HAZARDOUS GAS-MODEL EVALUATION WITH FIELD OBSERVATIONS

Fifteen hazardous gas models were evaluated using data from eight fiel d experiments. The models include seven publicly available models (AFT OX, DEGADIS, HEGADAS, HGSYSTEM, INPUFF, OB/DG and SLAB), six proprieta ry models (AIRTOX, CHARM, FOCUS, GASTAR, PHAST and TRACE), and two ''b enchmark'' analytical models (the Gaussian Plume Model and the analyti cal approximations to the Britter and McQuaid Workbook nomograms). The field data were divided into three groups-continuous dense gas releas es (Burro LNG, Coyote LNG, Desert Tortoise NH3-gas and aerosols, Goldf ish HF-gas and aerosols, and Maplin Sands LNG), continuous passive gas releases (Prairie Grass and Hanford), and instantaneous dense gas rel eases (Thorney Island freon). The dense gas models that produced the m ost consistent predictions of plume centerline concentrations across t he dense gas data sets are the Britter and McQuaid, CHARM, GASTAR, HEG ADAS, HGSYSTEM, PHAST, SLAB and TRACE models, with relative mean biase s of about +/- 30% or less and magnitudes of relative scatter that are about equal to the mean. The dense gas models tended to overpredict t he plume widths and underpredict the plume depths by about a factor of two. All models except GASTAR, TRACE, and the area source version of DEGADIS perform fairly well with the continuous passive gas data sets. Some sensitivity studies were also carried out. It was found that thr ee of the more widely used publicly-available dense gas models (DEGADI S, HGSYSTEM and SLAB) predicted increases in concentration of about 70 % as roughness length decreased by an order of magnitude for the Deser t Tortoise and Goldfish field studies. It was also found that none of the dense gas models that were considered came close to simulating the observed factor of two increase in peak concentrations as averaging t ime decreased from several minutes to 1 s. Because of their assumption that a concentrated dense gas core existed that was unaffected by var iations in averaging time, the dense gas models predicted, at most, a 20% increase in concentrations for this variation in averaging time.