Photochemical modeling is the central element of the air quality
modeling process. A photochemical model is used to calculate
pollutant concentrations.
The Role of a Photochemical Model in Air Quality Modeling
A photochemical model is used to:
- assess how sensitive pollutant concentrations are to changes in
various parameters including pollution emissions, meteorological
conditions, and initial and boundary conditions;
- assess the sensitivity of pollutant predictions to various
control scenarios (individual controls and combined controls); and
- determine whether various control scenarios actually result in
predicted attainment or achievement of target concentrations.
How Does a Photochemical Model Work?
A photochemical model represents the atmosphere of a modeled
area with a three-dimensional grid that may include thousands of
grid cells. Imagine each grid cell is a box. These boxes are
stacked one on top of another and vary in height, with the layers
of shorter boxes representing the parcels of air nearest the
ground. The model calculates concentrations of pollutants, such as
ozone, in each cell by simulating
- movement of air into and out of cells (advection and
dispersion);
- mixture of pollutants upward and downward among layers;
- injection of new emissions from sources such as point, area, mobile, and biogenic into each
cell; and
- chemical reactions based on chemical equations, pollution
precursors, and incoming solar radiation in each cell.
Major Photochemical Models
Following are major photochemical models used by the air quality
modeling community:
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