Atmospheric Chemistry Knowledge Gaps for Accidental Releases of Chlorine from Railcars
In an accident, the pressurized liquefied chlorine is released through a relatively large hole (perhaps about 10 cm), where 20 tons can be released in a few minutes or less. The flashing process at the source aperture produces a very dense chlorine cloud that is initially about 20 % gas and 80 % small (median diameter 20 to 30 μm) aerosol drops, with a temperature near the boiling point of chlorine (- 34 C). The first challenge is to identify samplers (in situ or remote) that can accurately measure the chlorine concentration in a two phase cloud at very high concentrations (exceeding 1 %). The second challenge is to measure the aerosol chemical composition and size distribution; there is uncertainty concerning how fast the chlorine evaporates and how the chlorine gas and aerosols interact with ambient water vapor and drops that have condensed in the very cold (-34 C) cloud. The third challenge is to measure the chemical conversion of chlorine to reaction products in the near field (where there is an aerosol cloud) and farther downwind (past a few hundred meters) where the cloud is all gas and is invisible. Since chlorine is very reactive with water and organic matter on surfaces, the fourth challenge is to measure removal of the chlorine at the ground surface and by vegetation. Of course in all cases, care is needed to avoid damage to the sampling instruments by the corrosive chlorine cloud.