## 3 Results
### Kruskal-Wallis Analysis  
To identify the impact of traffic on PM\({}_{10}\) pollution levels, a one-way analysis of variance using the Kruskal-Wallis test was performed [44]. Station B-3 is positioned along a wide avenue, while station B-6 is located in a canyon-like area with high traffic, surrounded by tall buildings. These differences are also highlighted by the PM\({}_{10}\) mass concentrations shown in Tables 4 and 5. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Period/Station** & **B-3** & **B-6** & **BV-1** & **BV-3** & **IS-1** \\ \hline BAU & 0.831 & 0.057 & 0.590 & 0.032 * & 0.804 \\ LOCKDOWN & 0.005 * & 0.044 * & 0.006 * & 0.000 * & 0.000 * \\ ALERT 1 & 0.021 * & 0.000 * & 0.002 * & 0.000 * & 0.000 * \\ ALERT 2 & 0.000 * & 0.000 * & 0.2158 & 0.007 * & 0.000 * \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The \(p\)-value of the Kruskal–Wallis test between the year 2020 and the period of Business as Usual in the reference years 2017–2019 and 2021–2022, through the differences of the traffic stations and the background stations from the metropolitan cities of Bucharest, Brasov, and Iasi. Asterisks indicate statistical significance. Figure 4: The frequency distributions obtained from the ratio between the traffic stations and the background stations (relative correction), which correspond to the B-6 station in Bucharest, carried out during the specified periods. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**STATION** & \multirow{2}{*}{**B-3**} & \multirow{2}{*}{**2020**} & \multirow{2}{*}{**2022**} & \multirow{2}{*}{**2020**} & \multirow{2}{*}{**2020**} & \multirow{2}{*}{**BV-1**} & \multirow{2}{*}{**2020**} & \multirow{2}{*}{**2022**} & \multirow{2}{*}{**2020**} & \multirow{2}{*}{**2022**} \\
**YEARS** & & & & & & & & & & \\
2020 & & 2.03 & 2.04 & 1.88 & 1.39 & 1.30 & 1.60 & 1.60 & 2.43 & 2.20 \\ LOCKDOWN & 1.58 & 1.87 & 1.45 & 1.63 & 1.26 & 1.16 & 1.29 & 1.31 & 1.40 & 1.87 \\ ALERT 1 & 1.80 & 1.79 & 1.58 & 1.63 & 1.15 & - & 1.35 & - & 1.74 & 1.59 \\ ALERT 2 & 1.78 & 1.80 & 1.57 & 1.73 & 1.24 & - & 1.38 & - & 1.89 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Variations of ratio PM\({}_{10}\) (relative correction) between traffic stations and the corresponding regional background stations in Bucharest, Brasov, and Iasi. Table 4 presents \(p\)-values after the Kruskal-Wallis statistical evaluation was applied to the differences between traffic stations and background stations for the metropolitan cities of Bucharest, Brasov, and Iasi. The analysis reveals that during the Business as Usual (BAU) period and the Lockdown period, there is a significant difference between the concentrations of PM\({}_{10}\) across all stations of interest. A similar behavior can be noticed during the Alert 1 and Alert 2 periods. These findings suggest that the restrictions imposed on traffic had a beneficial influence on PM\({}_{10}\) levels in the studied metropolitan areas. There are two exceptions. The first one was recorded during BAU at BV-3, which is located near the main train station of Brasov. The second one was observed during Alert 2 at BV-1 when there were no traffic restrictions. Table 5 illustrates the results of the Kruskal-Wallis statistical evaluation applied to the ratio between traffic stations and background stations for the metropolitan areas of Bucharest, Brasov, and Iasi. The analysis reveals that during the Business as Usual (BAU) period, the values are significantly higher compared with those observed during the Lockdown period. Subsequently, an increase in the ratio is observed during the Alert 1 and Alert 2 periods, coinciding with the resumption of traffic activities. These findings indicate that traffic restrictions have a notable influence on PM\({}_{10}\) levels in the studied metropolitan areas. Three exceptions occurred, namely for B-3 during Alert 1, IS-1 during Alert 1, and BV-1 during Alert 2.