Evaluation of the SAFE_AIR code against air pollution field and laboratory experiments

The SAFE_AIR model simulates the transport and diffusion of airborne pollutants using Gaussian plume segments and/or puffs. The model is able to deal with both non-stationary and inhomogeneous conditions. SAFE_AIR is an evolution of the AVACTA II model “recommended” by the US Environmental Protection Agency. With respect to AVACTA II, SAFE_AIR contains a number of improvements among which: the complete change of the wind field generation, different algorithms calculating the wind field spatial average providing the advection velocities of pollutant elements, and a more realistic description of deposition patterns. The model evaluation of some aspects of SAFE_AIR was performed using field data (from the KNRC Katrex experiments, flat terrain, convectively unstable and neutral conditions) and laboratory data (from the EPA wind tunnel Rushil experiments, two-dimensional schematic hill, neutral conditions). This paper contains a comprehensive description of the results of these model evaluations. These exercises allowed us to draw some conclusions coherent with what one could expect from a similar model. Among them: (1) the model performance is better when the wind tunnel experiments are simulated than when an open-air experiment is performed; (2) the simulation results are in better agreement with measurements performed in an almost neutral atmosphere than with those performed in unstable conditions. (3) the model has still some problems in simulating emissions near the ground, while the results relative to elevated emissions are more satisfactory; (4) the choice of the σ-function is the most critical among the options considered in our numerical experiments. We believe these conclusions both rather general and useful as far as the calibration and operational use of SAFE_AIR are concerned.