Introduction >> Modèle
The three-day forecasts and air quality maps published on a daily basis on the PREV’AIR server are the result of numerical simulations carried out with the help of so-called 3D eulerian deterministic models ("chemistry-transport" models). For periods of time ranging from several days to several months, these tools allow to calculate changes in photochemical and specific pollution in the lower layer of the atmosphere, on different spatial scales.
The models used in the PREV’AIR system have been developed by the project’s partners:
A three-dimensional area includes the low troposphere above the region being studied. The changes in pollutant concentrations in this three-dimensional area over the chosen period - influenced by weather conditions and emissions of pollutants into the atmosphere - are calculated in a deterministic manner, i.e., by linking temporal variations in such concentrations to:
A system of partial differential equations (PDE) translates - or "models" - this complex of phenomena into mathematical terms.
The PDE system that describes the transport and the physics-chemistry of atmospheric pollution is then numerically solved on calculating machines, with the help of a numerical scheme adapted to the type of PDE to be processed.
In the eulerian three-dimensionnal approach, EDPs are projected in each spatial direction..
The numerical resolution of the EDPs entails spatial discretisation of the considered three-dimensional area: this is described by a vertical and horizontal grid with a spatial resolution that depends on the following factors:
The concentrations calculated by the models in each grid cell are the average concentrations that would be observed if there were a perfect mix in each sector, which is rarely the case.
Likewise, numerical resolution of the PDEs entails temporal discretisation of the period being studied. Changes in concentrations of pollutants are calculated with a temporal resolution (defined by the time step) that depends on the spatial grid, the duration of the period in question and, obviously, on the properties of the pollutants, the computer resources and the input data.
In the deterministic chemistry-transport models used within the context of the PREV’AIR system, changes in pollutant concentrations over time are calculated by linking the variation of pollutant concentrations in the area over time to physical-chemical processes that increase (production processes) or reduce (loss processes) the concentration of a chemical compound in the atmosphere.
For example, the following physical-chemical processes are used in the CHIMERE-Continental model:
In order to calculate the flows of production or loss of pollutants related to physical-chemical processes, a certain amount of input data must be provided for the chemistry-transport models, particularly pollutant emissions and meteorological data. These input data must be obtained from a number of bodies.
AEAT/ENV/R/0545 report, Speciation of UK emissions of NMVOC, N.R. Passant, February 2002
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