Pollution characteristic and temporal trend of PM_2.5-bound heavy metals in a megacity of Northeast China: Implication for the effect of air pollution control policy

PM_2.5-bound heavy metals present significant health risks especially in cold regions due to the enhanced coal combustion for space heating. This study conducted a comprehensive one-year observation of PM_2.5-bound heavy metals in Harbin City, a megacity in Northeast China, from August 2021 to July 2022. The findings revealed that zinc (Zn) and chromium (Cr) were the dominant heavy metals, with concentrations of 47 ng/m³ and 41 ng/m³, respectively. Notably, the levels of Cr(VI) exceeded both the WHO guidelines and the Chinese Ambient Air Quality Standards. Except for Cr, the concentrations of other heavy metals were significantly higher during the heating period than the non-heating period. Based on source apportionment, traffic emission and coal combustion were recognized as the primary sources in both periods, while coal combustion was the dominant source in the heating period. Health risk assessments revealed non-carcinogenic risk from arsenic (As) for infants and toddlers, while carcinogenic risk for most elements surpassed the threshold of 10⁻⁶. Nickel (Ni) and Cr(VI) emerged as the principal carcinogenic elements during the heating period and non-heating period, respectively. A comparative assessment demonstrated a substantial decline in the concentrations of targeted heavy metals, with an overall reduction of 75.9% compared to 2013. Importantly, the heating period accounted for 65.6% of this reduction, suggesting the effectiveness of air pollution control policies, especially those aimed at reducing coal combustion emissions. Therefore, the control policy addressing the specific sources of heavy metals is essential.