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Overview of selection
and use of Indic Airviro
Air pollution modelling is
co-ordinated across the West Midlands region of the UK byThe
West Midlands Joint Pollution Working Group. This group became involved in
modelling for review and assessment during a First Phase task which was
supported by the UK National Government (a Department of the Environment
sponsored pilot study during 1996/97 to examine various aspects of the process
of review and assessment), studying ADMS Urban and INDIC Airviro.
The advanced model, INDIC Airviro, which is licensed
by the Swedish Meteorological Office (SMHI), was seen to meet the needs of the
conurbation better for the subsequent air quality work. Within the West
Midlands there are now several users of the Airviro suite. This use of compatible systems allows the
inter-exchange of information and has created a local support network and local
technical knowledge. The complex modelling work is completed on one system
based at Birmingham City Council offices. However, each authority also has a
service contract and support from SMHI in Sweden.
The
Airviro system differs from the other PC – based
models in that it requires a UNIX workstation and requires complex
physiographic and meteorological configuration by the software supplier. Unlike
most Gaussian models, which rely upon meteorological information collected from
a single site, the Airviro model describes a pattern
of small-scale winds based upon the surface characteristics. The model
interfaces with a sophisticated emissions database capable of accepting point
sources, area sources and line sources and detailed diurnal/seasonal/production
variations of emissions (both traffic and industrial). The Airviro
system can be applied using a number of Gaussian model options, and a street
canyon model option.
In summary it is a complete Air
Quality Management System, which 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 Indico package.
Emission Database
Emission data is stored within
the Emission Database (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
concentrates 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, phsyiography
and climatology as input data. The module produces air quality calculations in
the form of seasonal or yearly means or 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 of
air quality in Birmingham, all modelling and validation work was carried out
using the Gauss model. The Gauss model is based on a Langrangean
Gaussian formulation and is recommended for calculations on smaller scales, and
for 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. The data is then used to
determine other parameters of the boundary layer (e.g. boundary layer height,
adiabatic 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 wind field. Buildings
give a rougher surface, which creates more friction and hence a lower wind
speed that 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, The
Airviro Users Guide SMHI (1997).
The Future
Over the last decade the Airviro system has seen continuous development and rigorous
testing of its applications. The Airviro system now
used by Birmingham has recently been upgraded to the iAirviro
(Internet) platform. As the iAirviro system is fully
internet based access to the secure domain can be achieved from any computer
with an internet connection which has a number of benefits, including data
management and resolution of problems away from the office or allow home
working, for example. Further information relating to the iAirviro
applications can be seen on the iAirviro website www.airviro.smhi.se.
For more detailed information on
how Birmingham City Council have and continue to use modelling in the Review
and Assessment of air quality throughout Birmingham, you should visit the
Birmingham City Council web site, www.birmingham.gov.uk. |