Low temperature studies of the removal reactions of¹CH<inf>2</inf>with particular relevance to the atmosphere of Titan

Methylene, CH 2 , is one of the major photolysis products of methane by Lyman-α radiation and is involved in the photochemistry of the atmospheres of Titan and the giant planets. The kinetics of the reactions of the first excited state of methylene, 1 CH 2 , with He, N 2 , O 2 , H 2 and CH 4 have been measured over the temperature range 43–160 K by pulsed laser photolysis, monitoring 1 CH 2 removal by laser induced fluorescence. Low temperatures were obtained with either a pulsed Laval expansion (43–134 K) or a, slow flow reaction cell (160 K). The rate coefficients for the reactions with N 2 , O 2 , H 2 and CH 4 all showed a strong negative temperature dependence. In combination with other literature data, the rate coefficients can be parameterised as: k He (43 < T/K < 800) = (1.90 ± 0.23) × 10 −12 × (T/298) 1.74±0.16 × exp ((88±23)/ T ) k N 2 (43 < T/K < 800) = (2.29 ± 1.12) × 10 −12 × (T/298) −2.15±1.38 × exp ((-74±96)/ T ) + (3.91 ± 0.78) × 10 −11 × exp ((-469±114)/ T ) k O 2 (43 < T/K < 300) = (6.16 ± 1.09) × 10 −11 × (T/298) −0.65±0.14 k H 2 (43 < T/K < 800) = (1.10 ± 0.04) × 10 −10 × (T/298) −0.40±0.06 × exp ((11.1±6.9)/ T ) k CH 4 (43 < T/K < 475) = (8.20 ± 0.46) × 10 −11 × (T/298) −0.93±0.10 × exp ((-20.5±12.8)/ T ) For the reactions of 1 CH 2 with H 2 and CH 4 , the branching ratio for quenching to ground state, 3 CH 2 , vs chemical reaction was also determined at 160 and 73 K. The values measured (H 2 : 0.39 ± 0.10 at 160 K, 0.78 ± 0.15 at 73 K; CH 4 : 0.49 ± 0.09 at 160 K, 0.64 ± 0.19 at 73 K) confirm trends of an increased proportion of reactive loss with increasing temperature determined at higher temperatures. The impacts of the new measurements for Titan's atmosphere have been ascertained using a 1D chemistry and transport model. A significant decrease (∼40%) in the mixing ratio of ethane between 800 and 1550 km is calculated due to the decrease contribution of methyl production from the reaction of 1 CH 2 with CH 4 , with smaller increases in the concentrations of ethene and acetylene. Ethene production is enhanced by more methylene being converted to methylidene, CH, and the subsequent reaction of CH with CH 4 to generate ethene. Photolysis of ethene is the major route to acetylene formation.