Night time thermal profiles on the urban heat island and in the surrounding rural area
Circulations induced by the island of heat can interact in complex ways with other local circulations (sea-land breezes, valley breezes).
The effects of the night heat island on air quality may be significant, but it is not known a priori if they are positive or negative. The presence of a turbulent nocturnal layer, dominated by a thermal inversion at high altitude, dilutes the local effect of polluting emissions distributing them throughout the urban area, but can thus favor the chemical reactions that give rise to the formation of secondary particulate.
In the lower diagram, the circulations induced by the heat island.
Circulations induced by the heat island can recall cleaner air from the countryside, but they can also converge the polluted air of industrial areas or peripheral road arteries towards the center.
Source: Urban microclimate: impact of urbanization on local climatic conditions and mitigation factors
By Giovanni Bonafè