physical conditions. The physical conditions heavily affect the wind
speed and especially the wind direction inside the street (Britter
and Hanna, 2003; Oke, 1996). The classical example is the street
vortex flow that governs the pollutant distribution inside the street
canyon. When wind flows are close to perpendicular to the street
canyon a spiral or helical type flow develops within the canyon.
Within the vortex flow relatively clean air from rooftop height is
drawn down at the windward face of the street, across the road at
street level, in the reverse of the wind direction at rooftop, bringing
pollutants in the road to leeward face of the canyon (Britter and
Hanna, 2003). The concentration inside the urban street canyon
can be considered as the result of two contributions, one from
emissions from local traffic in the street itself and one from background
pollution entering the street canyon from above roof level.