The association of chemical composition particularly the heavy metals with the oxidative potential of ambient PM_2.5 in a megacity (Guangzhou) of southern China

Atmospheric fine particulate matters (PM_2.5) can cause adverse health effects through the generation of reactive oxygen species (ROS), which is normally characterized by the oxidative potential (OP). However, the particulate components that are mainly responsible for the ROS-induced OP remain controversial and warrant further investigation, especially in megacities where high exposure exists and particulate composition is complex. In this study, we measured the OP of PM_2.5 using the dithiothreitol (DTT) assay with and without chelation of metals in a megacity in southern China, Guangzhou, in January and April. We explored the correlations between OP and various chemical components in PM_2.5, including water-soluble ions, organic carbon (OC), elemental carbon (EC), and metal elements. There are strong correlations between OP^DTTv (volume-normalized) and concentrations of PM_2.5, OC, and EC, while the correlations between OP^DTTm (mass-normalized) and mass-normalized water-soluble ions, OC, EC or metal elements are weak. The OP values with chelation were reduced by ∼90%, indicating that water-soluble heavy metals were the major contributors to OP of PM_2.5 in Guangzhou. On the other hand, correlations between OP^DTTm and OC improved significantly after the chelation of heavy metals, implying that OC explains the variance of OP^DTTm although its contribution to OP is much smaller than that of heavy metals. We postulate that there might be synergetic effects between water-soluble heavy metals (which contribute most to OP) and OC (which explains the variance of OP) in ROS generation by PM_2.5. The findings of the current study provide a better understanding on the critical components in PM_2.5 and potential synergism that might be responsible for health effects in urban areas.