Low Temperature Rate Coefficients for Outer Planet Atmospheres

Sri International, Menlo Park CA

Investigators

Abstract

AST 0074140 Smith Recent observations from the Infrared Space Observatory have provided the first measurements of the mixing ratio of the methyl radical, CH 3 , in the atmospheres of Saturn and Neptune. CH 3 is produced by photolysis of CH 4 and is the key photochemical intermediate leading to complex organic molecules on the giant planets and moons. However, the observed CH 3 infrared emissions are much weaker than predicted by current photochemical models. Several modelers favor using a rate coefficient for CH 3 + CH 3 +M--> C 2 H 6 + M for the temperature range 140-190 K that is at least 10 times larger than the value at 296 K, the lowest temperature at which laboratory measurements are available. Reliable experimental and theoretical pressure-dependent rate constants are required for this reaction and for others at low temperatures and pressures to model hydrocarbon, methyl, nitrile, and CO concentrations and to derive atmospheric transport rates. Dr. Smith and his colleagues will combine a theoretical approach with selected low temperature-low pressure recombination rate measurements in the laboratory to solve this modeling problem. The theoretical computations will provide consistent rate parameters to describe the data and potential energy surfaces and will cover a wider temperature range (30-3000K) than the current expressions, which were designed for combustion modeling purposes and extrapolate unreliably to low temperatures. Experiments will be conducted at 200K and lower, using a laser photolysis, resonance-enhanced multiphoton ionization (REMPI) technique in a low-pressure cell to provide key missing low-temperature data. A three-year program of research will be conducted to improve low-temperature hydrocarbon kinetics parameters in models of planetary atmospheres, emphasizing (1) examination of mechanisms in existing photochemical models with sensitivity analysis to identify important uncertainties, (2) theoretical parameterization of the temperature-dependent rates of CH 3 + CH 3 + M --> C 2 H 6 + M and other key steps, and (3) low temperature and pressure measurements of these reaction rates for input to the parameterizations. Reliable values for these rates must be made available to accurately model outer planet atmospheric photochemistry and to properly interpret old and new planetary data, including the derivation of transport rates. This award is funded through the Planetary Astronomy Program. ***