Source apportionment of black carbon (BC) from fossil fuel and biomass burning in metropolitan Milan, Italy

In this study, the temporal variations of sources of black carbon (BC) concentrations were evaluated in the metropolitan area of Milan, Italy, during three distinct seasons over 2017–2018. We measured BC concentrations using Aethalometers at two sampling sites, one in the city center of Milan, and one in the less densely populated suburb of Bareggio, approximately 14 km to the west of Milan's urban center. PM samples were also collected for elemental carbon (EC) and 14C analyses. The Aethalometer model was used to apportion BC concentrations to the fossil fuel combustion (BCff) and biomass burning (BCbb) originated BC. Additionally, radiocarbon 14C analysis was performed on the PM samples, allowing us to refine our estimates of the contributions of both BCff and BCbb to total BC concentrations and assess the dependence of the mass absorption cross section (MAC) of BC on its source. Overall, our results indicated that the annually averaged BC concentrations were higher at the Bareggio site (2763 ± 1050 ngm−3) than at the Milan site (1921 ± 876 ngm−3). The Aethalometer model results demonstrated that in all of the sampling seasons, fossil-fuel-originated BC (BCff) concentrations were slightly higher in Milan (e.g., a summertime average of 1045 ± 150 ng/m3) than in Bareggio (e.g., a summertime average of 940 ± 89 ng/m3); however, black carbon (BCbb) concentrations from biomass burning were considerably higher at Bareggio (e.g., a wintertime average of 3284 ± 713 ng/m3) than in Milan (e.g., a wintertime average of 1154 ± 103 ng/m3). Diurnal variation plots indicated that both in Milan and Bareggio, BCff peaked during the morning and, to a lesser extent, afternoon traffic rush hours, while BCbb peaked during nighttime when residential wood burning for heating purposes is prominent. Our results also highlight the significant impact of residential wood burning on ambient BC concentrations in the Milan metropolitan area