At local sites, e.g. in streets, the NO2 pollution is governed partly by the local

emission of NO and NO2. It is important again to emphasis the non-linearity of the

governing chemistry for NO2 . During most periods NO2 levels near urban streets are

limited by the ozone levels and not by the NOx releases in the street. This means that

the very local scale phenomenon of high NO2 concentrations in a street is coupled to

large scale phenomena, since high ozone concentrations are caused by long range

transport. However, in streets, short term peak concentrations may occur and here the

short term limit value for protection of health is the most critical.


In order to construct a model tool for determining the effects of different control

strategies, local scale dispersion and chemistry of nitrogen compounds and ozone, as

well as long range transport of ozone (and to some extend NOx ) must be taken into

account. The only practical way to construct such a tool is to combine a transportchemistry

model for long range transport to a local scale model for a single street.

However, the long range transport model also has to be able to account for the

dispersion and chemistry on an urban scale of a few kilometres.


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