Formaldehyde-to-nitrogen dioxide ratio (FNR) analysis for ozone sensitivity: a case study over bangladesh using OMI data

The ratio of space-based column HCHO to NO₂ (FNR) is widely used to assess the chemical sensitivity of near-surface ozone (O₃) formation in NOₓ-limited, VOC-limited, or mixed NOₓ-VOC-limited regimes. This study represents the first attempt to analyse FNR over Bangladesh using OMI (Ozone Monitoring Instrument) data. In the absence of significant emission control measures (ECMs), three natural scenarios were considered as proxies for ECMs: the 2020 Covid-19 lockdown, highly precipitated days (> 20 mm), and weekends. Results show FNR serves as a reliable proxy for O₃ sensitivity. Spatially, FNR values typically range from 0–1 (VOC-limited regime), 1–2 (mixed regime), and > 2 (NOₓ-limited regime). During the lockdown and highly precipitated days, most of Bangladesh exhibited an NOₓ-limited regime (FNR > 2), except Dhaka, which showed a mixed regime during lockdowns. A Pearson correlation analysis shows that FNR is positively correlated with surface temperature and precipitation, with Random Forest predictions strongly matching observed FNR values (r = 0.96). The O₃/NO_y ratio analysis further supports the FNR-based ozone sensitivity classification. These findings suggest that ECMs significantly shifted ozone formation to a NOx-limited regime, with FNR being strongly influenced by reductions in NO₂ and increased HCHO production driven by rising surface temperatures.