Aerosol spectral optical properties in the Paris urban area, and its peri−urban and forested surroundings during summer 2022 from ACROSS surface observations

Abstract. The complex refractive index (CRI, n−ik) and the single scattering albedo (SSA) are key parameters driving the aerosol direct radiative effect. Their spatial, temporal, and spectral variabilities in anthropogenic−biogenic mixed environments are poorly understood. In this study, we retrieve the spectral CRI and SSA (370−950 nm wavelength range) from in situ surface optical measurements and number size distribution of submicron aerosols at three sites in the greater Paris area, representative of the urban city, its peri−urban and forested rural environments. Measurements were taken as part of the ACROSS (Atmospheric ChemistRy Of the Suburban forest) campaign in June−July 2022 under diversified conditions: 1) two heatwaves leading to high aerosol levels; 2) an intermediate period with low aerosol concentrations; 3) an episode of long−range transported fire emissions. The retrieved CRI and SSA exhibit an urban–to–rural gradient, whose intensity is modulated by the weather conditions. A full campaign average CRI of 1.41–0.037i (urban), 1.52–0.038i (peri−urban), 1.50−0.025i (rural) is retrieved. The imaginary part of the CRI (k) increases and the SSA decreases at the peri−urban and forest sites when exposed to the influence of the Paris urban plume. Values of k>0.1 and SSA<0.6 at 520 nm are related to black carbon mass fraction larger than 10 %. Organic aerosols are found to contribute to more than 50 % of the aerosol mass and up to 10 % (urban), 17 % (peri−urban) and 22 % (forest) of the aerosol absorption coefficient at 370 nm. A k of 0.022 (370 nm) was measured at the urban site for the long−range transported fire episode.