Saharan dust flux and deposition rate near the Gulf of Guinea

It has been estimated that 240 ± 80 Tg of Saharan dust are transported annually from Africa over the Atlantic Ocean to far away places such as the Amazon Basin and the Caribbean during the summer months of June to August. There are, however, few direct measurements of the dust transport towards the Gulf of Guinea (5◦N) during the winter months of December–March. In this study, the Saharan dust flux and deposition to the Gulf of Guinea during the Harmattan (winter) season are estimated using the geographical area of Ghana (lying between latitudes 5◦ and 12◦N) as the reference location. The flux and deposition rates were determined during the Harmattan dust episodes of 2002 and 2005 by measuring surface dust concentrations concurrently at two locations in northern and central Ghana. The average particle number concentration in 2002 was 21 cm−3 while in 2005, it was 30 cm−3 (although a few daily mean values were as high as 60 cm−3). The corresponding mean mass concentrations were 543 μg m−3 and 1383 μg m−3 in 2002 and 2005, respectively. The deposition rates for the two winter seasons were estimated at 13 and 31 t km–2 yr–1, respectively, corresponding to a dust deposition thickness of 5 μm in 2002 and 12 μm in 2005. The transit time for the dust aerosol to travel a distance of 320 km between the two measurement sites in a north–south direction was determined experimentally to be of the order of 1 d, a result which could serve as an early warning indicator for severe dust outbreaks travelling from higher to lower latitudes in the region. The results of this study may also be compared with any future simulation of the African dust plume towards the Gulf of Guinea