  Formation of organic aerosols driven by photochemical reactions has been
observed and suggested in CH4-containing atmospheres, including Titan and early
Earth. However the detailed production and growth mechanisms of organic
aerosols driven by solar far ultraviolet (FUV) light remain poorly constrained.
We conducted laboratory experiments simulating photochemical reactions in a
CH4-CO2 atmosphere driven by the FUV radiations dominated by the Lyman-{\alpha}
line. In the experiments, we analyzed time variations in thickness and infrared
spectra of solid organic film formed on an optical window in a reaction cell.
Gas species formed by FUV irradiation were also analyzed and compared with
photochemical model calculations. Our experimental results show that the growth
rate of the organic film decreases as the CH4/CO2 ratio of reactant gas mixture
decreases, and that the decrease becomes very steep for CH4/CO2 < 1. Comparison
with photochemical model calculations suggests that polymerizations of
gas-phase hydrocarbons, such as polyynes and aromatics, cannot account for the
growth rate of the organic film but that the addition reaction of CH3 radicals
onto the organic film with the reaction probability around 10-2 can explain the
growth rate. At CH4/CO2 < 1, etching by O atom formed by CO2 photolysis would
reduce or inhibit the growth of the organic film. Our results suggest that
organic aerosols would grow through CH3 addition onto the surface during the
precipitation of aerosol particles in the middle atmosphere of Titan and early
Earth. On Titan, effective CH3 addition would reduce C2H6 production in the
atmosphere. On early Earth, growth of aerosol particles would be less efficient
than those on Titan, possibly resulting in small-sized monomers and influencing
UV shielding.
