## 3 Results and Discussion
### Analysis of Heavy Metal Content  
Dustfall has an extremely complex composition of chemical elements due to its source, migration and transformation processes, and sedimentation processes [12], and it often contains a large number of metal elements [44, 46, 47]. Table 2 shows the contents of the metal elements in the dustfall and background samples [48]. The Fe and Zn contents are the highest in the samples of 2019, with average values of 22,2859.800 mg/kg and 2933.821 mg/kg, respectively. The average value of Mn is 784.988 mg/kg. The elements of Cr, Cu, and Pb are similar in content, with the average values being 220.456 mg/kg, 222.860 mg/kg, and 229.604 mg/kg, respectively, and there are obviously larger and different fluctuation ranges. The content of Ni is the lowest, with an average value of 65.126 mg/kg. Therefore, the extremely high content of Fe and Ni can be corroborated by the preliminary conclusions of the high content of ferromagnetic minerals in the samples of December 2019. In the samples of December 2020, the contents of Fe and Cu elements are also relatively high, with average values of 711.668 mg/kg and 143.528 mg/kg, respectively. The contents of Pb, Ni, and Zn are similar, with the average values being 100.813 mg/kg, 98.152 mg/kg, and 91.343 mg/kg, respectively. The Cr element content is the lowest, with the average value of 43.615 mg/kg. From the perspective of the overall change trend, it can be found that the metal content of the samples in 2019 is higher than that in 2020. In addition, all the samples in 2019 and 2020 contain a large amount of Fe, Ni, and Cu. Figure 3: Temporal and spatial distribution characteristics of the magnetic parameters. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**mg/kg** & **Cr** & **Cu** & **Fe** & **Mn** & **Ni** & **Pb** & **Zn** \\ \hline Range & 58.3383-825.500 & 41.667-473.500 & 11.02810-588.965.220 & 243.158-1941.250 & 2.632-354.500 & 1448.000-16.905 & 16.3342.211-321.429 \\ \hline Range & 133.333–26.944 & 4425.83–1.003 & 4801.527–54.962 & 401.220–20.565 & 107.995–89.077 & 140.564–84.480 & 403.501–0.200 \\ Mean & 220.466 & 222.860 & 55.383.384 & 264.988 & 68.126 & 229.864 & 290.832.821 \\  & 43.015 & 143.528 & 711.668 & 74.157 & 98.152 & 100.813 & 91.343 \\ \hline Median & 193.250 & 213.233 & 36.726.500 & 762.171 & 60.451 & 144.029 & 221.264 \\  & 31.944 & 85.340 & 326.272 & 28.427 & 98.142 & 98.236 & 67.087 \\ \hline SD & 203.343 & 106.190 & 856.914 & 286.909 & 49.139 & 222.385 & 2754.301 \\  & 23.259 & 517.181 & 984.392 & 85.840 & 2.119 & 8.273 & 88.881 \\ \hline Background [48] & 179.680 & 128.730 & 40.695.000 & 902.400 & 4679.790 & 812.360 & 2112.580 \\ \hline \end{tabular} Note: The gray parts represent 2019, the whole parts represent 2020.  
\end{table} 
Table 2: Heavy metal element content in dustfall. Figure 4 shows the variation characteristics of the heavy metal content of the samples in different districts before and after COVID-19 outbreak. The peak values of heavy metal elements Fe, Zn, and Cu in 2019 are mainly concentrated in downtown areas such as the BS, XH, HP, and CN districts. Among them, Fe elements may come from soil, metal smelting, and brake pad wearing [49]. Cu and Zn elements may come from traffic emissions, such as automobile fuel, galvanized materials, the use of zinc compounds in rubber products, the consumption of vehicle tires and exhaust, gas, etc. [49; 50; 51]. The high values of Fe, Zn, and Cu in the study area are mainly due to the large flow of people, the denser traffic, and the burning of gasoline. The contents of Mn, Cr, and Pb are relatively distributed with a slight fluctuation range, and mainly concentrated in downtown areas, such as the BS (which was an industrial area before 2017), XH, and CN districts which are areas with high traffic flow and intensive human activities (industrial waste gas, motor vehicle exhaust, and building dust). Cr, Pb, and Mn elements may derive from the wear of brake pads, tires, or other components [52].