3.1.1 Objectives
for Siting
Representativeness
has been defined as "the extent to which a set of measurements taken in
a space-time domain reflects the actual conditions in the same or different
space-time domain taken on a scale appropriate for a specific
application" [10]. The space-time and application aspects of the
definition as relates to site selection are discussed in the following.
In
general, for use in air quality modeling applications, meteorological data
should be representative of conditions affecting the transport and
dispersion of pollutants in the “area of interest” as determined by the
locations of the sources and receptors being modeled. In many instances,
e.g. in complex terrain, multiple monitoring sites may be required to
adequately represent spatial variations in meteorological conditions
affecting transport and/or dispersion.
In
steady-state modeling applications, one typically focuses on the
meteorological conditions at the release height of the source or sources, or
the plume height in the case of buoyant sources. Representativeness for
steady-state modeling applications must necessarily be assessed in concert
with the steady-state assumption that meteorological conditions are constant
within the space-time domain of the application; as typically applied,
measurements for a single location, somewhere near the source, are assumed
to apply, without change, at all points in the modeling domain. Consistency
would call for site selection criteria consistent with the steady-state
assumption; i.e., to the extent possible, sites should perhaps be selected
such that factors which cause spatial variations in meteorological
conditions, are invariant over the spatial domain of the application,
whatever that might be. Such factors would include surface characteristics
such as ground cover, surface roughness, the presence or absence of water
bodies, etc. Similarly, the representativeness of existing third-party data
bases should be judged, in part, by comparing the surface characteristics in
the vicinity of the meteorological monitoring site with the surface
characteristics that generally describe the analysis domain.
Representativeness
has an entirely different interpretation for non-steady-state modeling applications which commonly employ three
dimensional gridded meteorological fields based on measurements at multiple
sites. The meteorological processors which support these applications are
designed to appropriately blend available NWS data, local site-specific
data, and prognostic mesoscale data; empirical relationships are then used
to diagnostically adjust the wind fields for mesoscale and local-scale
effects [11], [12] . These diagnostic adjustments can be
improved through the use of strategically placed site-specific
meteorological observations. Support for such applications is provided to
the extent that this guidance can be used for selecting sites to monitor the
significant meteorological regimes that may need to be represented in these
applications. Site selection for such applications (often more than one
location is needed) falls in the category of network design and is beyond
the scope of this document. Model user’s guides should be consulted for
meteorological data requirements and guidance on network design for these
applications.
3. SITING AND EXPOSURE
3.1 Representativeness
3.1.1 Objectives for Siting
3.1.2 Factors to Consider
3.2 Simple Terrain Locations
3.2.1 Speed and Wind Direction
3.2.2 Temperature, Temperature
Difference, and Humidity
3.2.3 Precipitation
3.2.4 Pressure
3.2.5 Radiation
3.3 Complex Terrain Locations
3.3.1 Wind Speed
3.3.2 Wind Direction
3.3.3 Temperature Difference
3.4 Coastal Locations
3.5 Urban Locations
3.6 Recommendations