...
Emission factor (EF). Past and future developments in anthropogenic emissions are estimated on the basis of projected changes in activity and emissions per unit of activity. The equation for this emission factor approach is:
where Emission is the emission of the specific gas or aerosol; Activity is the energy input or agricultural activity; r is the index for region; i is the index for further specification (sector, energy carrier); EF-base is the emission factor in the baseline; and AF is the abatement factor (reduction in the baseline emission factor as a result of climate policy). The emission factors are time-dependent, representing changes in technology and air pollution control and climate mitigation policies. The emission factor is used to calculate energy and industry emissions, and agriculture, waste and land-use related emissions. Following the equation, there is a direct relationship between level of economic activity and emission level. Shifts in economic activity (e.g., use of natural gas instead of coal) may influence total emissions. Finally, emissions can change as a result of changes in emission factors (EF) and climate policy (AF).
...
Table 5.1: Atmospheric emissions calculated in IMAGE, by source and by method applied (from the IMAGE 3.0 documentation)
A brief overview is presented here, for more information see the IMAGE 3.0 web page.