## 3 Results
### PM\({}_{10}\) Assessment Based on Absolute and Relative Correction  
To minimize the influence of various confounding factors when averages are used to compare air pollution by PM\({}_{10}\), we employed the methodology described in Section 2. Figure 3 illustrates the variation in PM\({}_{10}\) mass concentrations at traffic stations after subtracting background levels (as described in the Section 2) to compare the differences between 2020 and 2022, during which restrictions were imposed due to the COVID-19 pandemic. As the year 2020 was divided into four periods based on the restrictions implemented by national authorities in response to the pandemic, we analyzed the average PM\({}_{10}\) concentrations for each period in 2020 and the corresponding periods in 2022 to assess the impact of these restrictions on urban air pollution. The annual average PM\({}_{10}\) concentrations were calculated using the available daily data for each year. Figure 3 presents the frequency distribution of the difference between the background station and the traffic station for the B-6 station in Bucharest. The levels in 2020 are higher than those in 2022 during the BAU period, which is the timeframe prior to the implementation of COVID-19 pandemic restrictions. During the Lockdown period, the values were lower in 2020 compared with 2022 due to the imposed traffic restrictions. The same trend was maintained for the subsequent Alert 1 and Alert 2 periods. To observe the evolution of the restrictions imposed by government authorities, only one station was used. Table 2 demonstrates that during the BAU period of the pandemic year 2020, the pollution levels were high. Further, due to the Lockdown restrictions, the degree of pollution decreased and remained lower during the Alert 1 and Alert 2 periods compared with the reference year 2022. A comparison could not be made for Brasov stations BV-1 and BV-3 during Alert 1 and Alert 2, as well as for Iasi station IS-1 during Alert 2, due to the lack of data for processing from the year 2022. Figure 4 illustrates the frequency distribution of the ratio between the traffic station B-6 and the background station B-8 in Bucharest. Ratio values greater than one indicate that at a certain measurement site, the concentrations are higher compared with the background values. During the Business as Usual (BAU) period, which is the time frame prior to the imposition of COVID-19 pandemic restrictions in 2020, the values are higher compared with those in 2022. In the Lockdown period, the values were lower in 2020 due to the implemented traffic restrictions, in contrast to the year 2022. The same trend was observed for the subsequent Alert 1 and Alert 2 periods. Table 3 demonstrates that in the first BAU period of the pandemic year 2020, the pollution levels were high in Bucharest. However, due to the Lockdown restrictions, the degree of pollution decreased and remained lower in the Alert 1 and Alert 2 periods compared with the reference year 2022. A comparison could not be made for Brasov stations BV-1 and BV-3 during Alert 1 and Alert 2, as well as for Isai station IS-1 during Alert 2, owing to the absence of data for processing from the year 2022. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**STATION** & **B-3** & & \multicolumn{3}{c}{**B-6**} & \multicolumn{3}{c}{**BV-1**} & \multicolumn{3}{c}{**BV-3**} & \multicolumn{3}{c}{**IS-1**} \\
**YEARS** & **2020** & **2022** & **2020** & **2022** & **2020** & **2022** & **2020** & **2022** & **2020** & **2022** \\ \hline BAU & 15.65 & 9.37 & 13.07 & 6.89 & 1.87 & 2.83 & 4.88 & 8.01 & 20.83 & 17.86 \\ LOCKDOWN & 10.48 & 12.28 & 8.01 & 9.79 & 0.85 & 1.05 & 1.54 & 4.28 & 7.14 & 15.02 \\ ALERT 1 & 10.26 & 11.08 & 7.02 & 8.97 & 1.57 & - & 4.18 & - & 8.68 & 8.77 \\ ALERT 2 & 12.47 & 13.75 & 9.13 & 12.28 & 2.95 & - & 5.31 & - & 13.89 & - \\ \hline \hline \end{tabular}
\end{table}