per animal unit (AU) per year. EPA and the USDA have different definitions of AU (see Appendix B). Throughout this report, the EPA definition is used.

Emission factors are usually derived from calculations based on measured data. Actual measurements of concentrations and flow rates yield a value for an emission rate, the mass of a substance emitted per unit time (e.g., kilograms of ammonia [NH3] per year). Sometimes it is more appropriate to measure the flux of an emitted substance, the mass emitted per unit area of the source per unit time (e.g., kilograms of NH3 per hectare-year). An emission rate can be estimated from flux measurements by integrating emissions over the whole area of the emitting source. Emission rates for an AFO can be estimated from emission factors through the simple expression in Equation 2-1:

ER = AU × EF,

(Eq. 2-1)

where ER is the emission rate, AU is the number of animal units associated with the source, and EF is the emission factor in units of mass per AU per unit time. Equation 2-1 illustrates that the uncertainty contained in the numerical values selected for AU and EF are also present in the derived values for ER.

The main goal of the approach outlined by EPA (2001a) is to develop a method for estimating emissions at the individual AFO level that reflects the different kinds of animal production units commonly used in commercial-scale animal production facilities. Specifically, the approach attempts to subdivide the populations of AFOs according to the different production or manure management systems that are commonly used and to develop emission factors for model farms characterized by the processing steps. Assignment of emission factors to each of the individual processing steps within a model farm leads to an estimate of the annual mass of emissions. An estimate of the emissions from an individual AFO can then be made by associating it with the proper model farm, accounting for the AUs housed there, and adding the contributions from the processing steps (housing, manure storage, and land application).

The central assumption of this approach is that the individual processing steps within each identified manure management system are the principal factors that influence emissions. In other words, although there is inherent variability in emissions within each processing step that constitutes a manure management system, the act of subdividing the AFO population into model farms succeeds in decreasing this inherent variability to the point that single emission factors for individual processing steps, when combined, can adequately describe emissions from a model farm and thus from individual AFOs that are assigned to a given model farm category. It is further implied in this approach that the dominant factor controlling the magnitude of the calculated emissions is the number of AUs housed and not other unaccounted-for or unknown factors. This also explains the emphasis on finding the correct

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