Trace metals and magnetic particles in PM_2.5: Magnetic identification and its implications

Magnetic measurement was combined with geochemical analysis to investigate the trace metal pollution of PM_2.5. The study was carried out in Nanjing, China, where the average PM_2.5 concentrations in summer and winter in 2013-2014 were 66.37 and 96.92 μg/m³, respectively. The dominant magnetic mineral in PM_2.5 had a low-coercivity pseudo-single domain and consisted of magnetite and hematite. Iron-oxide magnetic particles comprised spherical as well as angular particles. Stable Pb isotopic ratio determinations showed that Pb in summer samples derived from coal emissions while the main sources of winter samples were smelting industry and coal emissions. The magnetic properties of the particles correlated strongly with trace metals derived from anthropogenic activities, such as industrial emission, coal combustion, and traffic vehicle activities, but poorly with those derived from natural sources. In the multiple linear regression analysis, Cr and Fe had higher correlation coefficients (training R > 0.7) in contrast to the low training R of As, Cd, Ni, Sr, and Ti (<0.5) determined using the PM_2.5 concentrations and magnetic parameter values as the decision variables. Our results support the use of environmental magnetism determinations as a simple and fast method to assess trace metals in urban particulate matter.