Integrating retrievals of volcanic cloud characteristics from satellite remote sensors: a summary

Volcanic eruptions are events that rapidly and suddenly disperse gases and fine particles into the atmosphere, a process most conveniently studied from the synoptic satellite perspective, where remote sensing offers a practical tool for spatial and temporal measurements. Meteorological satellites offer approximately 20 years of archived data, which can be analysed for measurements of masses of SO2and fine volcanic ash in spatial two–dimensional arrays and integrated with other meteorological data. The satellite data offer a tool to study volcano–atmosphere interactions in a quantitative way. They provide information of unique value for understanding the fate and transport of fine silicates with significant health hazards and for addressing the problem of volcanic cloud hazards to jet aircraft. Studies of satellite data have demonstrated the following. (1) Volcanic clouds from convergent plate boundary volcanoes contain large and variable excesses of SO2.(2) The second day of atmospheric residence for volcanic clouds has significantly higher SO2than the first, suggesting that early volcanic H2S may be converting to SO2.(3) Complete conversion of SO2to sulphate in the stratosphere occurs at an efolding rate of approximately 120 days. SO2loss from stratospheric volcanic clouds occurs at an e–folding rate of approximately 35 days, and the SO2loss rate for volcanic clouds which only barely reach the stratosphere is rapid (efolding only a few days). The latter limits the stratospheric aerosol build–up from smaller eruptions.(4) Fine volcanic ash (with diameters of less than ca.25μm) in drifting volcanic clouds retrieved after 10 h or more appear to represent a small fraction (less than 2 of the total mass) of the total mass of magma erupted, and also a small fraction (less than 20) of the total mass of fine ash erupted. This is probably explained by the fact that the total mass is greatly reduced by aggregation processes within the volcanic cloud.(5) The amounts of fine ash decrease faster in volcanic clouds of larger eruptions, supporting the self–removal processes suggested by Pinto et al.in 1989.(6) Evidence for a strong role of ice in the fallout and aggregation of volcanic cloud ash is considerable.(7) In many cases, volcanic clouds separate into higher SO2–rich portions and lower ash–rich portions. The two portions follow different trajectories and the lower, ash–rich portions are affected by interactions with moist tropospheric air.