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

Atmospheric lifetimes are evaluated for the fully fluorinated compound s CF4, C2F6, c-C4F8, C6F14, and SF6 using a two-dimensional transport and chemistry model which includes removal by electrons and ions in th e mesosphere and lower thermosphere. Laboratory measurements of the pe rtinent reaction rates were carried out at thermal energy for free ele ctrons and for the atmospheric ions O+, O-2(+) O-2(-), O-2(-), NO+, H3 O+, NO3-, and CO3-. Atmospheric removal by electrons reduces the lifet imes of c-C4F8 and SF6 from about 3200 years to 1400 and 800 years, re spectively, only if the respective product anions C4F8- and SF6- do no t subsequently regenerate the parent neutral compounds. Atmospheric re moval 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 i n the atmosphere for these two compounds, but their lifetimes are gove rned by removal at the Earth's surface in high-temperature combustors. While we show that the lifetimes of c-C4F8 and SF6 may be significant ly shorter than previously estimated, these compounds remain extremely long-lived with significant global warming potentials.