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United States Department of Agriculture

Agricultural Research Service

Title: Dispersion Modeling of Agricultural Low Level Point Sources

Authors
item Wanjura, John - TEXAS A&M UNIVERSITY
item Parnell, JR., Calvin - TEXAS A&M UNIVERSITY
item Lacey, Ronald - TEXAS A&M UNIVERSITY
item Shaw, Bryan - TEXAS A&M UNIVERSITY
item Buser, Michael

Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: July 28, 2003
Publication Date: November 5, 2003
Citation: Wanjura, J.D., Parnell, Jr., C.B., Lacey, R.E., Shaw, B.W., Buser, M.D. 2003. Dispersion modeling of agricultural low level point sources. American Society of Agricultural Engineers. Paper No. 034117. St. Joseph, MI:ASAE.

Interpretive Summary: Industrial Source Complex Short Term Version 3 (ISCST3) is the current EPA-approved dispersion model for low level point sources. ISCST3 uses the Gaussian dispersion equation with Pasquill-Gifford horizontal and vertical dispersion parameters. According to the literature, the Gaussian dispersion equation, in conjunction with the Pasquill-Gifford dispersion parameters, provides 10 minute average PM concentrations. A model was developed to determine downwind concentrations based solely on the Gaussian dispersion equation and corresponding parameters using 10 days of meteorological data. This model was compared to the ISCST3 model using the same 10 days of meteorological data. Results of the comparison showed that the hourly concentrations predicted by ISCST3 matched the maximum 24-hour concentrations predicted by the Gaussian dispersion equation. This time scale deviation resulted in the ISCST3 model over predicting the results of the Gaussian dispersion equation 24-hour concentration by a factor of 2.3. The ISCST3 concentration over prediction was due to the fact that ISCST3 does not account for wind direction changes within one hour intervals.

Technical Abstract: Industrial Source Complex Short Term Version 3 (ISCST3) is the current EPA-approved dispersion model for low level point sources. ISCST3 uses the Gaussian dispersion equation with Pasquill-Gifford horizontal and vertical dispersion parameters. The Gaussian dispersion equation, in conjunction with the Pasquill-Gifford dispersion parameters, provides 10 minute average PM concentrations. However, developers of ISCST3 assumed that this equation and corresponding parameters resulted in 60 minute average concentrations. Results from this research show that this assumption can cause the downwind concentrations predicted by ISCST3 to be over predicted by an average of 2.3 times the actual concentration. There are numerous examples of cotton gins that have been inappropriately regulated based upon the errors outlined in this paper. It has been demonstrated here that a few basic changes, based upon science, can have a great impact on the predicted downwind concentrations from low level point sources.

Last Modified: 12/26/2014
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