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exhaust velocities of the tailpipe emissions and the traffic wake-induced turbulence.
Primary turbulence is meaningful only in stagnant conditions (especially in the case of
cold-running) otherwise the atmospheric turbulence dominates the process. The plume
is transported with the wind speed at the emission height. A well-mixed zone can
occur behind a vehicle due to the thermal and mechanical turbulence.
To sum up the effects of initial and atmospheric turbulence a coordinate system
transformation has to be carried out [2] . A horizontal component of the initial
diffusion parameters is directed to the road axis, and a horizontal component of
atmospheric dispersion is directed to the wind flow.
Description of CALINE4 Model
CALINE4 is a Gaussian type dispersion model for predicting air pollution concen-
trations near roadways [3] . Each roadway link analyzed in the model is treated as a
sequence of short segments (Fig. 5 ). Each segment is treated as a separate emission
source, which produce a plume of pollutants dispersing downwind. Pollutant con-
centrations at any specific location are calculated using the total contribution from
overlapping pollution plumes originating from the sequence of roadway segments.
CALINE4 use the mixing zone concept (Fig. 6 ). The model treats the region
directly over the highway as a zone of uniform emissions and turbulence. This is
designated as the mixing zone, and is defined as the region over the traveled way
(traffic lanes; not including shoulders) plus 3 m on either side.
Fig. 5 Gaussian plume and roadway segments
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