Is fossil fuel combustion still a major contributor to atmospheric aerosol carbonaceous fractions in the Po Valley? Results from a ¹⁴C-based source apportionment

¹⁴C measurements on the carbonaceous fractions of atmospheric aerosol are an important tool for source apportionment. In this paper, a ¹⁴C-based source apportionment study was carried out on samples collected during winter 2021 at an urban background site in the Po Valley, one of the main pollution hot-spot areas in Europe. The samples were prepared using MISSMARPLE (MIlan Small-SaMple Automated Radiocarbon Preparation LinE for atmospheric aerosol), a recently developed sample preparation line for ¹⁴C measurements on atmospheric aerosol carbon fractions, specifically targeting small samples (about 50 μgC). ¹⁴C-based source apportionment was performed separately for elemental carbon (EC) and organic carbon (OC), after suitable optimization of model parameters. As this is the first source apportionment study using MISSMARPLE-prepared samples, our results were compared with other tracers and source apportionment outcomes obtained through parallel methodologies, demonstrating strong correlations in all tests (R > 0.87). The source apportionment results showed that fossil fuel combustion remains the main source of EC in the investigated area (60%), while modern contributions generally dominate the OC fraction (66%). However, some episodes were identified where fossil fuel combustion was the dominant contributor also to the OC fraction (up to 66%). During these episodes, the importance of secondary OC formation from fossil fuel combustion was highlighted through the EC-tracer approach. Thus, fossil fuel combustion is still a significant source of carbonaceous aerosol in the Po Valley during the winter. Both primary emissions and gaseous precursors must be targeted by future abatement policies to effectively reduce pollution in the area.