Climate and, in particular, ventilation (air flow and mixing height) are as important in
determining atmospheric concentrations as are emission rates. Thus the characteristics
of the structure of the wind fields and the dynamic evolution of the boundary layer are
key elements in the understanding of urban atmospheric pollution.
In urban areas the ventilation processes cannot be described because complex terrain
and thermal disturbance modify them. For example it can result in the formation of
heat islands, which often form around urban areas, and can hinder an efficient
exchange of air with the surroundings. In such conditions, air pollutants are recirculated
in the boundary layer and concentrations can become high. In the recirculated
air masses, photochemical reactions may occur and worsen the situation.
This is especially the case in the Mediterranean area, where land-sea breezes causing
recirculation , are both common and strong.
The dispersion capacity of the atmosphere has been described from measurements of
ambient short-lived radioactivity levels associated with the decay of radon (Allegrini
et al., 1994). The use of this technique in several urban areas has shown that the
occurrence of high concentrations of nitrogen dioxide may be associated with radical
reactions, taking place in the lower boundary layer during stable atmospheric
conditions. When the atmosphere is unstable, most NO2 is formed through the
titration of NO with O3. In that case, the maximum expected concentration of NO2 is
given by the O3 concentration in the free troposphere, the layer above the boundary
layer (Febo et al., 1996).
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castellano: DISPER CUSTIC DESCAR RADIA italiano:
deutsch: DIS CUS DES RAD
castellano: DIS CUS DES RAD english: DIS CUS DES RAD
português: DIS CUS DES RAD italiano: DIS CUS DES RAD
français: DIS CUS DES RAD