Odor impact assessment in the plume – A validation using tracer gases in two spatial farm configurations

Livestock facilities and biogas plants pose major challenges for odor assessment due to their spatial extent and the heterogeneity of areal sources. Methods for assessing odor impact in such situations have to overcome points of criticism, such as the lack of reliability and subjectivity in sensory analysis. The aim of this study was to validate an improved procedure for investigating odor plumes by trained assessors. In addition to the widespread approach, which focuses on odor frequency, we combined the odor parameters of intensity and frequency. Due to the relevance of weak and mixed odors, very weak (i.e., perceptible) odors were included, rather than focusing only on recognizable odors (clear, distinct perception). On two farms, a tracer gas approach was implemented to provide an objective measure of dispersion. Comparable spatial patterns in odor parameters (frequency, frequency-weighted odor intensity) and tracer gas concentrations provide a number of key findings to consider in odor assessment of areal sources. Two spatial source configurations were studied—the animal part and the biogas part nested or spatially separated—and discriminated by dosing two different tracer gases. In nested configurations, tracer gases mix homogeneously in the plume and therefore only the combined source can be mapped. By contrast, for spatially separated sources, each position in the plume receives an individual exposure to tracer gases and odor, depending on source arrangement, wind direction, and the adjacent buildings. The presented approach can be extended to reliably track and assign more complex situations. The improved procedures will support objectifying odor impact assessment and pave the way for developing appropriate mitigation strategies.