PM2.5 and PM10 oxidative potential at a Central Mediterranean Site: Contrasts between dithiothreitol- and ascorbic acid-measured values in relation with particle size and chemical composition

In this study, PM2.5 airborne particulate matter was collected over a full year at a costal site of the Central Mediterranean Sea and analysed for its chemical composition and oxidative potential (OP), determined by the dithiothreitol (DTT) and the ascorbic acid (AA) assays. In autumn-winter, the volume normalized oxidative OP (OPV) were 0.29 +/- 0.03 nmol min(-1) m(-3) and 0.21 +/- 0.03 nmol min(-1) m(-3) for the DTT (OPVDTT ) and AA (OPVAA) assay, respectively. In spring-summer the OPVDTT values were higher than OPVAA responses, i.e., 0.19 +/- 0.02 nmol min(-1) m(-3) vs. 0.09 +/- 0.01 nmol min(-1) m(-3). Overall, marked seasonality was observed with higher values in Autumn-Winter (AW) than in Spring-Summer (SS), i.e., 1.5 and 2.3 times increase for OPVDTT and OPVAA, respectively. In the cold season, the OPV activity was broadly correlated with metals and carbon species, such as K+, NO3-, Ba, Cd, Cu, Fe, Mn, P, V, OC, EC, Acetate, Oxalate and Glycolate (p < 0.05). This suggested the main contribution of a mixed anthropogenic source, consisting of the biomass burning (K+, OC and EC) and traffic (Ba, Cu, Fe, Mn, V, EC) emissions. In SS, OPV was significantly correlated with only few species i.e., OC, EC, Cu, and NO3-, suggesting main association with the mixed anthropogenic and the reacted dust sources. For each sampling day, PM2.5 and PM10 samples were simultaneously collected and analysed to investigate the variation of the OP activity in relation with the particle size and chemical composition. OPVDTT in values exhibited a poor particle-size dependence, with similar values close to 0.20 +/- 0.04 nmol min(-1) M-3 in both fractions. This could be explained by the association of OPVDTT with species mainly accumulated in the fine fraction, i.e., OC, POC and EC and K+. Otherwise, the OPVAA responses exhibited a clear particle-size dependence, with significantly higher values for PM10 than for PM2.5, i.e., 0.35 +/- 0.06 vs. 0.21 +/- 0.03 nmol min(-1) M-3 in AW and 0.23 +/- 0.04 vs. 0.09 +/- 0.01 in SS. This may be supported by the strong correlation of OPVAA with Cu and Fe, which were most abundant metals in the PM10 fraction. The data of specific monitoring days were investigated in detail to better highlight the impact of some individual redox active species on the OP(V)(DTT)and OPVAA responses.The oxidative potential of PM2.5 and PM10 samples was assessed with Dithiothreitol and Ascorbic Acid assays: the variation of OP responses was related with the PM size and chemical composition.