Spatial and seasonal variability of equivalent black carbon and its source contributions across Croatia

Black carbon (BC) is a major light-absorbing component of atmospheric particulate matter with well-documented effects on climate and human health, yet it remains poorly characterised across south-eastern Europe. This study presents the first systematic multi-station analysis of equivalent BC (eBC) source apportionment in Croatia, based on four years (2022–2025) of measurements from a national network of eight AE33 seven-wavelength aethalometers at five urban-background and three rural-background stations spanning continental, sub-Mediterranean, and Mediterranean environments. Fossil-fuel (eBC_ff) and biomass-burning (eBC_bb) fractions were resolved using the two-component aethalometer model. Annual mean eBC ranged from 0.28 μg m –3 at the cleanest rural station (Polača) to 1.92 μg m –3 at the most polluted urban station (Slavonski Brod), where winter biomass-burning fractions reached 73% and concentrations approached those of larger Balkan agglomerations such as Bucharest. A pronounced coastal–continental contrast was observed: continental urban stations (Slavonski Brod, Osijek, Zagreb) showed strong winter maximum dominated by residential combustion (winter eBC_bb of 62–73%), whereas coastal Mediterranean stations showed weak seasonality (eBC_bb below 34% year-round) and persistent traffic dominance. The eBC_ff component was stable and season-independent across all urban stations, identifying road traffic as a persistent, non-seasonal source, while diurnal profiles resolved traffic (twin rush-hour peaks) and evening residential-heating fingerprints. These findings establish an empirical baseline for BC in a data-sparse region of Europe and indicate that effective air quality management requires geographically differentiated strategies: prioritising residential energy transition at continental inland cities, where solid-fuel heating dominates the winter burden, while addressing traffic emissions at coastal urban centres.