CALPUFF, HPAC, and VLSTRACK evaluation with the Over-Land Alongwind Dispersion (OLAD) field data

The CALPUFF (California Puff), HPAC (Hazard Prediction and Assessment Capability), and VLSTRACK (Chemical/Biological Agent Vapor, Liquid and Solid Tracking) transport and dispersion models were evaluated with the Over-Land Alongwind Dispersion (GLAD) field data. OLAD involved nearly instantaneous line releases of sulfuric hexafluoride (SF6) from truck or aircraft on a mesoscale field site at Dugway Proving Ground, Utah. Meteorological data were measured at 16 surface sites, at one radiosonde site, and at one pibal site. SF6 concentrations were mainly measured by the whole-air samplers along three sampling lines. In general, all three models underpredicted the maximum dosages across a sampling line. CALPUFF, HPAC, and VLSTRACK predictions indicated a factor of 1.8, 2.0, and 3.3 mean underprediction, respectively; and a factor of two to seven random scatter for all three models, with HPAC having the smallest scatter, The fraction of predictions within a factor of two of observations was about 30% for CALPUFF, 25% for VLSTRACK, and about 50% for HPAC. The underprediction might be due to the model formulations for the vertical dispersion coefficient and the cloud advective speed.