During the design of monitoring networks account needs to be taken of both the
number of sites and their location. It must reflect the need to measure exposure of
humans and the environment. All site groups in Table 3.1 should be considered for
inclusion in the network.
Since automatic monitoring is very expensive, it will sometimes be preferable to use
automatic monitoring in combination with indicative monitoring (passive sampling)
and/or air quality modelling to more cost effective assess the extent of high
concentration areas and the spatial variability of pollutants.
Since the limit values for exposure of humans will be for 1 hour and annual
concentrations, the measurements and other assessment tools should be applied for
these averaging times. The highest peak values of NO2 occur in street canyons, where
traffic is the main source and it is most likely that the short term value will be
exceeded here. The residence time is generally short in street canyons, but many
people may be present, when the concentration is high. The long term exposure may
be more critical at residential or suburban areas.
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