Linking Satellite and Ground Observations of NO2 in Spanish Cities: Influence of Meteorology and O3

In Spain, several major cities face high rates of avoidable deaths due to NO2 exposure. Understanding NO2 atmospheric dynamics is essential to support public health efforts and policymaking. Recent satellite products have proven useful in characterizing urban atmospheric composition in various regions. This study compares NO2 concentration data from in situ air quality monitoring networks and the Sentinel-5P TROPOMI satellite in Spain’s three largest cities (Madrid, Barcelona, and Valencia), alongside O3 levels —due to its close photochemical relationship with NOx—wind speed and direction, temperature, relative humidity, and solar radiation. Data from 2022 were analyzed using Pearson correlation coefficients and Principal Component Analysis (PCA) to identify key relationships and patterns. Results showed a consistent negative correlation between NO2 and O3, wind speed, temperature, and solar radiation. Differences between in situ and satellite data were more pronounced in coastal cities, influenced by wind patterns and urban morphology (Madrid: r = 0.86, v = 1.34 m/s; Valencia: r = 0.68, v = 2.97 m/s; Barcelona: r = 0.65, v = 8.04 m/s). These insights enhance the understanding of NO2 behavior in urban environments and support the use of remote sensing to estimate surface-level pollution in areas lacking ground-based monitoring infrastructure. This is the first study in Spain to jointly evaluate NO2 from satellite and in situ data across multiple cities, linking pollutant concentrations with meteorological and chemical drivers to improve surface-level estimation strategies and support air quality assessment in under-monitored areas.