Following the massive 1883 Krakatoa volcanic eruption, a new atmospheric optical phenomeon was identified by Rev. S. E. Bishop. This inconspicuous one-ringed corona, or aureole, was immediately linked to the global spread of volcanic debris injected into the stratosphere, but little refinement in the mechanisms responsible for Bishop's ring has since been made. On the basis of our combined studies of sulfuric acid droplet-freezing theory and polarization (0.694-μm) lidar measurements of Bishop's ring aerosols from the June 1991 Mt. Pinatubo eruption that show average linear depolarization ratios of ∼0.05, it appears that this solar diffraction phenomenon is caused by accumulations of nonspherical sulfuric acid tetrahydrate (SAT) particles. The diffraction-theory aureole-derived SAT particle radius of ∼0.8 μm is consistent with the freezing of the large mode of volcanic acid droplets created by coagulation, which, according to theory, is necessary for concentrating a sufficient insoluble mass to promote heterogeneous drop freezing at temperatures below approximately —65°C.
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