## 3 Results and Discussion
### Source Apportionment PMF  
Four factors were obtained from the PMF analysis. Figure 5 shows the factor profiles and attributed emission sources. The first factor was characterized by Cl\({}^{-}\) (81% of the mass), NO\({}_{3}\)\({}^{-}\) (27% of the mass), K (19% of the mass), Na\({}^{+}\) (16% of the mass). It was attributed to solid fuel combustion and the de-icing of roads in winter. Cl, S, Cd, Cr, and Br come from coal combustion [42,43]. K and Cl\({}^{-}\) are the major compounds emitted from wood, straw, and other agricultural residues burning and are related to biomass burning [14,23,42,44]. The contribution of this source to the mass of the PM\({}_{10}\) was equal to 15 ug/m\({}^{3}\) and 2.9 ug/m\({}^{3}\) in 2018 and 2020, respectively. The contribution decreased by 82% in 2020 compared to 2018. The most important lowering of this emission source contribution to the PM\({}_{10}\) mass was observed in February and March. In these months, people heat their houses. This may be related to the introduction of the ban on the use of coal and wood for heating purposes in September 2019 in Krakow. The difference in the contribution to solid fuel combustion in 2018 and 2020 was very small in April and May (between 0.2 and 6.3 ug/m\({}^{3}\)) when higher ambient temperatures were observed. The second factor was attributed to secondary inorganic aerosols (SIAs) and the main chemical species were NH\({}_{4}\)\({}^{+}\) (84% of the mass), S (33% of the mass), NO\({}_{3}\)\({}^{-}\) (27% of the mass), Br (31% of the mass), Pb (24% of the mass), and Co (17% of the mass). The SIA contribution was reduced from 15.5 ug/m\({}^{3}\) in 2018 to 2.3 ug/m\({}^{3}\) in 2020. It was reduced by around 84% during the entire analyzed period. In February and March, it decreased by 98% and 72%, respectively. In April it lowered by 35% and in May it increased by 90%. But the contribution of SIAs in April and May was very low and was in the range of 1.4-3.0 ug/m\({}^{3}\). SIAs are produced in the atmosphere by chemical reactions involving SO\({}_{2}\), NO\({}_{x}\), and NH\({}_{3}\). (NH\({}_{4}\))2 SO\({}_{4}\) and NH\({}_{4}\)NO\({}_{3}\) were the dominant components of SIAs in PM\({}_{10}\), which could be chemically formed by atmospheric reactions of NH\({}_{3}\) with SO\({}_{2}\) and NO\({}_{x}\) emitted from high-temperature combustion processes using sulfur-containing fuels such as coal and  
Figure 4: Enrichment factors for elements at Aleja Krasiriskiego traffic-dominated station. heavy fuel oil. The \(\text{SO}_{4}{}^{2-}/\text{NO}_{3}{}^{-}\) ratio was in the range 0.35-7.74 and 0.35-4.60 in 2018 and 2020, respectively. A higher ratio was observed for 2018 than for 2020. In particular, the main sources of \(\text{SO}_{2}\) and NOx were coal-fired power plants, industrial boilers, and vehicular exhaust. The lowering of the above-mentioned ratio in 2020 can be connected to the lowering of the contribution of coal combustion. All elements included in this source are of dominantly or at least partly anthropogenic origin, as was confirmed by the analysis of the enrichment factors (Section 3.2.) The third factor characterized by Sr (70% of the mass), Rb (70% of the mass), Zn (66% of the mass), Fe (65% of the mass), K (67% of the mass), Ca\({}^{2+}\) (58% of the mass), Cu (62% of the mass), Mn (60% of the mass), Ti (62% of the mass), S (42% of the mass), Na\({}^{+}\) (39% of the mass), Pb (58% of the mass), and Br (48% of the mass) was attributed to traffic/industry/construction work. Ti, Mn, Sr, Zr, Al, Ca, and Fe come from the resuspension of road dust [45; 46]. Zn, Cu, Ba, Sb, and Fe are related to the wearing of brakes and tires [46; 47; 48]. Ti, Sr, and Fe may be emitted from the abrasion of roads, Fe, Mn, and Co from brake pads and Mn, Fe, and Co from tires [14] The contribution of this source to the PM10 mass was equal to 23.7 \(\mu\)g/m\({}^{3}\) and 23.1 \(\mu\)g/m\({}^{3}\) in 2018 and 2020 during the entire analysis period, respectively.