Measurement of aerosol concentrations in Gaborone

Environmental aerosol particles in Gaborone originate mainly from desert dust, bushfires, biomass burning and vehicle emissions. Measurements of aerosol size and number concentrations were carried out at the University of Botswana campus in Gaborone from September 1999 to April 2000 using two automatic laser scattering particle counters. Particles were monitored in eight size ranges from 0.1 urn to 5.0 um. The mean daily particle concentrations were found to vary from about 200 per cm3 on clear visibility days to 1400 cm3 on hazy, low visibility days. Particle concentrations were noticeably higher during peak vehicle traffic hours. Measurements during one busy period showed a six to sevenfold increase in the total particle concentration. Although local conditions such as wind speed played a significant role, the results suggest that the overall background natural aerosol concentration is determined principally by more global factors. A hand-held counter with a minimum detectable particle size of 0.3 urn was used to monitor the size and number concentrations at five typical locations around the city. The results indicate considerable variation ranging from low to high near the Gaborone dam and busy traffic circles respectively.