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Overview
of the selection of AQM tool / system
The West Midlands Joint Pollution Working
Group became involved in modelling for
review and assessment during a First Phase 1) task studying ADMS Urban and INDIC Airviro. INDIC Airviro, which is
licensed by the Swedish Meteorological Office
(SMHI), was seen to meet the needs of the
conurbation better for the subsequent
work.
The use of INDIC Airviro
Within the West Midlands, there are now three users of the Airviro suite,
Birmingham City Council, Coventry City Council and Sandwell
Metropolitan
Borough Council. The use of compatible systems allows the
inter-exchange of
information and has created a local support network
and local technical knowledge.
Each authority also has a service contract
and support from SMHI in Sweden.
Carbon monoxide (CO) and nitrogen dioxide
(NO2) modelling,
was completed on one Airviro system based in
Birmingham City Council offices.
The Airviro
system is a complete Air Quality Management System as it includes the following
functional blocks:
• emission surveying and modelling
within the Emission Database (EDB)
• dispersion modelling with
the Dispersion module
• monitoring data collection, analysis and presentation
with the INDIC package.
Emission Database (EDB)
Emission data is stored within the EDB.
Databases within the model allow it to store
emission characteristics from an unlimited number
of sources of four different types:
• point sources - the emission is assumed to be concentrated
in a small area such as that from a single point like a stack. Information
describing the stack conditions must be given for use in the dispersion model.
• area sources - emission is assumed to be distributed over
a rectangular area and released uniformly
• line sources - the emission is assumed to be evenly
distributed along a line and normally used as an approximation for describing
roads
• grid layers - smaller or less significant emission sources
are combined into a grid and represent background emission levels.
The EDB stores both static and dynamic
information allowing the emission to be
defined through, time and temperature variation,
different road types describing traffic
patterns, vehicle characteristics and speed
dependent emission factors, as well as other features that help to convert
obtainable data into emission figures.
The Dispersion
Model
The dispersion model uses information
about weather, emissions, topography and
climatology as input data. The module produces air
quality calculations in the form of
seasonal, yearly means, percentiles or hourly data
for specified time periods. The
user has a choice of different types of
models; the Gauss model, the Grid model and
the Canyon model.
For review and assessment exercise all modelling and validation work was
carried out using the Gauss model. The Gauss
model is based on a Langrangean
Gaussian formulation and recommended for calculations
on smaller scales, and for
areas where the topography is reasonably flat.
Prior to any dispersion calculation the wind field is
calculated. The wind field
calculation utilises meteorological
data to calculate stability (based on Monin-Obukhov
length) and turbulence within the boundary layer. This data is then used to
determine other parameters of the boundary layer (eg boundary layer height, diabatic
heating, potential temperature distribution at ground level
and the free wind field).
Dispersion calculations are then applied to simulate
the distribution of ground level
pollutants over urban or industrial areas. The model simulates
one-hour mean, steady state pollution concentrations and operates at best over
a distance of 100 m to 2 km.
The Gauss model does not resolve individual buildings.
Instead, surface structures
enter the model through local roughness values, and through the wind field.
Buildings
give a rougher surface, which creates more friction and hence a lower wind
speed, which influences dispersion.
The model, its internal computations and assumptions
including its advantages and
disadvantages over Gaussian plume models are outlined in more
detail in the user
documentation, SMHI (1997).
(1 First Phase was a Department of the Environment
sponsored pilot study during 1996/97 to examine various aspects of the process
of review and assessment. For a summary of all work carried
out for the First Phase see NETCEN (March 2000).
Further
Reading
More information on the Methodology of the Birmingham Air
Quality Assessment can be seen at :
www.birmingham.gov.uk/GenerateContent?CONTENT_ITEM_ID=6463&CONTENT_ITEM_TYPE=0&MENU_ID=10428
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