One-year measurements (October 2016–September 2017) of aerosol optical properties in the Athens urban environment were used in this study, supported by chemical composition of the non-refractory submicron aerosol fraction acquired with an Aerosol Chemical Speciation Monitor (ACSM) for closure purposes. Both spectral scattering (bsca) and absorption (babs) coefficients exhibit a pronounced annual variability with higher values (63.6 Mm-1 at 550 nm and 41.0 Mm-1 at 520 nm, respectively) in winter, due to increased carbonaceous emissions from domestic heating and the shallow mixing layer trapping aerosols near the surface. Much lower values (33.5 Mm-1 and 22.9 Mm-1 for bsca and babs, respectively) are found during summer, indicating rather aged aerosols from regional sources. The estimations of the dry spectral single scattering albedo (SSA), scattering (SAE) and absorption (AAE) Ångström exponents focus on the exploration of the seasonality of the urban aerosols. The high SAE (~2.0) and low SSA (0.62 ± 0.11) values throughout the year indicate dominance of fine-absorbing aerosols from fossil-fuel combustion, while the high AAE (~1.5) in winter suggests enhanced presence of biomass-burning aerosols. Pronounced morning and late evening/night peaks are found in both bsca and babs during winter, in coincidence with morning traffic rush hour and enhanced residential wood burning in the evening, while in the other seasons the diurnal patterns flatten out. Wind speed strongly affects the aerosol loading and properties in winter, since for winds below 3 m s-1, a high increase in bsca and babs is observed, consistent with low dilution processes and haze/smog conditions. Our closure experiments indicate a good agreement (R2 = 0.91, slope = 1.08) between reconstructed and measured bsca values, and reveal that organic matter contributes about half of the sub-micron mass in winter, followed by sulfate (~40%) and nitrate (10% only in winter) aerosols.