HR: 0830h
AN: SA51A-0482 [PDF]
TI: Laboratory Studies of Ice Growth in the Presence of Oxygen Atoms
AU: * Morgan, C G
EM: christopher.morgan@sri.com
AF: SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AU: Boulter, J E
EM: james.boulter@sri.com
AF: SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AU: Marschall, J
EM: jochen.marschall@sri.com
AF: SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025 United States
AB:
In the mesopause region, where noctilucent clouds (NLCs) form and polar summertime echoes are present, atomic oxygen is the
dominant reactive species. Observations by Gumbel {\it et al.} (1998) reveal sharp gradients and distinctive minima in
oxygen atom concentration coinciding with observed NLC layers. These observations suggest an interaction between oxygen
atoms and NLC particles. Recent laboratory studies conclude that the uptake coefficient of atomic oxygen on ice is not large
enough to change the gas-phase concentrations in the mesosphere lower thermosphere (MLT) region (Murray and Plane, 2003).
However, the question of whether or not atomic oxygen can affect the formation and growth of ice has not been experimentally
addressed.
To gain insight into possible interactions between atomic oxygen and ice surfaces, we directly measure ice growth rates at
temperatures associated with the summertime mesopause region (110-150 K), with and without exposure of the growing ice layer
to partially dissociated oxygen. A liquid nitrogen cooled cryostat is used to control the temperature of a gold mirror in a
high vacuum chamber. Water vapor, either from the residual background or from an introduced source, is allowed to condense
on the mirror. A microwave discharge is used to partially dissociate an oxygen stream, which is sampled into the chamber
through a small orifice facing the gold mirror. Grazing angle Fourier transform infrared reflection absorption spectroscopy
(FTIR-RAS) is used to monitor the rate of ice growth.
Preliminary results at 130 K indicate that the ice growth rate in the presence of oxygen slows when the microwave discharge
is activated and the ratio of water to oxygen is low. For H$_{2}$O/O$_{2}$ = $\sim$0.3 %, at a total chamber pressure of
about 7 $\mu$Torr, the growth rate reduction amounts to 24$\pm$9 %. Changes in the FTIR-RAS absorption profile of the OH
stretching vibrations are also noted, which may indicate changes in ice morphology. Both results suggest that the presence
of atomic oxygen influences how ice forms and grows, though more extensive experimentation is required to solidify this
conclusion. This testing is underway and results will be presented and discussed. \\ \\
Gumbel, J., D. P. Murtagh, P. J. Espy, and G. Witt, "Odd Oxygen measurements during the Noctilucent Cloud 93 rocket
campaign," {\it Journal of Geophysical Research}, Vol. 103, No. A10, 1998, pp. 23,399-23,414. \\ \\
Murray, B. J, and J. M. C. Plane, {\it personal communications}, 2003
DE: 0320 Cloud physics and chemistry
DE: 0340 Middle atmosphere--composition and chemistry
DE: 1610 Atmosphere (0315, 0325)
SC: SPA - Aeronomy [SA]
MN: 2003 Fall Meeting