The main properties of the wind flow in an urban area and especially in a street

canyon are well known. When the wind direction is perpendicular to the street

direction, a vortex is generated in the street canyon, whereby the wind flow at street

level is opposite to the flow above the roof level. Such a wind circulation results in a

characteristic dependence of pollution in the street on wind direction. Concentrations

on the windward side are much lower than those on the leeward side.

The simple and very important fastest chemical processes in the local scale involving

NO, NO2 and O3 are well known. The residence time of the air in the street canyon

can be estimated and from this the transformation to NO2 of the emitted NO from the

traffic in the street.

However, many slower processes are insufficiently understood to allow their current

use in model simulations, e.g. the formation of radicals and the transformation of

VOCs in a large urban area or a region.

The total description of local air quality by measurements requires many monitoring

stations. However, many suitable models exist for the calculation of air pollution

dispersion in local scale, and for fast chemical reactions, providing all relevant urban

background air quality data and local meteorological parameters are available. Consequently, it is

not necessary to measure at all highly

polluted streets. Only measurements at typical streets are necessary and the results

transferred to other streets by model estimates. These tools are very important for air

quality planning.


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