ROS-generation potential of Humic-like substances (HULIS) in ambient PM in urban Shanghai: Association with HULIS concentration and light absorbance.

Ambient fine particulate matter (PM) can cause adverse health effects through the generation of reactive oxygen species (ROS) after inhalation. Humic-like substances (HULIS) are major constituents contributing to the ROS-generation potential in organic aerosols. In this study, PM samples in urban Shanghai during autumn and winter (2018-2019) were collected. Mass-normalized ·OH generation rate in surrogate lung fluid (SLF) was used to denote the intrinsic ROS-generation potential of PM or of the HULIS isolated from PM. In this study, ROS-generation potential of PM decreased with increasing ambient PM concentration due to higher percentage of inorganic components in high PM event. Same trend was observed for the ROS-generation potential of unit mass of HULIS, which was higher when HULIS and PM concentrations were both relatively lower. The HULIS with high ROS-generation potential but low concentration (High-ROS/Low-Conc HULIS) were likely produced by the atmospheric aqueous-phase reactions during nighttime or under high relative humidity conditions, not from biomass burning emissions or the photochemical pollution products. The association between ROS-generation potential and light absorption properties of HULIS was studied as well. The High-ROS/Low-Conc HULIS also showed stronger light absorbance than the other HULIS. Our results implied the potentially important roles that HULIS species might play in atmospheric environment and human health even when the PM pollution is low.