Environ. Sci. Technol. 2006, 40, 2242-2246
Atmospheric Lifetime and Global Warming Potential of a Perfluoropolyether CORA J. YOUNG,† MICHAEL D. HURLEY,‡ TIMOTHY J. WALLINGTON,‡ AND S C O T T A . M A B U R Y †,* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6, and Ford Motor Company, Mail Drop SRL-3083, Dearborn, Michigan 48121
Perfluoropolyethers (PFPEs) are a family of perfluorinated fluids used mainly in industrial applications. Lower molecular weight commercial PFPE fractions have boiling points ranging between 55 and 270 °C, and have the potential to escape into the atmosphere. To improve our understanding of the atmospheric chemistry of PFPEs, a distilled fraction of a commercial mixture containing perfluoropolymethylisopropyl ethers (PFPMIEs) was introduced into an atmospheric chamber system. Relative rate techniques were used to determine upper limits for the rate constants for reactions of OH and Cl with PFPMIE in 700 Torr of air at 296 K. The reactivity of PFPMIE with Cl was less than 2 × 10-17 cm3 molecule-1 s-1, while the reactivity with OH was less than 6.8 × 10-16 cm3 molecule-1 s-1, indicating low reactivity in the troposphere. Consequently, the lifetime of PFPMIE should be limited by transport to the mesosphere, where photolysis by Lyman-R radiation at 121.6 nm will be efficient. By analogy to perfluorinated alkanes, the lower limit for the total atmospheric lifetime is 800 years. PFPMIE was shown to have instantaneous radiative forcing of 0.65 W m-2 ppb-1, which corresponds to a global warming potential on a 100 year time scale of 9000 relative to CO2 and 1.95 relative to CFC-11.
Introduction Recognition of the adverse environmental impact of chlorofluorocarbon (CFC) and Halon release into the atmosphere has led to an international effort to replace these compounds with environmentally acceptable alternatives. Perfluoropolyethers (PFPEs), along with hydrofluoroethers (HFEs), have been used as replacements for CFCs as heat transfer fluids. They are also used for electronic reliability testing. These compounds lack Cl, so they do not contribute to the catalytic destruction of the ozone layer. However, PFPEs and HFEs may be associated with other environmental risks. Within the Earth’s atmosphere, there are no major components that absorb significant thermal radiation between approximately 750 and 1250 cm-1, creating a window through which heat emitted from the Earth can escape. This region is termed the “atmospheric window”, and anthropogenic compounds in the atmosphere that absorb within this region have the ability to block the escape of terrestrial * Corresponding author phone: 416-978-1780; fax: 416-978-3596; e-mail:
[email protected]. † University of Toronto. ‡ Ford Motor Company. 2242
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 7, 2006
FIGURE 1. General structure of PFPMIEs. radiation. Radiative forcing is a measure of the ability of a compound to disrupt the energy balance of the earth, for example, by absorbing IR radiation within the atmospheric window. Fluorinated ethers can contribute to global warming, because both the C-F and C-O bonds absorb in this range. PFPEs and HFEs would be expected to have high radiative forcings, as they have multiple C-F and C-O bonds. Several studies have focused on the atmospheric fate of HFEs, and they have been shown to have lifetimes that vary between days and hundreds of years (1-5). These compounds degrade in the atmosphere through the abstraction of a hydrogen atom by hydroxyl radicals. PFPEs have no abstractable hydrogens, nor do they have any sites for OH addition. Thus, it is likely that their lifetimes will be much greater than those of HFEs. The atmospheric lifetimes of PFPEs are expected to ultimately be limited by photolysis in the upper atmosphere. A long lifetime, in combination with high radiative forcing, results in a high global warming potential (GWP) for a compound. Perfluoropolymethylisopropyl ethers (PFPMIEs) are sold as mixtures according to their boiling point. The fraction selected here boils at 70 °C, has an average molecular weight of 410 and is composed primarily of CF3OCF(CF3)CF2OCF2OCF3 (molecular weight ) 386), with smaller amounts of CF3OCF(CF3)CF2OCF2OCF2OCF3 (molecular weight ) 452) and longer-chain PFPMIEs (Figure 1). Despite the high molecular weight, PFPMIE is volatile and is expected to escape into the atmosphere. Currently, there are no available data concerning the atmospheric fate of PFPEs. To provide such data, the atmospheric chemistry of PFPMIE was investigated. Specifically, the following information was determined using smog chamber/FTIR techniques and by analogy to other long-lived perfluorinated compounds: (i) the kinetics of reactions with chlorine atoms and hydroxyl radicals, (ii) the infrared spectrum, (iii) the atmospheric lifetime, and (iv) the global warming potential.
Experimental Section Chemical Preparation. A commercial mixture of Galden HT70, obtained from Solvay Solexis (Thorofare, New Jersey), was purified using fractional distillation to remove any hydrogen-containing impurities. Purity of the distillate was confirmed using 1H NMR, where no evidence of hydrogen impurities was observed. The distillate was still a mixture, because fractional distillation could not isolate a single PFPE. The mixture was composed mainly of CF3OCF(CF3)CF2OCF2OCF3, with smaller amounts of CF3OCF(CF3)CF2OCF2OCF2OCF3 and longer-chain analogues. Kinetics. Experiments were performed in a 140 L Pyrex reactor interfaced to a Mattson Sirus 100 FTIR spectrometer. The reactor was surrounded by 22 fluorescent blacklamps (GE F15T8-BL), which were used to photochemically initiate the experiments. Chlorine atoms were produced by the photolysis of molecular chlorine:
Cl2 + hv f Cl + Cl 10.1021/es052077z CCC: $33.50
2006 American Chemical Society Published on Web 02/24/2006
OH radicals were produced by the photolysis of CH3ONO in air:
CH3ONO + hv f CH3O(•) + NO CH3O(•) + O2 f HO2 + HCHO HO2 + NO f OH + NO2 In relative rate experiments, the following reactions take place:
OH/Cl + reactant f products It can be shown that
(
ln
OH/Cl + reference f products
)
[reactant]t0 [reactant]t
)
(
)
[reference]t0 kreactant ln kreference [reference]t
where [reactant]t0, [reactant]t, [reference]t0, and [reference]t are the concentrations of the reactant and reference at times t0 and t and kreactant and kreference are the rate constants for the reactant and the reference. Reaction mixtures for k(Cl + PFPMIE) consisted of 88 mTorr of PFPMIE, 1.7 Torr of Cl2, and 2.6 mTorr of CF2ClH in 700 Torr of N2 diluent. Mixtures for k(OH + PFPMIE) consisted of 88-176 mTorr of PFPMIE, 100-240 mTorr of CH3ONO, and 1.5-2.6 mTorr of C2H2 in 700 Torr total pressure of air diluent. All experiments were performed at 296 ( 1 K. CH3ONO was synthesized by the dropwise addition of concentrated sulfuric acid to a saturated solution of NaNO2. All other reagents were obtained from commercial sources. C2H2 was selected as the reference compound in the OH relative rate experiments because it is the least-reactive compound whose loss can be monitored with high precision in the present system. Concentrations of reactants and products were monitored by FTIR spectroscopy. IR spectra were derived from 32 coadded interferograms with a spectral resolution of 0.25 cm-1 and an analytical path length of 27.1 m. To check for the unwanted loss of compounds via heterogeneous reactions, reaction mixtures were left to stand in the chamber for 60 min without irradiation; there was no observable (
