Cloud Water Chemistry at Sonnblick Observatory with a Focus on Organic Acids

Organic acids are, besides inorganic constituents, important components of cloud water samples. Typical analytes include monocarboxylic, dicarboxylic, tricarboxylic, and aromatic carboxylic acids. They are either scavenged from the gas and aerosol phase or formed within cloud droplets via chemical reactions in the aqueous phase. Concentrations in cloud water depend on meteorological conditions, air mass origin, and cloud properties, including cloud type and liquid water content. Organic acids influence cloud chemistry, facilitate cloud droplet formation, and increasingly contribute to the acidity of cloud water.

We present the results of two cloud water sampling campaigns conducted in August 2024 and May 2025 at Sonnblick Observatory (3106 m a.s.l.) in the Austrian Alps. The sampling campaigns were conducted within the framework of ACTRIS activities. Depending on the season, sampling comprised warm clouds as well as mixed-phase clouds, including periods coinciding with mineral dust events. Chemical analyses of organic acids were performed using ion chromatography with conductivity and mass spectrometry detection. Further analyses included inorganic ions, pH, conductivity, and selected carbohydrates.

One part of the evaluations focuses on methodological topics, such as the comparison of different analytical set-ups used for the analysis of organic acids. The advantage of mass spectrometry detection is demonstrated by the analysis of several coeluting substances that cannot be resolved by routine gradient ion chromatography. Additionally, the ion concentrations of organic acids observed during the two sampling campaigns are shown and discussed with respect to the overall chemical composition and meteorological conditions. Finally, the contribution of organic acids to the overall acidity of cloud water is discussed and evaluated in comparison with earlier measurements from the 1990s conducted at Sonnblick Observatory.