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

An experimental setup to study aerosol hygroscopicity is proposed based on the temporal evolution of attenuated backscatter coefficients from a ceilometer colocated with an instrumented tower equipped with meteorological sensors at different heights. This setup is used to analyze a 4.5-year database at the ACTRIS SIRTA observatory in Palaiseau (Paris, France, 2.208 E, 48.713 N; 160m above sea level). A strict criterion-based procedure has been established to identify hygroscopic growth cases using ancillary information, such as online chemical composition, resulting in 8 hygroscopic growth cases from a total of 107 potential cases. For these eight cases, hygroscopic growth-related properties, such as the attenuated backscatter enhancement factor fbeta (RH) and the hygroscopic growth coefficient y, are evaluated. This study shows that the hygroscopicity parameter y is negatively correlated with the aerosol organic mass fraction but shows a positive correlation with the aerosol inorganic mass fraction. Among inorganic species, nitrate exhibited the highest correlation. This is the first time that hygroscopic enhancement factors are directly retrieved under ambient aerosols using remotesensing techniques, which are combined with online chemical composition in situ measurements to evaluate the role of the different aerosol species in aerosol hygroscopicity.This work was supported by the Andalusia Regional Government through project P12-RNM-2409; the Spanish Ministry of Economy and Competitiveness through projects CGL2013-45410-R, CGL2016-81092-R, and CGL2017-83538-C3- 1-R; the Excellence network CGL2017-90884-REDT; the Juan de la Cierva grant IJCI-2016-29838; and the University of Granada trough the Plan Propio Program P9 Call-2013 contract and project UCE-PP2017. Andrés Esteban Bedoya-Velásquez has been supported by a grant for PhD studies in Colombia, COLCIENCIAS (Doctorado Nacional – 647), associated with the Physics Sciences program at the Universidad Nacional de Colombia, Sede Medellín, and the Asociación Universitaria Iberoamericana de Postgrado (AUIP). Financial support for EARLINET was provided through ACTRIS-2 Research Infrastructure Project EU H2O20 (grant agreement no. 654109), particularly for staying 2 weeks at SIRTA observatory and developing this investigation (TNA 3 SIR AHEAAARS). The authors gratefully acknowledge the FEDER program for the instrumentation used in this work. Juan Antonio Bravo-Aranda has received funding from the Marie Sklodowska-Curie Action Cofund 2016 EU project – Athenea3i under grant agreement no. 754446