Composition of the stratospheric “sulfate” layer

Spectacular sunsets are seen over much of the world following violent volcanic eruptions, and the brilliant displays may occur for many years (Figure 1). Especially noteworthy were those following the eruption of Krakatoa in the East Indies in 1883 and of Gunung Agung in Bali in 1963. The optical effects result from the scattering of sunlight by fine volcanic particles. Other recent eruptions that have been shown to inject particles—volcanic gases such as hydrogen sulfide and sulphur dioxide which by chemical reactions form particles—and entrained air into the stratosphere include Awu in 1966 and Fernandia in 1968. Of course there are small, but not zero, concentrations of particles in the stratosphere even following prolonged periods of little volcanic activity. These particles come from many sources, as discussed later in this article.