Stations used for
direct assessment of urban air quality
The number and types of
stations to be established in the urban area is part of the network design
process (see topic: How to design Urban monitoring networks, and what methods to use ?).
It has then also been
decided in which parts of the urban area the stations will be located. This
part of the location process can be termed "macro-locating", a term
also referred to in the Directives, with some guidance.
The actual detailed
locating of the station within the given area is then termed
"micro-locating".
Important
considerations re. micro-locating:
·
The location must be
representative of a certain area around it, so its measurements won't be
representing only the very small area where it is located. The EoI Guidance report as well as the Directives and the
EUROAIRNET criteria report give guidance of the minimum requirements on area
of representativeness.
·
The location of the
air intake (or probe) relative to nearly dominating sources. See EoI and the Directives for guidance and
·
The intake (or probe)
for air to be sampled by the instruments must be placed according to given
criteria, so that the sampled air is not influenced unduly by very local
effects (such as distance to walls, vegetation, etc.)
Problems with
micro-locating typically arises regarding availability of space for a station cabin
at the desired location, and availability of electric power, phone lines (not
so important now as mobile phones can be used), security and easy access. Such
problems may result in compromises between the ideal and possible locations to
be chosen.
An important aspect of
the representativeness of a station location, in addition to its representativeness
area, is its representativeness for the exposure
situations it is supposed to represent: How does the station represent similar
areas of exposure in the city? A good assessment of the representativeness
area would require special monitoring and modelling studies. The need for such
studies must be evaluated in each case.
Stations used for
evaluation of dispersion models
In addition to the
above criteria and problems, the locating of stations for model testing has to
fulfil the following criteria:
Its location must be
representative of an area which corresponds to the spatial resolution of the
dispersion model. For instance.
·
For a grid model which gives a calculated value
which represents the average of e.g. a 1 km2 area, the station must
be located such that it also represents the average concentration of the km2
area around it, which corresponds with the actual location of the grid used in
the model. The model also represents the concentration in a certain height, or
rather, the average value (height wise) of a layer of air of a given height
(e.g. the lowest 30 m of the atmosphere). The combination of the location and
height of the air inlet for the instruments must also be chosen so as to
represent a similar average value, height wise.
·
For a subgrid model,
e.g. a line source model which calculates the concentrations near
streets/roads, the station location must reflect the situation which is
represented by the model. Typically, such a model gives the concentrations for
an idealised type of street/building configuration (e.g. continuous building
facades of similar height, both sides, although Computational Fluid Dynamics
(CFD) models exist which can calculate concentrations for more complex
configurations) along the street section, away from influence from complicating
effects near intersections. Typically, such stations should be located away
from intersections in street sections with fairly homogeneous building
topography. |