Daily Levels of the Harmattan Dust near the Gulf of Guinea over 15 Years: 1996-2011

The Saharan dust which is transported over many countries in West Africa near the Gulf of Guinea (5°N) during the northern winter, known as the Harmattan period, is presented. The Harmattan dust phenomenon has influence on the climate of the region. The dust has been studied over a 15-year period, between 1996 and 2011, using a location at Kumasi in central Ghana (6° 40′N, 1° 34′W). The suspended Saharan dust particles have been sampled in situ by an optical particle counter, and the particle size and concentrations within the particle size range, 0.5–25 μm were analysed. The highest daily average particle diameter, number and mass concentrations during January-February reached 3.17 μm in Harmattan 2005, 148 particles/cm 3 in Harmattan 1997 and 6199 μg/m 3 in Harmattan 2005 respectively. It was found that the daily mean size has increased from D=1.01 µm in the period, 1996-2000, to D=1.76 µm in 2001-2011. The mass concentration has also increased from M=370 µg/m 3 over the period, 1996-2000 to M=1262 µg/m 3 in the period, 2001-2011. The increased particle size and corresponding concentrations are likely due to increased stronger winds. The diurnal characteristics of the peak Harmattan which is a period of 4-13 consecutive days of highest concentration within January – February, were also anlaysed over the 15 years. This allows the study of the trends of the physical characteristics of the suspended dust.