Modeling of 2008 Kasatochi volcanic sulfate direct radiative forcing: assimilation of OMI SO₂ plume height data and comparison with MODIS and CALIOP observations

Abstract. Volcanic SO₂ column amount and injection height retrieved from the Ozone Monitoring Instrument (OMI) with the Extended Iterative Spectral Fitting (EISF) technique are used to initialize a global chemistry transport model (GEOS-Chem) to simulate the atmospheric transport and lifecycle of volcanic SO₂ and sulfate aerosol from the 2008 Kasatochi eruption, and to subsequently estimate the direct shortwave, top-of-the-atmosphere radiative forcing of the volcanic sulfate aerosol. Analysis shows that the integrated use of OMI SO₂ plume height in GEOS-Chem yields: (a) good agreement of the temporal evolution of 3-D volcanic sulfate distributions between model simulations and satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP), and (b) an e-folding time for volcanic SO₂ that is consistent with OMI measurements, reflecting SO₂ oxidation in the upper troposphere and stratosphere is reliably represented in the model. However, a consistent (~25%) low bias is found in the GEOS-Chem simulated SO₂ burden, and is likely due to a high (~20%) bias of cloud liquid water amount (as compared to the MODIS cloud product) and the resultant stronger SO₂ oxidation in the GEOS meteorological data during the first week after eruption when part of SO₂ underwent aqueous-phase oxidation in clouds. Radiative transfer calculations show that the forcing by Kasatochi volcanic sulfate aerosol becomes negligible 6 months after the eruption, but its global average over the first month is −1.3 Wm⁻², with the majority of the forcing-influenced region located north of 20° N, and with daily peak values up to −2 Wm⁻² on days 16–17. Sensitivity experiments show that every 2 km decrease of SO₂ injection height in the GEOS-Chem simulations will result in a ~25 % decrease in volcanic sulfate forcing; similar sensitivity but opposite sign also holds for a 0.03 μm increase of geometric radius of the volcanic aerosol particles. Both sensitivities highlight the need to characterize the SO₂ plume height and aerosol particle size from space. While more research efforts are warranted, this study is among the first to assimilate both satellite-based SO₂ plume height and amount into a chemical transport model for an improved simulation of volcanic SO₂ and sulfate transport.