First insights into PM_2.5-bound photoinitiators in the Yangtze River Delta, China

Photoinitiators (PIs) are high-production-volume chemicals with multiple recognized toxicity endpoints. However, their occurrence and environmental fate in ambient fine particulate matter (PM_2.5) remain poorly understood. This study investigated PM_2.5-bound PIs from four structural categories in the Yangtze River Delta, China. It provides the first evidence of their occurrence in ambient PM_2.5, with concentrations ranging from 154 to 2352 pg/m³ (geometric means: 550-607 pg/m³). The compositional profiles exhibited distinct spatial heterogeneity, with phosphine oxide-type PIs dominating in Jiaxing and more mixed compositional signatures observed in Shanghai and Hangzhou. Seasonal patterns showed higher winter and lower summer concentrations, consistent with lower-temperature particulate retention and meteorological variability. Most PI-NO₂ associations became non-significant after temperature adjustment, indicating that they were not independent of seasonal temperature covariation. The detection of hydroxylated derivatives of benzophenone (BP) and 4-methylbenzophenone provided evidence consistent with atmospheric transformation of PIs. Integrated exposure estimates and bioactivity predictions indicated that estrogen receptor (ER) activity was the dominant contributor to predicted inhalation-related bioactivity risks. BP, triphenylphosphine oxide, and 2-isopropylthioxanthone were the main ER-related contributors in Shanghai, Jiaxing, and Hangzhou, respectively. These findings identify PIs as an overlooked class of PM_2.5-bound contaminants relevant to risk assessment and air quality management.