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The Benefits and Challenges of Using HAP 491 with Crack for HVAC Design and Analysis

The ISCST model is designed for use in predicting concentrations using averaging periods from one hour to one year. This model utilizes discrete hourly meteorological data. The ISCLT model is designed for use in predicting annual-average concentrations. This model utilized meteorological data in the format of a STAR summary. The STAR summary is a joint-frequency distribution of wind speed, wind direction, and stability classification, processed from discrete hourly observations. The use of this meteorological data summary enables the ISCLT dispersion model program to calculate ambient concentrations much faster than ISCST because dispersion calculations are performed for a small number of meteorological categories rather than for every hour of the year. The ISCLT and ISCST use identical equations for calculating ambient concentrations, with the exception of several changes necessary for the incorporation of the STAR summary.

Hourly Analysis Program 491 Crack


Two quality-assured sets of meteorological data were evaluated for use in this analysis: 1) a 14-year composite annual joint frequency distribution of wind speed, wind direction, and stability class (STAR) data processed from the National Weather Service (NWS) hourly surface observations at the County Airport, located approximately four miles from the plant boundary; and 2) a two-year composite STAR data set processed from 1990 and 1991 Regional Planning Commission (RPC) continuous observations at another County Airport location, approximately three miles form the plant boundary. The RPC data were selected for use in the majority of the analyses due to the continuous nature of the observations and the use of measured mixing heights. However, to examine the sensitivity of the risk estimates to changes in the meteorological data set, the ISCLT dispersion model was run with identical inputs, varying only the meteorological data. At nearby locations, predicted concentrations using RPC data were 25 to 100% higher than predicted concentrators using the NWS data. Using the RPC data, concentrator isopleths would extend farther to the east and are more rounded. using the NWS data, the isopleths would show more of a northsouth bias (Radian, 1992a).


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