Comment on acp-2021-766

The evaluation of black carbon (BC) sources is very important, especially in environmental sciences. This study shows how the contributions of biomass burning and fossil fuel/traffic to PM2.5 mass can be assessed. MABI was used for this purpose and gave the possibility to measure the transmission of light at different wavelengths. Absorption coefficients were calculated from measurements data and recalculated for concentrations of eBC. The samples of PM2.5 fraction were collected from February 1, 2020 to March 27, 2021 every third day in Krakow, Poland (50°04' N, 19°54'47" E). The concentrations of equivalent BC (eBC) from fossil fuel/traffic and biomass burning were in the range 0.82–11.64 μg m−3) and 0.007–0.84 μg m−3, respectively. At the same time, PM2.5 concentrations varied from 3.14 to 55.24 μg m−3. It means that about 18 % of PM2.5 mass belongs to eBC and 11.3 % of this value comes from biomass burning. The eBC contribution is the significant part of PM2.5 mass and we observed seasonal variation of the eBC concentration during the year with the peak in winter. The contribution of biomass burning to PM2.5 mass is more stable during the whole year. The eBC concentration during workdays is a bit higher than during weekend days but biomass burning is similar for both days (work and weekend taken as the mean for the whole period).