Insight into the characteristics and source apportionment of PM_2.5 oxidative potential over Xi'an, China

The oxidation potential (OP) of PM_2.5 increased the prevalence of respiratory and pulmonary diseases of human health. In this study, high-time-resolution sampling was conducted in summer and winter in Xi'an, and the dithiothreitol (DTT) assay was employed to quantify PM_2.5 OP. The results showed that winter PM_2.5 OP was more twice than summer, and nighttime OP was higher than daytime both in summer and winter. In summer, the mass-normalized DTT consumption rate (DTT_m) exhibited two peaks at 4:00–8:00 and 12:00–16:00. In winter, high DTT_m appeared during the nighttime periods (20:00–4:00). Functional group revealed that peak areas of R-ONO₂, aromatic C=C, and C=O from ketones, quinones and amides had consistent trends with DTT values in summer, confirmed that nitrogenous organic aerosols enhanced the oxidative activity. While on winter night, the high peak areas of O-H, aliphatic C-H, polysaccharide C-O, and deformation of aromatic C-H coincided with DTT_m variation, inferred these function groups had an important effect on OP. The Multi-Layer Perceptron (MLP) model results revealed that vehicle emission was the largest contributor to DTT activity both in summer (33 %) and winter (34 %). In summer, vehicle emission contributed the most between 16:00–20:00 and 0:00–4:00. Secondary formation contributed significantly during daytime (31 %–34 %). Vehicle emission contributed the most between 12:00–16:00 and 0:00–4:00 in winter. Combustion emissions contributed significantly during night-time (29 %–36 %). This study links OP with chemical components and source contributions, offering critical insights into health risks associated with PM_2.5 exposure.