The photograph on the left shows a sunrise over Pecos, Texas, as photographed from the space shuttle. Clouds (blue-white) and the stratospheric aerosol layer (red) are clearly visible in this image. The SAGE II (Stratospheric Aerosol and Gas Experiment) instrument aboard the ERBS satellite measures the obscuration of the sun by the Earth's atmosphere from a geometry similar to that in this image. The dependence of the obscuration on the color of light and altitude permits us to derive vertical profiles of aerosol, ozone, and other trace species.
By screening out sunlight, the stratospheric aerosol layer affects the atmospheric energy balance and hence climate. See NASA's fact sheet on Volcanoes and Global Cooling. The layer can also alter chemical cycles and perturb ozone levels.
The stratospheric aerosol layer is sustained by natural emissions of carbonyl sulfide (OCS) through biogenic processes. Carbonyl sulfide is relatively stable can mix into the stratosphere where it is photochemically broken down resulting in the formation of microscopic droplets of sulfuric acid.
Another sulfur-containing gas, sulfur dioxide (SO2), is normally too reactive to reach the stratosphere, instead it is rained out (as acid rain downwind of its sources). Volcanic eruptions, however, can inject SO2 directly into the stratosphere where it too undergoes tranformation into sulfuric acid. But most volcanic eruptions do not penetrate into the stratosphere. In fact only a small number of eruptions in this century have had a significant impact on the Junge Layer. The most recent example is the June 1991 eruptions of Mt. Pinatubo. The impact of volcanoes on the Earth System was dramatically demonstrated by the eruption of Mt. Pinatubo in the Philippines. The eruptions had near-global effects on weather and climate via the introduction of sulfur dioxide and aerosols into the atmosphere. Satellite observations showed that the stratospheric aerosol layer was significantly enhanced for over three years. Many parts of the world experienced a drop in average temperature of approximately 1 degree Fahrenheit in 1992 compared to the 30-year average.
In addition the links above,
other interesting sites are:
NASA's Goddard Institute for Space Studies (GISS) page with graphs depicting the optical thickness (opacity) of the Junge Layer
List of large and largest known volcanic eruptions
Volcano World with everything you want to know about volcanoes.
The story behind Edvard Munch's well-known painting "The Scream"