Termolecular ion-molecule reactions in Titan's atmosphere

We have applied information from the thermochemistry of cluster ions to a model of the Titan atmosphere. Using laboratory thermodynamic data on the clustering of ions with both nitrogen and methane from the literature, we find that substantial clustering of ions with the nitrogen atmosphere occurs from about 100 km down to the surface. The temperature and number density profiles of the various chemical species in the atmosphere of Titan were also taken from the literature. A specific analysis of the clustering of HCNH+ with nitrogen was performed. It was found that for HCNH+ substantial clustering will occur in a nitrogen atmosphere at the temperatures and pressures found in the Titan tropopause. The thermochemical data of the three ions: HCNH+, N2H+, and CH5+ are very simular. By inference the N2H+ and CH5+ ions are also expected to undergo substantial clustering. Assuming no factors other than equilibrium thermodynamics to control the cluster size, the maximum average cluster size found was 65.5 at the tropopause. At the surface, the cluster size decreased to about 25. (C) 2001 Elsevier Science (USA).