Post Date: 25 June 2026

Oxidative Potential of PM2.5 by Acellular Ascorbic Acid Assay and Its Relationship with PM2.5 Chemical Composition

Abstract:

Fine particulate matter (PM2.5) is a major air pollutant associated with adverse respiratory and cardiovascular effects. However, particle mass concentration alone does not adequately reflect the toxicity as health impacts of PM2.5 strongly depends on the chemical composition and redox activity. Oxidative potential (OP), defined as the capacity of PM to deplete antioxidant and generative reactive oxygen species (ROS), has therefore been proposed as a more health-relevant metric. This thesis characterized the acellular oxidative potential of ambient PM2.5 in urban Hong Kong using an ascorbic acid (AA) assay and investigated its relationship with water-soluble (WS) chemical components, episodic event, and AA depletion kinetics.

Daily PM2.5 samples were collected in Tsuen Wan, Hong Kong, from 1 October 2024 to 30 September 2025. OP was quantified as both volume-normalized OP (OPAAv) and mass-normalized OP (OPAAm). WS-metals and water-soluble organic carbon (WSOC) were also measured. Over the full sampling period, PM2.5 ranged from 2.83 to 59.67 μg/m3, with annual mean of 15.17 ± 9.51 μg/m3. OPAAv ranged from 0.037 to 4.39 μM/min/m3, with annual mean of 0.34 ± 0.32 μM/min/m3, whereas OPAAm ranged from 0.0030 to 0.17 μM/min/μg with annual mean of 0.023 ± 0.015 μM/min/μg. OPAAv exhibited clear seasonal pattern, with mean values of 0.44 in fall 2024, 0.56 in winter 2024, 0.30 in spring 2025 and 0.18 μM/min/m3 in summer 2025. In contrast, OPAAm varied less strongly by season, with corresponding means of 0.027, 0.025, 0.019 and 0.022 μM/min/μg.

Among measured chemical species, WS-copper (WS-Cu) showed the strongest association with OPAAv (r = 0.86, after outlier removal), but weaker for OPAAm (r = 0.37). Strong annual correlations were also observed between OPAAv and WS-As (r = 0.77), WS-Pb (r = 0.76), and WSOC (r = 0.76). Seasonal analysis showed that correlation between OPAAv and WS-Cu remained strong in fall 2024 (r = 0.89) and winter 2024 (r = 0.90).

Two pollution episodes further demonstrated the composition dependence of OP. During the Lunar New Year firework event on 29 January 2025, WS-Cu and WS-Ba increased sharply to 72.65 and 212.44 ng/m3 respectively, driving OPAAv and OPAAm to annual maxima of 4.39 μM/min/m3 and 0.17 μM/min/μg respectively. In contrast, during the dust storm (13–15 April 2025), PM2.5 surged from 18.88 to 51.76 μg/m3. Yet OPAAv dropped from 0.36 to 0.26 μM/min/m3 and OPAAm from 0.017 to 0.0052 μM/min/μg. Dilution experiments confirmed WS-Cu as the primary driver of AA depletion, while the chemical matrix, including WSOC, modulated the kinetic behaviour.

Overall, this thesis demonstrates that AA-based OP in Hong Kong PM2.5 is governed more strongly by chemical composition, particularly WS-Cu than by PM mass alone. These finding supports the use of OP as a complementary metric to PM2.5 mass in assessing the potential health relevance of ambient PM.