The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations Interim Report (2002) / Chapter Skim
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2. Determining Emission Factors
2. Determining Emission Factors
Pages 23-53
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From page 23... ...
emissions estimates are needed at the individual AFO level (Finding 2~; (2) it is not practical to measure emissions at all individual AFOs (Finding 3~; (3)
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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 unaccountedfor or unknown factors.
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Here the committee emphasizes the data quality that can be assigned to measurements of emissions, and to subsequently derived emission rates and emission factors. This discussion is placed in the context of the five specific questions from the EPA.
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Reported emission rates in excess of nutrient excretion rates should be viewed with suspicion; they may indicate measurement conditions atypical of normal operation, or a fatal flaw in the overall experimental design or instrumentation used in the study. Precision Assigning an estimate of precision to measurements of concentrations emitted from different components in a manure management system is not a simple task.
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From page 27... ...
Assigning an estimate of precision to an emission factor for an individual AFO is more challenging than assigning it to a set of concentration and airflow measurements. The relative uncertainty associated with emission factors from individual AFOs can be obtained by remembering that emission factors are an estimate of emissions of particulate matter (PM)
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From page 28... ...
Values of CV of 10 percent or greater are probably much more realistic for real AFOs. Continuing with the example of 60 percent of the excreted nitrogen emitted as NH3, the range in uncertainty in emissions, and therefore calculated emission factors, associated with a 10 percent CV can be calculated directly based on the amount of nitrogen excreted and the number of animal units housed.
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There are methods for deriving estimates of variance from highly autocorrelated data (Code of Federal Regulations, 2001~. PUBLISHED LITERATURE What are the strengths, weaknesses and gaps of published methods to measure specific emissions and develop emission factors that are published in the scientific literature?
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From page 30... ...
Expressing NH3 emission factors on a per annum and per AU basis facilitates calculation of total air emissions and accounts for variation due to size of AFOs, but it does not account for some of the largest sources of variation in emissions. Clearly, there is a great deal of variation in reported measurements among AFOs represented by a single model.
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The current approach ignores these mass balance considerations, and simply adds the emissions using emission factors determined separately for each subsystem. Dividing the total manure nitrogen that leaves the farm by the total nitrogen excreted can identify some potential overestimation of emission factors.
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Emissions can be estimated from the fraction of the applied fertilizer nitrogen emitted as NOX, but the flux varies strongly with land use and temperature. Vegetation cover greatly decreases NOX emissions (Civerolo and Dickerson, 1998~; undisturbed areas such as grasslands tend to have low emission rates, while croplands can have high rates.
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Using emission data from swine operations to estimate emission factors for other species such as dairy and poultry is not scientifically sound. Outside manure storage, such as storage in tanks or anaerobic lagoons, can be important sources of H2S emissions.
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states that "emission factors for N2O were not found in the literature," a large body of research exists on N2O emissions from livestock, manure, and soils. Time constraints prevent a thorough review of the literature, but this section condenses the main points of a few recent papers and attempts to summarize the state of the science.
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Manure Management Several recent studies indicate that N2O emissions from manure can be large (Jarvis and Pain, 1994; Bouwman, 1996; Mosier et al., 1996; Intergovernmental Panel on Climate, 2001~. For example, Jungbluth et al.
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From page 36... ...
as approximately 2.1 Tg N/yr. These estimates are based on an assumed average fraction of manure nitrogen converted to N2O and are subject to variability due to temperature, moisture content, and other environmental factors in a manner similar to soil emissions.
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Much research was overlooked since a number of papers and reports describing CH4 emission rates can be found in the literature. Fleesa et al.
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appeared in a specialty conference proceedings (non-peer reviewed) , and it is not clear how the emission rates were derived.
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. For PM, unlike most other air pollutants, emission factors developed for use in emission inventories and for dispersion modeling can, ideally, be reconciled using receptor modeling techniques.
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The applicability of these efforts to VOC emissions from AFOs is unknown at this time. Ongoing studies to determine emission rates of VOCs were not included in EPA (2001a)
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This analysis suggests that exposure factors can account for differences observed in VOC flux rates, VOC air concentrations, and odor intensities. Therefore, the equation used to model the emission factor for VOCs in EPA (2001a)
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on odor or odorant emission rates, flux rates, and emission factors are lacking for most livestock species (and for different ages and housing) and are needed for the development of science-based abatement technologies.
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(2001~. CHARACTERIZING VARIABILITY How should the variability in emissions be characterized that is due to regional differences, daily and seasonal changes, animal life stage, and different management approaches?
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. Increases in mean ambient temperature are expected to increase gaseous emission rates from several components of the model farms, including manure storage and land to which manure has been applied.
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Failure to account for short-term cycles in an experimental design used to characterize emissions could result in significant systematic error in a derived emission factor, when extrapolated to a one-year time period. Individual AFOs are essentially a collection of different biological systems each operating with its own hourly, daily, and seasonal cycles.
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From page 46... ...
However, given the paucity of emissions data deemed valid for the development of emission factors to characterize the model farms, it is not possible to determine to what extent such cycles may have impacted published emission measurements. As noted earlier, averaging published emission factors does not compensate for the presence of systematic bias that may be present as a result of a failure of the experimental design to account adequately for such cycles.
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At a minimum the data displayed in Figure 2-1 demonstrate that the same sampling scheme may not be applicable to all swine confinement units and that measurements of emissions may have to be weighted to account for differences in animal age. Management Optimal management is vital to the success of individual AFOs for the production of quality animals, and should also result in decreased emissions.
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From page 48... ...
Selecting an emission factor based on data from one region (e.g., the southeastern United States) and extrapolating it to other regions or even to other animal types is questionable at best, and must necessarily introduce systematic bias into the derived emission rates for individual AFOs.
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This section addresses the broader issue of uncertainty associated with published emissions data and their use in deriving emission factors. An example of the uncertainty associated with published emission rates from AFOs is illustrated in Table 2-2 adapted from Tables 9 and 10 in a recent review paper (Arogo et al., 2001)
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In addition, large uncertainties associated with emission rates for the principal components of a manure management system reduce the probability of documenting success in the application of emission reduction technologies. As a first approximation, estimates of the variance associated with emission rates, such as those in Table 2-2 can be obtained using normal
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, and reinforces the argument that the uncertainty associated with published values of emission rates (or flux) cannot be ignored when deriving emission factors.
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attempts to reduce the uncertainty in deriving emission factors for individual AFOs by subdividing the overall AFO population according to the manure management systems used. Subdivision of large sample populations into smaller subsets is an acceptable procedure to reduce uncertainty (i.e., improve sample quality)
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From page 53... ...
All other things being equal, reported values for emissions based on a relatively large number of observations should be given greater weight than those derived Tom relatively few observations. As presented in this chapter, a wide range of factors can influence air emissions of gases, PM, and other substances Tom AFOs.
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