The “regression analysis” approach uses standard least-squares multivariate regression equations to relate measures of air emissions to various factors that are hypothesized to affect them (e.g., number of animals, animal type and production system, productivity, housing, manure management, weather, climate). Once the equations are verified, they are used to gauge the importance of various factors that determine emissions and to estimate air emissions from other AFOs based on their individual characteristics. The weakness of this approach is the assumption that the within-factor variation of emissions is small relative to the among-factors variation for any category of operation. In addition, the current database for estimating regression equations is very limited and a major effort would be required to obtain them.

The “process-based” approach follows the fate of relevant elements (e.g., nitrogen, carbon and sulfur) step by step through the animal feeding process and identifies the chemical transformations that take place. It provides estimates of the characteristics and amount of air emissions that occur at each step as controlled by a mass balance approach (i.e., the emission of an element from the system, or from a part of the system, is equal to the input of that substance minus any accumulation that might occur). The advantages and limitations of this approach are described in some detail later in this chapter.

The goal of the U.S. Environmental Protection Agency (EPA, 2001a) is “to develop a method for estimating [air] emissions at the individual farm level that reflects the different animal production methods that are commonly used at commercial scale operations.” The approach was intended to provide estimates of total annual air emissions from AFOs for defined geographic areas by kind of animal and manure handling practices for each of eight kinds of emissions. It did this with a model farm construct that provides estimates of average annual emissions per EPA animal unit (AU) for twenty-three model farms (two for beef, eight for dairy, two for poultry-broilers, two for poultry-layers, two for poultry-turkeys, five for swine, and two for veal; EPA, 2001a). Each model is defined by three variable elements that describe manure management practices for typical large AFOs: (1) confinement and manure collection system, (2) manure management system, and (3) land application. The manure management system is further subdivided into solids separation and manure storage activities. Insofar as combinations of these elements are regionally distinctive, the model farms also reflect regional variations in air emissions.

Model farms, as described by EPA (2001a), are useful for aggregating emission rates across diverse sets of AFOs. A model farm can be used to represent the average emissions across some geographic area over some period of time per unit capacity of a class of farms (e.g., all pig farms in the United States that use an