## 3 Results and Discussion
### Characteristics of Magnetic Parameters
\begin{table}
\begin{tabular}{c|c c c c c c c} \hline  & \(\chi_{\rm H}\) (\(\mathbf{0}^{-5}\) m\({}^{3}\)/kg) & \(\chi_{\rm H}\)\% & \(\chi_{\rm ARM}\) (\(\mathbf{0}^{-5}\) m\({}^{3}\)/kg) & **SIRM (\(\mathbf{0}^{-5}\) Am\({}^{2}\)/kg)** & **HIRM (\(\mathbf{0}^{-5}\) Am\({}^{2}\)/kg)** & \(\chi_{\rm ASR}\)/kg/ & \(\chi_{\rm ARM}\)/(\(\mathbf{10}^{-3}\) Am\({}^{-1}\)) \\ \hline Range & 54.950-2374.250 & 0.081-437.37 & 148.743-3485.71 & 8517.054-2255.487.342 & 48.000-25925.123 & 0.640-3322 & 0.002-0.038 \\  & 100.106-3386.369 & 0.775-26.480 & 2.3787-4992.721 & 41.813-1917.250 & 44.473-1137.701 & 0.007-4.392 & 0.001-14.401 \\ \hline \multirow{2}{*}{Mean} & 718.521 & 4.354 & 12958.62 & 9299.646 & 1842.858 & 1.950 & 0.015 \\  & 672.017 & 4.392 & 880.937 & 56,255.653 & 368.269 & 1.437 & 0.222 \\ \hline \multirow{2}{*}{Median} & 597.224 & 2.825 & 1008.586 & 73.628214 & 944.866 & 1.826 & 0.014 \\  & 574.541 & 2.524 & 679.978 & 47.335.63 & 353.007 & 1.443 & 0.014 \\ \hline \multirow{2}{*}{SD} & 427.320 & 5.872 & 744.207 & 50.1049.71 & 3623.146 & 0.582 & 0.006 \\  & 517.162 & 4.427 & 776.471 & 37,426.130 & 216.888 & 0.795 & 1.719 \\ \hline \multicolumn{2}{l}{Background} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\ \([31]\) & 29.100 \(\pm\) 9.800 & 2.100 & 103.300 & 237.100 & 34.00 & 3.550 & 0.436 \\ \hline \end{tabular} Note: The gray parts represent 2019, the white parts represent 2003.  
\end{table} 
Table 1: Magnetic measurement results of samples before and after COVID-19 outbreak. Furthermore, when the concentration of MD and PSD grains dominates in dust (between 5% and 6%), and there are no existent SP grains, \(\chi_{\rm fd}\)% can be used to represent the concentration of MD and PSD grains [32; 33; 34]. When \(\chi_{\rm fd}\)% \(>\) 10%, it is suggested that there are a significant number of SP particles [35]. In this study, the mean value of \(\chi_{\rm fd}\)% is 4.354 in 2019 and 4.392 in 2020, both less than 5%, and when combined with the median values (2.825 in 2019 and 2.524 in 2020) and \(\chi_{\rm fd}\)% background value (2.100) [31], it is indicated that MD and PSD grains constitute the majority of ferromagnetic mineral grains. The ratio of the magnetic parameters mainly reflects the grain size of the magnetic minerals. The ratios of \(\chi_{\rm ARM}/\chi_{\rm Hf}\) and \(\chi_{\rm ARM}/\)SIRM can signify the grain size of magnetic minerals, which decrease with the increase in grain size [27]. \(\chi_{\rm ARM}/\chi_{\rm Hf}\) indicates the size of ferromagnetic mineral particles, with higher ratios reflecting more SSD particles and lower ratios indicating more MD or superparamagnetic particles (SP) [26; 36]. \(\chi_{\rm ARM}/\)SIRM also indicates the particle size variation of magnetic particles, with lower ratios reflecting more PSD and MD particles [37]. The mean values of \(\chi_{\rm ARM}/\chi_{\rm Hf}\) are 1.950 in 2019 and 1.437 in 2020. The difference between the \(\chi_{\rm ARM}/\chi_{\rm Hf}\) values before and after the COVID-19 outbreak is slight, showing the particle size in 2020 as slightly coarser than that in 2019. In addition, the \(\chi_{\rm ARM}/\chi_{\rm Hf}\) values of the dustfall are significantly lower than the background value (3.550) [31], indicating that the grain size of the magnetic particles in the dustfall is significantly coarser. In addition, the mean values of \(\chi_{\rm ARM}/\)SIRM are 0.015 \(\times\) 10\({}^{-3}\) m/A in 2019 and 0.222 \(\times\) 10\({}^{-3}\) m/A in 2020, both of which are lower than the background value (0.240 \(\times\) 10\({}^{-3}\) m/A) [31]. In conclusion, the mean values of \(\chi_{\rm ARM}/\chi_{\rm Hf}\) and \(\chi_{\rm ARM}/\)SIRM of the samples are less than 10 and 0.50 \(\times\) 10\({}^{-3}\) m/A, respectively, indicating that the ferrimagnetic minerals are dominated by PSD and coarser MD grains in the study area. Figure 2 shows the Dearing plot and King plot of the dustfall [32; 38].