The Stratospheric Aerosol Layer
a.k.a. The Junge Layer
While searching for cosmic dust and debris from nuclear bomb tests, Christian Junge discovered in 1960 a layer of microscopic
aerosol
particles between the tropopause and about 18 miles (30 km)
altitude. These particles are composed of
sulfuric acid and water
and are formed by the chemical transformation of sulfur-containing gases .
This layer is called the Junge Layer or the Stratospheric Aerosol Layer.
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"