Physical characteristics of Saharan dust near the Gulf of Guinea

The particle size, number and mass concentrations of Saharan dust during three successive Harmattan seasons in 1996, 1997 and 1998 in southern Ghana (6 degrees 40'N, 1 degrees 34'W) are reported. The transport distance for the dust was approximately 3000 km from the edge of the source region in the Sahara Desert and 150 km north of the Gulf of Guinea (5 degrees N). The fluctuations in the physical characteristics of dust, in consonance with the seasonal migration of the Inter-tropical Discontinuity, are quantified. The peak particle number concentration recorded in 1997 (150 cm(-3)) is approximately two orders of magnitude greater than the minimum background aerosol concentration (1 cm(-3)). Average mass concentrations are calculated and evidence is provided that even at low latitudes and at great distances from the dust production zones, the Saharan dust influence on the particulate mass content of the atmosphere over West Africa is important. Aerosol mean diameters were 0.96, 1.16 and 1.02 mu m for the 1996, 1997 and 1998 Harmattan seasons, respectively, with average mass concentrations reaching a peak of 134 mu g m(-3). In all cases, the mean aerosol diameter is observed to be smaller during the Harmattan period than during the rest of the year, an observation which is consistent with the transport of great quantities of small particles during the Harmattan season. It is shown that within the 1-5 mu m diameter range, the Harmattan particle size distribution remains essentially stable regardless of the change in seasonal dust concentrations. (C) 2000 Elsevier Science Ltd. All rights reserved.