ELECTRON-MICROSCOPE STUDIES OF MT-PINATUBO AEROSOL LAYERS OVER LARAMIE, WYOMING DURING SUMMER 1991

Stratospheric aerosol layers resulting from the June 1991 eruptions of Mt. Pinatubo were first observed over Laramie, Wyoming in July 1991. Atmospheric particles were collected from these layers during three balloon flights in July and August using cascade impactors. Analytical electron microscope analysis of the aerosol deposits indicated that a large majority (> 99%) of the fine particles in all three samples were collected as submicrometer aqueous H2SO4 droplets, which changed to (NH4)2SO4 particles over time. Other particles observed in the aerosol were larger, and consisted of supermicrometer sulfate particles and composite sulfate/crustal particles which ranged up to approximately 10-mu-m in size. Peak aerosol concentrations for r > 0.15-mu-m diameter particles (determined by optical particle counters) in the layers were higher for the July flights than for the August sounding. This was reflected in the electron microscope results, which showed that the July impactor samples had particulate loadings on the fine particle stages which were 20-30% higher than those from the corresponding substrate from the August sample. A detailed analysis of the fine sulfate aerosol was performed to assess whether the sulfate particles contained small condensation nuclei. Nearly all analyzed sulfate particles showed no evidence of a solid or dissolved nucleus particle, which suggests that the volcanic H2SO4 aerosol formed through homogeneous nucleation processes. These data support heated-inlet optical particle counter data from the balloon flights which suggest that 95-98% of the volcanic particles were aqueous H2SO4.