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Formation of organic aerosols driven by photochemical reactions has beenobserved and suggested in CH4-containing atmospheres, including Titan and earlyEarth. However the detailed production and growth mechanisms of organicaerosols driven by solar far ultraviolet (FUV) light remain poorly constrained.We conducted laboratory experiments simulating photochemical reactions in aCH4-CO2 atmosphere driven by the FUV radiations dominated by the Lyman-{\alpha}line. In the experiments, we analyzed time variations in thickness and infraredspectra of solid organic film formed on an optical window in a reaction cell.Gas species formed by FUV irradiation were also analyzed and compared withphotochemical model calculations. Our experimental results show that the growthrate of the organic film decreases as the CH4/CO2 ratio of reactant gas mixturedecreases, and that the decrease becomes very steep for CH4/CO2 < 1. Comparisonwith photochemical model calculations suggests that polymerizations ofgas-phase hydrocarbons, such as polyynes and aromatics, cannot account for thegrowth rate of the organic film but that the addition reaction of CH3 radicalsonto the organic film with the reaction probability around 10-2 can explain thegrowth rate. At CH4/CO2 < 1, etching by O atom formed by CO2 photolysis wouldreduce or inhibit the growth of the organic film. Our results suggest thatorganic aerosols would grow through CH3 addition onto the surface during theprecipitation of aerosol particles in the middle atmosphere of Titan and earlyEarth. On Titan, effective CH3 addition would reduce C2H6 production in theatmosphere. On early Earth, growth of aerosol particles would be less efficientthan those on Titan, possibly resulting in small-sized monomers and influencingUV shielding.

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Experimental study of heterogeneous organic chemistry induced by far  ultraviolet light: Implications for growth of organic aerosols by CH3  addition in the atmospheres of Titan and early Earth