On the chemistry of stratospheric SO₂ from volcanic eruptions

Abstract

The April 4, 1982, eruption of El Chichón in southern Mexico introduced several megatons of gaseous sulfur into the stratosphere, mostly as SO 2 or H 2 S. Available kinetic information indicates that H 2 S is converted to SO 2 much faster than SO 2 is oxidized to sulfate by homogeneous gas phase reactions. The process by which SO 2 is chemically converted to sulfate is not precisely known, and various reaction mechanisms have been proposed. The rate‐limiting step of SO 2 oxidation is believed to be reaction with the OH radical to form the HOSO 2 radical. Depending on the fate of HOSO 2 , odd hydrogen may or may not be regenerated in the overall oxidation sequence. A decrease in OH due to volcanic SO 2 should be observable through the gaseous species that depend on OH, and in particular SO 2 itself. One‐ and two‐dimensional model results are presented that estimate the photochemical effects of the SO 2 from the El Chichón volcano under various assumptions of HOSO 2 to sulfate conversion. It is found that the chemical lifetime for the volcanic SO 2 would be greater than 100 days for a large portion of the cloud if HOSO 2 does not regenerate odd hydrogen during conversion to sulfate and if heterogeneous losses of SO 2 are not competitive. However, observations of sulfate particle formation and SO 2 imply a chemical lifetime of 30–40 days, which is consistent with HOSO 2 conversion regenerating odd hydrogen. The implications of this finding for the problem of sulfate formation in the polluted troposphere is briefly reviewed.