27-10-2012, 03:21 PM
Rate Constant for the Reaction CH3 + CH3 Yields C2H6 at T = 155 K and Model Calculation of the CH3 Abundance in the Atmospheres of Saturn and Neptune
ABSTRACT
The column abundances of CH3 observed by the Infrared Space Observatory (ISO) satellite on Saturn and Neptune were
lower than predicted by atmospheric photochemical models, especially for Saturn. It has been suggested that the models
underestimated the loss of CH3 due to poor knowledge of the rate constant k of the CH3 + CH3 self-reaction at the low
temperatures and pressures of these atmospheres. Motivated by this suggestion, we undertook a combined experimental and
photochemical modeling study of the CH3 + CH3 reaction and its role in determining planetary CH3 abundances. In a
discharge flow-mass spectrometer system, k was measured at T = 155 K and three pressures of He. The results in units of cu
cm/molecule/s are k(0.6 Torr) = 6.82 x 10(exp -11), k(1.0 Torr) = 6.98 x 10(exp -11), and k(1.5 Torr) = 6.91 x 10(exp -11).
Analytical expressions for k were derived that (1) are consistent with the present laboratory data at T = 155 K, our previous
data at T = 202 K and 298 K, and those of other studies in He at T = 296-298 K and (2) have some theoretical basis to provide
justification for extrapolation. The derived analytical expressions were then used in atmospheric photochemical models for
both Saturn and Neptune. These model results reduced the disparity with observations of Saturn, but not with observations
of Neptune. However, the disparity for Neptune is much smaller. The solution to the remaining excess CH3 prediction in the
models relative to the ISO observations lies, to a large extent, elsewhere in the CH3 photochemistry or transport, not in the
CH3 + CH3 rate.
ABSTRACT
The column abundances of CH3 observed by the Infrared Space Observatory (ISO) satellite on Saturn and Neptune were
lower than predicted by atmospheric photochemical models, especially for Saturn. It has been suggested that the models
underestimated the loss of CH3 due to poor knowledge of the rate constant k of the CH3 + CH3 self-reaction at the low
temperatures and pressures of these atmospheres. Motivated by this suggestion, we undertook a combined experimental and
photochemical modeling study of the CH3 + CH3 reaction and its role in determining planetary CH3 abundances. In a
discharge flow-mass spectrometer system, k was measured at T = 155 K and three pressures of He. The results in units of cu
cm/molecule/s are k(0.6 Torr) = 6.82 x 10(exp -11), k(1.0 Torr) = 6.98 x 10(exp -11), and k(1.5 Torr) = 6.91 x 10(exp -11).
Analytical expressions for k were derived that (1) are consistent with the present laboratory data at T = 155 K, our previous
data at T = 202 K and 298 K, and those of other studies in He at T = 296-298 K and (2) have some theoretical basis to provide
justification for extrapolation. The derived analytical expressions were then used in atmospheric photochemical models for
both Saturn and Neptune. These model results reduced the disparity with observations of Saturn, but not with observations
of Neptune. However, the disparity for Neptune is much smaller. The solution to the remaining excess CH3 prediction in the
models relative to the ISO observations lies, to a large extent, elsewhere in the CH3 photochemistry or transport, not in the
CH3 + CH3 rate.