Aerosol complex refractive index retrieval in the Paris urban area and its forested surroundings during the ACROSS field campaign: variability and constraint for direct radiative effect estimation in regional models
Authors: Di Antonio Ludovico , Di Biagio Claudia , Beekmann Matthias , Gratien Aline , Formenti Paola , Bauville Astrid , Bérge Antonin , de Brito Joel Ferreira , Cazaunau Mathieu , Chevaillier Servanne , D’Anna Barbara , De Haan David Owen , Favez Olivier , Gaimoz Cecile , Garret Olivier , Hawkins Leila N. , Kammer Julien , Language Brigitte , Maisonneuve Franck , Močnik Griša , Monod Anne , Noyalet Gael , Pereira Diana , Perrier Sebastien , Petit Jean-Eudes , Pronovost Drew , Riffault Véronique , Riley Sydney , Riva Matthieu , Shahin Marwa , Siour Guillaume , Temime-Roussel Brice , Yu Chenjie , Zapf Pascal , Foret Gilles , Doussin Jean-François , Cantrell Christopher , Michoud Vincent 
Published: February 2023

The complex refractive index (CRI) is one of the key parameter driving aerosol spectral optical properties and direct radiative effects (DRE). Its value and spectral variation under different conditions, such as anthropogenic&#8722; and biogenic&#8722;dominated environments and anthropogenic&#8722;biogenic mixing situations, remains not fully understood. As a consequence, oversimplified representations of aerosol optical properties are generally used in climate models. Therefore, measurements of aerosol CRI in different environments and their inclusion in models are needed. The field observations from the ACROSS campaign, performed in June-July 2022 in the Ile de France region, are used in this study to deepen the knowledge of aerosol optical properties, aiming to improve the aerosol representation in the CHIMERE model and provide the best constraint for DRE simulations. Measurements obtained both at the Paris city center and the Rambouilllet rural forest sites during ACROSS are considered, in order to explore the CRI variability from anthropogenic&#8722;dominated to biogenic&#8722;dominated environments, including anthropogenic&#8722;biogenic mixing situations. The CRI retrievals at seven different wavelengths, performed by combining the Mie theory with optical and size distribution measurements, are representative of different atmospheric conditions, aerosol loadings as well as type and chemical compositions. In fact, the June-July 2022 period was characterized by highly diversified weather conditions: 1) two strong heatwaves, promoting SOA build-up and favoring the export of the Paris pollution plume towards the forest site; 2) Saharan dust events transported from the upper atmosphere to the ground; 3) biomass burning episode; 4) periods with reduced anthropogenic influence. The CRI retrievals under these different conditions and their link to particulate chemical composition is investigated. Hence, the CRI dataset presented here constitutes a unique dataset from which models can benefit to validate and constrain simulations and DRE estimations, under both urban and biogenic emissions influence. These data, in conjunction with those from the aircraft observations during ACROSS, are used to initialize and perform sensitivity studies on the aerosol DRE, using the CHIMERE&#8722;WRF coupled model, the OPTSIM model for the aerosol optical properties and the Rapid Radiative Transfer Model for GCMs (RRTMG).Keywords: Complex refractive index, direct radiative effect, aerosol mixing, urban, forest