Transformation of size distributions of emitted particles in streets

Dispersion and transformation of particulate matter in streets are studied by using a combination of a street pollution dispersion model, the Operational Street Pollution Model OSPM. and a particle transformation model. The particle model implements a very fast implicit method for solving the changes in particle size distribution due to coagulation, condensation and dilution processes. Using available measurements of emitted particle size distributions, impact analysis of the various transformation processes affecting the size distribution of particles was performed. For this purpose, a plume model simulating dilution of diesel exhaust has been developed and coupled with the particle coagulation model. The results show that due to the rapid dilution of the exhaust plume, the coagulation is not significant. Growth of the emitted particles due to condensation of water vapour appears also to be marginal, but this conclusion depends critically on the assumption of particle hygroscopicity. The freshly emitted diesel particles are believed to be less hygroscopic. Analysis of recent street measurements of particle size distributions in the range 0.2]20 mm reveals in general very poor correlation with the street traffic, but strong dependence on the relative humidity. This indicates that the particles measured are not freshly emitted and aged aerosols are dominant. Calculations with the street pollution dispersion model OSPM, using the diesel exhaust emissions as the only source, show that the mass concentrations of emitted particles in the street are predicted to be significantly lower than the measured concentrations.