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The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations Executive Summary Concern with possible environmental and health effects of air emissions generated from animal feeding operations (AFOs) has grown with the increasing size, geographic concentration, and suburbanization of these operations in what was formerly rural, sparsely populated agricultural land. This interim report, prepared at the request of the Environmental Protection Agency (EPA), evaluates the current knowledge base and approaches for estimating air emissions from AFOs. The issues regarding emissions from AFOs are much broader than the interests of any one federal agency. In recognition of this, the U.S. Department of Agriculture (USDA) joined EPA in the request for this study. Generating reasonably accurate estimates of air emissions from AFOs is difficult. The operating environment for these farms is complex. The species of animals are varied (e.g., swine, beef and dairy cattle, poultry), and farm practices differ not only between species, but also among farms for each species. The operations vary in size (this report is concerned with AFOs as defined by EPA; see Appendix B) and differ by region across the country. The chemical composition of the emissions varies depending on animal species, feeding regimes and practices, manure management practices, and the way in which the animals are housed. Much of the air emissions come from the storage and disposal of the manure (the term here is used to mean both urine and feces, and may also include litter or bedding materials) that is part of every AFO, but some also comes from dust produced by the handling of feed and the movement of animals on manure, as well as from the animals themselves. Meteorologic
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The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations conditions, of course, are an important factor. Estimates of emission rates generated in one type of AFO may not translate readily into others. EPA has a variety of needs for accurate estimation of air emissions from AFOs. Increasing pressure has been placed on the agency to address these emissions through the Clean Air Act and other federal regulations, and EPA has indicated the need to do so in the future. Also pressing, EPA is under court order to establish new water quality rules by December 2002. The current study will focus on ways to estimate these emissions prior to December 2002 to additionally help assure that rules aimed at improving water quality do not have negative impacts on air emissions. This interim report is intended to provide findings to date on a series of specific questions from EPA regarding the following general issues: identifying the scientific criteria needed to ensure that estimates of air emission rates are accurate, the basis for these criteria in the scientific literature, and the uncertainties associated with them. It also includes an assessment of the emission estimating approaches in a recent report Air Emissions From Animal Feeding Operations (EPA, 2001a). Finally, it identifies economic criteria needed to assess emission mitigation techniques and best management practices. The committee has answered the following sets of questions in the interim report within the confines of the Statement of Task (see Appendix A): What are the scientific criteria needed to ensure that reasonably appropriate estimates of emissions are obtained? 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? How should the variability due to regional differences, daily and seasonal changes, animal life stage, and different management approaches be characterized? How should the statistical uncertainty in emissions measurements and emissions factors be characterized in the scientific literature? Are the emission estimation approaches described in the EPA report Air Emissions from Animal Feeding Operations (EPA, 2001a) appropriate? If not, how should industry characteristics and emission mitigation techniques be characterized? Should model farms be used to represent the industry? If so, how? What substances should be characterized and how can inherent fluctuations be accounted for? What components of manure should be included in the estimation approaches (e.g., nitrogen, sulfur, volatile solids [see Appendix B])? What additional emission mitigation technologies and management practices should be considered? What criteria, including capital costs, operating costs, and technical feasibility, are needed to develop and assess the effectiveness of emission mitigation techniques and best management practices?
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The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations The goal of EPA (2001a) was to“develop a method for estimating emissions at the individual farm level.” To accomplish this, EPA (2001a) developed a set of 23 model farms (see Appendix D) intended to represent the majority of commercial-scale AFOs. Each model farm included three variable elements: a confinement area, manure management system, and land application method. The manure management system was subdivided into solid separation and manure storage activities. Given the specific nature of the questions answered, the committee has not yet addressed some of the broader issues related to AFOs. To the extent possible, these will be addressed in its final report, which will build on the findings of this interim report and include a more detailed response to the committee’s full Statement of Task (see Appendix A). The need for further discussion of some issues in the final report is indicated in various places in this report. These issues fall in eight broad categories: (1) industry size and structure, (2) emission measurement methodology, (3) mitigation technology and best management plans, (4) short- and long-term research priorities, (5) alternative approaches for estimating emissions, (6) human health and environmental impacts, (7) economic analyses, and (8) other potential air emissions of concern. This interim report represents the consensus views of the committee and has been formally reviewed in accordance with National Research Council (NRC) procedures. In answering these questions and addressing its Statement of Task (Appendix A), the committee has come to consensus on eight findings for the interim report. The basis of these findings is discussed more extensively in the body of the report. Finding 1: Proposed EPA regulations aimed at improving water quality may affect rates and distributions of air emissions from animal feeding operations. Discussion: Regulations aimed at protecting water quality would probably affect manure management at the farm level, especially since they might affect the use of lagoons and the application of manure on cropland or forests. For example, the proposed water regulations may mandate nitrogen (N) or phosphorus (P) based comprehensive nutrient management plans (CNMPs). AFOs could be limited in the amount of manure nitrogen and phosphorus that could be applied to cropland. If there is a low risk of phosphorus runoff as determined by a site analysis, farmers will be permitted to overapply phosphorous. However, they will still be prohibited from applying more nitrogen than recommended for crop production. Many AFOs (those currently without CNMPs) likely will have more manure than they can use on their own cropland, and manure export may be cost prohibitive. Thus, AFOs will have an incentive to use crops and management practices that employ applied nitrogen inefficiently (i.e., volatilize ammonia) to decrease the nitrogen remaining after storage or increase the nitrogen requirement for crop production. These
