Atmospheric Dispersion of Gases and Odours from Animal Production Systems

Abstract

Animal production systems are major agricultural sources of the greenhouse gases, methane and nitrous oxide, of pollutants such as ammonia and hydrogen sulfide, and of odours. Verifying inventory estimates of their production, and monitoring, regulating and reducing their emission all require the ability to make precise, non-obtrusive measurements of their fluxes, and in the case of odours, an ability to predict concentrations close to the source. However, these requirements present special problems. The source areas are often small or irregularly shaped, the sources may be distributed heterogeneously, and emissions may be transient. Odours present an additional problem in that it is difficult to quantify their intensity since they are usually due to a mixture of more than 100 volatile organic compounds. The use of tracer gases to follow odour dispersion is discussed, but quantifying odour intensity is a pressing problem. The paper considers measurement techniques appropriate for sources of different shapes and sizes. Most attention is given to small and irregularly shaped sources for which conventional, large-area micrometeorological approaches are inapplicable: line-sources, strips, small plots, and waste storage areas such as ponds and manure piles. Mass balance techniques appear to be particularly useful for these purposes. Various applications of the methodology are considered in some detail. The advent of open-path gas lasers that can measure concentrations rapidly and over distances of hundreds of meters is seen as a significant development. Finally, the paper considers the use of a Lagrangian dispersion model that allows emissions from sources of any geometry to be linked to concentrations at any location downwind. In forward mode, it can predict downwind concentrations for given source scenarios, while in backward mode, it can infer fluxes from measurements of concentration and wind speed at just one height downwind. It is a very attractive tool for use in many dispersion problems relevant to the theme of the paper.