O(¹D) Kinetic Study of Key Ozone Depleting Substances and Greenhouse Gases

A key stratospheric loss process for ozone depleting substances (ODSs) and greenhouse gases (GHGs) is reaction with the O­(¹D) atom. In this study, rate coefficients, <i>k</i>, for the O­(¹D) atom reaction were measured for the following key halocarbons: chlorofluorocarbons (CFCs) CFCl₃ (CFC-11), CF₂Cl₂ (CFC-12), CFCl₂CF₂Cl (CFC-113), CF₂ClCF₂Cl (CFC-114), CF₃CF₂Cl (CFC-115); hydrochlorofluorocarbons (HCFCs) CHF₂Cl (HCFC-22), CH₃CClF₂ (HCFC-142b); and hydrofluorocarbons (HFCs) CHF₃ (HFC-23), CHF₂CF₃ (HFC-125), CH₃CF₃ (HFC-143a), and CF₃CHFCF₃ (HFC-227ea). Total rate coefficients, <i>k</i>_T, corresponding to the loss of the O­(¹D) atom, were measured over the temperature range 217–373 K using a competitive reactive technique. <i>k</i>_T values for the CFC and HCFC reactions were >1 × 10^–10 cm³ molecule^–1 s^–1, except for CFC-115, and the rate coefficients for the HFCs were in the range (0.095–0.72) × 10^–10 cm³ molecule^–1 s^–1. Rate coefficients for the CFC-12, CFC-114, CFC-115, HFC-23, HFC-125, HFC-143a, and HFC-227ea reactions were observed to have a weak negative temperature dependence, <i>E</i>/<i>R</i> ≈ −25 K. Reactive rate coefficients, <i>k</i>_R, corresponding to the loss of the halocarbon, were measured for CFC-11, CFC-115, HCFC-22, HCFC-142b, HFC-23, HFC-125, HFC-143a, and HFC-227ea using a relative rate technique. The reactive branching ratio obtained was dependent on the composition of the halocarbon and the trend in O­(¹D) reactivity with the extent of hydrogen and chlorine substitution is discussed. The present results are critically compared with previously reported kinetic data and the discrepancies are discussed. 2D atmospheric model calculations were used to evaluate the local and global annually averaged atmospheric lifetimes of the halocarbons and the contribution of O­(¹D) chemistry to their atmospheric loss. The O­(¹D) reaction was found to be a major global loss process for CFC-114 and CFC-115 and a secondary global loss process for the other molecules included in this study