Importance of Recent CH3 Recombination Rate Measurements for Methane Photochemical Models

In the stratospheres of the outer planets methane (CH₄) is photolyzed by the solar ultraviolet into a suite of radicals methylidyne (CH), methylene (CH₂), and methyl (CH₃). Subsequent photochemical reactions convert those radicals into the more familiar and well loved C₂ hydrocarbons, which are the sources for the more complex hydrocarbons that are just now being detected. These hydrocarbons are important for controlling the thermal structure of the stratospheres and as precursors for the observed stratospheric haze. The recent observations of IR emission from CH₃ in the stratospheres of Neptune, Saturn, and now Jupiter (Cassini-CIRS) provide an important window early on into this chemical chain. The dominant sink of methyl is recombination to form ethane (CH₃ + CH₃ + M -> C₂H₆ + M). Since this reaction is three-body, the methyl loss rate, and thus the methyl abundance, is dependent upon both the temperature and the pressure where the methyl is formed. This sensitivity can then be exploited by a photochemical model to determine the pressure level to where the hydrocarbons are mixed. However, this technique is predicated upon knowing the sources and sinks for the CH₃. Recent low temperature experiments (155-296 K) for CH₃ + CH₃ in a helium atmosphere have been reported and with the aid of detailed chemical kinetic calculations provide information on the collisional energy transfer in the reaction. Using the calculated rate coefficients and the experimentally determined energy transfer model we have used various methods to parameterize the pressure dependence for this reaction. The expressions are then used to predict rate coefficients outside the temperature and pressure regions used for the "calibration". The sensitivity of the photochemical model predicted CH₃ abundance to these expressions for the rate coefficients will be presented.