Selective ultra-trace atmospheric passive sampling of gaseous perfluoroalkyl carboxylic acids

Abstract

Acidic pollutants have long been known to have detrimental impacts on remote ecosystems. They have been subject to assessment through long-term monitoring with low cost methodologies, such as passive sampling. Atmospheric oxidation is becoming recognized as a source of persistent perfluorinated carboxylic acids (PFCAs) derived from volatile poly- and perfluorinated alkyl substance (PFAS) precursors. In this work, nylon substrate passive air samplers that are selective for atmospheric acids are described. Their PFCA suitability is verified through extensive quality assurance and control tests, including controlled chamber tests for sampling rates. The sampling rate for gaseous trifluoroacetic acid (TFA; C2) was established experimentally, with ultra-trace detection limits at parts per quadrillion by volume mixing ratios (ppqv; 10^-15 mol mol^-1) for a sampling period of one week. Sampling rates for the C3-C6 homologues of the PFCA family were derived from two diffusion theories of varying complexity. Proof-of-concept measurements were performed at urban, rural continental, and remote marine sites in Canada for over a year. All PFCA homologues from C2-C6 were detected above the method detection limits, with abundance decreasing with increasing chain length and from urban to rural to remote locations. Atmospheric abundance was dominated by TFA, which also showed trends consistent with known sources of precursors and atmospheric oxidation chemistry. This new PFCA-specific passive air sampling method represents a promising new option for the study of PFCA formation, transport, and fate in the atmosphere.