Heterogeneous Reactions of Methane with Cl Radicals on Large Ar Clusters.

Heterogeneous chemistry on the surfaces of atmospheric particles has a wide impact on the properties and composition of the Earth's atmosphere. In laboratory studies, clusters can represent proxies to atmospheric aerosols and help to discern the individual steps in reactions on or in aerosols. We investigate the reactivity of Cl and CCl radicals with methane on argon clusters using the pickup method. For radical generation, we built a new pyrolysis source partially adapting the design of radical sources that utilize the supersonic expansion into a heated silicon carbide tube. Large Ar, ≈ 110, clusters were generated in a supersonic expansion, and CH molecules were embedded in the clusters via a pickup process followed by the uptake of the radicals produced in the pyrolysis source. The analysis of the mass spectra recorded under different experimental conditions (i.e., with the pyrolysis ON and OFF and with only one or both reactants) allowed us to identify various products of the radical reactions on Ar. We propose a sequence of reactions based on the reaction energetics. It starts with the hydrogen abstraction from CH by a Cl radical resulting in HCl and CH followed by a halogenation step where CCl molecules react with the available CH radicals, yielding CHCl. By analogy, the CHCl enters another hydrogen abstraction by Cl, producing HCl and the CHCl radical, which again undergoes a halogenation step with CCl, generating CHCl. Further reaction of CHCl with Cl terminates the sequence by the production of HCl and CHCl.