## 2 Materials and Methods  
### Study Areas and the Monitoring Stations  
In this study, three metropolitan areas--Bucharest, Brasov, and Iasi--were analyzed for the period from 1 January 2017 to 31 December 2022 to assess PM\({}_{10}\) levels in relation to different traffic restrictions implemented during the COVID-19 pandemic (Figure 1). Bucharest, the capital of Romania, covers an area of over 200 km\({}^{2}\) and is located on an open plain with a temperate continental climate typical of the region. The city has a population of approximately 2,149,018 [32]. Bucharest experiences relatively mild winters with minimal snowfall and high temperatures and hot summers with low rainfall. The city frequently encounters high pollution episodes, primarily due to emissions from power plants, heavy traffic, numerous small-scale industries, and domestic activities. Additionally, seasonal agricultural practices, such as biomass burning on the outskirts of the municipality, further contribute to the city's air pollution [4; 33]. Previous studies have also highlighted the impact of long-range transport pollution, particularly dust, on Bucharest's air quality [34]. This study utilized data series from two traffic-type monitoring stations, Mihai Bravu (B-3) and Cercul Militar (B-6), selected for their distinct urban locations. The B-3 station is located along a wide avenue, while the B-6 station is situated in a canyon-like area surrounded by tall buildings. These locations represent different urban topographies and traffic conditions, offering a comprehensive perspective on air pollution in Bucharest. Additionally, data from the regional background station, Balotesti (B-8), were included in the analysis. The B-8 station is located in a plain area, 24 km from the city center, and is not directly influenced by traffic emissions or urban activities (Table 1). By comparing data from the B-3, B-6, and B-8 stations, this study aimed to differentiate between urban and regional air pollution levels and to evaluate the impact of traffic and urban activities on air quality in Bucharest. Brasov, a city with approximately 283,912 inhabitants [32], is the capital of the county bearing the same name, located in central Romania within the Transylvania region. The city has a temperate continental climate and is surrounded by mountains, leading to slightly lower temperatures in its valley areas. Brasov is the third most polluted city in Romania and is currently facing an infringement procedure initiated by the European Union due to significant pollution issues. The main contributors to the city's pollution include road traffic, non-compliant construction sites, and industrial activities. Air quality monitoring in Brasov is conducted at seven stations, including one EMEP (European Monitoring and Evaluation Programme) station. To evaluate the impact of urban structure, traffic, and industrial activities on air quality in Brasov, this study considered data from two traffic stations with different characteristics: BV-1, located in a canyon-like area surrounded by high-rise buildings, and BV-3, situated in a less densely built area near the train station. Additionally, data from the BV-4 station, a suburban background station located approximately 8.5 km outside the city, was included in the analysis (Table 1). This multi-station approach allows for a comprehensive assessment of how urban structure, traffic, and industrial activities affect air quality in Brasov.