Volcanoes and Ozone

Ozone is a gas made of three oxygen atoms. Ozone is bluish in color and harmful to breathe. Most of the Earth's ozone (about 90%) is in the stratosphere. The stratosphere is a layer in the atmosphere from about 10km to about 50km in altitude. Ozone is important because it absorbs specific wavelengths of ultraviolet radiation that are particularly harmful to living organisms. The ozone layer prevents most of this harmful radiation from reaching the ground.

As concern grew over depletion of ozone in the stratosphere scientists examined the role of volcanoes. They noted that the gases emitted by most eruptions never leave the troposphere, the layer in the atmosphere from the surface to about 10km.

Hydrogen chloride released by volcanoes can cause drastic reductions in ozone if concentrations reach high levels (about 15-20 ppb by volume)(Prather and others, 1984). As the El Chichon eruption cloud was spreading, the amount of HCl in the cloud increased by 40% (Mankin and Coffey, 1984). This increase represents about 10% of the global inventory of HCl in the stratosphere. Other large eruptions (Tambora, Krakatau, and Agung) may have released almost ten-times more HCl into the stratosphere than the amount of chlorine commonly present in the stratosphere (Pinto and others, 1989). At least two factors reduce the impact of HCl, chlorine appears to be preferentially released during low-levels of volcanic activity and thus may be limited to the troposphere, where it can be scrubbed by rain. Hydrogen chloride may also condense in the rising volcanic plume, again to be scrubbed out by rain or ice. Lack of HCl in ice cores with high amounts of H2SO4 (from large eruptions) may indicate ambient stratospheric conditions are extremely efficient at removing HCl. Thus, most HCl never has the opportunity to react with ozone. No increase in stratospheric chlorine was observed during the 1991 eruption of Mt. Pinatubo.

Volcanoes account for about 3% of chlorine in the stratosphere. Methyl chloride produces about 15% of the chlorine entering the stratosphere. The remaining 82% of stratospheric chlorine comes from man-made sources, mostly in the form of chlorofluorocarbons.

Although volcanic gases do not play a direct role in destroying ozone they may play a harmful indirect role. Scientists have found that particles, or aerosols, produced by major volcanic eruptions accelerate ozone destruction. The particles themselves do not directly destroy ozone but they do provide a surface upon which chemical reactions can take place. This enhances chlorine-driven ozone depletion. Fortunately, the effects from volcanoes are short lived and after two or three years, the volcanic particles settle out of the atmosphere.

Study of ozone amounts before and after the 1991 eruption of Mt. Pinatubo show that there were significant decreases in lower stratospheric ozone (Grant and others, 1994). The amount of ozone in the 16-28 km region was some reduced by 33% compared to pre-eruption amounts. A similar reduced amount of ozone was measured in the summer of 1992.

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