Interactions of atmospheric gases and aerosols with the monsoon dynamics over the Sudano-Guinean region during AMMA

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 long-range pollution transport to CO or PM_2.5 concentrations is important. The contribution of desert dust PM_2.5 concentration decreases from ~38% in May to ~5 % in July. The contribution of biomass burning PM_2.5 concentration from Central Africa increases from ~10 % in May to ~52 % in July. The anthropogenic contribution is ~30 % for CO and ~10 % for PM_2.5 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 PM_2.5 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.