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Introduction to Air Quality Modeling: Photochemical Modeling

Photochemical modeling is the central element of the air quality modeling process and is used to simulate pollutant concentrations.

Photochemical Grid Model in Air Quality Modeling

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.

A photochemical grid model is used to:

  • assess how sensitive pollutant concentrations are to changes in various parameters including emissions of pollutants, 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.

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 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 characterized. Another example is emissions from industrial sources, cars and trucks, locomotives, ships, and the many other sources that emit chemicals that can participate in ozone formation. A third example is the chemistry: photochemical grid models simulate the photochemical reactions that result in formation of ozone. “Photochemical” reactions are molecular reactions triggered by exposure to sunlight, and some of these reactions create extremely reactive molecular fragments called “radicals”. These radicals can react with volatile organic compounds and nitrogen oxides, eventually leading 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 cells near the ground typically only about 30 meters thick increasing to a few kilometers 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.

Boundary Conditions

Although our photochemical modeling is focused upon Texas, the modeling must account for emissions and the transport of pollutants from outside of Texas and United States. On the edges of our largest modeling domain, the concentrations of pollutants are defined as boundary conditions. The boundary conditions vary by hour for each specific modeling episode and are set vertically from the surface to the highest model layer many kilometers into the atmosphere. The TCEQ obtains boundary conditions from the output of a global-scale chemical transport model such as the Goddard Earth Observing System Chemistry model (GEOS-Chem) , maintained by Harvard and NASA.

Major Photochemical Models

The two photochemical models most used by the air quality modeling community are: