Aging of Urban Traffic Fine Particles Modulates Toxicity Mechanisms and Oxidative Stress Responses

Vehicular emissions are an important source of urban air pollution that can age through atmospheric processes, but how their health effects change during atmospheric aging remains poorly understood. To simulate realistic urban emissions, we sampled air in a parking garage where cold-start and urban emissions coexist. Following aging in an oxidation flow reactor (OFR), we studied the cellular and acellular effects of the fresh and aged aerosols. Photochemical aging in the OFR increased secondary organic aerosol (SOA) mass by up to 6.9-fold, the oxygen-to-carbon (O:C) ratio from 0.26 to 0.79, and both the intrinsic oxidative potential (OPm) and the volume-normalized oxidative potential (OPv). In contrast, the peroxide-sensitive DCF activity decreased following aging. Cellular exposure to aged aerosol extracts increased cellular reactive oxygen species (ROS) formation in lung (A549) and liver (HepG2) cells, whereas fresh extracts strongly reduced cellular respiration, particularly in HepG2 cells. Cell painting analysis showed distinct morphological profiles. Exposure to aged extracts showed greater oxidative stress and DNA damage responses than those of exposure to fresh extracts. These findings suggest that atmospheric aging shifts the toxicity of fresh emissions from suppression of mitochondrial respiration toward oxygenated organics that can drive intracellular oxidative stress and DNA damage phenotypes. In conclusion, atmospheric aging of cold-start and urban vehicular emissions contributes to enhanced SOA formation and leads to distinct biological outcomes that depend on the degree of oxidative aging.