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 trajectory parcels and tracking them over 8 years. We simulate the evolution of the water vapor plume from the January 2022 Hunga eruption. The simulation suggests a lag time of 1.4 years and the decay time 2.35 ± 0.05 years, producing a stratospheric lifetime of 3.75 ± 0.05 years. From Microwave Limb Sounder observations, we estimate the Hunga water vapor plume decay time to be 2.6 ± 0.75 years and the lifetime to be 4.0 ± 0.75 years which is in good agreement with our model calculations. Overall, we find that injected material lifetime increases with altitude and decreases with the latitude. If polar stratospheric cloud formation is a 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. We use the observed lifetime of the Hunga aerosol plume and gravitational settling rate to estimate a particle median radius of ∼0.3 µm in agreement with other estimates. Our calculations are also relevant to geoengineering plans for modifying the stratospheric albedo, where sustained stratospheric aerosol concentrations are envisioned.