I/. &EFK United States Environmental Protection Agency Environmental Sciences Research ^* Laboratory '. Research Triangle Park NC 27711 Research and Development EPA-600/S3-82-078 Oct. 1982 Project Summary Estimating Concentrations Downwind from an Instantaneous Puff Release William B. Petersen This document provides an ap- proach to estimating concentrations downwind of an instantaneous puff release. Dispersion of the puff is described by the Gaussian puff equa- tion, using the dispersion parameters presented in Meteorology and Atomic Energy. The primary purpose of this work is to provide estimates of the instantaneous peak concentration or average concentration through the use of simple equations and nomo- grams. Example problems are pro- vided to demonstrate the use of the equations and nomograms. A com- puterized Gaussian puff model is also presented. This Project Summary was devel- oped by EPA's Environmental Sciences Research Laboratory. Research Tri- angle Park. NC, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ' ordering information at back). Introduction This document is provided m re- sponse to the need for making disper- sion estimates of accidental releases of hazardous chemicals. Every year there are news stories of industnal-or trans- portation-related spills requiring the evacuation of nearby residents. The main objective of this work is to provide the air pollution specialist a meth- odology for estimating peak concen- trations downwind of an instantaneous release. The equations, nomograms, and methodology described in this docu- ment were not designed for the fireman at the site of an accidental spill. He will not have the time or the technical back- ground to use the equations and nomograms properly. The factors af- fecting his decision to order an evacua- tion will be based primarily on the size of the spill, and the nature and toxicity of the substance. However, material provided in this document would be valuable to the air pollution specialist who is assisting the fire department in estab- lishing procedures to determine toxic corridors. The dispersion algorithms described would provide the basis for a model catering to the specific needs of the user A computer algorithm de- scribed in this document could also be adapted for on-site use at a chemical plant using real time meteorology to predict downwind concentrations. Approach The document is composed of three major sections. The first part of Section 2 describes a technique for estimating peak concentrations of instantaneous puff releases as a function of atmos- pheric stability and downwind distance. This discussion was designed so that within minutes after a spill, estimates of peak concentration could be provided to the agencies directly concerned with the immediate health and welfare of the public. These initial estimates could be provided without the aid of a computer. The remainder of Section 2 is concerned * US GOVERNMENT PRINTING OFFICE 1982-559-017/0856 ------- with estimating average concentrations and exposure. Section 3 provides a discussion of a generalized Gaussian puff equation with the same assump- tions and limitations of the approach described in Section 2. The equations in Section 3 are the basis for a computer model described in Section 4. The modeling effort is significantly simplified if only the magnitudes of the concentrations must be predicted, without regard to exactly where the concentrations will occur. If concentra- tion estimates are to be made at specific points, it is important to account for puff trajectory; and, because methods for determining the trajectory of the puff contain deficiencies, estimating con- centrations at point locations is very difficult. Releases are seldom point sources, but are more typically small area sources. Small area source re- leases can be modeled to some extent by using initial dispersion parameters. The initial horizontal dispersion is typically calculated by dividing the initial horizontal dimension of the area source by 4.3. This method will give reasonable concentration estimates at downwind distances greater than about five times the horizontal dimension of the source. As with the lateral dispersion, the initial vertical dispersion parameter is typi- cally evaluated by dividing the initial height of the source by 2.15. Many sources are not truly instan- taneous. However, if the travel time is long compared to the release time, then the puff model can be used to estimate concentrations. If the travel time to a receptor is on the same order or smaller than the release time, the Gaussian plume model can be used. Where the release time is finite but smaller than the travel time, the instantaneous puff model will provide a worst-case esti- mate of peak concentrations for the same total release. It is difficult to evaluate the effect on average concen- trations over a given sampling time for a finite release using simple models, since the puff is distorted in the downwind direction. Recommendations The model estimates are appropriate for travel times where the atmosphere can be assumed to be steady state. A number of factors affect the length of time the atmosphere will remain approximately steady: season of the year, latitude, synoptic conditions, and local meteorology such as sea breeze and terrain flows. During periods of transition (such as sunrise or sunset), the stability, mixing height, wind profile, wind speed, and wind direction are likely to change; therefore, particular care should be exercised in character- izing the atmosphere during these time periods. For situations where the travel time of the puff is greater than an hour, the factors which affect the dispersion and trajectory of the puff should be incorporated in the concentratioh estimates. The Gaussian instantaneous puff model is applicable for neutrally buoy- ant nonreactive releases. The dis- persion of a small negatively buoyant release can be modeled using the puff equation, provided the material mixes rapidly with the ambient air and disperses like a neutrally buoyant source. The EPA author William B. Peterson is with the Environmental Sciences Research Laboratory, Research Triangle Park, NC 27711. The complete report, entitled "Estimating Concentrations Downwind from an Instantaneous Puff Release," (Order No. PB 82-261 959; Cost: $9.00, subject to change) will be available only frorh: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer cay? be contacted at: Environmental fdfences Research Laboratory U. S'Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Postage and Fees Paid Environmental Protection Agency EPA 335 Official Business Penalty for Private Use $300 PS 0000329 ------- |