Seasonal characteristics of spectral absorption and BC source apportionment at a background site in the southern Balkans

Carbonaceous aerosols constitute a major component of the lower atmosphere in both urban and rural environments, originating from a variety of natural sources and anthropogenic combustion processes. This study examines the seasonal variability of BC and its fractions related to fossil fuel combustion (BC_ff) and biomass burning (BC_bb), along with the spectral absorption characteristics associated with BC and BrC. Aethalometer (AE-33) measurements were performed and analysed at a continental background site in NW mountainous Greece (Kozani, 40.27 ^oN, 21.76 ^oE, 768 m a.m.s.l) throughout the year 2023. The measuring station, located within the University of Western Macedonia campus, is mostly affected by regional background aerosol plumes with different optical and physicochemical characteristics. Major contributing sources include emissions from nearby lignite-fired power plants, the long-range transport of polluted air masses from the Balkan region, and, to a lesser extent, local emissions such as traffic within the university campus—where private vehicle use is restricted—and domestic heating in nearby villages (1–2 km away), particularly during the winter season. Furthermore, secondary aerosol formation plays a role in modifying the local aerosol burden. Hourly BC concentrations ranged from ~0.1 μg m^-3 to ~2.2 μg m^-3, with higher concentrations noted during winter due to enhanced residential biomass burning for heating. The BC_bb is about 50% during winter and much lesser (~25%) during summer, reflecting the absence of combustion processes and dominance of fossil-fuel sources, although the summer BC concentrations are low. Absorption due to BrC is mostly detected during winter, while its summer values are significantly lower. The contribution of BrC to the total absorption recorded is about 44% at 370 nm during winter, dropping to 16% during summer. This seasonal contrast reflects the influence of biomass combustion in the winter and the dominance of secondary organic aerosol formation and naturally occurring sources of BC during the summer.