Dispersion and transformation of particulate matter in streets are studied
by using a combination of a street pollution dispersion model, the Operatio
nal Street Pollution Model (OSPM) and a particle transformation model. The
particle model implements a very fast implicit method for solving the chang
es in particle size distribution due to coagulation, condensation and dilut
ion processes. Using available measurements of emitted particle size distri
butions, 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 rapi
d 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 mu m reveals in general very poor c
orrelation with the street traffic, but strong dependence on the relative h
umidity. This indicates that the particles measured are not freshly emitted
and aged aerosols are dominant. Calculations with the street pollution dis
persion model OSPM, using the diesel exhaust emissions as the only source,
show that the mass concentrations of emitted particles in the street are pr
edicted to be significantly lower than the measured concentrations. (C) 199
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