JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D1, PAGES 1287-1294, JANUARY 20, 1995 Effects of electron and ion reactions on atmospheric lifetimes of fully fiuorinated compounds Robert A. Morris, Thomas M. Miller,•, 2 A. A. Viggiano, and John F. Paulson Phillips Laboratory, Geophysics Directorate, Ionospheric Effects Division (GPID), Hanscom AFB, Massachusetts Susan Solomon and George Reid NOAA Aeronomy Laboratory, Boulder, Colorado Abstract. 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 mesosphereand lower thermosphere. Laboratory measurements of the pertinent reaction rates were carried outat thermal energy for freeelectrons and for theatmospheric ions O+ O•- O- O•-, NO+ , H3 O+, NO•-, and CO•-. Atmospheric removal by electrons reduces the lifetimesof c-C4F8 and SF6 from about 3200 years to 1400and 800 years, respectively, only if the respectiveproduct anionsC4F•- and SF•- do not subsequently regenerate the parent neutral compounds. Atmospheric removal by ion reactions is minor or negligible, withthelargest effect (-5%) being removal of C6F14 by O•-. Removal of CF 4 and C2F 6 by O+ is probably themost important single destruction process in the atmospherefor these two compounds,but their lifetimes are governed by removal at the Earth's surface in high-temperaturecombustors.While we show that the lifetimes of c-C4F8 and SF6 may be significantly shorter than previously estimated, these compoundsremain extremely long-lived with significantglobal warming potentials. Introduction Fully fluorinated compounds(perfluorocompounds) repre- sent a class of chemicals which include many extremely inert species--inert to both chemical reaction and photolysis. Well known examplesinclude SF6 and the perfluorocarbons CF 4 and Teflon (polytetrafluoroethylene). SF6 is used as a gaseous dielectric, as a semiconductor etchant, and as an atmospheric tracer. Long-term observations of SF6 in the atmosphere have recently been reported and reveal that the atmospheric SF6 concentration has increased by 2 orders of magnitude between 1970 and 1992 [Maiss and Levin, 1994]. CF 4 and C2F 6 are released into the atmosphere as by- products of aluminum manufacture and are also used in plasma processing. Several perfluorocompoundshave been proposed as substitutes for the ozone-destroying chloroflu- orocarbons and halons; proposed substitutesinclude SF6, C3F8, c-C4F8, C4Flo, C5F12 , and C6F14. Perfluorocom- pounds were suggested, in part, because they are believed not to harm the stratospheric ozone layer and were therefore considered environmentally friendly. Recently, scientists at the National Oceanic and Atmo- spheric Administration (NOAA) [Ravishankara et al., 1993] reported that perfluorocompounds have very long atmo- spheric lifetimes (of the order of millennia) and thus signifi- JUnder contract to Orion International Technologies, Albuquer- que, New Mexico. 2Also at Department of Physics andAstronomy, University of Oklahoma, Norman. Copyright 1995 by the American Geophysical Union. Paper number 94JD02399. 0148-0227/95/94JD-02399505.00 cant global warming potentials (GWP). The GWP is a measure of the relative efficiency of a gas to contribute to the greenhouse effect through absorption of infrared radiation (per kilogram emitted). The absolute global warming de- pends on this and on total emission. The long lifetimes are the result of the extreme inertness toward the processes which remove most trace chemicals from the atmosphere, i.e., photodissociationand reaction with radicals. Ravishan- kara et al. [1993] determined that the most important atmo- spheric removal processes for most of these very inert compounds are mesospheric photolysis and reaction with O•D, buttheydidnotconsider removal by electron andion reactions which may occur in the mesosphere and lower thermosphere (except in the case of SF6 for which a slow rate of removal by electrons was assumed). Another recent study [Ko et al., 1993] has reported a possible lifetime of 25,000 years for SF6. The model used in the Ko et al. study extends to 60 km and hence does not include mesospheric chemistry. In most cases, ions and electrons play essentially no role in controlling the atmospheric concentrations of trace spe- cies. This is the result of the relatively low concentrations of electrons and ions compared with reactive neutral species and the effects of photolysis. Reactions with electrons and ions are likely to be significant sinks only for those com- pounds which are otherwise extremely stable against chem- ical and photolytic degradation. The perfluorochemicals fall into this class of very inert compounds. Despite the rela- tively low atmospheric concentrations of charged species, the fact that ions and especially electrons can have large reaction rate constants may lead to significant removal rates for some of the otherwise inert perfluorocompounds. Rate 1287