Impact of water uptake on fluorescence of atmospheric aerosols: insights from Mie–Raman–fluorescence lidar measurements

Abstract. This study investigates the impact of water uptake by particles during hygroscopic growth on aerosol fluorescence properties, using multiwavelength Mie–Raman–fluorescence lidar measurements conducted at the ATmospheric Observation at LiLLe (ATOLL) observatory (Laboratoire d'Optique Atmosphérique, University of Lille) between 2021 and 2024. During certain episodes, we observed a systematic height-dependent decrease in the fluorescence backscattering coefficient within the well-mixed planetary boundary layer. This phenomenon begins at relatively low relative humidity (RH ∼ 50 %) simultaneously with a decrease in the particle depolarization ratio. However, the rapid growth of the aerosol backscattering coefficient at high RH is not mirrored by the same rate of fluorescence reduction. This distinct behavior suggests a nonlinear relationship between water uptake and fluorescence suppression, likely indicating that water-induced quenching effects operate independently of bulk hygroscopic growth. Furthermore, we demonstrate the capability to retrieve particle volume and surface area density from single-wavelength extinction coefficients during strong hygroscopic growth episodes, validated against full 3β+2α lidar measurements. The values of the conversion factors for urban aerosol and smoke at 355 and 532 nm, together with associated uncertainties, are presented.