Characterisation of hygroscopic aerosol processes using remote sensing and in situ techniques at the MeteoSwiss Payerne station

Abstract. This study presents a comprehensive assessment of aerosol hygroscopicity during the HYGRO19 field campaign conducted at the MeteoSwiss station in Payerne (Switzerland) from June to December 2019. The campaign provided a unique experimental framework by combining continuous Raman lidar observations with vertically resolved in situ measurements from balloon-borne Compact Backscatter Aerosol Detector coupled with RS41 Vaisala radiosondes (COBALD-RS41), ground-based tandem nephelometers, and complementary aerosol chemical analyses, enabling aerosol hygroscopicity to be evaluated using three independent approaches. Two contrasting aerosol regimes were observed: a summer period characterised by relatively high aerosol loading and frequent influence of long-range transported particles, and a second period dominated by low-pressure systems, persistent cloudiness, and reduced aerosol concentrations. Throughout these different atmospheric conditions, the Raman Lidar for Meteorological Observations (RALMO), which continuously provides independent profiles of aerosol backscatter, water vapour mixing ratio, and temperature during both daytime and nighttime, identified more than 200 well-mixed hygroscopic aerosol layers. Median hygroscopicity parameters were f_β (355 nm) = 2.64 ± 0.64 and γ_β (355 nm) = 0.70 ± 0.18. Despite differences in measurement principle, wavelength, and sampling technique, Raman lidar retrievals showed good agreement with COBALD-RS41 and tandem nephelometer observations. Statistical analyses revealed no systematic dependence of aerosol hygroscopicity on altitude, with comparable values in the atmospheric boundary layer and free troposphere. Enhanced hygroscopic growth was generally associated with a larger contribution of inorganic aerosol species, whereas no clear dependence on particle size was identified. These results demonstrate that combining remote sensing and in situ observations provides a robust framework for characterising aerosol hygroscopicity under ambient atmospheric conditions.