## 3 Results and Discussion
### Air Quality
The decrease in traffic was noticed, especially at the CJ-1 traffic station, where a constant reduction in PM\({}_{10}\) concentrations was recorded. In CJ-1, the PM\({}_{10}\) levels decreased from 38.6 \(\upmu\)g/m\({}^{3}\) in 2009 to 32.1 \(\upmu\)g/m\({}^{3}\) in 2019. A constant decrease was also remarked at the CJ-4 industrial station, where the annual average concentrations reduced from 28.4 \(\upmu\)g/m\({}^{3}\) in 2009 to 15.7 \(\upmu\)g/m\({}^{3}\) in 2019; a possible explanation could be the decrease in economic activities after the 2008 recession. The monthly PM\({}_{10}\) (Figure 3a) was the highest in December 2009 (44.4 \(\upmu\)g/m\({}^{3}\)) and the lowest in May 2019 (7.4 \(\upmu\)g/m\({}^{3}\)). Generally, the monthly PM\({}_{10}\) concentrations were higher in the cold seasons than in the warm seasons. A possible explanation for high PM\({}_{10}\) levels in winter could be the accumulation of PM in the low atmosphere due to the reduction of mixing layer thickness, determined by meteorological conditions. Household heating and the use of solid road deicing products during winter could also have increased the PM\({}_{10}\) emissions [1]. Both the maximum and the 90 th percentile of monthly PM\({}_{10}\) (Figure 3b) showed decreasing trends, confirming the lowering of the value and frequency of high concentrations. In the case of the 90 th percentile, the higher values were measured between October and March, and a decreasing trend in monthly PM\({}_{10}\) from 2009 to 2019 was observed. Figure 2: Temporal evolution of PM\({}_{10}\), temperature and wind speed in Cluj-Napoca between 2009 and 2019. To assess the spatial variability of PM\({}_{10}\) concentration in Cluj-Napoca, the average PM\({}_{10}\) for each monitoring station was calculated for the periods in which all four stations functioned. The average PM\({}_{10}\) concentrations were comparable for the CJ-2 (urban) and CJ-3 (suburban) stations, with values of 26.8 ug/m\({}^{3}\) and 26.7 ug/m\({}^{3}\), respectively. These values were almost two times higher than the average PM\({}_{10}\) concentrations found for the CJ-1 (traffic, 13.6 ug/m\({}^{3}\)) and CJ-4 (industrial, 15.1 ug/m\({}^{3}\)) stations. The low PM\({}_{10}\) values were measured by the industrial and traffic background stations, which confirms that industry and traffic are responsible for a lower share of PM\({}_{10}\) pollution than the other urban pollution sources, such as heating and constructions/demolitions. The obtained annual PM\({}_{10}\) values were comparable to those reported by [PERSON] et al. (2014) in London (26-44 ug/m\({}^{3}\)), Athens (23-51 ug/m\({}^{3}\)) and Madrid (29-39 ug/m\({}^{3}\)) [31]. Slightly lower PM\({}_{10}\) multiannual (2000-2006) average values were obtained by [PERSON] et al. (2014) in nine French cities [35]. The PM\({}_{10}\) values measured in Cluj-Napoca were lower than those from Bucharest [48,49]. The average monthly \(AQI\) (Figure 3c) ranged between 15 (May 2019) and 87 (December 2009), indicating good to moderate air quality. However, the 90 th percentile (Figure 3d) ranged between 22 (May 2019) and 147 (January 2010), indicating that there were months when the air quality was unhealthy for sensitive population groups. Similar to the PM\({}_{10}\) values, good air quality was found in the warm months, the air quality decreasing in the cold months. Furthermore, an air quality improvement from 2009 to 2019 was observed, especially for the cold months. The number of months with average \(AQI\) above 50 decreased from eight months in 2010 to four months in 2019, while instances of 90 th percentile of \(AQI\) above 100 were identified in four months in 2010, but in only one month between 2011 and the present. Figure 3: Temporal variation of (**a**) average of PM\({}_{10}\) concentration, (**b**) 90 th percentile of PM\({}_{10}\) concentration (ug/m\({}^{3}\)), (**c**) average of air quality index (\(AQI\)) and (**d**) 90 th percentile of air quality index (\(AQI\)) in Cluj-Napoca city for 2009–2019.