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The year 2020 was marked by the start of SARS-COV2 virus pandemic. As globally, in Romania the social, economic and transport activities were also restricted at various levels during the year. There were many restrictions and even blockages of some activities at national level. This has led to mass concentrations of air pollutants considerably lower and the improvement of the air quality. was easily noticed. For present study, three metropolitan areas of Bucharest (population 2,161,347 inhabitants), Brasov (population 289,502 inhabitants) and Iasi (population 376,180 inhabitants) were analysed for the period 01. January 2019- 31. December 2020. These cities often face problems of exceeding the air quality limit values imposed by the European legislation. Bucharest is constantly monitored by eight stations (type: traffic, urban, regional, suburban, industrial), Brasov is monitored by five stations (type: traffic, urban, industrial, suburban, regional) and Iasi is monitored by six stations (type: traffic, urban, industrial, rural, suburban). The air pollution monitoring data are those provided by the National Air Quality Monitoring Network (RNMCA). Using RNMCA observations, a synthetic data base consisting of daily time series of mass concentrations of major traffic pollutants, NO and PM10, was created and analysed for traffic stations and reference stations of each city. Data were analysed statistically over four time intervals: Business as Usual (01.01-15.03), Lockdown (16.03-15.05), Alert status with traffic restrictions (Alert 1) (16.05-15.08) and Alert status with normal traffic (Alert 2) (16.08- 31.12). Information about the inhabitants was taken from the website of the National Institute of Statistics.The study shows that in 2020 the pollution levels in the Bucharest metropolitan area were considerably lower for the whole year than in 2019, and during the state of emergency (Lockdown) and the state of alert the pollution levels with PM10 were the lowest in the year. The same observations are valid for the metropolitan areas of Brasov and Iasi. With the relaxation of the restrictions and the entry into the fourth time interval (Alert 2), it was observed that the levels have started to increase by the year 2020. After analysing the NO data according to PM10 at the traffic stations in Bucharest, we determined a lower Pearson's coefficient in 2020 compared to 2019.For the other cities the data cannot be concrete because there are no enough data to draw a firm conclusion for the entire year 2019.Acknowledgements:GBB was supported by the University of Bucharest, PhD research grant. SS and GI thanks the support from NO Grants 2014-2021, EEA-RO-NO-0423 project, contract no 31/2020.Ground-based air pollutant data and meteorology by site were extracted from the public available Romanian National Air Quality Database, www.calitateaer.ro.Data on the number of inhabitants were extracted from the database of the National Institute of Statistics, www.insse.ro

Present research contributes to scientific knowledge concerning spatial and temporal variation of major air pollutants with high resolution at the country scale bringing statistical information on concentrations of NOx, O3, CO, SO2 and particulate matter with an aerodynamic diameter below 10 μm (PM10) and below 2.5 μm (PM2.5) during the pandemic year 2020 using an observational data set from the Romanian National Air Quality Network in seven selected cities spread out over the country. These cities have different level of development, play regional roles, might have potential influence at European scale and they are expected to be impacted by different pollution sources. Among them, three cities (Bucharest, Brașov, Iași) appear frequently on the list of the European Commission with reference to the infringement procedure that the European Commission launched against Romania in the period 2007-2020 regarding air quality.Air pollutant data was complemented with local meteorological parameters at each site (atmospheric pressure, relative humidity, temperature, global solar radiation, wind speed and direction). Statistics of air pollutants provide us with an overview of air pollution in main Romanian cities. Correlations between meteorological parameters and ambient pollutant levels were analyzed. Lowest air pollution levels were measured during the lockdown period in spring, as main traffic and non-essential activities were severely restricted. Among exceptions were the construction activities that were not interrupted. During 2020, some of selected cities experienced few pollution episodes which were due to dust transport from Sahara desert. However, in Bucharest metropolitan area, some cases with high pollution level were found correlated with local anthropogenic activity namely, waste incinerations. Air mass origins were investigated for 72 hours back by computing the air mass backward trajectories using the HYSPLIT model. Dust load and spatial distribution of the aerosol optical depth with BSC-DREAM8b v2.0 and NMBM/BSC-Dust models showed the area with dust particles transport during the dust events.The obtained results are important for investigations of sources of air pollution and for modeling of air quality. Acknowledgment:The research leading to these results has received funding from the NO Grants 2014-2021, under Project contract no. 31/2020, EEA-RO-NO-2019-0423 project. NOAA Air Resources Laboratory for HYSPLIT transport model, available at READY website https://www.ready.noaa.gov and the Barcelona dust forecast center for BSC-DREAM8b and NMBM/BSC-Dust models, available at: https://ess.bsc.es/bsc-dust-daily-forecast are also acknowledged. The data regarding ground-based air pollution and meteorology by site was extracted from the public available Romanian National Air Quality Database, www.calitateaer.ro.