Long-Term (2011–2019) Trends of O₃, NO₂, and HCHO and Sensitivity Analysis of O₃ Chemistry over the GBM (Ganges–Brahmaputra–Meghna) Delta: Spatial and Temporal Variabilities

Decadic (2011–2019) column NO₂, HCHO, and surface O₃ of nine different locations were studied over the GBM (Ganges–Brahmaputra–Meghna) delta. Column-based HCHO/NO₂ ratios were employed to characterize ozone formation sensitivity. Level-3 gridded retrievals from Ozone Monitoring Instrument (OMI) were utilized to identify whether the studied areas would benefit more from reducing NO_x emissions (NO_x-limited) or VOC emissions (VOC-limited). Formaldehyde (HCHO) and nitrogen dioxide (NO₂) were chosen as markers of VOCs and NO_x, respectively. The long-term monthly trends in the column NO₂, HCHO, and surface O₃, respectively, showed slopes of 3.59 × 10¹² ± 5.41 × 10¹² molecules cm^–2 month^–1, 6.47 × 10¹² ± 3.61 × 10¹² molecules cm^–2 month^–1, and 0.02 ± 0.02 ppb month^–1. Seasonal variations of O₃ showed similarities to those of NO₂. Strong north-to-south gradients of −2.52 × 10¹⁴ molecules cm^–2 degree^–1, −4.07 × 10¹⁴ molecules cm^–2 degree^–1, and 1.004 ppb degree^–1 were observed for NO₂, HCHO, and O₃, respectively. The seasonal variation in meteorological parameters was also examined to demonstrate effects on the ozone-forming sensitivity through a correlation study. Metropolitan cities (Dhaka, Chattogram, and Kolkata), big cities (Rajshahi, Khulna, and Barishal) except for Birbhum (16%), and oceanic sites were found to be the NO_x-limited regime (64–98%). Increasing NO_x sensitivity in all of the areas suggests that local NO_x emission mitigation policies will improve the air quality and reduce ambient O₃ in the GBM delta region.