In January 1993, the Earth's average stratospheric ozone concentration was the lowest on record. Although the ozone layer has since recovered, the cause of this reduction has interested and concerned scientists. Recently, this ozone event has been linked to the June 1991 eruption of the Philippine volcano Mount Pinatubo.1 One of the major atmospheric effects of this eruption was the addition of 15-30 MT sulfur dioxide (SO₂). This excess SO₂ has been linked to the abnormally low ozone levels. However, the actual ozone depletion was less than scientists expected for this amount of SO₂. In this paper, this phenomena will be explained. To explain this, the three major effects of SO₂ on the ozone layer will be discussed. Then, the results of two studies will be reported: short-term (one to two months after eruption) computer modeling and long-term (three to seven months after eruption) computer modeling.

It is known that during the first two months after an eruption, SO₂ affects the ozone layer both positively and negatively. SO₂ depletes the ozone layer by reducing the solar flux because it absorbs 180 nm-390 nm. This is the same range for the photolysis of O₂, which is necessary for ozone production. Since photolysis is reduced, ozone production is also reduced.

SO₂ also increases stratospheric ozone concentrations, as illustrated by the following equations. In this SO₂, by reaction with ultraviolet light, produces an ozone precursor (O). Effectively, SO₂ catalyzes the formation of ozone.

SO₂ + hv = SO + O (wavelength<220nm)

SO + O₂ = SO₂ + O

2(O + O₂ + M = O₃ + M)

3O₂ = 2O₃

In short-term computer modeling study, it was found that immediately after the eruption, at an altitude of 25 km, no ozone depletion takes place. The photolysis reduction effect and the catalyzing effect cancel each other out. Immediately below the cloud, though, ozone is reduced because of the reduced photolysis of O₂ (due to absorption by SO₂).

However, after two months, most SO₂ is converted to sulfuric acid by reaction with hydroxyl radicals (OH). This condenses into aerosols in the atmosphere. This is known as the aerosol effect. Nitrogen oxides (NO_x =NO, NO₂, NO₃, and N₂O₅) react with the surface of the aerosols to form nitric acid (HNO₃). Normally, NO_x reacts with ozone-depleting Cl and ClO to form less ozone-depleting compounds. However, because the sulfuric acid aerosol removes NO_x, the ozone layer becomes more sensitive to Cl and ClO. In this case, the ozone concentration decreases.

This long-term situation was verified using three computer models. First, a situation was studied three months after the eruption, assuming the SO₂ cloud was confined to the tropics. It was also assumed that SO₂ acted as a greenhouse gas and caused slight stratospheric heating. In this case, it was found that the concentration of ozone-depleting radicals increased by 25-50% at 20 km. Second, the same situation was studied without heating. In this case, NO₂ was decreased by 40% and ClO was increased by a factor of 2.3. Lastly, this was studied seven months after eruption, assuming that the cloud was evenly dispersed over the Earth. In this case, NO₂ concentration was reduced by 30-35% at 20-25 km.

As shown, SO₂ has been found to not change ozone concentration at 25 km in the first two months after a volcanic eruption. Then, in all three long-term computer modeling studies, the ozone concentration decreases, due to the aerosol effect. These facts agree well with the stratospheric ozone data collected. For this reason, a mechanism to explain the effect of volcanic SO₂ on the ozone layer has been found.