Diurnal and intra-urban particle concentrations in relation to windspeed and stability during the dry season in three African cities.

The spatial and temporal variations of PM(2.5), PM(10) and TSP in three African cities of different sizes (Dar es Salaam, Ouagadougou and Gaborone) were investigated using portable particle counters. Three different areas (downtown, green residential and traditional residential) and a reference site were designated in each of the cities in order to detect intra-urban and temporal variability. Morning, noon and night measurements were conducted in the urban areas while observations at reference stations were made continuously over the field periods. A clear diurnal pattern in particle concentrations was found in inland Gaborone and Ouagadougou, with morning and night peaks where the latter was the highest. However, in coastal Dar es Salaam the night peak was almost absent due to delayed stabilisation of the air. Particle concentrations at the Ouagadougou reference station were extreme. The direct contribution of vehicle emissions are of secondary importance since the PM(2.5)/PM(10) ratios are low (0.1-0.3). Much of the particles are supposed to be soil particles that are entrained in the air by daytime high windspeeds followed by nighttime subsidence as the air is stabilised and windspeed decreases. However, in all three cities, resuspension are important as areas with a network of unpaved roads showed the highest concentrations of suspended particles. Generally, the central business district had the lowest concentrations of particulate matter.