## 4 Conclusions  
COVID-19 has had an impact on urban particulate pollution, as well as people's lives. In this study, samples obtained before and after the COVID-19 outbreak in Shanghai were subjected to magnetic, chemical, and scanning electron microscopy investigation. The pollution characteristics and the magnetic responses of heavy metals were compared and studied. The main conclusions are as follows:  
1. PSD and MD ferrimagnetic minerals predominate in the dustfall in Shanghai. The dustfall samples in 2019 had a higher magnetic mineral concentration than those in 2020. Magnetic parameter peaks are concentrated in the districts of XH, CN, and BS, which have high levels of population mobility, heavy industrial activity, and significant traffic flow. The epidemic prevention and control measures had a significant impact on the magnetic characteristics of the dustfall. 2. Fe has the largest concentration of heavy metals, followed by Pb, Zn, Mn, and Cu. Due to constraints on human activity following the COVID-19 epidemic, the metal composition of the dustfall samples in 2020 is much lower than that of 2019. This \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameter** & **Cr** & **Cu** & **Fe** & **Mn** & **Ni** & **Pb** & **Zn** \\ \hline \multirow{2}{*}{Xif} & 0.013 & 0.015 & 0.554 ** & 0.001 & 0.037 & 0.716 ** & 0.004 \\  & 0.378 * & 0.009 & 0.139 * & 0.420* & 0.048 & 0.300 * & 0.070 \\ \hline \multirow{2}{*}{XARM} & 0.016 & 0.020 & 0.018 & 0.001 & 0.040 & 0.801 ** & 0.007 \\  & 0.826 ** & 0.001 & 0.989 ** & 0.801 ** & 0.522 ** & 0.137 & 0.034 \\ \hline \multirow{2}{*}{SIRM} & 0.002 & 0.003 & 0.123 & 0.002 & 0.008 & 0.674 ** & 0.001 \\  & 0.958 ** & 0.089 & 0.799 ** & 0.814 ** & 0.827 ** & 0.574 ** & 0.240 * \\ \hline \multirow{2}{*}{XARM} /\(\chi_{\rm{If}}\) & 0.777 ** & 0.874 ** & 0.001 & 0.997 ** & 0.816 ** & 0.551 ** & 0.916 ** \\  & 0.563 ** & 0.453 * & 0.411 * & 0.559 ** & 0.304 * & 0.629 ** & 0.369 * \\ \hline \multirow{2}{*}{XARM} /SIM & 0.935 ** & 0.735 ** & 0.458 * & 0.422 & 0.916 ** & 0.564 ** & 0.748 ** \\  & 0.353 * & 0.474 * & 0.577 ** & 0.229 * & 0.388 * & 0.116 * & 0.890 ** \\ \hline \multirow{2}{*}{XARM} & 0.664 ** & 0.270 * & 0.625 ** & 0.281 * & 0.674 ** & 0.793 ** & 0.151 \\  & 0.072 & 0.761 ** & 0.313 * & 0.039 & 0.735 ** & 0.678 ** & 0.792 ** \\ \hline \hline \end{tabular} Note: N = 140; ** correlation is significant at the 0.01 level (two-tailed); * correlation is significant at the 0.05 level (two-tailed). The gray parts represent 2019, the white parts represent 2020.  
\end{table} 
Table 4: Summary of Spearman coefficients of metal and magnetic parameters (N = 140). directly correlates to a relative weakening of metal pollution. Moreover, since Fe, Zn, Cr and Cu are concentrated in areas of intense human activity, such as BS, HP, and CN, heavy metal pollution is more severe in these areas. 3. \(\chi_{\text{H}}\), \(\chi_{\text{ARM}}\) and SIRM before and after COVID-19 outbreak have different correlations with Cr, Fe, Zn, Pb, Mn, and Cu. The contents of Fe, Pb, Cr, and Zn are significantly correlated with the magnetic parameters, further proving the indication of magnetic parameters on the heavy metal pollution of atmospheric particulate matter. 4. Although this study compared the contamination characteristics and magnetic responses of heavy metals in fallout before and after the COVID-19 outbreak in Shanghai, the sampling density is still not high and the testing means are limited, and it is proposed to take additional samples and adopt more testing and analytical means to analyze and study the samples subsequently. Conceptualization, G.W. and F.R. ; methodology, G.W. ; software, X.G. and Q.X. ; validation, Q.X., X.G. and X.Z. ; formal analysis, Q.X. ; investigation, Q.X. ; resources, Q.X. ; data curation, W.Y. ; writing--original draft preparation, G.W. and Q.X. ; writing--review and editing, F.R. ; visualization, Z.J. ; supervision, F.R. ; project administration, G.W. ; funding acquisition, G.W. All authors have read and agreed to the published version of the manuscript.