## 3 Results and Discussion  
### Chemical Analysis  
Figure 3 shows the daily PM\({}_{10}\) mass concentrations at the traffic-dominated station during the two sampling periods. Table 1 presents the minimum, maximum, and mean \(\pm\) SD of the PM\({}_{10}\) and element and ion concentrations at the traffic-dominated station in 2018 and 2020. It also presents a ratio of the concentrations in 2018 to the concentrations in 2020. Table S1 includes monthly concentrations of the PM\({}_{10}\) and element and ion concentrations at a traffic-dominated station in 2018 and 2020. The PM\({}_{10}\) decreased from 74 \(\pm\) 29 \(\mathrm{\SIUnitSymbolMicro m^{3}}\) to 37 \(\pm\) 13 \(\mathrm{\SIUnitSymbolMicro m^{3}}\) at the traffic station in 2018 and 2020, respectively. It was lowered by 61%, 54%, 34%, and 42% in February, March, April, and May in 2020 compared to 2018, respectively (Table S2). Table S2 shows the concentration of PM\({}_{10}\) before the COVID-19 pandemic (in 2018 and 2017-2019) and during the pandemic (2020) in different cities. [PERSON] et al. observed a slightly lower drop in the PM\({}_{10}\) concentrations than those observed by [PERSON] et al. for Krakow in the same period [32]. The PM\({}_{10}\) lowering in February can be caused primarily by one reason. It is the introduction of a ban on the use of coal and wood for residential heating purposes in Krakow in September 2019. In March, two reasons can be observed. One is the introduction of the ban, and the second is the COVID-19 pandemic which started in March 2020 in Europe. In April and May, the differences between 2018 and 2020 were around 30-40%. It may be related to the limited activity of people during the COVID-19 pandemic, and especially the limited transport in Krakow. [PERSON] et al. observed smaller differences in the PM\({}_{10}\) between 2018 and 2020 in March, April, and May in other Polish cities, namely Warszawa, Wroclaw, Lodz, and Gdansk [32]. [PERSON] et al. obtained a 51% and 48.7% reduction of the PM\({}_{10}\) in the Valencia traffic station and Wuhan in March and April, respectively [41]. Strong differences in the concentration were observed for Cl. The 2018/2020 ratio was equal to 3.9. Its concentration decreased by 80%, 82%, and 20% in February, March, and May, respectively, while in April it increased by 31%. This element is an indicator of solid fuel combustion and the de-icing of roads in winter. Therefore, it can be expected that the emission related to solid fuel combustion will be lowered by a similar factor for 2020 compared to 2018. The concentrations of Cu, Zn, Ca, and Fe were lowered by 30-40% in April and May 2020. These elements are indicators of traffic. It could be caused by restrictions in activity during the COVID-19 pandemic. The concentration of K and Na\({}^{+}\) in 2018 was 2.3 times higher than in 2020. The NH\({}_{4}\)\({}^{+}\) concentration was 2.5 times lower in 2020 than in 2018. K is an indicator of biomass burning, and NH\({}_{4}\)\({}^{+}\) is a tracer of coal combustion, while Na\({}^{+}\) is connected to road dust and soil or salt. The following chemical species had similar concentrations at the station in both years: Ca, Ti, Mn, Ni, Rb, Sr, PO\({}_{4}\)\({}^{3-}\), Mg\({}^{2+}\), K\({}^{+}\), and Ca\({}^{2+}\). The 2018/2020 ratio was in the range 0.9-1.2 for the species mentioned above. The ratio varying between 1.5 and 1.9 was for S, NO\({}_{3}\)\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), Br, and Zn. The ratio for Fe, Co, and Cu was found to be equal to 1.4. The Cr concentration was higher in 2020 than in 2018. This element can come from the steel industry.