## 4 Results
### Air-Quality Analyses  
To test the hypothesis that the positive impact of the proposed underpass will be less than assumed by the government and the planning team, we examined spatial and temporal trends in air quality before and during construction, a period that overlapped with the Covid pandemic. If the hypothesis is correct, air quality in the city should not change significantly. In order to understand and analyze the evolution of air quality in the city of Oradea before the investment, during and after the construction works of the underpass, we used data of the extended aerosol measurements from NASA's EOS platforms, MERRAero, images collected by the monitoring instrument from Sentinel-2 [37] and data provided by the EPA and EEA air-quality sensor network. As mentioned earlier, the National Environmental Protection Agency conducts air-quality measurements through a network of sensors. At the Oradea Municipality level, there are three monitoring stations BH-1 (Dacia Boulevard) for measuring urban air quality, BH-2 (Matei Corvin Street) for measuring industrial pollution, and BH-3 (Nufarul Street)  
Figure 7: Traffic in Oradea on 17 May 2021 at 2 p.m. (made by the authors with Google Maps). for assessing traffic-related pollution. All of these are unfortunately marked as \"station for which no data have been reported\" on the EEA website [35]. However, the annual average values of NO2 pollution are available and are presented in Table 1.  
Analysis of data provided by the EEA for NO2, one of the main traffic-related pollutants, shown in the table above, indicates that the average annual values in Oradea for two of the sensors in 2020, the year of the COVID-19 pandemic, were significantly lower than in 2019 and 2021. In accordance with the Air Quality Health Risk Assessments in 2020, an index of health risk from NO2 pollution in urban areas of Bihor County was calculated, which was also lower than in 2019 due to the lower traffic volume [35]. The following figures show the images acquired by Sentinel 2 and visualized with EOSDA Land Viewer for the monitoring period proposed in this study. Figure 8 shows atmospheric penetration as a combination of SWIR2, SWIR1, and NIR bands during the four periods of February 2020, (b) April 2020, (c) September 2020, and (d) October 2020. A visual comparison of these four images shows a clearer image for April 2020 that could be assimilated with a clear atmosphere. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Sensor** & **2019** & **2020** & **2021** \\ \hline BH-1 & 27.84 & 21.53 & 27.71 \\ \hline BH-2 & 33.43 & 24.05 & 26.62 \\ \hline BH-3 & N/A & 25.70 & 18.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Annual mean value for NO2 (ug/m3), measured by the three sensors in Oradea [35]. Figure 8: Atmospheric penetration. (**a**) February 2020, (**b**) April 2020, (**c**) September 2020, and (**d**) October 2020 (made by the authors). Figure 9 presents the atmospheric removal of a band combination of SWIR2, NIV, and green for (**a**) February 2020, (**b**) April 2020, (**c**) September2020, and (**d**) October 2020. Similar to the previous case, the visual comparison of these four images shows a clearer picture for April 2020, which can be equated with less pollution in the atmosphere. The second study is based on the calculation of the atmospheric particulate matter, or particulate matter (PM), with diameter of 2.5 \(\upmu\)m (1.0 \(\upmu\)m) or less called PM2.5. This type of particulate matter is generated by all types of pollution, including traffic pollution and dust brought by the wind. The PM2.5 is calculated in MERRA-2 by using the formula which was adapted from [20] as the following:  
\[ \begin{array}{c}\text{Surface Mass Concentration of PM2.5 (units: kg m}^{-3})=\text{BCSMASS + OCSMASS}\\ +\text{SO4 SMASS}*(132.14/96.06)+\text{DUSMASS25}+\text{SSSMASS25},\end{array}