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 NO 2, HCHO, and surface O 3 of nine different locations were studied over the GBM (Ganges–Brahmaputra–Meghna) delta. Column-based HCHO/NO 2 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 (NO 2 ) were chosen as markers of VOCs and NO x, respectively. The long-term monthly trends in the column NO 2, HCHO, and surface O 3, respectively, showed slopes of 3.59 × 10 12 ± 5.41 × 10 12 molecules cm –2 month –1, 6.47 × 10 12 ± 3.61 × 10 12 molecules cm –2 month –1, and 0.02 ± 0.02 ppb month –1 . Seasonal variations of O 3 showed similarities to those of NO 2 . Strong north-to-south gradients of −2.52 × 10 14 molecules cm –2 degree –1, −4.07 × 10 14 molecules cm –2 degree –1, and 1.004 ppb degree –1 were observed for NO 2, HCHO, and O 3, 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 O 3 in the GBM delta region.