Each of those components is described below.

Emission Inventories

The first step in developing an emission-control strategy for a criteria pollutant is to develop an inventory of pollutant emissions that lists all sources of the pollutant or its precursor and the rate at which each source emits the pollutant to the atmosphere. EPA has specified a general procedure for emissions-inventory development that categorizes emissions into four source types: stationary, mobile, biogenic, and geogenic (EPA 2003f). Stationary sources are further divided into major stationary and area sources. Major stationary sources are defined as stationary sources having emissions that exceed a minimum or threshold level, which varies depending on the pollutant. Stationary sources that fall below the threshold level are merged as area sources. The reporting requirements for major stationary sources are more detailed than those for area sources (65 Fed. Reg. 33268 [2000]). Mobile sources include on-road and nonroad vehicles and sources such as lawn and garden, recreational, construction, and marine equipment; aircraft; and locomotives. Biogenic sources include plants, trees, grasses, and agricultural animals and crops, and geogenic sources include gas seeps, soil wind erosion, geysers, and volcanoes. Inventories are generally developed using a combination of the direct measurements and emission models described below (EPA 1998b).

Continuous Emissions Monitoring Systems

The most direct way to determine the rate at which a pollutant is emitted into the atmosphere is through emissions monitoring. However, the large number and varying types of sources that generally exist for a given pollutant make this impractical and only the largest major stationary sources are generally equipped with continuous emissions monitoring (CEM) systems.

As a result of the requirements in the Acid Rain Title of the 1990 CAA Amendments, increasing numbers of electrical utility boilers are now using CEM systems. The data from them are posted quarterly on the internet, providing hourly emissions as well as values averaged quarterly and annually (EPA 2003g). Because of the use of CEM systems, the emission rates from these sources are generally viewed as being among the most accurate of all known rates in the United States.

Emission Estimation Models

Models for estimating emissions have been developed and made available by EPA for selected sources, particularly area sources for which mea-

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