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Carbon monoxide, CO, and fine atmospheric particulate matter, PM2.5, are analyzed over the Guinean Gulf coastal region using the WRF-CHIMERE modeling system and observations during the beginning of the monsoon 2006 (from May to July), corresponding to the Africa Multidisciplinary Monsoon Analysis (AMMA) campaign period.

Along the Guinean Gulf coast, the contribution of longrange pollution transport to CO or PM2.5 concentrations is important. The contribution of desert dust PM2.5 concentration decreases from similar to 38% in May to similar to 5% in July. The contribution of biomass burning PM2 : 5 concentration from Central Africa increases from similar to 10% in May to similar to 52% in July. The anthropogenic contribution is similar to 30% for CO and similar to 10% for PM2.5 during the whole period.

When focusing only on anthropogenic pollution, frequent northward transport events from the coast to the Sahel are associated with periods of low wind and no precipitation. In June, anthropogenic PM2.5 and CO concentrations are higher than in May or July over the Guinean coastal region. Air mass dynamics concentrate pollutants emitted in the Sahel due to a meridional atmospheric cell. Moreover, a part of the pollution emitted remotely at the coast is transported and accumulated over the Sahel.

Focusing the analysis on the period 8-15 June, anthropogenic pollutants emitted along the coastline are exported toward the north especially at the beginning of the night (18: 00 to 00: 00 UTC) with the establishment of the nocturnal low level jet. Plumes originating from different cities are mixed for some hours at the coast, leading to high pollution concentration, because of specific disturbed meteorological conditions.