This study aims to ascertain the oxidative potential (OP) of PM_2.5 in Chiang Mai (CM) City, Northern Thailand. Chiang Mai suffers from severe air pollution, which poses a health risk. Dithiothreitol assay (DTT) was used to analyze the OP of 53 samples of PM_2.5 filters collected between January and April 2021 using a medium-volume air sampler with a flow rate of 100 L/minute for 24 hours every other day. We analyzed components of PM_2.5, including carbonaceous content [i.e., organic carbon (OC), elemental carbon (EC), water soluble organic carbon (WSOC)], eight water-soluble inorganic ions (WSIIs), and twenty-one metal components.

Our study found that OP^DTTv (volume-normalized DTT activity) in PM_2.5 had an average of 0.13 ± 0.01 nmol/min/m³ and OP^DTTm (mass-normalized DTT activity) had an average of 2.44 ± 0.24 pmol/min/μg. OP^DTTv was moderately correlated with carbonaceous components (r = 0.44 - 0.50, P <0.01), WSII components (r = 0.41 - 0.55, P <0.01), and metal components (r = 0.40 - 0.48, P <0.01). No significant positive correlation between thesePM_2.5 components and the OP^DTTm was found in this study. Interestingly, moderate positive correlations were observed between OP^DTTv and potassium (K, K⁺) and WSOC, indicating that these sources were primarily derived from biomass combustion tracers and secondary organic aerosols, respectively. Moderate positive correlations were also observed between OP^DTTv and secondary ions (NO₃^-, NH₄⁺). The redox-active nature of NO₃^- produced by gases and free radicals led to OP^DTTv associations with secondary ions. Furthermore, associations were found between OP^DTTv and transition metals such as copper (Cu) and iron (Fe), which contribute to generating oxidative stress.

Our study showed that the OP of PM_2.5 is dominated by carbonaceous components from burning biomass, secondary organic aerosols, and transition metals. Further OP studies of other chemical components in PM_2.5 should also be explored to estimate more potential health risks.