Generation of Aerosols by an Urban Fire Storm

Particles are being studied that were generated during a fire storm in Hiroshima, Japan. The fire storm was initiated by the atomic bomb detonated on August 6, 1945. These particles were extracted from streaks of "black rain" found on a plaster wall and may be representative of the aerosols that lead to a "nuclear winter." Gamma spectroscopy measurements indicate the presence of naturally occurring radionuclides 40 K, 226 Ra, 228 Ra, 232 Th, and 234 Th, along with the fission product 237 Cs. Sooty particles of varying sizes down to the submicrometer range have been found using optical photomicrography. These particles have been studied using X-ray-induced X-ray fluorescence, and Ca, Sr, Ba, Fe, and Zn have been detected with elemental composition ratios representatave of Hiroshima soil. Particle size measurements using electron and optical microscopy and laser-flow cytometry indicate that the particles have a number mean diameter of 1.8 μm, while 30% are less than 1 μm in diameter. Thus the residence time of the smaller-size fractions in the atmosphere might be long, if these particles are not removed by physical processes such as oxidation or rainout. The presence of graphite as a component of these particles is suggested by electron photomicrographs and has been confirmed using Raman spectroscopy, surface ionizaton mass spectroscopy, and electron scattering for chemical analysis. The results of electron scattering studies indicate, however, that the carbon is primarily amorphous, suggesting that graphite occurs only as short-range order in the amorphous particles.