Once the emissions inventory is completed, including emissions estimates for all sources, a correlation between the existing total level of emissions and the existing monitored levels of ambient air pollution can be made. It must then be determined how much emissions must be reduced to result in the needed reduction in the monitored level of air pollution. However, there is not a direct linear relationship between emissions and actual pollutant concentrations, and many factors influence these concentrations. Air quality modeling or some analytical method may be required to determine the total needed emissions reductions.
Once the total emissions reductions target is determined, control strategies must be determined to demonstrate how emissions are to be reduced to that level. Projected changes in population, industrial activity, travel, and other social and economic factors must be evaluated, and control strategies must reflect any projected growth to ensure that emissions are reduced to the actual level needed.
The selected measures will be some combination of stationary, area, on-road, and off-road controls. The selected measures result in a certain level of emissions reductions from each source category. This allocation of emissions reductions is intended to allow for States to determine the fastest and most cost-effective way of reaching the level of total emissions estimated to result in attainment.
In addition, sometimes the adopted control strategies may actually result in more emission reductions than what are needed for attainment. This produces a "growth" or "safety" margin that can give communities flexibility in accounting for future growth and economic activity.