During major volcanic eruptions, sulphur dioxide gas is emitted and typically reaches the stratosphere. There, it reacts chemically with OH from water vapor to form sulphuric acid droplets, also known as sulphate aerosols. These aerosols have an important climate impact for several years before re-entering the lower atmosphere. The aerosols also contribute to ozone change.
Ozone Changes Without Chlorine
Before the 1970s, there were much smaller amounts of chlorine in the atmosphere and atmospheric chemistry was simpler. Nonetheless, chemical reactions took place in the aerosol droplets, of which the most significant is:
N2O5 + H2O --> 2HNO3 ............ R1
The main effect of reaction R1 is to transform nitrogen oxides (collectively known as NOx) into the long time-scale chemical nitric acid (HNO3). On recalling that nitrogen oxides are a major catalytic destroyer of ozone, the effect of volcanic aerosols is to decrease the amount of ozone destruction. That is, ozone is actually increased by a small amount (about 10% locally and 1 or 2% in the global average) as explained by Tie and Brasseur (1995). This was what was observed for a short period following the eruption of Agung (1963).
Ozone Changes With chlorine
After the 1970s, the amount of chlorine from chlorofluorocarbons increased, reaching its peak in the stratosphere at about the year 2000. Although the above reaction R1 still occurs on volcanic aerosols, it is no longer the most important, and those reactions which ultimately give rise to the Antarctic ozone hole also occur, mainly:
HCl + ClNO3 --> Cl2 + HNO3 ......... R2
Now the picture is reversed: the original chemicals HCl and ClNO3 have a long time scale and are unreactive towards ozone. The resulting chemical Cl2 breaks down readily into chlorine atoms and reacts directly with ozone.
Hence, the net effect of volcanoes on ozone is an increase due to NOx production, and a decrease due to Cl production. Whichever process dominates determines the net effect on ozone.
For the eruptions of El Chichon (1982) and Pinatubo (1991), an increasing net ozone loss has been seen, due to higher amounts of chlorine in the atmosphere. For Pinatubo, the ozone loss averaged globally to be about 3% (WMO, 2011, Chapter 2). While small, this influence complicates the estimate of ozone depletion from other sources.
Not All Volcanoes are Alike
Many other issues complicate the prediction of ozone change due to volcanic eruptions. In addition to the amount of sulphur emitted and the altitude it reaches in the atmosphere, another important factor is simply the latitude of the volcano.
Tropical volcanoes have much more influence than others because material is emitted into the tropics of the stratosphere. From there, the sulphate aerosols can be lofted upwards and polewards and have a longer period of time in the stratosphere. Within a few years, the aerosols re-enter the lower atmosphere and are absorbed into clouds and rained out.
Middle and high latitude eruptions by comparison produce aerosols which last less than a year in the stratosphere and have little overall influence on ozone behavior. However, even smaller eruptions can lead to significant short term ozone loss, particularly if accompanied by the emission of hydrogen chloride. One example is the Hekla (Iceland) eruption in 2000 (Millard et al., 2006).
Future Volcanic Eruptions
Clearly, major eruptions of tropical volcanoes are difficult to predict. There is an obvious concern that with chlorine likely to remain high in the atmosphere for several decades more, any tropical eruption would cause some ozone loss. Even moderate eruptions are now suspected of contributing to the amount of aerosol in the stratosphere over the last decade (Vernier et al., 2011)
If a major eruption were to occur, it would be important to put this into the proper perspective. Not only would the ozone loss be expected but it would not mean that the important steps made to ban the uses of CFCs would have been in vain.
References
Millard et al. (2006), Halogen emissions from a small volcanic eruption: Modeling the peak concentrations, dispersion and volcanically induced ozone loss in the stratosphere, Geophys. Res. Lett., 33, L19815, doi:10.1029/2006GL026959.
Tie and Brasseur (1995), The response of stratospheric ozone to volcanic eruptions: Sensitivity to Atmospheric chlorine loading, Geophys. Res. Lett., 22, 3035-3038, doi:10.1029/95GL03057.
Vernier et al. (2011), Major influence of tropical volcanic eruptions on the stratospheric aerosol layer during the last decade, Geophys. Res. Lett., 38, L12807,doi:10.1029/2011GL047563
WMO (2011), Scientific Assessment of Ozone Depletion 2010
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