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The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations practices may increase nitrogen volatilization to the air. The committee was not informed of specific regulatory actions being considered by EPA (beyond those addressed in the Federal Register) to meet its December 2002 deadline for proposing regulations under the Clean Water Act. Finding 2: In order to understand health and environmental impacts on a variety of spatial scales, estimates of air emissions from AFOs at the individual farm level, and their dependence on management practices, are needed to characterize annual emission inventories for some pollutants and transient downwind spatial distributions and concentrations for others. Discussion: Management practices (e.g., feeding, manure management, crop management) vary widely among individual farms. Estimates of emissions based on regional or other averages are unlikely to capture significant differences among farms that will be relevant for guiding emissions management practices aimed at decreasing their effects. Information on the spatial relationships among individual farms and the dispersion of air emissions from them is needed. Furthermore, developing methods to estimate emissions at the individual farm level was the stated objective of EPA's recent study (EPA, 2001a). Finding 3: Direct measurements of air emissions at all AFOs are not feasible. Nevertheless, measurements on a statistically representative subset of AFOs are needed and will require additional resources to conduct. Discussion: Although it is possible in a carefully designed research project to measure concentrations and airflows (e.g., building ventilation rates) to estimate air emissions and attribute them to individual AFOs, it is not practical to conduct such projects for more than a small fraction of AFOs. Direct measurements for sample farms will be needed in research programs designed to develop estimates of air emissions applicable to various situations. Finding 4: Characterizing feeding operations in terms of their components (e.g., model farms) may be a plausible approach for developing estimates of air emissions from individual farms or regions as long as the components or factors chosen to characterize the feeding operation are appropriate. The method may not be useful for estimating acute health effects, which normally depend on human exposure to some concentration of toxic or infectious substance for short periods of time. Discussion: The components or factors used to characterize feeding operations are chosen for their usefulness in explaining dependent variables, such as the mass of air emissions per unit of time. The emission factor method, which is based on the average amount of an emitted substance per unit of activity per year (e.g., metric tons of ammonia per thousand head of cattle per year), can be useful in estimating annual regional emissions inventories for some pollutants,
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The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations provided that sufficient data of adequate quality are available for estimating the relationships. Finding 5: Reasonably accurate estimates of air emissions from AFOs at the individual farm level require defined relationships between air emissions and various factors. Depending on the character of the AFOs in question, these factors may include animal types, nutrient inputs, manure handling practices, output of animal products, management of feeding operations, confinement conditions, physical characteristics of the site, and climate and weather conditions. Discussion: The choice of independent variables used to make estimates of air emissions from AFOs will depend on the ability of the variables to account for variations in the estimates and on the degree of accuracy desired, based on valid measurements at the farm level. Past research indicates that some combination of the indicated variables is likely to be important for estimates of air emissions for the kinds of operations considered in this report. The specific choices will depend on the strength of the relationships for each kind of emission and each set of independent variables. Finding 6: The model farm construct as described by EPA (2001a) cannot be supported because of weaknesses in the data needed to implement it. Discussion: Of the nearly 500 possible literature sources for estimating emissions factors identified for EPA (2001a), only 33 were found by the report's authors to be suitable for use in the model farm construct. The committee judged them to be insufficient for the intended use. The breadth in terms of kinds of animals, management practices, and geography in this model farm construct suggests that finding adequate information to define emission factors is unlikely to be fruitful at this time. Finding 7: The model farm construct used by EPA (2001a) cannot be supported for estimating either the annual amounts or the temporal distributions of air emissions on an individual farm, subregional, or regional basis because the way in which it characterizes feeding operations is inadequate. Discussion: Variations in many factors that could affect the annual amounts and temporal patterns of emissions from an individual AFO are not adequately considered by the EPA (2001a) model farm construct. The potential influences of geographic (e.g. topography and land use) and climatic differences, daily and seasonal weather cycles, animal life stages, management approaches (including manure management practices and feeding regimes), and differences in state regulations are not adequately considered. Furthermore, aggregating emissions from individual AFOs using the EPA (2001a; not a stated objective) model farm construct for subregional or regional estimates cannot be supported for similar
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The Scientific Basis for Estimating Air Emissions from Animal Feeding Operations reasons. However, with the appropriate data identified there may be viable alternatives to the currently proposed approach. Finding 8: A process-based model farm approach that incorporates “mass balance” constraints for some of the emitted substances of concern, in conjunction with estimated emission factors for other substances, may be a useful alternative to the model farm construct defined by EPA (2001a). The committee plans to explore issues associated with these two approaches more fully in its final report. Discussion: The mass balance approach, like EPA’s model farm approach, starts with defining feeding operations in terms of major stages or activities. However, it focuses on those activities that determine the movement of nutrients and other substances into, through, and out of the system. Experimental data and mathematical modeling are used to simulate the system and the movement of reactants and products through each component of the farm enterprise. In this approach, emissions of elements (such as nitrogen) cannot exceed their flows into the system.
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