During the past decade it has become clear that the long-lived stratospheric clouds produced by volcanic eruptions are composed largely of sulphuric acid aerosols1,2. The amount of sulphur-rich volatiles (for example, SO2, H2S) injected into the stratosphere by an explosive eruption is, therefore, a critical determinant of its atmospheric impact3,4. The small-volume eruptions of Mt Agung in 1963 and El Chichón in 1982 both generated substantial stratospheric aerosol clouds, despite the fact that they erupted ≲0.5 km3 of magma. Comparison of data from direct measurements of stratospheric optical depth, Greenland ice-core acidity, and volcanological studies shows that such relatively small, but sulphur-rich, eruptions can have atmospheric effects equal to or even greater than much larger sulphur-poor eruptions. These small eruptions are probably the most frequent cause of increased stratospheric aerosols.
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