DETERMINATION OF ODOR SOURCES FOR CONTROL
The successful solution of odor pollution problems first requires an assessment of the nature and extent of the problems. A plant or facility that emits odor from a single source (such as ii stack or vent) requires less effort than a plant with a variety of odor sources that may number in the hundreds. In the latter case, there is a need to assess the problem in such terms as to achieve maximum reduction in odor at a minimal control cost. Quantitative and accurate measurements of the various emission sources and their contribution to the odor in the ambient air must be determined in order to identify and rank the various sources that should receive priority for control. Accurate measurements of odor strength relative to concentration are also useful for defining the degree of control necessary for the single-source situation. Attempts to relate source odor measurements with ambient odors by the use of diffusion equations have met with varying degrees of success.1.2 Wohlers,l utilizing an evacuated bulb technique for sampling stack gases from a petroleum coking plant, a kralt paper mill, an anion-garlic dehydrating plant, and a rotogravure printing plant, found that odor measurements at the stacks did not agree in all cases with the calculated dilutions at the distances in the field where the odors were noted. Hogstrom2 demonstrated that a fluctuating plume dispersion model can be used to give realistic estimates of odor frequencies around a point source when the odor thresholds of the chimney gases (sulphate pulp factory) are determined by sampling and sensory methods described by Lindvall.3 Our experience in sampling and evaluating odors has indicated that existing sampling techniques such as ASTM syringe method have seriously underestimated the magnitude of odors emitted by a factor in the range of 2OO-2O,OOO.4 We believe that inadequate sampling and evaluation techniques are the major reasons for reported “failures” of dispersion estimates in predicting ambient odors. In our technique, we use trained odor chemists to carry out odor surveys of the surrounding areas to define the odor types and intensities perceived off-site and to conduct in-plant surveys to identify all significant odor sources. T o supplement this, we have developed a source-sampling detailed laboratory odor-evaluation technique to allow comparison of the various odor sources and to make use of predictions based on dispersion equations. We have observed reasonably good correlation between predicted odor intensities from the source analyses determined in the laboratory and those observed by our analysts in the field. OFF-SITE SURVEY
A method for predicting odor frequencies from a point source.
Abstract It is demonstrated that a fluctuating plume dispersion model can be used to give realistic estimates of odour frequencies around a point source, when the odour threshold of the chimney gases is determined by sensory methods ( Lindvall , 1970). The theoretical calculations are compared with observational results from a field experiment at and around a sulphate pulp factory in Sweden. Trained observers were used, making a large number of instantaneous observations of odour in the outdoor air at a variety of locations around the factory.
A statistical approach to the air pollution problem of chimney emission
A method is described that not only gives one single concentration value but the expected concentration frequency distribution at an arbitrary point in the vicinity of the emitting chimney. Unlike the method of gifford (1959) the concentration frequency distribution obtained comprises the full range of meteorological conditions at a given place and not only the short term variations in concentration during one particular meteorological situation. 
 
In making the principal mathematical formulation of the method no assumptions whatsoever are needed about the mechanism of dispersion. In the practical application on the other hand, full use is made of the detailed knowledge of the dispersion process. 
 
First a “basic case” is treated, the chief characteristics of which are: isolated stack, situated on a flat surface of uniform roughness; sampling time about an hour. The following information is found necessary for solving a concrete problem of this kind: 
1. 
(1) plant data, viz. rate of emission, gas volume and temperature, chimney height and exit diameter; 
 
2. 
(2) geographical site data, viz. roughness length; 
 
3. 
(3) meteorological data, viz. wind direction frequencies and statistics of “dispersion categories” (stability and wind speed). Applicable formulae are discussed and also how statistics of “dispersion categories” can be obtained by evaluating data from radiosonde stations. 
 
 
 
 
Certain deviations from the basic case are also treated in some detail; limited mixing height, the effect of large heat content on the dispersion parameters, the effects of buildings and topography, ground level release and sampling times other than an hour. 
 
A thorough test of the described method is presented. Nine months statistics of 2-hr SO2 concentration (2602 values) measured at a point situated 750 m from a sulphuric acid plant are compared with theoretically obtained statistics. The result strongly supports the method presented.