A 10-year monitoring of PM_2.5-bound nickel in Xi'an, China: Effects of source control on its trend and

A large data set of PM_2.5-bound nickel (Ni) concentration has been obtained from daily filter sample collection in the urban of Xi'an, China over a 10-year period (2004-2013). The Ni concentration trend, pollution sources, and the potential health risks associated with Ni were investigated. The average Ni concentration was 5.9±5.5 ng m^-3, ranging from <0.08 (MDL) to 88.1 ng m^-3. The Ni concentrations increased from 2004 to 2008, but then decreased significantly. The obvious decrease of Ni concentration and related health risks confirmed the effectiveness of pollution controls and air quality management policies from the government in China. In order to verify a hypothesis: Ni emission sources have a very strong relationship with the factory work arrangement (work-rest schedule) and the traffic pattern (rush-non rush hours), the comparison of long-term distributions of Ni concentrations were examined between workday and non-workday periods. Both of the high ratio of workdays to non-workdays and the elevated enrichment factors of Ni on workdays can be explained by the fact that the factories reduced their outputs and traffic decreased in the non-workday period. Ni cancer risk was 1.1×10^-6 for all ages in both workday and non-workday periods, which was higher than the threshold value of 10^-6, demonstrating that carcinogenic Ni was a concern to the residents. The implementation of motor vehicle and industrial emissions control policies and advanced technologies have led to the benefit to human health seen in our long-term monitoring.