MINERAL AEROSOLS DISTRIBUTION AND ITS RADIATIVE EFFECTS OVER AFRICA USING GROUND – BASED AND SATELLITE OBSERVATIONS

Abstract

Monthly, seasonal and yearly variations of mineral aerosols were analysed at twenty nine (29) African stations, with emphasis on how these variations affect the aerosol direct radiative balance in the four regions of Africa (West, North, East and South). This was achieved by using eight (8) years in situ Aerosol Robotic Network (AERONET) Aerosol optical depth (AOD) observations and the Moderate resolution infrared spectro – radiometer (MODIS) over land (TERRA) and ocean (AQUA) at 10 X 10 km spatial resolution. This thesis aims to characterize mineral dust in Africa using optical and radiative properties of AOD such as, Angstrom exponent, single scattering albedo and radiative forcings. Means, standard deviations, variance and the HYSPLIT (Hybrid Single Particle Langrangian integrated trajectory) 4.8 model back – trajectories were used to examine the transport of aerosols to the study area. Results show that mean and standard deviation of aerosol optical depth for the eigth (8) years period from AERONET data at the 440 nm spectral channel were; 0.562±0.2 in West Africa, 0.255±0.132 in North Africa, 0.224±0.074 in East Africa and 0.319±0.219 in South Africa respectively. The variability in AOD were estimated at 36%, 32%, 33% and 69% respectively. The Ångstrӧm Exponent α870-440 show the dominant aerosols type in West Africa is a mixture of coarse dust particles and biomass burning (0.5 < α < 1.0). Southern African sites are biomass burning aerosols (α > 1.0) while East and North Africa are mixed. Five (5) day HYSPLIT 4.8 model back – trajectories at 500, 1000 and 1500 meters AGL (Above Ground Level), traces the aerosols in Ilorin (West Africa) during the intensive dust outbreaks to the dust source region of the Bodele depression in Chad Republic. Some near surface (500m AGL) contribution to the dust arriving at this site occur from Northern Sudan and Southern Libya. Blida (Northern Africa) is fossil fuel burning from Spain and Marine sea-spray from the North Atlantic. Mongu (South Africa) and Nairobi (East Africa) at altitudes of 1000m and 1500m AGL, aerosols are of biomass burning with marine sea spray aerosols at 500m. Mean ADRF at TOA in West Africa during all aerosols events was (−20.1±17.55) Wm−2, while at the surface (−77.53±42.35) Wm−2, thus, producing an atmospheric mean ADRF (ADRF Atmosphere) value of (+57±25)Wm−2. In Southern Africa, the highest mean ADRF at surface was (-66.24±39.65) W m -2 and TOA was -16.66 ± 8.42 W m -2. The aerosol radiative forcing efficiency (ARFE) was -131.4 and -30.37 Wm-2 per unit optical depth at surface and TOA. The highest ADRF values at East Africa occurred during the JJA peak biomass burning preparatory to the agricultural season with a magnitude of -47.67±19.0 Wm-2 at the surface and -16.89±7.76 Wm-2 at TOA. The forcing efficiencies were -146.52 at the surface and -52.29 Wm-2 per unit optical depth at the TOA. These results suggest cooling of the surface and absorption of solar radiation in the atmosphere, leading to significant atmospheric warming in all the regions considered.