When less burning does not mean cleaner air: Decoupling of burned area, fire-related PM2.5 emissions, and population exposure in West Africa

Fire activity plays a central role in West African ecosystems. However, substantial uncertainties remain in how changes in burned area (BA) are reflected in fire-related particulate emission estimates and, through their influence on air quality, in population exposure. This study provides a two-decade regional assessment of trends in BA, fire-related PM2.5 emissions, climate variables, and population exposure across West Africa from 2002 to 2023. BA declined significantly across most Sudanian and Sahelian savannas, with several countries experiencing reductions exceeding 50 %. In contrast, fire-related PM2.5 emissions showed weaker or non-significant declines and increased in parts of the coastal and forest–savanna mosaic, where agricultural and peri‑urban open burning is common. These results indicate that BA alone is no longer a reliable proxy for open biomass-burning emissions in West Africa. A decoupling index was developed to identify locations where BA and fire-related PM2.5 emissions evolve independently. Approximately 7 % of the domain falls into strict or lenient decoupling categories, encompassing an estimated 551 000 residents. Decoupling is concentrated in agricultural mosaics and peri‑urban landscapes, consistent with shifts in fire use, land-use practices, and fuel types associated with higher emissions per unit area burned. At the annual scale, fire-related PM2.5 emissions are positively associated with temperature in many countries and negatively associated with precipitation in semi-arid regions, suggesting that climate variability modulates emission patterns even where BA declines. Population exposure analyses reveal contrasting trends: the population-weighted PM2.5 indicator decreases over time (Theil-Sen slope of -0.34 kt person-1 yr-1), while total exposure increases in most countries due to rapid population growth, often occurring alongside urban expansion and changing emission environments. Despite declining fire extent, total exposure increases significantly in 11 countries, highlighting emerging spatial inequalities in fire-related air pollution. Overall, the findings show that reductions in BA do not necessarily translate into proportional reductions in fire-related emissions or population exposure. Fire management and air-quality strategies in West Africa should therefore move beyond BA metrics alone and explicitly consider emission intensity and demographic dynamics.