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| Titel |
Technical Note: On the possibly missing mechanism of 15 μm emission in the mesosphere–lower thermosphere (MLT) |
| VerfasserIn |
R. D. Sharma |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 4 ; Nr. 15, no. 4 (2015-02-17), S.1661-1667 |
| Datensatznummer |
250119437
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| Publikation (Nr.) |
 copernicus.org/acp-15-1661-2015.pdf |
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| Zusammenfassung |
| 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. |
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