## 2 Materials and Methods
### Experimental Data
The same situation applies to Iasi, but here, the first year concentrations were reported was 2017, increasing slightly every year, and in 2021, there were no significant data. Moving on to Cluj-Napoca, as can be seen in Figure 4, no significant values were highlighted until 2016. After this interval, variations in concentrations were recorded, suddenly rising to 2.59 [mg/m\({}^{3}\)] in 2020, and in 2021, the values became insignificant again. A more special situation is presented by the report on arsenic pollution in Timisoara because, unlike the other 3 cities, arsenic concentrations were recorded here every year, between 0.34 and 1.45 [mg/m\({}^{3}\)]. Figure 4: Average annual concentration of As in the four cities (Brasov, Cluj-Napoca, Iasi, and Timisoara). Figure 3: Average annual concentration of CO in the four cities (Brasov, Cluj-Napoca, Iasi, and Timisoara). Furthermore, the maximum and minimum concentrations of each year, for each city and pollutant, depending on the alert threshold, are represented. Figure 5 shows the values for PM\({}_{2.5}\), the threshold value being 17 [\(\mathrm{\mu g/m^{3}}\)]. As can be seen, in the case of all 4 cities, the maximum values for each year exceeded the alert threshold. A constant trend can be noticed for Cluj-Napoca, Iasi, and Timisoara, but for Brasov, there are variations from 49.87 [\(\mathrm{\mu g/m^{3}}\)] to 198.31 [\(\mathrm{\mu g/m^{3}}\)]. For Cluj-Napoca and Timisoara, the concentrations were lower, unlike in Iasi, where PM\({}_{2.5}\) pollution ranged between 80 and 120 [\(\mathrm{\mu g/m^{3}}\)]. The minimum annual measured concentrations were below the alert threshold value for each case. Figure 5: (**a**) Maximum concentrations of PM\({}_{2.5}\) mean daily values depending on the alert threshold; (**b**) minimum concentrations of PM\({}_{2.5}\) mean daily values depending on the alert threshold. The carbon monoxide alert threshold sets a value of 7 [mg/m\({}^{3}\)], which was not exceeded in any of the four cities during the last 11 years (2011-2021). The concentrations remained in a constant line, with there being several variations in Brasov and Cluj-Napoca, the maximum concentrations being 3.45 [mg/m\({}^{3}\)] and 3.56 [mg/m\({}^{3}\)], respectively. Additionally, in the case of the cities of Iasi and Timisoara, the trend of decreasing values in recent years can be best observed (Figure 6). Regarding arsenic, a pollutant whose alert threshold is 4.2 [mg/m\({}^{3}\)], there were only two situations in which this number was exceeded, namely, in Cluj-Napoca in 2017 and 2020, with concentrations of 5.76 [mg/m\({}^{3}\)] and 4.91 [mg/m\({}^{3}\)], respectively. In the other 3 cities, the alert threshold limit was not exceeded. As can be seen in Figure 7, a special situation  
Figure 6: (**a**) Maximum concentrations of CO mean daily values depending on the alert threshold; (**b**) minimum concentrations of CO mean daily values depending on the alert threshold. was highlighted in Timisoara, where arsenic was present every year and the concentrations recorded varied, increasing to 2.54 [ng/m\({}^{3}\)] in 2021, compared to 1.58 [ng/m\({}^{3}\)] in 2020. The last type of graph was aimed at highlighting the predominant pollutant for each city by calculating the aggregate index, which represents the average of the concentrations for all 11 years (2011-2021), considering all the values from the previously mentioned monitoring stations [29; 35]. According to Figure 8, for lasi, the most significant values of PM\({}_{2.5}\) were reported, the index level being 22.81 \(\pm\) 0.83 [ug/m\({}^{3}\)].