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United States Department of Agriculture

Agricultural Research Service

Title: Influence of meteorological time frame and variation on horizontal dispersion coefficients in Gaussian dispersion modeling

Authors
item Fritz, Bradley
item Shaw, Bryan - TEXAS A&M UNIV
item Parnell, Calvin - TEXAS A&M UNIV

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 22, 2005
Publication Date: June 30, 2005
Citation: Fritz, B.K., Shaw, B.W., Parnell, C.B. 2005. Influence of meteorological time frame and variation on horizontal dispersion coefficients in Gaussian dispersion modeling. Transactions of the ASAE. 48:1185-1196.

Interpretive Summary: Small agricultural businesses including cotton gins, feed mills, and grain elevators must demonstrate that normal operations do not create off-site airborne concentrations of particulates exceeding the maximum levels established by the United States Environmental Protection Agency. Compliance demonstration, most commonly through mathematical dispersion models, is required for a business to receive a construction or operating permit and may be required at any time to prove continued compliance. This research examined the present method used to account for dispersion due to meteorological variations and developed a new methodology based on recorded meteorological data. A comparison of these methods demonstrates that application of the present method does not adequately account for plume spread due to variations in meteorological conditions and may result in over-estimates of downwind particulate concentrations. These results provide a basis for the development of a more appropriate dispersion model.

Technical Abstract: The air pollution regulatory process involves the permitting of sources of regulated pollutants. This process requires sources to demonstrate that the National Ambient Air Quality Standards (NAAQS) are not exceeded as a result of released pollutants. A determination of a facility's compliance with the NAAQS is more frequently being based on dispersion modeling estimates rather than ambient air sampling results. Current Gaussian based dispersion models do not adequately account for pollutant dispersion due to sub-hourly variations in wind speed and direction. This can result in over-estimates of downwind concentration and consequentially require costly additional control measures or denial of a construction or operating permit. This research focuses on developing a methodology to analyze the theoretical degree of dispersion within sub-hourly and hourly intervals. The methodology used to develop the presently used plume spread factors (Pasquill-Gifford [PG] dispersion parameters) is explored and used in developing a new methodology for estimating theoretical plume dispersion parameters based on recorded meteorological data. This comparison allowed for an appropriate application time period of the PG dispersion parameters to be determined, which in general, varied from 3 to 20 minutes. The most critical result of this research is that applying the PG dispersion parameters to a one-hour period is incorrect. This misapplication will result in concentration estimates based on insufficient plume spread, which will over-estimate downwind concentrations and result in inappropriate regulation of emitting sources.

Last Modified: 11/23/2014