In the air quality management work, “Airshed” is a common phrase used for describing the modeling domain – the length and breadth of an area to conduct emissions and pollution modeling.
How do we define this airshed?
How big or small should the airshed be, which can cover the needs of a city or a region in addressing the air pollution problem?
Are there any mathematical constraints to be considered when modeling?
There is no standard definition or rules for setting an airshed size.
The objective is to study the area’s air quality.
Designating an airshed size (x-y dimensions) for this area, is a subjective assessment and here is the only thumb rule for setting an urban airshed — The airshed must include any of the surroundings that influence the city’s air quality, such as large settlements like satellite cities, large point sources like power plants, cement plants, brick kiln clusters, etc.
The size is never the political/administrative boundary of the city under consideration. It is always about all the emission sources with the potential to influence the urban air quality of the political/administrative boundary.
A clear understanding of the city’s (and surroundings) geography and the emission strengths (including inventory) of various sources inside and outside the city limits is necessary.
In the modeling world, we have to keep the airshed size to something mathematically solvable and a size manageable by the (available) computational system.
- For urban-scale simulations: A typical grid-size is 1-km (approximately 0.01 degrees near the Equator) and a typical airshed size is 30 x 30 grids for small cities to 80 x 80 grids for big cities, in north-south and east-west directions. Always beyond the city’s political/administrative boundary. It is a good practice to set rounded number of grids to support parallel processing and other server configurations.
- For regional and sub-regional simulations: A larger grid resolution of 0.1º to 0.25º (~10 to 25 km) is typically used. This choice is often determined by the availability of regional emission inventories. Since most global emission inventories are available at a 0.1º resolution, many regional modeling systems are run at this resolution. While this is sufficient to capture most urban signatures, if high-resolution local inventories are available, specific areas (airsheds) can be embedded as nests within the larger domain for higher-resolution modeling. This approach allows for more detailed analysis in key areas while maintaining computational efficiency for the broader region.
Extracts from Beginners Handbook (see under XYZ)
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