Deposition Following Accidental Releases of Chlorine from Railcars

Chlorine releases to the atmosphere due to accidents involving railcars can be extremely hazardous to health, the environment, and man-made materials. Since the chlorine is released as a mixture of reactive gas and small (median diameter of 20-100 mu m) aerosols, and the initial cloud has a very high concentration (>10,000 ppm), deposition to the surface can be important. The various mechanisms include dry deposition caused by chemical reactions between the gas and the surface (ground, vegetation, or materials), dry or wet deposition of small aerosols, and gravitational settling and impaction of larger aerosols. The state-of-the art in gas deposition modeling is based on the resistance analogy, which has been widely used in deposition modeling of a variety of air pollutants. The resistance formula is reviewed and it is shown that, even though chlorine is relatively reactive, its deposition may be inhibited by the increased aerodynamic resistance in the very stable cloud. A method is suggested for parameterizing the effect of the dense cloud on the aerodynamic resistance. Deposition measurement methods planned for the Jack Rabbit II (JR II) chlorine release field experiments are reviewed, where up to 10 tons of pressurized liquefied chlorine will be released in several field trials.