Sources of PM_2.5 Oxidative Potential during Haze and Non-haze Seasons in Chiang Mai, Thailand

Dithiothreitol (DTT) assay is an acellular technique used to investigate the oxidative potential (OP) of chemical substances bound on PM, which may potentially lead to oxidative stress after exposure. In this study, the source contributions of 16 high priority polycyclic aromatic hydrocarbons (PAHs), designated by the United States Environmental Protection Agency (U.S. EPA), and 10 species of water-soluble inorganic ions bound on PM2.5 and their OP were investigated using DTT assay. The 24-hr ambient PM2.5 samples were collected throughout 2018-2019 and the analyzed OP was compared during haze episodes, which generally occurs in the dry season, and non-haze rainy season in the Chiang Mai-Lamphun basin. During haze episodes, DTTv activity was positively correlated with 4-5 rings PAHs including fluoranthene (Fla) pyrene (Pyr), benzo[a]anthracene (BaA), chrysene (Chr), benzo[b]fluoranthene (BbF) and benzo[k]fluoranthene (BkF) with coefficient ranging from 0.327 to 0.545, p = 0.002 to 0.009 (Pearson's correlation). Inorganic ions, particularly NH4+, SO42-, and NO3-, which are the tracers of secondary inorganic aerosol (SIA), were positively correlated with DTTv activity (r = 0.394 to 0.659. p = 0.000 to 0.047; Spearman's correlation). Positive matrix factorization (PMF) indicated the biomass burning factor had the highest contribution (57.9%) to PM2.5 during haze episodes, followed by SIA (26.2%), and vehicle exhausts (7.8%). Furthermore, multiple linear regression (MLR) showed that biomass burning was the highest contributor to DTTv (43.0%). These results suggest that during haze episodes, higher levels of PM2.5 and its chemical compositions play a crucial role on OP, particularly DTTv activity, which may induce oxidative stress in human body.