The rural background can be difficult to estimate, depending on the climatic

conditions. In northern Europe the dispersion of pollutants is mainly determined by

the air flow due to the prevailing wind and surface roughness. In the southern Europe

the dispersion is highly influenced by thermal convection and local wind flow

systems. On the large scale, especially in the southern part of Europe, the dispersion is

more complicated, due to complex flow structure. The long residence time may also

lead to more complicated chemical processes, which are not easy to describe (see

chapter 1.3). In these cases the models for dispersion and chemical reactions need

further development and present knowledge is mostly based on measurements.

The urban background can be measured by few measurement stations located away

from local sources, e.g. above the roof tops, in traffic free squares, in parks etc. The

stations should represent the air pollution level, without significant impact from a

single source (e.g. a local point source or a street) The representativity of such stations

for the whole urban area must be documented, for example by using passive samplers

to determine optimal locations. The optimal location is defined as the location that

represents the average concentration of a certain area.

The local air pollution depends very much on the local sources, e.g. the traffic in the

street or point sources and the local dispersion conditions, and micro-scale influence

on the aerodynamics of the site e.g. by the surrounding buildings.


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