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Aerosols in the Stratosphere

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.

[Graph showing stratospheric aerosols falling rapidly after 1992]

Source: NOAA CMDL Exit EPA

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.

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.

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