Catastrophic loss of stratospheric ozone in dense volcanic clouds

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Abstract
Rapid, localized loss of ozone is predicted to occur in the mid‐latitude and tropical stratosphere in the presence of very large concentrations of sulfate aerosols. Volcanic eruptions can increase the effective surface area of sulfuric acid so that heterogeneous reactions involving ClONO2, and secondarily N2O5, are able to suppress NOx abundances by more than a factor of 10 relative to gas phase chemistry. When NOx levels fall below a threshold, e.g., 0.6 ppb at 24 km in mid‐latitudes, the chlorine‐catalyzed loss of O3 proceeds at rates comparable to those during the formation of the Antarctic ozone hole, more than 50 ppb per day. If such losses occurred following the eruption of Mount Pinatubo in the most volcanically perturbed regions over the tropics and mid‐latitudes, this model predicts that they are driven primarily by the suppression Of NOx below these critical levels. The increase in stratospheric chlorine since El Chichon has made Mount Pinatubo more than twice as effective in causing rapid O3 loss. Overall global losses associated with a volcanic eruption are approximately linear in the amount of sulfate surface area, but depend critically on the rate of the CIONO2‐sulfate reaction.