The role of submicrometer aerosols and macromolecules in H₂ formation in the Titan haze

Previous studies of the photochemistry of small molecules in Titan's atmosphere found it difficult to have hydrogen atoms removed at a rate sufficient to explain the observed abundance of unsaturated hydrocarbons. One qualitative explanation of the discrepancy nominated catalytic aerosol surface chemistry as an efficient sink of hydrogen atoms, although no quantitative study of this mechanism was attempted. In this paper, we quantify how haze aerosols and macromolecules may efficiently catalyze the formation of hydrogen atoms into H₂. We describe the prompt reaction model for the formation of H₂ on aerosol surfaces and compare this with the catalytic formation of H₂ using negatively charged hydrogenated aromatic macromolecules. We conclude that the PRM is an efficient mechanism for the removal of hydrogen atoms from the atmosphere to form H₂ with a peak formation rate of ∼ 70 cm^-3 s^-1 at 420 km. We also conclude that catalytic H₂ formation via hydrogenated anionic macromolecules is viable but much less productive (a maximum of ∼ 0.1 cm^-3 s^-1 at 210 km) than microphysical aerosols.