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

15

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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