How does humidity affect lidar-derived aerosol optical properties, and how do they compare with CAMS?

Abstract. From May to August 2020 and during summer 2024, aerosol backscatter and relative humidity profiles were measured in the Paris region using the Water Vapour and Aerosol Lidar (WALI). The campaigns included observations on the Saclay plateau (48°42’42’’ N / 2°8’52’’ E) and in Paris (48°50’12’’ N / 2°20’10’’) during the 2024 Olympic Games. The high vertical (15 m) and temporal (15 min) resolution of WALI allow study of aerosol optical properties and water vapour under stable atmospheric conditions. This study focuses on characterizing aerosol hygroscopic growth using lidar derived backscatter coefficients as a function of relative humidity. Eight case studies were selected where the potential temperature gradient was neutral and the water vapor mixing ratio was constant with height. These include long range pollution transport from the Benelux region, low hygroscopic aerosol events, and a sea salt-pollution mixture episode. Hygroscopicity was assessed using CAMS model analyses and reanalyses, allowing attribution of chemical composition to observed optical changes. Good agreement was found between lidar-derived hygroscopic properties and CAMS-inferred aerosol types. Lidar growth factors ranged from 0.3 to 1.5, with higher values linked to sea salt presence, consistent with literature values. Aerosols hygroscopicity is also studied using Mie's theory, as aerosols can be considered nearly spherical. Differences between extinction and backscatter-based growth retrievals are interpreted through Hänel’s formalism. The results demonstrate the capability of Raman lidar to constrain aerosol hygroscopicity, offering valuable input to chemistry-transport models and helping to reduce uncertainties in climate projections related to aerosol-cloud interactions.