We present the first detailed analysis of low volatility organic compounds and non-volatile organic matter in cloud water samples using thermal desorption – proton transfer reaction - mass spectrometry (TD-PTR-MS). Cloud water samples were collected at the Sonnblick Observatory (3106 m a.s.l.) in the Austrian Alps in November-December 2022 using a single-stage droplet impactor. The samples were prepared by removing water and volatile compounds under vacuum before analysis using a TD-PTR-MS. We detected 693 ions above the limit of detection, and the 19 ions with the highest sample median concentration contributed to 30% of the total observed signal. The total mass of organic compounds measured was in the range 0.47-34 μmol L -1 cloud water, with a mean concentration (± 1 standard deviation) of 5.5 ± 8.0 μmol L -1 . The most abundant ions were nitrogen-containing fragments including CH 4 NO + (46.029 Th, 323 pg m -3 median), and CH 3 O 2 + (47.013 Th, 167 pg m -3 ), along with multiple compounds that have been observed in experiments investigating the degradation of biogenic compounds. Non-negative matrix factorization (NMF) using 4 factors was applied to cluster the measured compounds and identify possible chemical sources and processes. The four factor NMF solution explained 91.1% of data variance and we tentatively associate the most dominant contributions with aged biogenic volatile organic compounds, and occasional anthropogenic influence. • First detailed characterization of low volatility and non-volatile organic compounds in cloud water samples. • Nitrogen-containing fragments were the most abundant semi-volatile components in cloud water. • Aged organic compounds are the most important source of cloud organics. • Influences from biogenic sources and biomass burning were detected in high-altitude cloud samples.
Notø et al. (Fri,) studied this question.