Analysis of PM2.5 precursors by satellite products over Lazio region

Fine particulate matter with an aerodynamic diameter below 2.5µm (PM2.5) is widely recognized as one of the most harmful air pollutants due to the impact on human health, ecosystems, and climate. The importance of controlling PM2.5 concentrations has been reinforced by the European Air Quality Directive 2024, which aligns particulate matter standards more closely with World Health Organization guidelines. PM2.5 originates from both direct emissions and secondary formation processes involving gaseous precursors such as nitrogen dioxide (NO₂) and volatile organic compounds (VOCs). Secondary aerosols often dominate PM2.5 mass in urban and regional environments, making the characterization of the spatial and seasonal variability of these precursors essential for understanding formation pathways and supporting season-specific air quality management strategies.

Tropospheric vertical column densities (VCDs) of NO₂ and HCHO (used as a proxy for VOCs) were retrieved from the TROPOspheric Monitoring Instrument (TROPOMI) onboard Sentinel-5P. The results reveal pronounced seasonal variations in precursor concentrations. NO2 spatial distributions closely follow urban centers and major road networks, with higher winter concentrations driven by stable atmospheric conditions and increased emissions from heating and traffic, and lower summer levels reflecting enhanced photochemical processing and atmospheric mixing. In contrast, HCHO shows a more widespread seasonal pattern, with higher summer concentrations largely driven by intensified photochemical activity and biogenic emissions, with isoprene acting as a key local precursor. These seasonal dynamics are consistent with established atmospheric chemistry and emission patterns.

The application of k-means clustering enabled the identification of regions of interest, distinguishing highly polluted areas from cleaner backgrounds and highlighting urban and agricultural hotspots such as Rome, the Sacco Valley, and the Tiber Valley. Comparison with land cover data indicates that elevated pollution levels are associated with urban, industrial, and transportation-related emissions, while areas with natural vegetation exhibit greater mitigation capacity.

Aerosol optical depth (AOD) derived from the MAIAC algorithm applied to MODIS data from the AQUA and TERRA satellites was employed to investigate the relationship between gaseous precursors and particulate matter formation. The results indicate a distinct seasonal coupling between precursor gases and AOD. During winter, NO2 shows stronger associations with AOD, highlighting the dominant role of inorganic secondary aerosol formation. During summer, HCHO exhibits a closer relationship with AOD, pointing to the increased importance of photochemically driven secondary organic aerosol production.

Overall, satellite-based Earth observation provides a powerful complement to ground-based monitoring for investigating PM2.5 precursors and demonstrates strong potential to support the implementation of the 2024 European Air Quality Directive. By identifying spatial hotspots and seasonal drivers of precursor gases, this analysis supports the development of effective, season-specific emission reduction strategies and improves understanding of the atmospheric processes controlling air quality in the Lazio region.