flux using the eddy correlation method for another trace gas. Uncertainty in the determination of eddy diffusivity is a major source of uncertainty in the flux gradient methods (Lapitan et al., 1999). Like other micrometeorological methods, flux gradient techniques are subject to uncertainties associated with spatial and temporal heterogeneities.

This is an indirect technique that is based upon the surface energy budget. The Bowen ratio is the ratio of the sensible heat to the latent heat. This method does not require eddy flux measurements or stability corrections. It does require measurement of the incoming net radiation at the earth’s surface and the soil heat flux, as well as the concentration difference for the species of interest, along with concurrent flux and difference measurements of temperature, humidity, or other trace species. The Bowen ratio method fails under conditions of low energy availability such as during the night or during periods of precipitation. Like all of the micrometeorological methods, the Bowen ratio technique is subject to errors associated with the chemical and micrometeorological sensors and the spatial and temporal variability of fluxes within the source area of interest.

As opposed to the above micrometeorological techniques, the box model method works over smaller areas of fetch and so may be more applicable for some AFOs than the micrometeorological techniques discussed above. The method is a direct technique requiring no empirical relationships. In it, an imaginary box is constructed around some measurement volume so that the transfer of mass into and out of the walls of the box is measured (Shaw et al., 1998). In the most common configuration for flux measurements, the vertical profile of both the horizontal wind speed and the species concentration must be sampled through the entire downwind plume. The height of the plume, and thus the height to which sampling must be conducted, will depend on the atmospheric stability. The mass balance method assumes steady-state conditions, homogeneous horizontal winds, no other internal sources or sinks of the species of interest, and zero or known background concentrations. The box method is not suitable for highly reactive gases such as ammonia that may be rapidly deposited on surfaces. Other problems include disturbing the soil or vegetation to be measured (such as increasing the temperature) and thereby perturbing the rate of emissions. Also where turbulence or fast photochemistry strongly influence emissions or ambient concentrations, box methods must be used with caution.

In this method, upwind and downwind concentration measurements are made, and the emission rate is back-calculated from a dispersion simulation, usually based on a Gaussian dispersion algorithm, making assumptions regarding the

Front Matter (R1-R22)

Executive Summary (1-13)

1. Introduction (14-25)

2. Livestock Agriculture and Animal Feeding Operations (26-49)

3. Air Emissions (50-73)

4. Measuring Emissions (74-97)

5. Approaches for Estimating Emissions (98-128)

6. Government Regulations and Programs (129-151)

7. Improving Knowledge and Practices (152-168)

8. Conclusions (169-175)

References (176-194)

Appendix A: Statement of Task (195-197)

Appendix B: Acronyms and Glossary (198-204)

Appendix C: Bioaerosols (205-206)

Appendix D: Nitrogen and Sulfur Contents of Animal Products and Live Animals -- Sample Excretion Predictions (207-213)

Appendix E: Animal Units (214-216)

Appendix F: Ammonia Emissions from Manure Storage (217-221)

Appendix G: Regulatory Action Levels by Selected Atmospheric Pollutant (222-223)

Appendix H: Regulatory Action Levels by Regulatory Requirement and Action Status (224-225)

Appendix I: Emission Factors for a Feed Mill or Grain Elevator (226-227)

Appendix J: Public Meeting Agendas (228-233)

Appendix K: Geographic Distribution of Livestock and Poultry Production in the United States for 1997 (234-240)

Appendix L: Emisson Factors in Published Literature (241-254)

About the Authors (255-260)

Board on Agriculture and Natural Resources Publications (261-264)