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 (OLAD) 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.