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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 the number size distribution of submicron aerosols at three sites in the greater Paris area, representative of the urban city, as well as 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, and (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), and 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 a 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 value of 0.022 (370 nm) was measured at the urban site for the long-range-transported fire episode. 

Abstract. Alpha-dicarbonyl compounds are believed to form browncarbon in the atmosphere via reactions with ammonium sulfate (AS) in clouddroplets and aqueous aerosol particles. In this work, brown carbon formationin AS and other aerosol particles was quantified as a function of relativehumidity (RH) during exposure to gas-phase glyoxal (GX) in chamberexperiments. Under dry conditions (RH < 5 %), solid AS,AS–glycine, and methylammonium sulfate (MeAS) aerosol particles brown withinminutes upon exposure to GX, while sodium sulfate particles do not. When GXconcentrations decline, browning goes away, demonstrating that this drybrowning process is reversible. Declines in aerosol albedo are found to be afunction of [GX]2 and are consistent between AS and AS–glycineaerosol. Dry methylammonium sulfate aerosol browns 4 times more than dryAS aerosol, but deliquesced AS aerosol browns much less than dry AS aerosol.Optical measurements at 405, 450, and 530 nm provide an estimatedÅngstrom absorbance coefficient of -16±4. This coefficient andthe empirical relationship between GX and albedo are used to estimate anupper limit to global radiative forcing by brown carbon formed by 70 ppt GXreacting with AS (+7.6×10-5 W m−2). This quantity is< 1 % of the total radiative forcing by secondary brown carbonbut occurs almost entirely in the ultraviolet range.