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

Abstract

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.