During the 1984 ASCOT field study in Brush Creek, CO, two gaseous tracers were released into valley drainage flow at different heights at the same site on 5 nights. This paper describes simulations of surface concentration data from two tracer experiments, one elevated release and one surface release, which complement those reported by Rao et al. (1989, J. appl. Met. 28, 609-616). The integrated Gaussian puff model adapted by them has been improved and used for these new simulations. The improved model, VALPUFF, affords a physically realistic simulation of the mean transport for elevated plumes under highly variable emission conditions. The model simulations are evaluated by comparing predicted hourly concentrations with the corresponding observed values at 51 surface samplers. For the elevated release case, the observed mean concentration is 28 pl l-1, and the predicted mean is 31 pl l-1. Almost 85% of predictions are within a factor of 5 of observations, and 50% within a factor of 2. For the ground release case, the observed and predicted means are 128 and 183 pl l-1, respectively. About 53% of predictions are within a factor of 5 of observations, and 23% within a factor of 2. The effects of varying wind field inputs and turbulence parameterizations on model performance are tested using several variations of the VALPUFF model. Cumulative frequency distributions of the ratio of modeled and observed hourly concentrations, and other evaluation statistics, are presented for each model. Applying resampling methods, 95% confidence intervals for fractional mean bias and normalized mean square error are calculated for the predictions of each model and for differences between the predictions of various models. It is shown that the VALPUFF model that uses vertically-averaged winds (for elevated releases) and dispersion parameters based on on-site turbulence data gives the best overall performance.
