Observations of Volcanic Clouds in Their First Few Days of Atmospheric Residence: The 1992 Eruptions of Crater Peak, Mount Spurr Volcano, Alaska

Abstract

Satellite SO 2 and ash measurements of Mount Spurr's three 1992 volcanic clouds are compared with ground-based observations to develop an understanding of the physical and chemical evolution of volcanic clouds. Each of the three eruptions with ratings of volcanic explosivity index three reached the lower stratosphere (14 km asl), but the clouds were mainly dispersed at the tropopause by moderate to strong (20-40 m/s) tropospheric winds. Three stages of cloud evolution were identified. First, heavy fallout of large (1500 mm) pyroclasts occurred close to the volcano (!25 km from the vent) during and immediately after the eruptions, and the cloud resembled an advected gravity current. Second, a much larger, highly elongated region marked by a secondary-mass maximum occurred 150-350 km downwind in at least two of the three events. This was the result of aggregate fallout of a bimodal size distribution including fine (!25 mm) ash that quickly depleted the solid fraction of the volcanic cloud. For the first several hundred kilometers, the cloud spread laterally, first as an intrusive gravity current and then by wind shear and diffusion as downwind cloud transport occurred at the windspeed (during the first 18-24 h). Finally, the clouds continued to move through the upper troposphere but began decreasing in areal extent, eventually disappearing as ash and SO 2 were removed by meteorological processes. Total SO 2 in each eruption cloud increased by the second day of atmospheric residence, possibly because of oxidation of coerupted H 2 S or possibly because of the effects of sequestration by ice followed by subsequent SO 2 release during fallout and desiccation of ashy hydrometeors. SO 2 and volcanic ash travelled together in all the Spurr volcanic clouds. The initial (18-24 h) area expansion of the clouds and the subsequent several days of drifting were successfully mapped by both SO 2 (ultraviolet) and ash (infrared) satellite imagery.

Deposition and immersion-mode nucleation of ice by three distinct samples of volcanic ash

“…Thus, our results indicate that some volcanic plumes may not be overseeded with ice. Indeed, this has been directly observed in some volcanic plumes such as the 17 September 1992 eruption of Mt Spurr, where remote sensing measurements showed that ash mass dominated over ice mass (Rose et al, 2001). The current study suggests that immersion freezing, and therefore overseeding, may be dictated by the differences in the mineralogy of the crystalline material found in volcanic ash.…”

A review of volcanic ash aggregation

“…The proportion of fine ash involved in aggregation may be inferred from satellite observations of volcanic ash clouds (e.g., 1992 eruptions of Crater Peak, USA, Rose et al, 2001). In the first 30 minutes, >70 wt.% of erupted material falls out within 10s km of the volcano while in the next 24 hours another ~25 wt.% of total erupted mass falls out within 100s km to 1000s km dominantly through hydrometeor-enhanced aggregation of fine ash.…”

Composition and evolution of volcanic aerosol from eruptions of Kasatochi, Sarychev and Eyjafjallajökull in 2008–2010 based on CARIBIC observations

“…Also no background correction was made for the Eyjafjallajökull samples as mentioned earlier. An additional important aspect is initial processes in the volcanic cloud that lead to growth by aggregation and rapid sedimentation of fine ash particles (Rose et al, 2001). Such processes could be of importance during sampling in the Eyjafjallajökull volcanic clouds because they were only 1-2 days old.…”

“…Large particles sediment quickly while very fine ash particles (<15 µm) have been found to have a residence time of days to weeks in the UT/LMS (Rose and Durant, 2009;Niemeier et al, 2009). During the first 24 h after an eruption a rapid decrease of the fine ash (<25 µm) content of the volcanic cloud have been observed, likely caused by aggregation into larger particles with higher settling velocities (Rose et al, 2001). After this initial phase, ash concentrations decrease more slowly together with concentrations of SO 2 .…”