Vertical variation in magnetic particles within atmospheric PM in indoor dustfall in industrial cities of northern China

Magnetic particles greatly contribute to atmospheric particulate matter (PM) pollution. The concentration, particle size, and component changes in different functional areas of a city and during different seasons can be detected by environmental magnetic indicators. However, the regularity of changes in magnetic particles and their sources with increasing height indoors are unclear, and whether magnetic parameters can directly indicate atmospheric PM pollution is unclear. Therefore, we investigated indoor dustfall in a high-rise building in Xuzhou and then combined the experimental results with existing data from Lanzhou for further research. Here, we showed that in these industrial cities, there are two prevalent pollution zones. One is close to the ground at -5-18 m (lower pollution zone), and the other is located above -42.2 m (high-level pollution zone). We found that the lower pollution zones in both Xuzhou and Lanzhou contained coarse ferrimagnetic particles (mainly magnetite), which are likely related to vehicular emissions and some industrial activities. However, the highlevel pollution zone in Xuzhou was dominated by fine-grained ferrimagnetic particles (with a small amount of hematite) from industrial combustion processes rather than coarse magnetic particles from natural sources, while in Lanzhou, these two types of magnetic particles yielded combined effects. An ultrafine magnetic particle pollution zone resulting from vehicle emissions occurred only in Xuzhou, indicating that this source is not prevalent in the industrial cities of northern China. Moreover, our results demonstrated that the changes in the size and composition of magnetic particles are closely related to the different atmospheric PM types.