## 4 Atmospheric Chemistry of DA-FPEs  
The absence of chlorine atoms in HFEs makes them possess a negligible ozone depleting potential (ODP), but the presence of C-F and C-O bonds may absorb in the terrestrial infrared radiation region (800-1200 cm\({}^{-1}\)), and could play a significant role as a greenhouse gas. Therefore, considerable attention has been paid in recent years to perform both experimental and theoretical studies on atmospheric chemistry of HFEs. However, surprisingly, only relatively little information about DA-FPEs has been reported [36,37]. Since the only available data on DA-FPEs compounds are related to DM-FPEs with structure CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\) that only contain the -(CF\({}_{2}\)CF\({}_{2}\)O)- units, the discussion has been focused prevalently on the atmospheric chemistry of these compounds. Atmospheric Lifetimes and Decomposition Mechanism of \(\alpha\),\(\omega\)-Dialkoxperfluoropolyethers  
In 2004, the first understanding of the atmospheric chemistry for DM-FPEs was achieved by studying the kinetics and mechanism of CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\) (\(n\) = 1-3) oxidation which is initiated by Cl atoms and OH radicals in the atmosphere; the impact of their degradation products on the climate were also discussed [1]. Like most organic compounds, the primary tropospheric degradation of CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\) is mainly dominated by the attack of OH radicals [38]. Understanding of reaction kinetics is essential for the evaluation of the atmospheric lifetime of these molecules. In addition to the reaction with hydroxyl radicals (OH), organic compounds can also be removed from the atmosphere via photolysis, wet deposition, and reaction with NO\({}_{3}\) radicals, Cl atoms, and O\({}_{3}\)[38, 39, 40]. For saturated compounds such as CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\), reactions with NO\({}_{3}\) radicals and O\({}_{3}\) are typically too slow to be of importance [1]. The average concentration of Cl atoms in the troposphere is orders of magnitude less than that of OH radicals [41]. Reaction with Cl atoms will be a less significant atmospheric loss mechanism. Since ethers do not absorb at UV wavelengths \(>\) 200 nm, photolytic sink will be not be an important degradation pathway [42] either. Highly fluorinated molecules, such as CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\), are hydrophobic and wet deposition is unlikely to be of importance. In conclusion, the atmospheric lifetime of CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\) is determined by its reaction with OH. Smog chambers equipped with FTIR spectrometers were used to study the Cl atom and OH radical initiated oxidation of CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\) in 720 Torr of air at 296.3 K. Relative rate methods were used to measure _k_(Cl+CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\))) = 3.7 (\(\pm\)0.7) \(\times\) 10\({}^{-13}\) and _k_(OH+CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\)) = 2.9 (\(\pm\) 0.5) \(\times\) 10\({}^{-14}\) cm\({}^{3}\) molecule\({}^{-1}\)-s\({}^{-1}\). The relative rate method is widely used to measure the reactivity of OH radicals with organic compounds [43]. The hydrogen abstraction reaction from the terminal groups has been assumed, according to established literature data, as the rate-determining step for the decomposition of the entire molecule [44]. It is well known that -CF\({}_{2}\)CF\({}_{2}\)- units are unreactive toward Cl and OH radicals. Thus, the reactivity of CH\({}_{3}\)O(CF\({}_{2}\)CF\({}_{2}\)O)\({}_{n}\)CH\({}_{3}\) is confined to the C-H bonds on either end-groups of the molecule. Indeed, appropriate studies showed that the reactivity of the methyl group did not change noticeably by increasing the number \(n\) of -(CF\({}_{2}\)CF\({}_{2}\)O)- units from 1 to 3 [45].