## 3 Results and Discussion
### Air Quality during 2020 and Reference Years in Highly Populated Cities in Spain
Sulfur dioxide (SO\({}_{2}\)) showed uneven behavior during the time of confinement and throughout 2020, for example, in Barcelona, Valencia and especially in Sevilla, which almost showed their average values (Table 4). What is also noteworthy is the high concentration of this gas in Malaga, which quintupled Barcelona, although it is not a particularly industrial city (the data collection station is located next to a water treatment plant but far from the city's thermal power plant). Its maximum values were close to 20 \(\upmu\)g/m\({}^{3}\) on average in 24 h. In the past, the main anthropogenic source was the burning of fossil fuels (coal and oil) used in heating, electricity generation and thermal combustion vehicles [55; 56], which during the lockdown in Spain should be not considered, since the diesel fuel in Europe today is virtually sulphur-free. Therefore, it could be more related to the use of home electricity demands [57]. Finally, regarding the data obtained for PM\({}_{2.5}\), PM\({}_{10}\) and CO, the available data made them impossible to be compared in all the cities and years. Although PM\({}_{2.5}\) and PM\({}_{10}\) are constituted naturally by dust particles, sea salts, pollen or spores, they also have a clear urban origin in emissions from engine combustion and anthropic volatile organic compounds and construction with negative impacts on human health [58; 59]. Recently, several authors observed a clear relationship between an increase in PM\({}_{2.5}\) and PM\({}_{10}\) and an increase in air temperatures, due to their ability to absorb radiation and serve as a medium for chemical reactions. This is also the case for CO, which can be combined with oxygen to increase the generation of greenhouse effects. In the case of our analyzed data, a generalized decrease in the cases studied was noted. The local peculiarities derived from a longer period of rain in certain seasons could coincide with the confinement, although they were not observed at a general level in all seasons. Therefore, it can be concluded that there was also a reduction during 2020 and especially during the lockdown. Our results agree with other countries, particularly urban areas in China studied by several scholars during the last decade [60; 61; 62]. The most recent studies also confirmed that PM\({}_{2.5}\) and PM\({}_{10}\) showed the most significant changes during the first part of 2020 during the COVID-19 pandemic, as some scholars demonstrated in California [63] and the megacities of New Delhi [64] and Baghdad [65] due to the reduction of activities. It is worth highlighting that an investigation conducted in the three major French cities (Paris, Lyon and Marseille) demonstrated the possible correlation between these pollutants and deaths due to COVID-19. They considered the hypothesis that predetermined particulate concentrations could foster COVID-19 and make the respiratory system more susceptible to this infection [66]. It is also plausible that reduced pollution due to lockdowns would lead to short-term warming as a result of more incoming sunlight, rather than cooling. Several authors reported this short-term cooling effect of aerosol-causing pollution (e.g., [67]). Moreover, it would be interesting to see whether solar radiation, cloudiness or visibility data are available for these stations in the future to substantiate the effect of reduced pollution on sunlight reaching the surface. Therefore, we consider that it is key that green policies and strict regulations of contaminants spread out to the atmosphere must be further developed to reduce the potential increase in COVID-19 infections and guide recovery [68; 69].