| Summary: | Raman microspectroscopy and thermo-optical-transmittance (TOT) method were used to study airborne ambient soot collected at the suburban air monitoring station in southern Poland during the residential heating (January-February) and non-heating (June–July) seasons of 2017. Carbonaceous material constituted on average 47.2 wt.% of PM<sub>2.5</sub> during the heating season and 26.9 wt.% in the non-heating season. Average concentrations of OC (37.5 ± 11.0 μg/m<sup>3</sup>) and EC (5.3 ± 1.1 μg/m<sup>3</sup>) during the heating season were significantly higher than those in the non-heating season (OC = 2.65 ± 0.78 μg/m<sup>3</sup>, and EC = 0.39 ± 0.18 μg/m<sup>3</sup>). OC was a chief contributor to the TC mass concentration regardless of the season. All Raman parameters indicated coal combustion and biomass burning were the predominant sources of soot in the heating season. Diesel soot, which is structurally less ordered than soot from other sources, was dominant during the non-heating season. The D1 and G bands area ratio (D1<sub>A</sub>/G<sub>A</sub>) was the most sensitive Raman parameter that discriminated between various soot sources, with D1<sub>A</sub>/G<sub>A</sub> > 1 for diesel soot, and less than 1 for soot from coal and wood burning. Due to high daily variability of both TOT and Raman spectroscopy data, single-day measurements can be inconclusive regarding the soot source apportionment. Long-time measurement campaigns are recommended.
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