## 3 Results and Discussion  
### Characteristics of Magnetic Parameters  
In the application of the environmental magnetism, various environmental magnetism parameters can be comprehensively used to reflect the magnetic properties of matter. In this study, several key magnetic parameters were employed to investigate the particulate pollution. Among them, \(\chi_{\text{H}}\) generally indicates the distribution characteristics of the magnetic mineral content in the sample. SIRM is related to the total remanence-carrying minerals and controlled largely by stable single-domain (SSD) ferrimagnetic grain concentrations and the presence of canted antiferromagnetic minerals [26; 27]. \(\chi_{\text{ARM}}\) is generally particularly sensitive to single-domain particles (SD) and PSD particles, and is generally used to reflect the magnetic domain properties of magnetic particles [28; 29; 30]. HIRM is generally used to reflect the relative content changes of ferrimagnetic minerals and incomplete antiferromagnetic minerals. Table 1 shows the magnetic measurement results of samples before and after the COVID-19 outbreak. The magnetic parameters of the dustfall in December 2019 (before COVID-19 outbreak) are generally higher than those in December 2020 (after COVID-19 outbreak). The mean value of \(\chi_{\text{H}}\) is 718.521 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2019 and 672.071 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2020, ranging from 54.950 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg to 2374.250 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2019 and 100.105 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg to 3386.369 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2020. It is indicated that the magnetic minerals of the samples before and after the epidemic were mainly made of ferromagnetic minerals. The mean value of SIRM is 92,992.646 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg in 2019 and 56,255.653 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg in 2020, ranging from 8517.054 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg to 225,487.342 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg in 2019, and  
from 41.813 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg to 191,278.250 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg in 2020. SIRM values in 2019 and 2020 are both higher than the background value (237.100 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg), reflecting the high degree of ferrimagnetic mineral spatial enrichment in the samples before and after COVID-19 outbreak. The mean value of \(\chi_{\rm ARM}\) is 1295.862 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2019 and 880.937 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2020, ranging from 148.743 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg to 3485.171 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2019 and 23.787 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg to 4592.721 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2020. The mean value of HIRM is 1842.538 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2019 and 368.269 \(\times\) 10\({}^{-8}\) m\({}^{3}\)/kg in 2020, ranging from 48.060 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg to 25,925.123 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg in 2019 and 44.473 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg to 1137.701 \(\times\) 10\({}^{-5}\) Am\({}^{2}\)/kg in 2020. Thus, it can be concluded that the magnetic properties of dustfall in Shanghai were dominated by both ferromagnetic minerals and incomplete antiferromagnetic minerals [8].