## 3 Results and Discussion  
### Air Quality  
The basic statistics for PM\({}_{10}\) for the period 2009-2019, calculated based on the daily values of the operational monitoring stations, are presented in Table 1. The annual average, calculated based on daily values, ranged between 20.3 and 29.5 \(\mu\)g/m\({}^{3}\), this being below the annual limit value set by the EU legislation (40 \(\mu\)g/m\({}^{3}\)), but above the \(AQG\) set by the \(WHO\) (20 \(\mu\)g/m\({}^{3}\)) [4; 29]. The highest annual maximum was measured in 2010 (111 \(\mu\)g/m\({}^{3}\)), while the lowest annual maximum in 2012 (54.8 \(\mu\)g/m\({}^{3}\)). During the monitoring period, the annual average concentration was comparable, ranging between 20.3 and 29.5 \(\mu\)g/m\({}^{3}\), but the annual maximum almost halved from 111 \(\mu\)g/m\({}^{3}\) in 2010 to 62.6 \(\mu\)g/m\({}^{3}\) in 2019. The difference between the average and median annual PM\({}_{10}\) value decreased from 2009 to 2019, indicating a reduction of the days with high PM\({}_{10}\) values in favor \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**PM\({}_{10}\)** & **2009** & **2010** & **2011** & **2012** & **2013** & **2014** & **2015** & **2016** & **2017** & **2018** & **2019** \\ \hline Minimum & 2.0 & 6.35 & 2.7 & 5.8 & 6.7 & 7.0 & 7.5 & 2.7 & 3.2 & 7.9 & 3.6 \\ Maximum & 86.8 & 111.3 & 59.7 & 54.8 & 61.6 & 65.7 & 61.6 & 76.6 & 56.5 & 59.5 & 62.2 \\ Average & 27.4 & 29.5 & 23.2 & 24.0 & 24.4 & 24.9 & 25.7 & 22.0 & 22.2 & 24.4 & 20.3 \\ Standard deviation & 15.9 & 14.5 & 11.1 & 9.8 & 10.7 & 11.2 & 10.1 & 10.6 & 10.1 & 9.4 & 10.7 \\
10 th percentile & 9.8 & 14.2 & 11.4 & 12.2 & 12.8 & 11.8 & 14.4 & 9.7 & 10.4 & 13.8 & 8.7 \\
50 th percentile & 24.6 & 26.8 & 19.9 & 22.5 & 21.3 & 22.6 & 23.7 & 20.4 & 21.04 & 22.7 & 18.0 \\
90 th percentile & 51.6 & 47.6 & 40.9 & 38.1 & 39.2 & 40.1 & 39.9 & 36.5 & 38.1 & 37.6 & 35.3 \\ Skewness & 1.1 & 1.5 & 0.8 & 0.5 & 0.9 & 0.6 & 0.7 & 1.0 & 0.7 & 0.8 & 1.1 \\ Kurtosis & 0.6 & 4.0 & \(-\)0.2 & \(-\)0.6 & 0.4 & \(-\)0.2 & 0.1 & 2.3 & 0.17 & 0.5 & 1.4 \\ Days with PM\({}_{10}\) \(>\) 50 \(\mu\)g/m\({}^{3}\) & 29 & 26 & 4 & 1 & 6 & 6 & 5 & 3 & 3 & 5 & 9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic statistics of the annual PM\({}_{10}\) concentrations (\(\mu\)g/m\({}^{3}\)) in Cluj-Napoca for 2009–2019. of days with low PM\({}_{10}\) values. The positive skewness of the data also confirms this decrease. Highly skewed data were obtained for 2009, 2010 and 2019, the years that also have the highest numbers of days with PM\({}_{10}\) exceeding the daily limit value. The daily maximum PM\({}_{10}\) threshold value (50 \(\upmu\)g/m\({}^{3}\)) was not exceeded more than 35 times in the studied years. The highest number (29) of breaches occurred in 2009, of which 20 breaches occurred during October-December. With few exceptions, in each year, the majority of breaches occurred between October and March. The observed decreasing trend could be explained by the existence of some pollution reduction measures related to traffic restriction in the city center, and to the decrease of emissions from domestic heating following the increase in ambient temperatures and reduction of the number of frost days in Cluj-Napoca during the winters. Temperature fluctuation is the main factor that favors the increase in PM\({}_{10}\) concentrations, while the wind speed influences the dispersion of pollutants (Figure 2). Both temperature (r = \(-\)0.68) and wind speed (r = \(-\)0.44) are negatively correlated with the PM\({}_{10}\) concentration. The highest monthly average wind speed was recorded in spring (4.51 m/s), while the lowest monthly average wind speed was recorded in December (3.13 m/s). Generally, the highest PM\({}_{10}\) concentrations were recorded when the average monthly temperature was below 0 \({}^{\circ}\)C and the intensity of wind was low. The heavy traffic from the city center was redirected to the city outskirts, starting in 2011, when the ring road became functional.