## 3 Results and Discussion
### Air Quality during 2020 and Reference Years in Highly Populated Cities in Spain
This could be explained by Valencia possibly having more manufacturing plants or more commuters than the other cities, and therefore, the shutdown had a larger impact. In addition, we considered that the older population affecting the more elevated amount of the potential population's ability to work in Valencia may be related to the decrease in activities and an increase in air quality. However, longer analyses should be conducted to understand why they had such a large reduction while Sevilla registered the lowest reduction. Nitrogen dioxide (NO\({}_{2}\)) is a respiratory toxic gas that, in the outside air, is mainly derived from the oxidation of nitric oxide (NO) coming from vehicles in urban areas (especially diesel), but also from heating appliances and electricity generation [48]. For NO, recommended hourly limits of 200 \(\upmu\)g/m\({}^{3}\) and alert thresholds of about 400 \(\upmu\)g/m\({}^{3}\) for reducing human health issues and costs were highlighted by several scholars in highly populated cites [49; 50]. However, the Spanish Directive 2008/50/CE and RD 102/2011 lowered these values to 30 \(\upmu\)g/m\({}^{3}\) as a critical level for the protection of vegetation. The average values of the series (Table 3) indicate that Madrid was the city in which the highest values were normally reached (25.58 \(\upmu\)g/m\({}^{3}\)). Meanwhile, Malaga did not reach 10 \(\upmu\)g/m\({}^{3}\) (8165). Nevertheless, throughout 2020, the most drastic reductions overall were registered in Sevilla (which reduced its average emissions to 35.8% of the average of previous years) followed by Madrid (47.7%). On a monthly level, it should be noted that, except Madrid, important decreases for the averages of the months of confinement were recorded, especially during April. Therefore, we observed a decrease in pollution due to the reduction of activities [51]; although, this did not reflect lower COVID-19 spreadability [52; 53]. NO content is exponentially increased with temperature [54] which, observing the air temperature results obtained above, could be worrying. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Values (ug/m\({}^{3}\)) (\({}^{\star}\))** & **Madrid** & **Barcelona** & **Valencia** & **Sevilla** & **Malaga** \\ \hline
2020 & 20.94 & 21.05 & 21.14 & 21.89 & 27.91 \\ Annual values (10 years) & \(30.61\pm 0.79\) & \(34.88\pm 1.26\) & \(37.81\pm 1.75\) & \(27.1\pm 0.86\) & \(37.95\pm 2.57\) \\ Mar-20 & 22.61 & 24.45 & 22.97 & 24.25 & 28.93 \\ Av. March & \(31.8\pm 2.59\) & \(31.37\pm 4.52\) & \(33.91\pm 3.01\) & \(26.44\pm 2.19\) & \(34.81\pm 4.53\) \\ Apr-20 & 26.06 & 29.95 & 29.15 & 26.5 & 35.15 \\ Av. April & \(36.34\pm 3.08\) & \(36.46\pm 2.88\) & \(38.73\pm 2.36\) & \(29.65\pm 2.32\) & \(37.82\pm 5.13\) \\ May-20 & 28.64 & 27.19 & 28.74 & 28.37 & 34.81 \\ Av. May & \(39.07\pm 1.84\) & \(36.36\pm 1.38\) & \(38.51\pm 3.26\) & \(33.26\pm 2.18\) & \(41.42\pm 2.96\) \\ \hline \hline \end{tabular} (\({}^{\star}\)) MITECO (ica.miteco.es) use these units.  
\end{table} 
Table 2: Comparison of tropospheric ozone values in 2020 and the 10 years of data series. Av. = average. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Values (ug/m\({}^{3}\))** & **Madrid** & **Barcelona** & **Valencia** & **Sevilla** & **Malaga** \\ \hline
2020 & 12.2 & 11.66 & 6.15 & 5.78 & 5.63 \\ Annual values (10 years) & \(25.58\pm 2.03\) & \(13.79\pm 0.64\) & \(10.03\pm 2.56\) & \(16.18\pm 1.86\) & \(8.17\pm 0.54\) \\ Mar-20 & 10.12 & 10.97 & 4.61 & 5.11 & 5.05 \\ Av. March & \(24.99\pm 1.72\) & \(14.91\pm 3.02\) & \(12.13\pm 2.82\) & \(16.37\pm 2.15\) & \(7.46\pm 1.38\) \\ Apr-20 & 5.29 & 6.7 & 2.55 & 3.13 & 2.62 \\ Av. April & \(21.45\pm 1.09\) & \(13.71\pm 1.78\) & \(9.17\pm 3.0\) & \(15.85\pm 0.58\) & \(7.33\pm 0.44\) \\ May-20 & 5.96 & 7.08 & 3.67 & 3.59 & 4.07 \\ Av. May & \(21.22\pm 2.53\) & \(12.75\pm 1.43\) & \(6.99\pm 1.56\) & \(15.01\pm 2.2\) & \(7.41\pm 1.32\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of nitrogen dioxide values in 2020 and the 10 years of data series. Av. = average.