Trends and Challenges Regarding the Source-Specific Health Risk of PM 2.5 -Bound Metals in a Chinese Megacity from 2014 to 2020.

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/m 3 and from 2.4 to 1.1 μg/m 3 , 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.