Saharan dust flux and deposition rate near the Gulf of Guinea

Abstract

A B S T R A C T 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.

What do we know about effects of desert dust on air quality and human health in West Africa compared to other regions?

“…According to D'Almeida (1986), overall 60% of the total particles from the Sahara Desert are transported to the Gulf of Guinea. The number of particles, mass distribution, dust flows, the deposition rate and the mean size of the particles have been estimated in several countries, especially in Ghana (Afeti and Resch, 2000;Breuning-Madsen and Awadzi, 2005;Resch et al, 2007;Sunnu et al, 2008), Mali (McTainsh et al, 1997) and Nigeria (Anuforom, 2007). These studies show that the dust quantity (which varies from year to year) is greater in the northern parts of these countries and that the dust particles become finer in size as they move further south.…”

“…With a few rare exceptions (e.g. D 'Almeida, 1986;McTainsh, 1980), only recently have more authors begun to focus on the presence of desert dust, its physical characteristics and movement around the continent of Africa (Resch et al, 2007). Among other places, the dust is transported from the Sahara to the Gulf of Guinea by north-easterly trade winds in a south-westerly direction.…”

Desert dust impacts on human health: an alarming worldwide reality and a need for studies in West Africa

“…With a few rare exceptions (e.g. D'Almeida 1986; McTainsh 1980), only recently have more authors begun to focus on the presence of desert dust, its physical characteristics and movement around the continent of Africa (Resch et al 2007). Among other places, the dust is transported from the Sahara to the Gulf of Guinea by north-easterly trade winds in a southwesterly direction.…”

Multiyear Observations of the Tropical Atlantic Atmosphere: Multidisciplinary Applications of the NOAA Aerosols and Ocean Science Expeditions

“…A seasonal cycle is apparent in that the main dust plume axis translates northward (southward) during the boreal summer (winter) months, remaining to the north of the intertropical convergence zone (ITCZ). These trans-Atlantic dust outflows, in terms of westward extent and maximum intensity, also gradually decrease from a maximum during Northern Hemisphere summer (June-July) to a minimum during the autumn (October-November) before increasing again in the spring, whereas dust outflows into the Gulf of Guinea become prevalent during the months of DecemberMarch (as noted by Resch et al 2008). African smoke outflows associated with sub-Saharan biomass burning gradually become apparent south of the ITCZ in the boreal spring before achieving a maximum in late summer (July-September), then gradually decrease again, minimizing in winter.…”

“…African smoke outflows associated with sub-Saharan biomass burning gradually become apparent south of the ITCZ in the boreal spring before achieving a maximum in late summer (July-September), then gradually decrease again, minimizing in winter. Biomass burning in Africa also occurs north of the ITCZ during boreal winter months, resulting in some contribution to the Atlantic and Gulf of Guinea plumes during December-March (e.g., Resch et al 2008), and burning over the Yucatan Peninsula becomes prominent during April-May. Finally, there is also an increase in aerosol outflows off the east coast of the United States during spring to early autumn months (March-September), presumably the result of increased anthropogenic smog from the urban corridor.…”