A closer look into the contribution of atmospheric gas-phase pathways in the formation of perfluorocarboxylic acids

The potential synthesis of the notorious perfluorocarboxylic acids (PFCAs, CF₃(CF₂)_nC(O)OH) from the atmospheric degradation of fluorotelomer alcohols (FTOHs, CF₃(CF₂)_nCH₂CH₂OH), has been a focal point of debate with many intriguing questions appearing unanswered. In addition to transport by ocean currents, the atmospheric “precursor” pathway provides a likely source for the observed alarming load of PFCAs in remote regions; most notably the Arctic circle. However, pertinent smog chambers experiments that established the FTOHs → PFCAs route, were carried out under conditions that are not typically encountered in the ambient atmosphere (i.e., excessive or absence of NO/HO₂ concentrations). To underpin the underlying chemical phenomena and the plausible formation of PFCAs (and other perfluorinated compounds), we report herein a detail chemistry kinetic model into the OH-initiated decomposition of 4:2 FTOH as a model compound of fluorotelomer alcohols. The underlying aim is to illustrate pathways that govern the production of fluorotelomers acids (CF₃(CF₂)_nCH₂C(O)OH, FTCAs), fluorotelomer aldehydes (CF₃(CF₂)_nCH₂C(O)H, FTALs), perfluorinated aldehydes (CF₃(CF₂)_nC(O)H, PFALs), and PFCAs. Time-dependent molar yields of a large array of species were attained against representative atmospheric compositions that reflect urban, ocean, and arctic environments. A fast decarbonylation of the CF₃(CF₂)₃CO radicals significantly diminishes the gas-phase formation of the CF₃(CF₂)₃CO(OH) compound. Overall, it is concluded that the widely accepted formation of PFCAs in the atmosphere may take place through non-gas phase routes. Metals-rich silicate particles in aerosols and dust, could mediate formation of FTCAs and PFCAs in the atmosphere from FTOHs.