Quantifying the decay rate of volcanic sulfur dioxide in the stratosphere

Abstract. The injection of sulfur dioxide (SO₂) into the stratosphere and its subsequent oxidation to form sulfate aerosols after large volcanic eruptions can have profound effects on Earth’s climate. The lifetime of volcanic SO₂ in the stratosphere is thought to be determined by its gas-phase oxidation by the hydroxyl radical (OH); once oxidized, it goes on to form sulfate aerosols. However, it has also been suggested that heterogeneous oxidation on ash could also be important or even dominant, which would imply faster formation of aerosols at least in ash-rich plumes. Additionally, recent work uses an assumed exponential fit to determine the total SO₂ mass loading following large eruptions; the quality of this fit translates directly to the accuracy of the mass loading estimate. It is therefore of interest to examine how accurately the SO₂ lifetime can be determined from observations, and compare observations to models. Here we evaluate the SO₂ lifetime and its uncertainties following several significant eruptions using three different sets of satellite observations and compare these to the CESM-WACCM6 model. We show that defining an accurate baseline against which a volcanic injection can be quantified limits accuracy in the estimated lifetime for some satellite data sets. We find that uncertainties in lifetimes across different altitudes and eruptions make it difficult to attribute variations in lifetime to specific SO₂ removal processes for the events examined.