Low temperature studies of the removal reactions of 1CH2 with particular relevance to the atmosphere of Titan

Abstract

Methylene, CH2, 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, 1CH2, with He, N2, O2, H2 and CH4 have been measured over the temperature range 43–160 K by pulsed laser photolysis, monitoring 1CH2 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 N2, O2, H2 and CH4 all showed a strong negative temperature dependence. In combination with other literature data, the rate coefficients can be parameterised as:kHe(43 < T/K < 800) = (1.90 ± 0.23) × 10−12 × (T/298)1.74±0.16 × exp((88±23)/T)kN2(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)kO2(43<T/K<300) = (6.16 ± 1.09) × 10−11 × (T/298)−0.65±0.14kH2(43<T/K<800) = (1.10 ± 0.04) × 10−10 × (T/298)−0.40±0.06 × exp((11.1±6.9)/T)kCH4(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 1CH2 with H2 and CH4, the branching ratio for quenching to ground state, 3CH2, vs chemical reaction was also determined at 160 and 73 K. The values measured (H2: 0.39 ± 0.10 at 160 K, 0.78 ± 0.15 at 73 K; CH4: 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 1CH2 with CH4, 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 CH4 to generate ethene. Photolysis of ethene is the major route to acetylene formation.