Long-term aerosol optical hygroscopicity study at the ACTRIS SIRTA observatory: synergy between ceilometer and in-situ measurements

Abstract. An experimental setup to study aerosol hygroscopicity is proposed based on the time evolution of attenuated backscatter coefficients from a ceilometer co-located to an instrumented-tower equipped with meteorological sensors at different heights. This setup is used to analyse a 4.5-year database at the ACTRIS SIRTA observatory in Palaiseau (Paris, France, 2.208o E, 48.713o N, 160 m above sea level). A strict criterion has been established to identify hygroscopic growth cases using ancillary information such as on-line chemical composition, resulting in eight hygroscopic growth cases from a total of 120 potential cases. For the eight cases, the hygroscopic growth-related properties such as the attenuated backscatter enhancement factor f β (RH) and the hygroscopic growth coefficient γ are evaluated. This study evidence that the hygroscopicity parameter γ increases as the contribution of organic aerosols decreases. In this sense, organic mass fraction is anti-correlated with γ while it shows a positive correlation with the inorganic mass fraction. Among inorganic species, the higher correlation was found for NO 3 − . This is the first time that hygroscopic enhancement factors retrieved directly for ambient aerosols using remote sensing techniques are combined with on-line chemical composition in-situ measurements to evaluate the role of the different aerosol species on aerosol hygroscopicity.