Chemical analysis and source estimation of low volatility and non-volatile organic constituents in cloud water collected at Mt Sonnblick, Austria

Abstract

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 CH4NO+ (46.029 Th, 323 pg m−3 median), and CH3O2+ (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.