EFFECTS OF ELECTRON AND ION REACTIONS ON ATMOSPHERIC LIFETIMES OF FULLY FLUORINATED COMPOUNDS

Atmospheric lifetimes are evaluated for the fully fluorinated compounds CF4, C2F6, c-C4F8, C6F14, and SF6 using a two-dimensional transport and chemistry model which includes removal by electrons and ions in the mesosphere and lower thermosphere. Laboratory measurements of the pertinent reaction rates were carried out at thermal energy for free electrons and for the atmospheric ions O+, O-2(+) O-2(-), O-2(-), NO+, H3O+, NO3-, and CO3-. Atmospheric removal by electrons reduces the lifetimes of c-C4F8 and SF6 from about 3200 years to 1400 and 800 years, respectively, only if the respective product anions C4F8- and SF6- do not subsequently regenerate the parent neutral compounds. Atmospheric removal by ion reactions is minor or negligible, with the largest effect (similar to 5%) being removal of C6F14 by O-2(+). Removal of CF4 and C2F6 by O+ is probably the most important single destruction process in the atmosphere for these two compounds, but their lifetimes are governed by removal at the Earth's surface in high-temperature combustors. While we show that the lifetimes of c-C4F8 and SF6 may be significantly shorter than previously estimated, these compounds remain extremely long-lived with significant global warming potentials.