148
Air Pollution and Turbulence: Modeling and Applications
a pollutant retrieved by a receptor is calculated by summing the contributions Δc of
all the puffs emitted by all the sources present.
A crucial difference between expressions (5.16) and (5.25) should be noted: in the
latter a different diffusion term replaces the advection term, with the resulting disappearance of the mean wind velocity u from the expression. This means that in a puff
model the wind velocity infl uences the calculation of the concentration only in the
density of puffs in the region of diffusion (the lower the wind velocity, the closer the
puffs emitted by a source). For this reason, puff models, unlike the Gaussian model
described in Equation 5.16 (the concentration tends to the infi nity for u tending
toward zero), can be used to simulate diffusion in calm wind conditions.
Puff models are also suitable for the simulation of diffusion over complex
orographies: in this case a three-dimensional wind fi eld must be reconstructed from
orographic characteristics and available anemometric measurements.
5.6 SPECIAL ALGORITHMS UTILIZED IN GAUSSIAN
AND ANALYTIC MODELS
Gaussian and analytic models generally make use of algorithms based on empirical
considerations to describe complex real situations. The various models presented in
the literature are differentiated according to the algorithms adopted. Listed below
are some phenomena relative to such situations.
5.6.1 PLUME RISE
Once emitted into the atmosphere, plume tends to continue to rise due to is initial
momentum. If hot, the rise is also caused by Archimede’s force. In fact, warm fumes
are less dense and therefore lighter than the surrounding air. Different formulae are
presented in the literature to describe this phenomenon. The most commonly used
are those developed by Briggs (Briggs, 1975).
5.6.2 DOWNWASH
This phenomenon arises from the perturbation of the wide fi eld and turbulence
caused by structures present in the area of pollutant emission. There are two types of
downwash: stack downwash and building downwash. Stack downwash refers to the
lowering of the plume due to the perturbation of the wind fi eld caused by the source
itself. It is evaluated through an artifi cial decrease in the emission source height.
Building downwash is a distortion of the plume caused by buildings situated in
proximity of the source. The distortion is also evaluated through an empirical modifi cation of the “sigmas.”
5.6.3 PLUME TRAPPING
It consists of the entrapment of pollutants in a layer of the atmosphere close to the
ground due to a thermal inversion at a level higher than that of fume emission.
© 2010 by Taylor and Francis Group, LLC
Air Pollution and Turbulence: Modeling and Applications
a pollutant retrieved by a receptor is calculated by summing the contributions Δc of
all the puffs emitted by all the sources present.
A crucial difference between expressions (5.16) and (5.25) should be noted: in the
latter a different diffusion term replaces the advection term, with the resulting disappearance of the mean wind velocity u from the expression. This means that in a puff
model the wind velocity infl uences the calculation of the concentration only in the
density of puffs in the region of diffusion (the lower the wind velocity, the closer the
puffs emitted by a source). For this reason, puff models, unlike the Gaussian model
described in Equation 5.16 (the concentration tends to the infi nity for u tending
toward zero), can be used to simulate diffusion in calm wind conditions.
Puff models are also suitable for the simulation of diffusion over complex
orographies: in this case a three-dimensional wind fi eld must be reconstructed from
orographic characteristics and available anemometric measurements.
5.6 SPECIAL ALGORITHMS UTILIZED IN GAUSSIAN
AND ANALYTIC MODELS
Gaussian and analytic models generally make use of algorithms based on empirical
considerations to describe complex real situations. The various models presented in
the literature are differentiated according to the algorithms adopted. Listed below
are some phenomena relative to such situations.
5.6.1 PLUME RISE
Once emitted into the atmosphere, plume tends to continue to rise due to is initial
momentum. If hot, the rise is also caused by Archimede’s force. In fact, warm fumes
are less dense and therefore lighter than the surrounding air. Different formulae are
presented in the literature to describe this phenomenon. The most commonly used
are those developed by Briggs (Briggs, 1975).
5.6.2 DOWNWASH
This phenomenon arises from the perturbation of the wide fi eld and turbulence
caused by structures present in the area of pollutant emission. There are two types of
downwash: stack downwash and building downwash. Stack downwash refers to the
lowering of the plume due to the perturbation of the wind fi eld caused by the source
itself. It is evaluated through an artifi cial decrease in the emission source height.
Building downwash is a distortion of the plume caused by buildings situated in
proximity of the source. The distortion is also evaluated through an empirical modifi cation of the “sigmas.”
5.6.3 PLUME TRAPPING
It consists of the entrapment of pollutants in a layer of the atmosphere close to the
ground due to a thermal inversion at a level higher than that of fume emission.
© 2010 by Taylor and Francis Group, LLC