Calibration of the dispersion code SAFE_AIR using a release in nocturnal low wind conditions

The SAFE_AIR code simulates the transport and diffusion of airborne pollutants. This dispersion code is based on the advection of Gaussian segments and puffs driven by a 3D diagnostic wind model, able to deal with both nonstationary and inhomogeneous conditions. SAFE_AIR is an evolution of the AVACTA II code, a code "recommended" by the U.S. EPA. In this work, we applied SAFE AIR to a tracer project designed to collect diffusion data in the region of Ilo, Peru, a complex coastal area where large SO2 emissions from a copper smelter plant affect the local air quality. The field project is based on a large set of meteorological and tracer (SF6) field data collected in the area of interest under a variety of meteorological conditions and during different times of the day. Among the tracer experiments performed we simulated a nocturnal low wind conditions stack release. This release provided the best example of plume movement in the area and also contained the highest SF6 concentration measured during the entire field project. The strong time variability and inhomogeneity of the experimental conditions presented a challenge to the code. The issues we considered in this exercise concern the ability of the model to predict the temporal evolution of the tracer gas dispersion pattern. To do this, special attention was directed towards a detailed description of the nocturnal light wind field during the tracer release and the selection of the code wind speed value to discriminate between transport and calm conditions.