Coupling aerosol hygroscopicity and fluorescence using lidar and in situ observations during the 2024–2025 CHOPIN campaign

Aerosol hygroscopicity play a fundamental role on cloud activation, radiative transfer, and particle–light interactions, yet its impact on fluorescence properties remains poorly understood. During the CleanCloud Helmos OrograPhic site experimeNt (CHOPIN) campaign at Mount Helmos, Greece (38.0°N, 22.2°E; 1700–2314 m a.s.l.), aerosol hygroscopicity and fluorescence were investigated across two periods: autumn (Oct–Nov 2024) and spring (Apr–May 2025). The high altitude and strategic location of the site allow the observation of a wide variety of aerosol types, including Saharan dust, biomass burning smoke, urban pollution, and biogenic particles.A multi-wavelength elastic-Raman–fluorescence lidar (ATLAS-NEF) operating at 355, 387, 407 and 470 nm, provided vertically resolved aerosol optical properties (extinction, backscatter, lidar ratios) and water vapor mixing ratios, as well as fluorescence backscatter profiles.Hygroscopic growth factors were derived from Raman-based backscatter following Hänel-type parameterizations, supported by measurements (pressure, temperature, and relative humidity) from radiosondes, a tethered helikite, and Unmanned Aerial Vehicles (UAVs). Fluorescence quenching was quantified as a function of relative humidity and compared to the optical growth exponent γ, while the Wideband Integrated Bioaerosol Sensor (WIBS) and Multiparameter Bioaerosol Sensor (MBS) provided information on bioaerosol concentrations and types.Aerosol backscatter generally increased with relative humidity, while fluorescence decreased, indicating humidity-dependent quenching. Biogenic particles showed strong fluorescence but limited hygroscopic growth, whereas dust and urban aerosols were moderately hygroscopic with reduced fluorescence. Synergies with MBS and WIBS highlighted temporal variability in bioaerosol concentrations, linking lidar fluorescence changes to particle composition and aging. These results demonstrate that this synergistic approach provides a robust framework to assess humidity-driven changes in optical and microphysical properties, with implications for cloud formation and radiative forcing.