In March and April 1982, the El Chichón volcano in southern Mexico erupted violently, injecting vast quantities of ash and sulfurous gases directly into the stratosphere. While explosive eruptions are known for creating global cooling aerosol veils, El Chichón was unique because its magma was unusually rich in sulfur and halogens, specifically chlorine and bromine compounds. This unique magma composition triggered localized heterogenous chemical reaction cascades that severely depleted stratospheric ozone.
Once inside the stratosphere, hydrogen chloride (HCl) gas reacted on the surfaces of microscopic volcanic ash particles and sulfuric acid aerosol droplets. These heterogenous surface reactions converted inactive reservoir species of chlorine into highly reactive free radicals, such as chlorine monoxide. Driven by solar ultraviolet radiation, these chlorine radicals entered catalytic destruction cycles, where a single chlorine atom could destroy thousands of ozone molecules before being neutralized.
Furthermore, the immense surface area provided by the fine volcanic ash accelerated non-standard chemical pathways, resulting in record-low local ozone concentrations over equatorial and mid-latitude regions in late 1982. The detailed scientific observations gathered during the El Chichón event revolutionized atmospheric chemistry models, providing crucial real-world evidence for how aerosol surface area catalyzes halogen activation—insights that later proved essential for understanding the Antarctic ozone hole phenomenon.