Influence of meteorological time frame and variation on horizontal dispersion coefficients in Gaussian dispersion modeling
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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 overestimates 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 employed to develop the presently used Pasquill-Gifford (PG) dispersion coefficients is explored and used in developing a new methodology for estimating theoretical dispersion coefficients based on recorded meteorological data. This comparison allowed for an appropriate application time period of the PG dispersion coefficients to be determined, which in general varied from 3 to 20 min. The most critical result of this research is that universal application of the PG dispersion coefficients to a 1 h time period is incorrect. This misapplication will result in concentration estimates based on insufficient plume spread, which will overestimate downwind concentrations and result in inappropriate regulation of emitting sources.
