## 3 Results and Discussion
### Contents of Pollutants in Roof Runoff
The amount of the detected microorganisms may be also affected by the time elapsed from the beginning of rainfall to the time of sampling, as rain washes the air and the highest microbial concentrations are observed in the first flush of roof runoff [84; 93]. The other factor that may influence microbiological composition of roof runoff is the air-dust particle size, as particular biological objects are associated with various particle fractions [94]. It was demonstrated that the amount of microorganisms in the air was correlated with particular dust fractions [79; 95]. In this study, the pH values of roof runoff in Krakow were in the range of 6.3-7.6. Mean pH values were similar in all sampling periods between 2019 and 2020: 6.7 in the spring of 2019, 6.8 in the autumn of 2019, and 6.9 in the winter of 2020. In the period from 2019 to 2020, a significant decrease in the average values of most physio-chemical parameters (with sufficient data collection) was observed, and those were (in brackets: mean values for the spring of 2019 and the winter of 2020, respectively, in mg/L, unless otherwise stated): EC (31.3, 19.6 \(\upmu\)S/cm), N-NO\({}_{3}\) (0.337, 0.202), P-PO\({}_{4}\) (0.081, 0.023), Ag (0.0336, \(<\)LOQ), Al (0.0158, \(<\)LOQ), Ba (0.0049, 0.0030), Mo (0.0106, \(<\)LOQ), Pb (0.0051, \(<\)LOQ), and Sb (0.0016, \(<\)LOQ). Our results are in line with research by [PERSON] et al. [22] that indicated a constant decrease in the concentration of chemical components in air and precipitation in Krakow between 1996 and 2017. In addition, the results obtained by [PERSON] and [PERSON] [33] revealed the improvement of air quality in Krakow between 2005 and 2020. According to that publication, the reasons of that trend included industrial, economic, and social changes occurring in Krakow. A significant improvement in air quality, and associated roof runoff quality, in the period from May 2019 to February 2020, could also be related to the fact that a total ban on the use of solid fuels in boiler houses, stoves, and fireplaces was imposed in Krakow on 1 September 2019 [96]. Our research also showed that values of such parameters as EC, N-NO\({}_{3}\), SO\({}_{4}\)\({}^{2}\), SiO\({}_{2}\), Ca, K, Mg, Na, and Zn in roof runoff mainly depended on the location of sampling, rather than the season of the year. Pollutant concentrations in the roof runoff are also correlated with the amount of rainfall as heavy rainfall dilutes the precipitation. The relevant parameters were definitely higher in eastern Krakow (District 4--KR2), which is an industrial section of the city, than in other sections (Figure 1, Table S5). The only parameter that seems to be influenced by season rather than sampling location, was concentration of chlorides, which was the highest during the winter of 2020. That could have been probably caused by the use of salt for road de-icing [97; 98]. The results of the measurements, taken at the reference site in Mogilany (MOG--Figure 1, Table S5), revealed that the mean acidity of samples (pH = 6.1) was lower than that observed in the roof runoff water collected in Krakow. Higher mean concentrations of such parameters as Cl\({}^{\prime}\), N-NO\({}_{3}\), P-PO\({}_{4}\) that were observed in Mogilany, could have been caused by more intensive de-icing and fertilization outside the city. On the other hand higher mean contents of Ca, K, Pb, and Zn were observed in Krakow than in the reference site, which could have been related to intensive traffic and industrial sources of pollution [99]. A comparison with rainwater taken directly from the rooftop would give additional information on the quality of the rainwater. However, it was not possible to take such samples for comparative analysis in Krakow. Our mean results from the research conducted in 2019/2020 in Krakow were compared to the results obtained for other cities in Poland (Table 2), by the Chief Inspectorate of Environmental Protection in Poland [78].