Assessing nitrogen dioxide monitoring techniques: a comparative analysis of Sentinel-5 Precursor satellite and ground measurements in Catalonia

Abstract. Effective monitoring of air pollution is essential for the development of environmental and public health policies. Comprehensive air quality management requires precise tools and strategies to assess the spatial and temporal distribution of pollutants. This study investigates the correlation between nitrogen dioxide (NO2) concentrations detected by the Sentinel-5 Precursor (S5p) satellite and those measured at ground stations by Catalonia's official air quality monitoring network during 2022 and 2023. The methodology integrates satellite and surface data aligned in space and time. The relationship between both measurements is analyzed under different frameworks: (i) global, considering the entire territory; (ii) by geographic zone (urban, suburban, and rural, as well as inside and outside the Barcelona Metropolitan Area (BMA)); (iii) according to the type of stations (traffic, background, industrial); and (iv) at a seasonal level, covering different quarters of the year. Statistical tools are then used to identify patterns and differences based on zones, typology, and seasonality characteristics. The results show a moderate positive correlation at the global level, with r=0.66. By zones, the analysis reveals that suburban (r=0.66) and non-BMA zones (r=0.67) present stronger correlations compared to urban zones (r=0.55), traffic typology (r=0.61), or stations located in the BMA zone (r=0.42). Seasonally, the correlation peaks in winter (r=0.70) and autumn (r=0.66), periods with more stable atmospheric conditions for NO2 concentrations, while it is lowest in spring (r=0.61) and summer (r=0.57). These findings highlight the utility of the S5p satellite as a complement to ground-based networks in NO2 monitoring, while revealing the limitations of applying a direct relationship between both types of data at the regional level and across different geographic zones.