An emission model is a set of mathematical equations that represent the processes that occur when a facility generates a pollutant. This kind of estimation is similar to determining the fuel efficiency for your car. The manufacturer provides an average miles per gallon rating when you buy a car. Depending on many factors such as how you drive your car, your actual fuel efficiency may differ. The concentration of a pollutant decreases as it travels from the site of release because the pollutant spreads out.
Dispersion also depends on the terrain, whether it is on flat or mountainous land or in a valley.
Other factors can affect the concentration, or level of a pollutant at a given location. The amount of a pollutant at any one location can vary over time depending on the pattern of releases. For example, industrial processes can release some pollutants only at certain times and other pollutants continuously. The ground-level concentration near a facility is generally lower when a pollutant is released from high stacks because the pollutant is more diluted when it reaches the ground. Other factors that affect concentration include the temperature and speed of the gas released through the smoke stack and the location of places in the facility where it is released.