Effect of volcanic eruptions on stratospheric ozone and irradiance

Abstract

Major volcanic eruptions can inject large amounts of sulfur into the stratosphere, increasing the stratospheric aerosol layer for several years. In the last decades, this effect was observed after the eruptions of El Chichón, in 1982, and Mt. Pinatubo, in 1991, which had a considerable contribution to ozone depletion. The natural variability of atmospheric ozone occurs in time scales ranging from day to day, to decadal. Atmospheric circulation, chemistry, and radiative processes play important roles in the variability of ozone. Large volcanic eruptions reaching the stratosphere could cause significant temporary changes. The effect of a volcanic eruption on stratospheric ozone is largest when large chlorine and bromine levels are present in the atmosphere. How past volcanic eruptions have changed stratospheric ozone was studied using models. It was concluded that eruptions of El Chichon and Pinatubo resulted in globally total ozone losses and severe ozone depletion in the lower stratosphere. Earlier eruptions (Krakatau, Santa Maria and Agung) resulted in globally total ozone increases and almost no ozone changes in the stratosphere. This difference in ozone response was caused by the amount of ozone depleting substances present in the atmosphere, which increased the availability of chlorine and caused a change in the chemistry, during the last decades. In the mid-1990s, a maximum in stratospheric ozone depletion was observed, as consequence of ozone response to volcanic aerosols from Mt. Pinatubo, superimposed to the depletion produced by halocarbons. One of the reasons that make stratospheric ozone important is the attenuation of solar ultraviolet (UV) radiation while it trespasses the atmosphere. Radiation reaching the Earth‘s surface is influenced by scattering and absorption processes in the atmosphere and the ground. Sun-Earth distance, atmospheric gases and aerosols, solar zenith angle (SZA), clouds and surface albedo are the main factors that determine radiation at ground-level. The effect of volcanic eruptions on UV radiation is complex. On one side, depletion of stratospheric ozone results in an increase in UV radiation. In addition, volcanic eruptions increase the aerosol amount and change aerosol size composition, producing important variation in optical properties of the atmosphere. As a result, a decrease is usually observed in direct UV radiation, due to absorption produced by the emitted particles. At the same time, an increase in the diffuse radiation may be experienced, resulting from scattering increase. As consequence of these two processes the effect on global UV radiation is difficult to anticipate. In this chapter, we present a review of the effect of major volcanic eruptions on the ozone layer and UV radiation. We also introduce our own results on the effect of Puyehue-Cordon Caulle volcanic complex eruption, in 2011, which did not affect globally the ozone layer but produced a large regional impact on aerosols and UV radiation.