lead to vibration-to-translation kVT rate coefficients in the terrestrial atmosphere that are 1–2 orders of magnitude larger than those observed in the laboratory. It is pointed out that the efficient near-resonant rotation-to-vibration (RV) energy transfer process has a chance of being the unknown mechanism if very high rotational levels of N2, produced by the reaction of N and NO and other collisional processes, have a super-thermal population and are long lived. Since atomic oxygen plays a critical role in the mechanisms discussed here, it suggested that its density be determined experimentally by ground- and space-based Raman lidars proposed earlier.

Highlights

The 15 μm emission from CO2 is the dominant cooling mechanism in the mesosphere–lower thermosphere (MLT) region (Gordiets et al, 1982; Dickinson, 1984; Sharma and Wintersteiner, 1990; Wintersteiner et al, 1992; López-Puertas et al, 1992; Sharma and Roble, 2002)
The room temperature value of the rate coefficient kATM for the exothermic process derived by modeling the 15 μm emission, observed by the Spectral Infrared Rocket Experiment (SPIRE) (Stair et al, 1985) from the MLT region of the atmosphere, is 5×10−13 cm3 s−1 (Sharma and Nadile, 1981), 5.2 × 10−12 cm3 s−1 (Stair et al, 1985), 3.5 × 10−12 cm3 s−1 (Sharma, 1987), and (3 − 9) × 10−12 cm3 s−1 (Sharma and Wintersteiner, 1990)
A large value of kx requires the rate coefficient of the unknown mechanism to be equal to kx × (M volume mixing ratio (VMR))/(O VMR), where M is the species participating in the unknown mechanism

Summary

Introduction

The 15 μm emission from CO2 is the dominant cooling mechanism in the MLT region (Gordiets et al, 1982; Dickinson, 1984; Sharma and Wintersteiner, 1990; Wintersteiner et al, 1992; López-Puertas et al, 1992; Sharma and Roble, 2002). The room temperature value of the rate coefficient kATM for the exothermic process derived by modeling the 15 μm emission, observed by the Spectral Infrared Rocket Experiment (SPIRE) (Stair et al, 1985) from the MLT region of the atmosphere, is 5×10−13 cm s−1 (Sharma and Nadile, 1981), 5.2 × 10−12 cm s−1 (Stair et al, 1985), 3.5 × 10−12 cm s−1 (Sharma, 1987), and (3 − 9) × 10−12 cm s−1 (Sharma and Wintersteiner, 1990). To resolve the discrepancy between kATM and kLAB, Feofilov et al (2012) postulate that nonthermal, or “hot”, oxygen atoms, produced in the MLT region by photolysis of O2 and dissociative recombination of O+2 , etc., may serve as an additional source of CO2(v2) level excitation These authors have derived CO2 volume mixing ratio (VMR) parts per million by volume (ppmv) in the MLT region for the time of their experiment from atmospheric models as well as space-based observations. Molecule acts as a reservoir that takes energy from OH and stores it until it is preferentially released to CO2

Hypothesis

Thermal rotational levels

Nonthermal rotational levels

Findings

Conclusions