The impact of biomass burning on the oxidative potential of PM2.5 in the metropolitan area of Milan

Abstract In this study, we investigated the impact of biomass burning on the oxidative potential of PM2.5 in the metropolitan area of Milan, Italy. PM2.5 samples were collected on quartz filters during cold (December 2018–February 2019) and warm (May 2019–July 2019) seasons at the Municipality of Bareggio, a small town located approximately 14 km northwest of the Milan city center. The PM2.5 constituents were chemically analyzed, and its corresponding oxidative potential was measured by means of the dithiothreitol (DTT) assay. Total PM2.5 mass concentration was significantly higher in winter (71.82 ± 4.17 μg/m3) compared to summer (16.67 ± 0.27 μg/m3), mainly a result of enhanced biomass burning emissions combined with higher atmospheric stability and lower mixing during the cold season. The enhanced biomass burning activities during the winter period also resulted in very high polycyclic aromatic hydrocarbons (PAHs) concentrations (72.81 ± 16.59 ng/m3) which were more than 150-fold higher than the warm period values (0.40 ± 0.07 ng/m3). PAH concentrations were highly correlated with chemical markers of biomass burning (i.e., levoglucosan (R2 = 0.79), and K+/K (R2 = 0.87)) in the winter period. Spearman correlation analysis between DTT and PM2.5 chemical species showed a dominant role of secondary organic aerosols (SOA) and vehicular emissions in summer-time PM2.5 oxidative potential (i.e., the capacity of PM2.5 species to oxidize target molecules), while in the wintertime, the DTT values were highly correlated with chemical markers of biomass burning, vehicular activities, and re-suspended road dust. Multiple linear regression (MLR) analysis identified biomass burning (41%) as the dominant contributor to DTT, followed by SOA (20%), re-suspended road dust (18%), and vehicular emissions (16%). Our results underscore the importance of biomass burning to the overall oxidative potential of PM2.5 in the metropolitan area of Milan, urging the need to promulgate effective mitigation policies targeting these emissions.