Seasonal and Spatial Variations of the Oxidative Properties of Ambient PM(2.5) in the Po Valley, Italy, before and during COVID-19 Lockdown Restrictions

This study describes the chemical and toxicological characteristics of fine particulate matter (PM(2.5)) in the Po Valley, one of the largest and most polluted areas in Europe. The investigated samples were collected in the metropolitan area of Milan during the epidemic lockdown and their toxicity was evaluated by the oxidative potential (OP), measured using ascorbic acid (OP(AA)) and dithiothreitol (OP(DTT)) acellular assays. The study was also extended to PM(2.5) samples collected at different sites in the Po Valley in 2019, to represent the baseline conditions in the area. Univariate correlations were applied to the whole dataset to link the OP responses with the concentrations of the major chemical markers of vehicular and biomass burning emissions. Of the two assays, OP(AA) was found mainly sensitive towards transition metals released from vehicular traffic, while OP(DTT) towards the PM carbonaceous components. The impact of the controlling lockdown restrictions on PM(2.5) oxidative properties was estimated by comparing the OP values in corresponding time spans in 2020 and 2019. We found that during the full lockdown the OP(AA) values decreased to 80–86% with respect to the OP data in other urban sites in the area, while the OP(DTT) values remained nearly constant.