Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsa21a0225c&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SA21A-0225
Other
0300 Atmospheric Composition And Structure, 0310 Airglow And Aurora, 0340 Middle Atmosphere: Composition And Chemistry, 0342 Middle Atmosphere: Energy Deposition (3334)
Scientific paper
The hydroxyl radical is a key player in the chemistry and energetics of the middle terrestrial atmosphere, and several studies have investigated energy transfer processes between OH(υ) and atmospheric molecules. Nevertheless, a gap exists in our understanding of its interaction with oxygen atoms. Oxygen atoms are present at about 10% of the oxygen molecule concentration at ~95 km and about 1% at 88 km, so if their rate constant is significantly faster than that of O2 and N2, they will strongly influence the intensity and the vibrational distribution extracted from the OH(υ) emission. We report laboratory measurements of the total removal rate constants of OH(υ = 8, 9) by O(3 P) atoms and preliminary measurements on CO2. These measurements are required so that we can quantify the importance of these collisional processes in the modeling of atmospheric OH emissions and evaluate the chemical heating rate from measurements by the SABER instrument aboard the TIMED satellite. In the experiments, we generate O(3P) and OH(υ) by photodissociation of ozone at 250 nm in a mixture of ozone, nitrogen, hydrogen. The highly excited vibrational levels OH(υ = 7-9) are produced in the reaction of H atoms with ozone that has not been photodissociated. We monitor the temporal evolution of the OH(υ = 8 and 9) population by laser excitation via the \it B3Σ_u- \textendash \it X3Σ_g- (0,9) and (0,8) transitions near 237 nm and 226 nm, respectively, and subsequent detection of visible fluorescence emitted from the \it B3Σ_u^{- } \textendash \it A3Σ_u+ band, an approach developed previously in our laboratory [1]. By controlling the initial conditions of the experiments, we can extract the rate coefficient for OH removal by O atoms in the system. For direct analysis of the OH signal rise to yield accurate rate coefficients an extremely good signal-to-noise-ratio is required. However, a preferred approach involves comparison of the OH signal relative intensity changes when small amounts of other efficient collider gases are added. The steady- state population of OH(υ = 8 and 9) depends on the relative amount of O atoms with respect to other gases that deactivate these OH levels rapidly. Given that the rate coefficients for deactivation of OH(υ = 8 and 9) by CO2 is known [2,3], we can determine the rate coefficients for OH(υ = 8 and 9) + O with respect to that for OH(υ = 8 and 9) + CO2. Preliminary experiments indicate a total removal rate coefficient for OH(υ = 9) by O atoms exceeding 1 × 10^{-10} cm3s-1. We will present our results and discuss their atmospheric implications. This work was supported by the NASA Geospace Sciences and Planetary Atmospheres Programs. [1] A. D. Sappey, and R.A. Copeland, J. Chem. Phys. 93, 5741 (1990). [2] M. J. Dyer, K. Knutsen, and R.A. Copeland, J. Chem. Phys. 107, 7809 (1997). [3] B. R. Chalamala and R.A. Copeland, J. Chem. Phys. 99, 5807 (1993).
Copeland Richard A.
Kalogerakis Konstantinos S.
Mlynczak M. M.
Smith Graham P.
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