(173o) Chlorine and Ammonia Removal By Dry Deposition during an Accidental Release: Considering the Importance of the Boundary Layer Resistance

Many toxic industrial chemicals (TICs) such as chlorine and ammonia are very reactive with commonly encountered environmental materials. The amount of TIC material reacted in the environment is an important factor in determining the impact of a release. Typically characterized with a dry deposition rate in atmospheric dispersion models, the reaction rate has predominately been studied in field scale experiments at concentration levels typical for air pollution (ppm levels). When considering the removal of chlorine or ammonia by (dry) deposition in an episodic release, gas phase concentrations will be much higher than those associated with air pollution, and consequently, reaction rates of chlorine and ammonia with local vegetation, soil, and built-environment materials are expected to be much higher. In this work, standard engineering methods characterizing the rate of mass transfer from a fluid to a surface in conjunction with reaction at that surface are applied, and analysis of experimental data showed that reaction of chlorine and ammonia reach a limiting maximum. This study developed a model to account for boundary layer resistance and the impact of maximum deposition in a framework that could be incorporated in atmospheric dispersion models. The Controlled Environment Reactivity Test (CERT) apparatus was designed to expose selected environmental materials to chlorine and (separately) ammonia with initial concentration of (nominally) 1000 ppm. The gas phase concentration was measured as a function of time to determine relevant modeling parameters. The apparatus was built to control the velocity of the gas/air mixture over the experimental substrates with turbulence levels that are comparable to the atmosphere. Model parameters were found for the environmental materials tested. Ammonia proved much more reactive than chlorine, and consequently, the characterization of the boundary layer resistance proved important for the data analysis.