## 2 Materials and Methods  
### Site Boundaries and Brief Description  
The National Air Quality Monitoring Network performs continuous measurements of sulfur dioxide (SO\({}_{2}\)), nitrogen oxides (NO\({}_{\rm X}\)), carbon monoxide (CO), ozone (O\({}_{3}\)), particulate matter (PM\({}_{10}\) and PM\({}_{2.5}\)), benzene (C\({}_{6}\)H\({}_{6}\)), lead (Pb), arsenic (As), cadmium (Cd), nickel (Ni), and benzo(a)pyrene, and it includes 148 automatic air quality monitoring stations and 11 mobile stations distributed as follows: 30 traffic-type stations, 58 industrial stations, 37 urban background-type stations, 13 suburban-type stations, 7 stations of the regional background type, and 3 EMEP-type stations. By region, the monitoring stations cover 20 automatic stations in the North-East Region, 21 in the South-East Region, 27 in the South-Muntenia Region, 12 in the South-West Oltenia Region, 21 in the West Region, 18 in the North-West Region, 21 in the Central Region, and 8 in the Bucharest-Ilfov region [6]. In the last two years, several charts have been produced for the most polluted cities in Romania, and each time, among other cities, Iasi, Brasov, Cluj-Napoca, and Timisoara were found to be the most polluted. Table 1 presents the similarities of the four selected cities for this study.  
The monitoring stations considered for this study are presented in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**City** & **Population** & \begin{tabular}{c} **Number of Water** \\ **Treatment Plants** \\ \end{tabular} & \
\begin{tabular}{c} **Number of Wastewater** \\ **Treatment Plants** \\ \end{tabular} &
\begin{tabular}{c} **Number of Air Quality** \\ **Monitoring Stations** \\ \end{tabular} \\ \hline Brasov & 275,514 & 2 & 5 & 7 \\ Cluj-Napoca & 303,047 & 1 & 8 & 5 \\ Iasi & 318,871 & 8 & 4 & 6 \\ Timisoara & 306,462 & 3 & 1 & 7 \\ \hline \end{tabular}
\end{table}
Table 1: Similarities between the evaluated cities [33,34].  
Brasov is a tourist city in the center of the country and has 7 air quality monitoring stations, of which we chose to take the data recorded by 3 since they have the most complete set of measurements. BV-2 is an urban-type station and BV-4 is a suburban-type station, meaning that they aim to monitor the influence generated by pollutants within a radius of 1-5 km. The third station is EM-1, an EMEP-type station that deals with monitoring long-distance air quality in a cross-border context, and it is located at 1231 m elevation [6; 34]. Cluj-Napoca, in the northwest of the country, is also called the \"heart of Transylvania\" and is also a tourist city. Air quality monitoring is carried out through 5 stations. For this area, we chose the CJ-1 station (traffic type), which, as its name suggests, registers the influences generated by traffic on air quality, and CJ-2 and CJ-3, which are urban and suburban types, respectively [6; 34]. _The city of the 7 hills_, or the city of Iasi, is in the northeast of the country, and within its territory, there are 6 air quality monitoring stations. IS-2 and IS-5 are urban and suburban types, respectively, and IS-6 is a rural-type station [6; 34]. To the west of the country is Timisoara, a city known for its commercial, industrial, cultural, medical, and university activities. There are 7 stations here, of which TM-1 is a traffic-type station, TM-2 is an urban-type station, and TM-3 is a suburban-type station [6; 34].  
All monitoring stations are equipped with high-performance analyzers that continuously monitor selected air pollutants, including PM\({}_{2.5}\), As, and CO. The measurement methods used to determine pollutant concentrations are the reference methods, according to the EU environmental standards, as follows: for CO, the determination method is non-dispersive infrared spectrometry, and for PM\({}_{2.5}\), it is gravimetric and nephelometric determination. The reference method for measuring arsenic is in accordance with the SR EN 14902 standard, specifying absorption spectrometry with atomic or coupled plasma [34].