Comparison of Size Distribution of Ultra fine Particles in Roadside Atmosphere and Diesel Exhaust

The size distributions of ultra fine particles in diesel exhaust were measured in a dilution tunnel at a high dilution ratio, and the results were compared with roadside measurements. The size distribution in diesel exhaust showed a single mode with a peak around 50 nm; in roadside atmosphere there was a bimodal distribution with peaks around 20 and 100 nm. Coagulation of diesel particles in the atmosphere was examined with an exhaust gas dispersion chamber. Coagulation of particles in the atmosphere and transient driving conditions of vehicle may cause the difference in size distribution between bench test results and roadside measurements.