Investigating Ozone Formation Regimes in the Metropolitan Area of São Paulo Using Five Years of TROPOMI HCHO/NO2 Ratios

The Metropolitan Area of São Paulo (MASP), located in southeastern Brazil, faces significant air quality challenges due to its large vehicle fleet and complex fuel composition, including widespread ethanol use. Air pollution dynamics in this context are investigated, focusing on spatio-temporal variations in formaldehyde (HCHO) and nitrogen dioxide (NO2), and their role in ozone (O3) formation. High-resolution data from the TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor satellite are used to analyze HCHO and NO2 vertical column densities (VCDs) over a 5-year period (2019–2023). Results reveal high HCHO and NO2 VCDs over MASP, with spatial patterns related to land use and higher concentrations during the dry season, with HCHO mean VCD reaching 14.21 × 1015 molecules cm−2 and NO2 mean VCD reaching 8.91 × 1015 molecules cm−2. The Formaldehyde to Nitrogen dioxide Ratio (FNR) thresholds were derived based on observations from 24 CETESB surface O3 monitoring stations, providing region-specific constraints for O3 sensitivity classification in MASP, with lower and upper thresholds of 1.6 and 2.4. Based on these thresholds, the analysis indicates a predominance of VOC-sensitive conditions in the urban core, alongside transition and NOx-limited regimes in other areas.