2.1. Area of study

The study is focused on the south-central plateau of the Iberian Peninsula (Fig. 1 ). It is a zone with Mediterranean-continental climate, moderately industrialized, and whose most relevant pollution episodes are usually due to traffic emissions in cities under stagnation and high irradiance conditions (^{MITECO, 2018}).

Map of the study area and locations of air quality monitoring stations.

Madrid is the most densely populated city in Spain with 3.3 million inhabitants (^{INE, 2019}). Road transport is the most important source of urban pollution, although other sources such as residential heating also contribute to emissions. In this sense, road transport in Madrid was roughly responsible for 58% of NOx emissions and 52% of PM_2.5 emissions (^{DGSEC, 2018}). Castilla-La Mancha is located in the South-East, (Fig. 1), where Albacete is the largest city (173,000 inhabitants, ^{INE, 2019}) and it is placed 250 km south-east from Madrid. Puertollano, with 47,000 inhabitants (^{INE, 2019}), is located 240 km south-west of Madrid, and it has the highest concentration of heavy industry in the centre of the Iberian Peninsula with a refinery, petrochemical installations, a fertilizer factory and two power plants. Lastly, S. Pablo is a small village (1800 inhabitants, ^{INE, 2019}) at 170 km south-west of Madrid. It is a rural area with agriculture as the main activity and is considered as a remote continental area of central Spain.