Quantifying the decay timescale 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 removal of volcanic SO2 in the stratosphere is thought to be driven 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 the faster removal of SO2 and thus the 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 decay timescale can be determined from observations and to compare observations to models. Here we evaluate the SO2 decay timescale and its uncertainties following several significant eruptions using three different sets of satellite observations and compare these to the CESM2-WACCM6 model. We show that defining an accurate baseline against which a volcanic injection can be quantified increases the variability and uncertainty in the estimated decay timescale for some satellite datasets. While the typical decay timescale for SO2 is on the order of a few weeks to a month, we find that uncertainties across different altitudes and eruptions result in timescales that can vary by more than a factor of 2. This makes it difficult to attribute variations in the decay timescale to specific SO2-removal processes for the events examined.