Evaluation of Low Wind AERMOD Modeling Approaches for Tall-Stack Databases

摘要

The increased reliance upon modeling tools for determining 1-hour SO_2 NAAQS compliance for existing facilities is a new emphasis by EPA. One such tool used extensively in this regard is EPA's preferred dispersion model for short-range applications, AERMOD. AERMOD's handling of low wind speed conditions, especially for applications with only one level of meteorological data and no direct turbulence measurements or vertical temperature gradient observations, is the focus of this study. The analysis documented in this paper addresses evaluations for low wind conditions involving tall stack releases for which multiple years of concurrent emissions, meteorological data, and monitoring data are available. AERMOD was tested on three databases in default mode, with various low wind speed beta options, and using sub-hourly meteorological data. These databases included: 1) Mercer County, a North Dakota database featuring 5 SO_2 monitors in the vicinity of the Dakota Gasification Company's plant and the Antelope Valley Station power plant in an area of both flat and elevated terrain; 2) a flat-terrain setting database with four SO_2 monitors in the vicinity of Gibson Generating Station, and 3) a flat and complex terrain setting database with four SO_2 monitors near Mt. Tom Power Plant. The low wind beta options show a considerable improvement in model performance helping to reduce some of the over-prediction biases currently present in AERMOD when run with regulatory default options. The overall findings with the low wind speed testing on these tall stack databases indicate that AERMOD low wind speed options have a minor effect for flat terrain locations, but can have a very significant effect for elevated terrain locations. The performance of AERMOD using low wind speed options leads to improved consistency of meteorological conditions associated with the highest observed and predicted concentration events. The available sub-hourly modeling results using the Sub-Hourly AERMOD Run Procedure (SHARP results) are relatively unbiased for the North Dakota and Gibson databases.