## 2 Materials and Methods
### Experimental Data  
The air quality assessment was performed based on the data from the last 11 years, considering 3 pollutants: arsenic, PM\({}_{2.5}\), and carbon monoxide. Arsenic, which is part of the heavy metal category, can be found in PM\({}_{2.5}\) fractions that are known to be very dangerous and can enter the respiratory tract. Carbon monoxide, a toxic gas, can be lethal in high concentrations, reducing the ability to carry oxygen in the blood [6]. Their  
Figure 1: Location of air quality monitoring stations. measured concentrations for the last 11 years (2011-2021) in the 4 cities were obtained from the EPA public reports. For each of these cities, we considered the daily data recorded by 3 monitoring stations. To better observe the evolution of the air quality and its trend during the 11 years, statistical analyses were run, considering the minimum and maximum concentrations measured within each year. Where there are no data presented in the figures for a specific pollutant, year, or monitoring station, there were no measurements included in the report. After a deep analysis of the information reported, the data were organized into tables, for each monitoring station, year, and pollutant, for each city, resulting in 4 complex tables. For each pollutant, the annual averages measurements, and the maximum and minimum concentrations were considered as well. Based on these tables, we also produced the first set of graphs that show the average concentration for each year, station, and pollutant. The errors could not be calculated for this dataset because the values were taken from the reports validated by the Environmental Protection Agency (EPA). Figure 2 shows the evolution of air quality in the four cities during the 11 years (2011-2021), depending on the average annual concentration of PM\({}_{2.5}\). Through this representation, the evolution and variability of the values can be clearly visualized. For each city, there were concentrations recorded by only one monitoring station, as can be seen in Figure 2. Thus, in the case of Brasov, a downward trend of the concentrations recorded in recent years can be observed, reaching a maximum of 25.89 [\(\mathrm{\mu g/m^{3}}\)] in 2017, exceeding the legal limit for this year. This value was registered by the BV-2 station. The evolution of air quality in terms of particulate matter pollution for Cluj-Napoca was monitored by the CJ-2 station. Thus, a maximum annual average was reached in 2011, of 28.32 [\(\mathrm{\mu g/m^{3}}\)], also exceeding the legal limit calculated for one year. Regarding the city of lasi, the trend of decreasing concentrations in recent years has been absent for PM\({}_{2.5}\), with variation in the values between 20 and 25 [\(\mathrm{\mu g/m^{3}}\)], and with the highest concentration recorded for this pollutant among the four cities (28.70 \(\mathrm{\mu g/m^{3}}\)) in the 11 years. The lowest values among the 4 cities were registered in Timisoara, being between 9.94 and 21.50 [\(\mathrm{\mu g/m^{3}}\)], in 2016. All four monitoring stations that recorded PM\({}_{2.5}\) concentrations are urban types and are in the vicinity of residential areas. Regarding the average annual concentrations of carbon monoxide in the case of Brasov, Timisoara, and Cluj-Napoca (Figure 3), the values did not remain in a constant range, varying depending on the location of the stations, but within normal limits. For lasi,  
Figure 2: Average annual concentration of PM\({}_{2.5}\) in the four cities (Brasov, Cluj-Napoca, lasi, and Timisoara). the data from the IS-6 monitoring station were highlighted, which were higher than those from the other two stations. Even if there were some variations, the values were within normal limits for this city as well, with a maximum of 1.32 [mg/m\({}^{3}\)] in 2014. The representation of the average annual concentration for arsenic (Figure 4) shows that in Brasov, the stations recorded concentrations within normal limits starting in 2020; until then, there were no reported values.