Impacts of COVID-19 lockdown on PM2.5-bound polycyclic aromatic hydrocarbons in Hohhot, Northern China: characteristics, sources, and source-specific health risks.

Quantifying the impacts of reduction strategies on PM_2.5-bound polycyclic aromatic hydrocarbons (PAHs) is essential for reducing the health risks of PM_2.5. The COVID-19 lockdown provided an opportunity to reveal the quantitative relationship between lockdown measures and the health risks of PAHs. In this study, the characteristics, sources, and health risks of PAHs were investigated during the COVID-19 lockdown in Hohhot. The source-specific health risks of PAHs were assessed using a combination of incremental lifetime cancer risk models (ILCR) and positive matrix factorization (PMF). Compared with the pre-LD period (pre-LD, 87.41 ± 5.98 ng·m^-3), the total concentration of ∑PAHs during the lockdown period (LD, 32.52 ± 2.31 ng·m^-3) decreased by 62.8% in Hohhot. Coal combustion (51.5%), gasoline emissions (21.9%), diesel emissions (12.9%), industrial emissions (9.3%), and biomass burning (4.7%) were the predominant sources of PAHs in Hohhot. Except for male children, the ILCR of all groups exceeded the threshold for high health risks (1 × 10^- 4). Dermal contact is the predominant exposure pathway for carcinogenic risk. Compared with the pre-LD period, the ILCR values decreased by 62.5-62.7% during the LD period. The PMF-ILCR results indicated that industrial emissions (29.1%), coal combustion (28.4%), and diesel emissions (18.5%) were the main sources of ∑ILCR. A Monte Carlo simulation revealed that the cumulative carcinogenic risks at the 95th percentile of the six groups were 1.5-6.3 times the threshold of high health risk (1 × 10^- 4). These results emphasize that regulating industrial emissions and coal combustion is effective in reducing carcinogenic risks in industrial cities with large coal consumption.