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Photochemical modeling is the mathematical simulation of the chemical and meteorological processes associated with the formation of ozone.
Ground-level ozone is not directly emitted, but is formed through the chemical reaction of VOCs, NOx and sunlight. Photochemical models simulate this process to predict or describe ambient ozone concentrations. The inputs into these models are, at a minimum, primary pollutant concentration information and meteorological data. The more complicated models also incorporate topographic data. As an output, the photochemical models show both primary and secondary pollutant concentrations at a given location and time.
Lagrangian and Eulerian models are the most commonly used photochemical models. These categories describe groups of models using a given frame of reference, or coordinate system. The Eulerian models work from a grid that is fixed at the earth’s surface. The Lagrangian models follow a pollutant trajectory, where the pollutant is modeled as a well-mixed parcel (or box) of air moving through the trajectory. Most of the earlier photochemical models follow the Lagrangian approach, while the latest generation of photochemical models is largely Eulerian. Photochemical modeling is an extremely active area of research, and the models are continually being refined and updated.
Photochemical models are often a required component of state implementation plans (SIPs), to demonstrate the predicted effectiveness of a given policy. They are also used for scientific investigation and daily forecasting. From a management perspective, photochemical models are a critical component of controlling ozone concentrations.