Redox characteristics of size-segregated PM from different public transport microenvironments in Hong Kong

Exposure to airborne particulate matter (PM) has been associated with various adverse health effects, including severe pulmonary and cardiovascular effects. PM consists of different chemical components that vary with microenvironments in urban areas and pose challenges to assess personal exposure. In Hong Kong, more than 70% of the population commutes through roadway and railway public transport. This study aims to determine the oxidative potential and role of aerosol carbon and water-soluble metals in fine (d_p < 2.5 μm) and coarse PM (2.5 <d_p <10 μm) in public transport systems including underground (UG) subway, above-ground (AG) train, and buses (BUS). Metals such as Fe, Cr, Mo, Pb, Ni, and V from UG, AG, and BUS routes exhibited much lower solubility compared with ambient PM. The cell toxicity of PM in these transport microenvironments was also analyzed in vitro and compared with urban ambient environments. Strong positive associations were observed for reactive oxygen species (ROS) with water-soluble metals (Cr, Cu, Fe, Mn, Ni, V, Mo; R > 0.70) and organic and elemental carbon (OCEC) (R > 0.85) for UG and AG routes. In addition, PM from UG and AG routes generated 3–4-fold (in PM_2.5) and 40–50-fold (in coarse PM) less ROS compared to urban sites, suggesting PM in these public transport microenvironments may not be intrinsically redox active than in urban ambient, and water solubility of metals seems to have played an important role in it.