ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-13-1895-2013 Modeling of 2008 Kasatochi volcanic sulfate direct radiative forcing: assimilation of OMI SO<sub>2</sub> plume height data and comparison with MODIS and CALIOP observations Wang J. https://orcid.org/0000-0002-7334-0490 1 Park S. 1 Zeng J. 1 Ge C. 1 6 Yang K. https://orcid.org/0000-0003-0767-2451 2 4 Carn S. https://orcid.org/0000-0002-0360-6660 3 Krotkov N. https://orcid.org/0000-0001-6170-6750 4 Omar A. H. 5 Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, NE, USA Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD, USA Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA Science Directorate, NASA Langley Research Center, Hampton, VA, USA State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China 19 02 2013 13 4 1895 1912 Copyright: © 2013 J. Wang et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/13/1895/2013/acp-13-1895-2013.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/13/1895/2013/acp-13-1895-2013.pdf

Volcanic SO<sub>2</sub> 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 SO<sub>2</sub> 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 SO<sub>2</sub> 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 SO<sub>2</sub> that is consistent with OMI measurements, reflecting SO<sub>2</sub> 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 SO<sub>2</sub> 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 SO<sub>2</sub> oxidation in the GEOS meteorological data during the first week after eruption when part of SO<sub>2</sub> 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<sup>−2</sup>, with the majority of the forcing-influenced region located north of 20° N, and with daily peak values up to −2 Wm<sup>−2</sup> on days 16–17. Sensitivity experiments show that every 2 km decrease of SO<sub>2</sub> 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 SO<sub>2</sub> 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 SO<sub>2</sub> plume height and amount into a chemical transport model for an improved simulation of volcanic SO<sub>2</sub> and sulfate transport.

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