Continued assessment of chlorine reactivity with environmental materials for hazard assessment of large scale releases

Abstract

Many toxic industrial chemicals (TICs) such as chlorine and ammonia are reactive with commonly encountered environmental materials. The amount of TIC material reacted in the environment can be an important factor in the consequences of a release. Typically characterized with a dry deposition rate in atmospheric dispersion models, dry deposition has predominately been studied in field-scale experiments at concentration levels typical for air pollution (ppb or ppt levels), but much higher concentrations are relevant in hazard consequence assessment.For consequence assessment purposes, reduction of gas phase chlorine concentration (whether by chemical reaction or adsorption/desorption processes) will reduce the impact of a release. In previous experiments with chlorine, a kinetic model accounting for the maximum mass that can react at a surface (maximum deposition) was proposed and found to agree with available data (Spicer et al., 2021). The importance of maximum deposition will be most important for high chlorine concentrations and may not be possible to detect (or even be important) in air pollution studies. Furthermore, limitation by maximum deposition indicates that the reaction between chlorine and environmental surfaces is actually a second order chemical reaction, while in contrast, the standard (air pollution) approach for modeling dry deposition in atmospheric dispersion models is a first order reaction. The relationship between the two approaches is explored. Current atmospheric dispersion models do not limit the mass of chlorine (or any other contaminant) that can be removed by environmental surfaces. Consequently, there is a possibility of atmospheric dispersion models overestimating the removal of chlorine by dry deposition which would result in under-prediction of the potential hazard.As reported here, new experimental measurements with plant materials and materials characterizing built environments show that the rate of chlorine removal can be described as a first order, heterogeneous reaction provided the effect of maximum deposition is taken into account. The experimental apparatus used in the study was designed to expose selected materials to chlorine with initial concentration of (nominally) 1000 ppm in air with controlled gas velocity having turbulence levels that are comparable to the atmosphere. The materials tested in the two studies were intended to characterize typical environmental materials for various land use categories including built environments appropriate for consequence assessments.