Christy Remucal (@remucal.bsky.social)

High Time Resolution Ambient Observations of Gas-Phase Perfluoroalkyl Carboxylic Acids: Implications for Atmospheric Sources Atmospheric formation of persistent perfluoroalkyl carboxylic acids (PFCAs) is a route to global contamination, including drinking water sources. We present high time resolution measurements of C2–C6 PFCAs in ambient air made over 6 weeks each in winter and summer 2022 in Toronto, Canada. Observations were made using chemical ionization mass spectrometry with acetate ionization, with care taken to avoid system contamination. Measurements of trifluoroacetic acid (C2 PFCA) are reported at 1 min time resolution, while those for C3–C6 PFCAs are at 10 min time resolution. Variations with time and relationships to meteorology and other pollutants show evidence of secondary formation of all observed PFCAs except perfluoropropionic acid (C3 PFCA). High time resolution data allow these unique observations, allowing for improved process and source understanding moving forward. Mixing ratios of TFA were larger than predicted from models that describe TFA formation from known precursors, indicating additional atmospheric sources of this molecule have yet to be identified. https://pubs.acs.org/doi/10.1021/acs.estlett.4c00897