Stratospheric residence time and the lifetime of volcanic stratospheric aerosols

The amount of time that volcanic aerosols spend in the stratosphere is one of the primary factors influencing the climate impact of volcanic eruptions. Descriptions of stratospheric aerosol persistence vary, with many works quoting an approximately 1-year residence time for aerosol from large tropical eruptions, but other references to 1-2 year “lifetimes”. We introduce a framework for describing the evolution of global stratospheric aerosol after major volcanic eruptions and assess its persistence, based on analysis of global satellite-based aerosol observations, tracer transport simulations and simple conceptual modeling. We show that stratospheric residence time, which is estimated through passive tracer pulse experiments and is one factor influencing the lifetime of stratospheric aerosols, is strongly dependent on the injection latitude and height, with an especially strong sensitivity to injection height in the first four kilometers above the tropical tropopause. Time series of simulated stratospheric tracer fraction are best described by a simple model which includes a lag between the injection and initiation of removal from the stratosphere. A simple model including lagged decay, as well as a timescale for sulfate aerosol production, produces a best fit to global observations of stratospheric aerosol after the 1991 eruption of Mt. Pinatubo consistent with a stratospheric lifetime of about 24 months. We estimate the potential impact of observational uncertainties on this lifetime estimate and find it likely that the lifetime of Pinatubo stratospheric aerosol is 20 months or greater.