At about 7:15 a.m. on July 26, 1993, a safety valve ruptured on a railroad tank car containing approximately 95 tons of 35-grade oleum (H2S2O7), 35 % or 33 tons of which was sulfur trioxide (SO3). The release from the tank car was heated gaseous SO3, which immediately condensed into a fine sulfuric acid mist (H2SO4) with a median diameter of 1 micrometer. The final estimate of the sulfuric acid released in the first hour was 8,000 pounds (The estimated SO3 source rate was multiplied by 1.225, the ratio of the molecular weight of sulfuric acid to sulfur trioxide, to account for the addition of water vapor to the sulfur trioxide gas during its conversion to sulfuric acid mist).
Scientists at the Lawrence Livermore National Laboratory Atmospheric Release Advisory Capability (ARAC) provided dispersion modeling assistance to the California Office of Emergency Services and the Bay Area Air Quality Management District (Baskett, 1994). ARAC is a centralized federal program for assessing atmospheric releases of hazardous materials (Sullivan, 1993). ARAC employs a three-dimensional, diagnostic, finite-difference dispersion modeling system for estimating the consequences from accidental atmospheric releases. MATHEW (Mass-Adjusted Three-Dimensional Wind field), an Eulerian wind field code (Sherman, 1978), and ADPIC (Atmospheric Diffusion by Particle-In-Cell), a hybrid Eulerian-Langrangian dispersion model (Lange, 1978), form the core of the system. The ARAC and EPIcode data both assumed the effective deposition velocity was 1 centimeter per second for the sulfuric acid mist.
Baskett, R.L., Vogt, P.J., Schalk III, Pobanz, B.M., “ARAC Dispersion Modeling of the July 26, 1993 Oleum Tank Car Spill in Richmond, California,” UCRL-ID-116012, February 3, 1994.
Sullivan, J.T., Ellis, J.S., Foster, C.S., Foster, K.T., Baskett, R.L., Nasstrom, J.S., and Schalk III, W.W., "Atmospheric Release Advisory Capability: Real-Time Modeling of Airborne Hazardous Materials", Bulletin of the American Meteorological Society, pp. 2343-2361, 1993.
Sherman, C.A., “A Mass-consistent model for wind fields over complex terrain,” Journal of Applied Meteorology, 17, 312-319, 1978.
Lange, R., “A Three-dimensional Particle-in-cell Model for the Dispersal of Atmospheric Pollutants and its Comparison to Regional Tracer Studies,” Journal of Applied Meteorology, 17, 320-329, 1978.
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