The impact of biomass burning on the environmental aerosol concentration in Gaborone, Botswana

Biomass burning, in the form of savanna fires and firewood for cooking and warmth, is widespread during the dry winter months in Southern Africa. This study was carried out to investigate its impact on the environment in Gabo rone, Botswana, which is a small-sized city with very little pollution from industrial sources. Measurements of aerosol size and number concentrations were carried out at the University of Botswana campus in Gaborone from Sep tember 1999 to July 2000 using two automatic laser scattering particle coun ters. Particles were monitored in eight size ranges from 0.1 to 5.0 mum The mean daily particle concentrations were found to vary from about 200 cm(-3 ) on clear visibility days during the summer to a high of over 9000 cm(-3) on cold winter evenings, when there was a significant smoke haze over the c ity. Particle concentrations were noticeably higher during the winter than in the summer. During a typical winter day, the total particle concentratio n peaked between 18 and 23 h, often showing an increase of over four-fold f rom mid-morning minimum values. The aerosol number size distributions under various conditions were investigated and the corresponding surface area an d volume distributions were derived. In general, both the surface and volum e distributions were bimodal with peaks close to 0.2 mum and at 5.0 mum or greater. A hand-held counter with a minimum detectable particle size of 0.3 mum was used to monitor the size and number concentrations of aerosols acr oss the city. The results indicate a consistent pattern of maximum concentr ation in the highly populated areas close to the city centre, falling signi ficantly in the sparsely populated outlying areas by up to an order of magn itude during peak biomass burning, suggesting that much of the smoke partic les in the city are removed by wind. (C) 2001 Published by Elsevier Science Ltd.