Impact of the implementation of Madrid's low emission zone on NO2 concentration using Sentinel-5P/TROPOMI data

In recent years, several measures and policies have been introduced to safeguard healthy NO2 levels in urban areas, including the creation of low emission zones (LEZs). In November 2018, a traffic -restricted area known as the Special Protection Low Emission Zone (SPLEZ) was established in the core of the city of Madrid, embracing most of its central district. This study aims to evaluate whether the implementation of the LEZ has led to improvements in air quality by analyzing NO2 concentrations in various case scenarios. For this purpose, data from the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel -5 precursor (Sentinel -5P) were used. The pre- and post-LEZ implementation seasonal analysis (from July 2018 to May 2019) and the entirety of 2022 indicated that the epicenter of transport -related NO2 concentrations did not lie within its boundaries. However, NO2 levels within the SPLEZ peaked during winter, with heating and meteorological factors playing a significant role. Additionally, the findings from the case study of the COVID-19 pandemic demonstrated the influence of human activities on the NO2 concentration in both the SPLEZ and the entire metropolitan area. NO2 emissions decreased during the strict lockdown period and steadily increased as activities resumed. Satellite data was compared to surface -level measurements in each case study, resulting in a linear correlation coefficient of 0.72. TROPOMI data was found to closely align with surface -level observations as pollution levels increased, contrasting with better air quality scenarios. Despite certain limitations, it can serve as a valuable tool for providing complementary information to surface -level data, particularly in characterizing changes in pollution distribution patterns.