There is growing evidence that fine airborne particulates could play the most important role in determining health effects. The aim of this work was to investigate the number concentration and size distributions of particulates in the exhausts ofdiesel vehicles (mainly buses) of different ages and make, operating under different loads. Particle-sizing instruments used were the Scanning Mobility Particle Sizer (SMPS) and Aerodynamic Particle Sizer (APS). The average particle number concentrationof the exhausts was in the range (0.7-3.9) ×10{sup}7 cm{sup}-3 in the SMPS range (0.0075-0.304μm) and (0.3-32) × 10{sup}3 cm{sup}-3 in the APS range (0.5-30μm). In most cases, particle number concentrations increased with the increased power outputfrom the engine and, in both SMPS and APS ranges, varied significantly within each group of vehicles, but the differences between the groups were small. For individual vehicles, there was no relation between emissions in the smaller and larger particleranges. Emission characteristics did not appear to be correlated with engine model or age. The implications of these findings to particle emission testing and control as well as to exposure and risk analysis are discussed.
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