Assimilated Volcanic SO2 as a Source of Sulfate Aerosol Transported to the Stratosphere

Abstract:

The most powerful volcanic eruptions can inject significant quantities of sulfur directly into the stratosphere, but are sporadic in their occurrence. Many less dramatic volcanic events nevertheless produce significant plumes extending into the upper troposphere, which may provide a source for more limited transport across the tropopause. However, separating volcanic sulfates from other aerosol types in satellite retrievals is challenging, especially where clouds are also present. We therefore use assimilation of the GOME-2 volcanic SO₂ product, built into the ECMWF Integrated Forecast System (IFS) as run by the Copernicus Atmosphere Monitoring Service (CAMS) to produce operational global forecasts of atmospheric composition. This can provide a source for both oxidation to sulfate aerosol in the model's chemistry–aerosol module, and further vertical transport via the model dynamics.

We use several test cases of recent volcanic events to assess the impact of SO₂ assimilated in this way on aerosol reaching or formed in the lower stratosphere. We also show the sensitivity of this contribution to both prior assumptions regarding the vertical profile of the assimilated SO₂ column, and to uncertainty in the timescales for SO2-to-sulfate conversion in these regimes. In this way we demonstrate how an air-quality-focussed coupling of chemistry, aerosols, meteorology and data assimilation can also be used to gain insight into longer-timescale questions regarding stratospheric aerosol sources.