| Most volcanoes do not penetrate the stratosphere. In fact, only
a small number of eruptions have produced a significant amount of
aerosols in this century. (Note
that volcanic aerosols are totally unrelated to consumer aerosol products,
like hair spray, that have not used ozone-depleting substances since
the 1970s.) One example is Mt. Pinatubo, which injected 30 million
tons of aerosols into the stratosphere during its 1991 eruption in
the Philippines. That amount is represented by the peak in the graph
below. The topmost graph shows measurements from Barrow, Alaska; the
lower graph represents measurements taken at Mauna Loa, Hawaii .
These tiny particles can provide a surface where the ozone destruction
reactions take place very rapidly. Aerosols only have an effect
because of the currently high levels of stratospheric chlorine released
from ODS. They improve a chlorine
atom's effectiveness at destroying ozone molecules, producing a
short-lived spurt in ozone depletion.
![[Graph showing stratospheric aerosols falling rapidly after 1992]](/web/20060419004444im_/http://www.epa.gov/ozone/science/aerosol2.gif)
Source: NOAA
CMDL 
However, as the graph also shows, these particles were fairly
quickly removed from the stratosphere. A standard way to quantify
the longevity of a substance in the atmosphere is its "lifetime"
-- the time that it takes for an initial amount to be cut by about
two-thirds. More precisely, for an initial amount of 1000 tons injected
into the stratosphere, about 368 tons would be left after one lifetime.
Each subequent lifetime would reduce the remaining amount by about
63%. As the graph shows, the amount of aerosols in the stratosphere
dropped at about the same rate as would a substance with a lifetime
of about 11 months. Three years after the eruption, nearly all of
the Mt. Pinatubo aerosols were gone.
One disturbing point to note from the above graph is that it appears
to take much longer for aerosols to be removed from polar regions
than from tropical regions. The polar regions, particularly Antarctica,
are particularly susceptible to major drops in stratospheric ozone.
In comparison, CFC-12 has a lifetime of 100 years and CFC-11 has
a lifetime of 45 years (as reported in the Scientific Assessment
of Ozone Depletion: 2002). Hence, the impact of aerosols is
much shorter in duration than that of the CFCs. The fact that CFCs
have long lifetimes is one reason why it will take so long for the
ozone layer to recover after the production
phaseout.
Volcanoes & Ozone Depletion
Myths About Ozone Depletion
CFCs are Heavier Than Air, So They Can't Reach the Ozone Layer
Ozone Depletion Occurs Only In Antarctica
No Link Exists Between Ozone Depletion and Higher UV Levels |
