Stratospheric ozone changes from explosive tropical volcanoes: Modeling and ice core constraints

Major tropical volcanic eruptions have emitted large quantities of stratospheric sulphate and are potential sources of stratospheric chlorine although this is less well constrained by observations. This study combines model and ice core analysis to investigate past changes in total column ozone. Historic eruptions are good analogues for future eruptions as stratospheric chlorine levels have been decreasing since the year 2000. We perturb the pre‐industrial atmosphere of a chemistry‐climate model with high and low emissions of sulphate and chlorine. The sign of the resulting Antarctic ozone change is highly sensitive to the background stratospheric chlorine loading. In the first year, the response is dynamical, with ozone increases over Antarctica. In the high HCL (2Tg emission) experiment, the injected chlorine is slowly transported to the polar regions with subsequent chemical ozone depletion. These model results are then compared to measurements of the stable nitrogen isotopic ratio, δ 15N(NO3‐), from a low snow accumulation Antarctic ice core from Dronning Maud Land (recovered in 2016‐17). We expect ozone depletion to lead to increased surface ultraviolet (UV) radiation, enhanced air‐snow nitrate photo‐chemistry and enrichment in δ 15N(NO3‐) in the ice core. We focus on the possible ozone depletion event that followed the largest volcanic eruption in the past 1000 years, Samalas in 1257. The characteristic sulphate signal from this volcano is present in the ice‐core but the variability in δ 15N(NO3‐) dominates any signal arising from changes in UV from ozone depletion. Prolonged complete ozone removal following this eruption is unlikely to have occurred over Antarctica.