## 2 Materials and Methods
### Data Sources and Treatment  
In this research, we considered five of the most populated cities in Spain in 2020 (Source: Instituto Nacional de Estadistica -INE; Spanish National Statistics Institute): Barcelona (Vall d'Hebron 41\({}^{\circ}\)25\({}^{\prime}\)38.02\({}^{\prime\prime}\) N, 2\({}^{\circ}\)08\({}^{\prime}\)32.56\({}^{\prime\prime}\) E), Madrid (La Castellana, 40\({}^{\circ}\)26\({}^{\prime}\)23.61\({}^{\prime\prime}\) N, 3\({}^{\circ}\)41\({}^{\prime}\)25.34\({}^{\prime\prime}\) W), Malaga (El Atabal, 36\({}^{\circ}\)43\({}^{\prime}\)46.15\({}^{\prime\prime}\) N, 4\({}^{\circ}\)27\({}^{\prime}\)55.59\({}^{\prime\prime}\) W), Sevilla (Torrone, 37\({}^{\circ}\)23\({}^{\prime}\)51.74\({}^{\prime\prime}\) N, 6\({}^{\circ}\)00\({}^{\prime}\)02.92\({}^{\prime\prime}\) W) and Valencia (Politecnico, 39\({}^{\circ}\)28\({}^{\prime}\)46.46\({}^{\prime\prime}\) N, 0\({}^{\circ}\)20\({}^{\prime}\)14.84\({}^{\prime\prime}\) W). Out of the six most populated cities in Spain, Zaragoza could not be analyzed due to insufficient data (just one year, 2019, in addition to 2020). We extensively analyzed five urban climate stations with long and complete data series, made available by the Spanish Ministry for the Ecological Transition and the Demographic Challenge (MITECO), which is the department of the government of Spain responsible for developing government policy on the fight against climate change, prevention of pollution, and protecting natural heritage, biodiversity, forests, sea, water and energy for a more ecological and productive social model ([[https://www.miteco.gob.es/es/calidad-y-evaluacion-ambiental/temas/atmosfer](https://www.miteco.gob.es/es/calidad-y-evaluacion-ambiental/temas/atmosfer)]([https://www.miteco.gob.es/es/calidad-y-evaluacion-ambiental/temas/atmosfer](https://www.miteco.gob.es/es/calidad-y-evaluacion-ambiental/temas/atmosfer)) a-y-calidad-del-aire/calidad-del-aire/evaluacion-datos/datos/Datos_2001_2018.aspx) (accessed on 18 March 2021). These data belong to five climate stations throughout Spain in highly populated urban areas with the remarked human activities with (1) daily air temperature data organized by average, minimum and maximum values for the last 30 years (1930-present-day) and (2) diverse pollutants, including O\({}_{3}\), NO\({}_{2}\), SO\({}_{2}\), CO, PM\({}_{10}\) and  
Figure 2: Demographical characterization of the five most highly populated cities in Spain. (**A**) Total population. (**B**) Density. (**C**) Population pyramid. Source: Instituto Nacional de Estadistica (INE; Spanish National Statistics Institute). PM\({}_{2.5}\) for the last 10 years. Despite this, for the Andalusian cities (Malaga and Sevilla), the dataset had to be completed using temperature and pollutant values from the global quality platform air data ([[https://aqicn.org/data-platform/register/](https://aqicn.org/data-platform/register/)]([https://aqicn.org/data-platform/register/](https://aqicn.org/data-platform/register/))) (accessed on 18 March 2021), Red de Informacion Ambiental de Andalucia for Malaga and Sevilla (REDIAM) and Tutiempo.net ([[https://www.tutiempo.net/clima/espana.html](https://www.tutiempo.net/clima/espana.html)]([https://www.tutiempo.net/clima/espana.html](https://www.tutiempo.net/clima/espana.html))) (accessed on 18 March 2021). Considering the temporal and spatial representability of this approach, it was only valid for the near-urban environment of the sampled climate stations and surrounding areas with similar environmental conditions. They were the representative urban stations located in non-green areas with the longest and most complete available data. The interpolation procedure, and thus the extension to the rest of the city, was carried out due to the high difference in data between the purely urban stations and the stations located in green areas. However, there was no reason to think that urban areas close to green areas were less polluted than fully urban ones, based on the data consulted in this study.