Persisting volcanic ash particles impact stratospheric SO₂ lifetime and aerosol optical properties

Volcanic ash is often neglected in climate simulations because ash particles are assumed to have a short atmospheric lifetime, and to not participate in sulfur chemistry. After the Mt. Kelut eruption in 2014, stratospheric ash-rich aerosols were observed for months. Here we show that the persistence of super-micron ash is consistent with a density near 0.5 g cm^-3, close to pumice. Ash-rich particles dominate the volcanic cloud optical properties for the first 60 days. We also find that the initial SO₂ lifetime is determined by SO₂ uptake on ash, rather than by reaction with OH as commonly assumed. About 43% more volcanic sulfur is removed from the stratosphere in 2 months with the SO₂ heterogeneous chemistry on ash particles than without. This research suggests the need for re-evaluation of factors controlling SO₂ lifetime in climate model simulations, and of the impact of volcanic ash on stratospheric chemistry and radiation.