Distribution characteristics and sources of heavy metals and polycyclic aromatic hydrocarbons in PM_2.5 in an urban area of Taizhou City from 2017 to 2022

Background Fine particulate matter (PM_2.5) in ambient air is an important carrier of heavy metals and polycyclic aromatic hydrocarbons (PAHs), and long-term inhalation of heavy metals and PAHs poses a potential threat to human health.

Objective To analyze the ambient PM_2.5 concentrations and the distribution characteristics and sources of heavy metals and PAHs in PM_2.5 in an urban area of Taizhou City, aiming to provide basic data and a scientific basis for local air pollution management and population health prevention.

Methods From 9am to 8pm on the 10th to 16th of each month from 2017 to 2022 (23 h·d⁻¹), a monitoring point was set up on the top floor of a building in the main urban area of Taizhou to collect PM_2.5 samples (sampling flow rate: 2.3 m³·h⁻¹) and measure its concentration. The characteristics of 12 heavy metals and 16 priority PAHs in PM_2.5 were analyzed. The pollution sources of heavy metals and PAHs were determined by enrichment factor method and isomer characteristic ratio method, respectively.

Results From 2017 to 2022, the average daily concentration of PM_2.5 in this urban area ranged from 4 to 141 μg·m⁻³, and the median annual concentrations were lower than the national secondary standard limit. The seasonal concentration of PM_2.5 was as follows: winter>spring>summer and autumn, and the difference was statistically significant (H=134.077, P<0.01). The concentration of PM_2.5 in 2017.1 to 2020.1 was higher than that in 2020.2 to 2022.12 (Z=−3.989, P<0.01). The main heavy metal pollutants in this area were manganese (Mn), lead (Pb), arsenic (As), nickel (Ni), and chromium (Cr). Except for beryllium (Be) and mercury (Hg), the concentrations of other heavy metals had seasonal differences (P<0.01), higher in winter and spring than in summer and autumn. Except for manganese (Mn) and arsenic (As), the concentrations of other heavy metals were higher from 2017.1 to 2020.1 than from 2020.2 to 2022.12, and the difference was statistically significant (P<0.05). The results of enrichment factor indicated anthropogenic pollution sources of antimony (Sb), cadmium (Cd), and selenium (Se). The average daily concentration of PAHs ranged from 0.88 to 24.56 ng·m⁻³, with a seasonal trend of winter>spring>autumn>summer, and the difference was statistically significant (H=231.387, P<0.01). Among the 16 PAHs, the top pollutants were benzo b fluoranthene (BbF) (25.36%), indene 1,2,3-c,d pyrene (IcdP) (11.22%), and benzo g,h,i perylene (BghiP) (9.02%). The results of characteristic ratio method showed that the 16 PAHs pollutants were produced by a combined action of gasoline buring, coal burning, and biomass burning.

Conclusion In selected urban area of Taizhou City, the concentrations of PM_2.5, heavy metals, and PAHs all show a downward trend year by year at a relatively low level, and obvious seasonal distributions which are higher in winter and spring. Man-made pollution is the main source of heavy metals, and automobile exhaust emissions, industrial coal burning, and waste incineration may be the main sources of PAHs.