Abstract
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/cm3 in Harmattan 1997 and 6199 μg/m3 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/m3 over the period, 1996-2000 to M=1262 µg/m3 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.
Highlights
Atmospheric dust pollution is a common environmental problem in most parts of West Africa
The average daily particle number concentration of the atmospheric aerosol sampled during the Harmattan period (January –February) and selected background dust sampled during the non Harmattan period (April – October) are plotted against the days in the 15 years (Figure 2a)
The daily variations of the average size, number and mass concentrations of the Harmattan dust in January-February and the peak Harmattan dust have been determined over fifteen years
Summary
Atmospheric dust pollution is a common environmental problem in most parts of West Africa. In addition to the normal condition, the atmospheric environment in most parts of West Africa is characterized by abnormally high suspended and deposited natural dust levels everywhere during the northern winter season This occurrence is a result of the dusty northeast winds known as the Harmattan, which blows across the Sahara Desert through West Africa with the windblown dust particles entrained from the dust sources in the Sahara-Sahel regions. The Harmattan dust production and transport has been studied in various study areas such as the dust sources, soil texture and strengths, period of transport, wind patterns, climate and large scale weather features including the ITCZ, the nature of the deflation, transport, and deposition of the aeolian dust material, the dust effect on the environment, air quality, climate visibility, etc. In this paper, the daily dust particle concentration monitored over 15 years (1996-2011) is used to study the trend of the Harmattan dust phenomenon in Kumasi near the Gulf of Guinea
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