teristics, such as solids content and the presence of metals, is not well understood. In contrast to ammonia, little research has been conducted to quantify the differences between the ionization constants of sulfide in animal manure versus water. The mass-transfer coefficient KLS is a function of manure temperature, air temperature, wind velocity, and relative humidity. Arogo et al. (1999) developed a correlation for KLS for under-floor pit manure storage. More research is needed to define KLS for outside manure storage.
Whereas it is difficult and expensive to measure most emissions from AFOs directly, and the measurements may be of questionable accuracy, regulatory and management agencies seeking to mitigate emissions need reasonably accurate methods to estimate and attribute them to particular enterprises or activities. One approach is to develop emission factors for different animal production sectors, and estimate emissions as the product of the specific factor for that sector and the number of animals associated with the enterprise or geographic region. The committee found and reported in its Interim Report (NRC, 2002a) that existing data were not adequate at this time to determine accurate emission factors, and that a greater number of emission factors than originally proposed would be needed to explain the variation in emissions from different enterprises. The committee debated the cost and time requirements to conduct necessary studies and develop appropriate emission factors. The approach would require a considerable number of new measurements to represent all conditions (e.g., farm sectors, animal productivity levels, management choices, climates) that could affect emissions. Regression analysis would be used to determine empirically which factors were of greatest importance. After having considered the emission factor approach, the committee recommended an alternative: to estimate emissions using process-based models rather than strictly empirical models.
Process-based models would involve analysis of the farm enterprise through study of its component parts. The analysis would use mathematical modeling and experimental data to simulate conversion and transfer of reactants and products through the farm enterprise. In many cases, this approach would make use of mass balance equations to represent mass flows and losses of major elements through the system. Development of process-based models would still require research studies to obtain measurements of transformations (changes in form of relevant compounds) and transfers (changes in location of compounds), as well as emissions in animal enterprises. Nonetheless, the estimates would be based on the understanding of processes inherent to animal production. System-level models would be needed to predict the effect of management changes simultaneously on different compounds emitted and to estimate the effects of management changes on multiple farms associated with animal production either directly or indirectly (e.g., crop production).