Photochemical modeling is the central element of the air quality
modeling process. A photochemical grid model is used to calculate
pollutant concentrations.
The Role of a Photochemical Grid Model in Air Quality
Modeling
A photochemical grid 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 Grid Model Work?
A photochemical grid model is a computer model designed for
simulating air pollution episodes.
Air pollution scientists can investigate the causes of air
pollution with measurements of the ambient air. These measurements
are useful for determining how serious the air pollution problems
are, and how they occur. But in order to determine how to alleviate
complex air pollution problems, scientists need a tool that can
allow them to run experiments that test different strategies for
controlling air pollution. Currently, the most effective way of
testing control strategies is a type of computer model called a
photochemical grid model.
Photochemical grid models are intended to accurately depict the
ways in which air pollution forms, accumulates, and dissipates.
They accomplish this by simulating the processes within a city that
are most important in generating ozone pollution. For example,
photochemical grid models are driven by
meteorological models, similar to those used for weather
forecasting, so that the winds that carry pollutants around the
city are accurately simulated. Another example is the
emissions from industrial sources, cars and trucks, locomotives,
ships, and the many other sources that emit chemicals which can
participate in ozone formation. A third example is the chemistry:
photochemical grid models simulate the chemical reactions that can
result in formation of ozone. The models are called "photochemical"
because they can simulate the destruction of chemicals by sunlight.
The fragments of these chemicals can react with volatile organic
compounds and nitrogen oxides, which can eventually lead to ozone
formation.
A photochemical grid model simulates the atmosphere above a city
by dividing it into thousands of boxes, or individual grid cells.
These grid cells are typically a few kilometers wide (e.g., 4
kilometers by 4 kilometers). The vertical thickness of the cells
varies, with the shortest cells near the ground, and thicker cells
at the higher levels of the atmosphere. 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:
