Modification of a Gauss dispersion model for the assessment of odour sensation in the vicinity of livestock buildings.

The concentration of odoriphores can be handled like other volatile pollutants. The dispersion of airborne emissions can be described by dispersion models (eg Gaussian model). To apply a dispersion model to odour emissions, the odour concentration and the volume flow of the outlet air have to be known. The emission model is based on a steady-state balance of the sensible heat fluxes to calculate the indoor temperature and the related volume flow of the ventilation rate. By a simple model of the odour release the odour flow is assessed. The dispersion model calculates half hour mean concentrations. The sensation of odour, however, depends on the momentary odour concentration and not on a mean value over a long time of integration. Therefor the concentration fluctuation has taken into account. By the concept of the peak-to-mean ratio the calculated mean value is transformed to the expected peak concentration. The change of the variability of the mean concentration is expressed as a function of the time of travel, and the Lagrangian time scale which describes the turbulent diffusion, strongly depending on the stability of the atmosphere. The mean concentration of the dispersion model is modified by an amplification function depending on the stability, wind velocity and the distance. The highest amplifications can be expected for very unstable conditions with low wind velocity. INTRODUCTION Livestock farming is increasingly confronted with questions of environmental protection because of different kinds of pollutants brought into the atmosphere. One of them is odour which is a very important component because the acceptance of livestock farming in the vicinity can decrease due to an increase in odour sensation (Schiffman, 1998). The concentration of odoriphores can be handled like other volatile pollutants and can be measured by an olfactometer in odour units per volume (OU m). One odour unit per qubicmeter is defined as the concentration of odoriphores which results in an odour sensation by at least half of the members of a panel (CIGR, 1994). The odour sensation is triggered by the odour stimulus and characterised by intensity and frequency. To predict these parameters it is necessary to consider short-term fluctuations of odourant concentrations at the receptor point. Odour sensation can only be observed if the odourant concentration is higher than the odour threshold of the substances. Due to fluctuations an odour sensation can take place even if the mean odourant concentration is lower than the odour threshold. Previous investigations of the present authors concentrated on a comparison of the shape of the different power functions used by various guidelines to calculate the separation distance between livestock farms and residential areas with that derived via a Gauss model to conclude that the latter shows a stronger dependence from the odour emission than suggested by the guidelines (Piringer and Schauberger, 1999) as well as on the development and test of a steady-state balance model to calculate the indoor climate of livestock buildings (Schauberger et al., 1999, 2000ba). No diurnal variation of the odour emission was assumed in these publications. This paper * The authors are Gunther Schauberger, ASEA Member Engineer, Associate Professor, Institute of Medical Physics and Biostatistics, University of Veterinary Medicine Vienna, Vienna, Austria (Veterinarplatz 1, A 1210 Vienna, Austria; voice: +43 (1) 250 77 4306; fax: +43 (1) 250 77 4390; e-mail: gunther.schauberger@vu-wien.ac. at); Martin Piringer, Senior Researcher, and Erwin Petz, Computer Engineer, Central Institute for Meteorology and Geodynamics, Vienna, Austria. presented at the 2 International Conference on Air Pollution from Agricultural Operations, Iowa October 2000