Stratospheric Injection Lifetimes

Material injected to the stratosphere by volcanoes and pyrocumulonimbus clouds (pyroCBs) is observed to have different lifetimes depending on the altitude, latitude, season of the injection and removal processes. We adopt a framework that describes the stratospheric lifetime of injected material as the sum of lag and decay timescales and compute these quantities in tracer simulations by injecting hundreds of thousands of trajectory parcels and tracking them over 8 years.  We simulate the evolution of the Hunga water vapor plume from the January 2022 Hunga eruption. The simulation suggests the lag time would be 1.4 years and the decay time ~ 2.3 years, producing a stratospheric lifetime of ~3.7 years. From Microwave Limb Sounder observations, we estimate the Hunga lifetime to be 3.7±0.36 years which is in good agreement. Overall, we find that passive tracer lifetimes increase with altitude and decrease with the latitude.  If polar stratospheric cloud formation is a tracer loss process, the lifetime is shortened. Aerosol gravitational settling also shortens the lifetime and should be included especially for aerosols with < 0.5 µm radius. With the decay of Hunga aerosol plume, we use the lifetime and gravitational settling rate to estimate a particle median radius of ~0.3µm in agreement with other estimates.  Our calculations explain the different observed lifetimes for historic stratospheric injections and the changes in total stratospheric aerosols and water observed after the Hunga eruption. Our calculations are also relevant to geoengineering plans for modifying the stratospheric albedo where sustained stratospheric aerosol concentrations are envisioned.