Chlorine and Anhydrous Ammonia Concentrations Observed and Simulated in the Jack Rabbit Field Experiment, for Releases of 1 or 2 Tons in a 30 to 60 Second Period

Large amounts of chlorine released in a few minutes from pressurized liquefied storage such as a railcar may form a dense two-phase (gas plus small liquid aerosol drops) cloud at ground level. The time duration of release from the storage tank could be as small as a few seconds for a large hole with diameter exceeding 20 or 30 cm. But even if the release from the tank occurs in a few seconds, a dense two-phase cloud may remain in the area of the tank for many minutes if winds are light enough and/or there is a terrain depression. The 2010 Jack Rabbit (JR) field experiments at Dugway Proving Ground, Utah, demonstrated that a 30 to 60 s release of one or two tons of pressurized liquefied chlorine or anhydrous ammonia gas would result in an initial hold-up of about 30 min of the dense two-phase cloud at winds less than about 2 m/s. In this case, the chlorine gas would be slowly detrained from the surface of the two-phase cloud around the source location, following the 1990 theory of Briggs. The observed cloud hold-up time is proportional to the cube of the wind speed, in agreement with the theory. This detrainment process can be treated as an area source as far as downwind concentrations are concerned. We have carried out an analysis, considering instrument thresholds, mean biases, and uncertainties, of three types of JR concentration samplers (MiniRae, Jaz, and Canary) operated on arcs at distances from 25 to 500 m from the source. The resulting best estimates of arc-maximum observed 10 min averaged concentrations (important for health effects) at each distance are compared, assuming the two-phase cloud hold-up time as estimated using Briggs’ theory.