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
Theory and methods for odor evaluation
The typical study of the sense of smell is concerned with the effects of a single odor substance. Most of our knowledge therefore applies to pure substances and not complex gas mixtures 27,42,66. In real life, however, most odors are complex mixtures of various components in rather low concentrations. Although much is still unknown about the functioning of the olfactory system, the knowledge available gives an impressive picture of its capacity 22. For example, the ability to discriminate among odorous compounds is enormous even in the ever present odorous environment zs. Further, the sense can handle a wide range of stimulus concentrations and can often detect odor substances at the ppb level ~. On the other hand, the sense of smell is easily fatigued by continuous and constant odor exposure, though it recovers fast on the termination of exposure 38,42. The development of optimal methods for sensory odor evaluation using human observers presents both a challenge and a promise, regardless of whether the odor system is used as a measurement tool or as an indicator of environmental pollution. In the following, theories and methods for odor evaluation are discussed. Examples are given from laboratory as well as field research. Needless to say there is not much difference in the methodological requirements of basic and applied research.
Concentration fluctuations and peak-to-mean concentration ratios in plumes from a ground-level continuous point source☆
Diffusion data collected by 63 incremental samplers during four short duration, continuous releases of 85Kr are used to examine concentration fluctuations and peak-to-mean concentration ratios in the atmosphere. Cumulative frequency distributions and the intensity of short-term concentrations are shown to be a function of the relative crosswind position within the mean plume. Peak-to-mean concentration ratios are shown as a function of relative crosswind position within the plume and the ratio of the durations of the mean and the peak (time ratio). 
 
The relationship between the intensity of concentration fluctuations and the relative crosswind position is similar to that reported for laboratory data. The relationship between peak-to-mean ratios and the time-ratio was found to differ from the simple power law relationship suggested by several workers. Previous estimates of the peak-to-mean/time ratio relationship for positions near the mean plume axis are shown to be too low.