Possible effects of volcanic eruptions on stratospheric minor constituent chemistry

Although stratosphere penetrating volcanic eruptions have been infrequent during the last half century, periods have existed in the last several hundred years when such eruptions were significantly more frequent. Several mechanisms exist for these injections to affect stratospheric minor constitutent chemistry, both on the long-term average and for short-term perturbations. These mechanisms are reviewed and, because of the sensitivity of current models of stratospheric ozone to chlorine perturbations, quantitative estimates are made of chlorine injection rates. It is found that, if chlorine makes up as much as 0.5 to 1% of the gases released and if the total gases released are about the same magnitude as the fine ash, then a major stratosphere penetrating eruption could deplete the ozone column by several percent. The estimate for the Agung eruption of 1963 is just under 1% an amount not excluded by the ozone record but complicated by the peak in atmospheric nuclear explosions at about the same time. The long-term contribution to stratospheric CIX by volcanic eruptions is estimated as ∼0.1 ppbv for the period 1900–60 and ∼1 ppbv for the much more volcanically active period 1780–1840. All of the estimates are subject to large uncertainties, perhaps a factor of 2 or 3 on the high side and a factor of 10 or more on the low side.