## 3 Results and Discussion
### Analysis of Heavy Metal Content
The results further verify that human activities (metal manufacturing, steel production, smelting activities) have a significant interference effect on the enrichment effect of metals. Additionally, the pollution changed from single stable pollution to compound pollution. The high contents of Fe, Zn, and Cu elements in the samples of December 2020 were mainly concentrated in the YP, HK and HP districts, mainly due to the fact that these areas are located close to the central ring of Shanghai, with more residential areas and relatively frequent human activities. The contents of Mn, Cr, and Pb show a basically similar spatial distribution trend, and the contents of the samples in the downtown area such as the YP, HK, HP, and XH districts are significantly higher than those in other regions with less human activities. It can be seen that human factors have a certain influence on the heavy metal pollution of particulate matter. Combining the element measurement results of the dust samples in 2019 and 2020, it can be found that the metal element contents in 2019 (before COVID-19) are significantly higher than those in 2020 (after COVID-19 outbreak). For instance, the concentrations of Fe, Cr, Cu, and Zn in 2019 are significantly higher than those in 2020, and in particular, the Fe content changes significantly before and after the epidemic occurrence. It is further proved  
Figure 4: Spatial variation characteristics of the heavy metal content before and after COVID-19 outbreak. that the significant difference in air quality before and after COVID-19 outbreak is likely to be closely related to human activities, for example, human activities were obviously restricted after the epidemic and the flow of people and vehicles were reduced to a certain extent, resulting in the relative weakening of heavy metal pollution. Moreover, the district variation trends of the metal content in the samples of 2019 and 2020 are basically similar; especially, Fe, Cu and Zn are the most important pollution elements, and their peaks are mainly concentrated in the PT, XH, HP districts due to various human activities. However, the contents of Mn, Cr, and Zn metals showed a spatially different distribution, which was most likely caused by the impact of COVID-19 on human activities, such as industrial action and transportation in space [53]. On the whole, the heavy metal contents of the dustfall in 2019 were higher than those in December 2020. Based on the district distribution characteristics of the heavy metals, it is further confirmed that the pollution status of the downtown areas (PT, HP, XH, PD) is significantly higher than that in the suburbs. The closer to the downtown area, the greater the traffic volume, the greater the proportion of human influence factors, and the more obvious the degree of disturbance to the environment, further indicating that the degree of heavy metal pollution is closely related to human factors. This conclusion is highly consistent with the district distribution characteristics of the magnetic parameters; that is, particle magnetic domain size, particle type, metal content, and type are all closely related to human activities.