SEASONAL POLLUTION PATTERNS IN WEST AFRICA: A REVIEW OF ATMOSPHERIC CHEMICAL PROCESSES DURING THE MONSOON AND HARMATTAN

The West African atmosphere is a dynamic chemical reactor, governed by the dramatic seasonal alternation between the dry, dust-laden Harmattan winds from the Sahara and the humid, rain-bearing West African Monsoon. These regimes create profoundly different environments for the emission, transformation, and removal of pollutants, yet a synthesized understanding of the resulting seasonal chemistry remains scattered across discipline-specific studies. This narrative review consolidates current knowledge to elucidate the distinct atmospheric chemical processes that define pollution patterns in West Africa. During the Harmattan season, the atmosphere is dominated by long-range transport of mineral dust and biomass burning aerosols, which interact with anthropogenic pollutants under intense solar radiation, driving distinctive photochemical pathways and leading to severe particulate matter (PM) episodes. In contrast, the monsoon season introduces high humidity, frequent precipitation, and active biosphere emissions. This shifts the chemical regime towards aqueous-phase processes, efficient wet deposition, and complex oxidation chemistry fueled by biogenic volatile organic compounds (BVOCs), often resulting in lower aerosol burdens but a more reactive gaseous environment. We critically examine the interactions and feedbacks between meteorology and chemistry, highlight the unique challenges of observation and modeling in this under-instrumented region, and identify key gaps in our understanding of transition periods and integrated climate-air quality impacts. This synthesis underscores the necessity of considering West African pollution through a lens of seasonal duality, providing a crucial foundation for improved regional air quality management, health impact assessments, and climate modeling.