Trends and Challenges Regarding the Source-Specific Health Risk of PM_2.5-Bound Metals in a Chinese Megacity from 2014...

Identifying the health risk of PM_2.5 is essential for urban air pollution control. In 2013, China announced the ever-strict national Air Pollution Prevention and Control Action Plan, and its health benefit should be evaluated to provide reference for future policymaking. In this study, we conducted a seven-year (2014-2020) continuous observation of PM_2.5 in Shenzhen, the third largest city in China, which has relatively good air quality. The results showed that the annual mean PM_2.5 and total concentration of 21 associated metals dropped from 37.7 to 18.5 μg/m³ and from 2.4 to 1.1 μg/m³, respectively. Combining methods for source apportionment and health risk assessment, we found that the total carcinogenic risk (CR) of five hazardous metals (Cd, Cr, Ni, Co, and Pb) showed a clear decreasing trend. However, the total CR (1.8 × 10^-6) in 2020 still exceeded the widely acceptable risk level (i.e., 1 × 10^-6), with the primary contributor changing from industrial emissions (61%) to vehicle emissions (63%). Further analysis indicated that the CR of vehicles mainly came from Cr and Ni released by braking and tire wearing and has fluctuated in recent years, highlighting a great challenge of controlling nonexhaust emissions of vehicles (including electric cars) in the future.