Another reason is that data produced by use of emission factors can be either representative of actual emissions or completely non-representative. This means a high degree of judiciousness must come into play in the application of emission factors.
Emission factors have become essential in meeting requirements of regulations such as the GHG reporting rule, the toxics release inventory, and employee exposure standards. In addition, EPA and state agencies use emission factors in compiling emission inventories, developing emission control strategies, determining the applicability of permitting programs, and related applications. The regulated community can independently use emission factors for a variety of purposes, including estimating the results of pollution reduction efforts.
EPA defines an emission factor as a “representative value that attempts to relate the quantity of a pollutant released to the atmosphere with an activity associated with the release of that pollutant.” In most cases, these factors are simply averages of all available data of acceptable quality and are generally assumed to be representative of long-term averages in the source category (i.e., a population average).
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EPA emphasizes that emissions factors in AP-42 are neither EPA-recommended emission limits (e.g., best available control technology [BACT] or lowest achievable emission rate [LAER]) nor standards (e.g., national emission standards for hazardous air pollutants [NESHAP] or new source performance standards [NSPS]). Further, use of these factors by regulators as source-specific permit limits and/or as emission regulation compliance determinations is not recommended by EPA. The Agency notes that, as an average of a range of emission rates, approximately half the subject sources will have emission rates greater than the emission factor and the other half will have emission rates less than the factor. “As a result, a permit limit using an AP-42 emission factor would result in half the sources being in non-compliance,” says EPA.
Using emission factors to estimate emissions from a single source is also prone to error. “Between-source variability,” even among similar individual sources, can be large, depending on process, control system, and pollutant, notes EPA. Although the causes of this variability are considered in emission factor development, this type of information is seldom included in emission tests used to develop AP-42 factors, states EPA. “As a result, some emission factors are derived from tests that may vary by an order of magnitude or more,” says the Agency. “Even when the major process variables are accounted for, the emission factors developed may be the result of averaging source tests that differ by factors of five or more.” For example, differences in design of air pollution control devices may cause differing emission characteristics.
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EPA recommends that before applying AP-42 emission factors to predict emissions from new or proposed sources or to make other source-specific emission assessments, the user review the latest literature and technology to be aware of circumstances that might cause such sources to exhibit emission characteristics different from those of other, typical existing sources. Care should be taken to ensure that the subject source type and design, controls, and raw material input match those of the sources(s) identified in AP-42. Also short-term emissions from a single specific source often vary significantly because of fluctuation in process operating conditions, control device operating conditions, raw materials, ambient conditions, and other such factors.
These factors are generally not accounted for in testing that is used to develop emission factors. Emission factors are typically developed to represent long-term average emissions, so testing is conducted at normal operating conditions. Therefore, using emission factors to estimate short term emissions will cause even greater uncertainty.
Given their limitations, emission factors should be employed only when more reliable emissions data, such as information collected from source-specific emission tests, continuous emissions monitoring, or materials balancing are not available.