Chemical and oxidative properties of fine particulate matter from near-road traffic sources

The toxicity associated with the fine particulate matter (PM_2.5) has not been well studied, particularly in relation to the emissions from on-road vehicles and other sources in low- and middle-income countries such as India. Thus, a study was conducted to examine the oxidative potential (OP) of PM_2.5 at a roadside (RS) site with heavy vehicular traffic and an urban background (BG) site in Mumbai using the dithiothreitol (DTT) assay. Simultaneous gravimetric PM_2.5 was measured at both sites and characterized for carbonaceous constituents and water-soluble trace elements and metals. Results depicted higher PM_2.5, elemental carbon (EC), and organic carbon (OC) concentrations on the RS than BG (by a factor of 1.7, 4.6, and 1.2, respectively), while BG had higher water-soluble organic carbon (WSOC) levels (by a factor of 1.4) and a higher WSOC to OC ratio (86%), likely due to the dominance of secondary aerosol formation. In contrast, the measured OP^DTTv at RS (8.9 ± 5.5 nmol/min^/m³) and BG (8.1 ± 6.4 nmol/min/m³) sites were similar. However, OP^DTTv at BG was higher during the afternoon, suggesting the influence of photochemical transformation on measured OP^DTTv at BG. At RS, OC and redox-active metals (Cu, Zn, Mn, and Fe) were significantly associated with measured OP (p < 0.05), while at BG, WSOC was most strongly associated (p < 0.05). The coefficient of divergence (COD) for PM_2.5, its chemical species, and OP^DTTv was >0.2, indicating spatial heterogeneity between the sites, and differences in emission sources and toxicity. The estimated hazard index (HI) was not associated with OP^DTTv, indicating that current PM_2.5 mass regulations may not adequately capture the health effects of PM_2.5. The study highlights the need for further studies examining PM_2.5 toxicity and developing toxicity-based air quality regulations.