Understanding O₃ formation regimes in higher spatio-temporal resolution using daily derived FNR with TROPOMI HCHO and NO₂ retrievals

This study examined O₃ formation regimes in the Aegean Region (Türkiye), a complex environment with diverse pollution sources, including urban, industrial, and biogenic emissions. Satellite-derived Sentinel-5P TROPOMI formaldehyde (HCHO) and nitrogen dioxide (NO₂) retrievals were analyzed along with routine ground-level O₃ and NO₂ station measurements to evaluate the spatial and temporal variability of O₃ production, using the formaldehyde-to-nitrogen dioxide ratio (FNR) as an indicator of O₃ sensitivity to its precursors. Satellite observations showed high NO₂ (∼4–5 × 10¹⁵ molecules cm⁻²) over industrial areas and elevated HCHO (∼1.2 × 10¹⁶ molecules cm⁻²) over forested and coastal regions. Region specific FNR ranges were estimated for O₃ formation regimes. Monthly FNR values indicated that NO_x-limited regimes increased during summer with consistent NO_x-saturated hotspots around polluted regions, while transitional to NO_x-saturated regimes were found in Apr and Oct. Significant correlations were found between monthly MDA8O₃ and HCHO (R = 0.50-0.89∗∗∗) levels in stations. Ozone formation with significantly high levels in the Aegean Region mainly occurred throughout an extended season including Apr and Oct compared to the ozone season (May-Sep). However, there are regions with NO_x-saturated regime such as Izmir and Aydin exhibited consistently high NO₂ levels even in summer months. The findings showed significant variation in the region that future ozone control strategies need to be planned according to local conditions.