Improvements to Stratospheric Aerosol Microphysics in E3SM and Their Impact on Scattering and Absorption Efficiencies Following Mt. Pinatubo

Here we validate volcanic aerosol microphysical evolution in version 2 of the Department of Energy (DOE) Energy Exascale Earth System Model (E3SMv2) with a volcanic prognostic aerosol (PA) treatment, while changes in effective particle size are tied to resulting scattering and absorption efficiencies. E3SMv2-PA reasonably reproduces stratospheric aerosol lifetime, mass burden, and aerosol optical depth following the Mt. Pinatubo eruption (June 15^th, 1991) when compared to remote sensing observations and the interactive chemistry-climate model, CESM2-WACCM6. Global stratospheric aerosol size distributions identify the nucleation and growth of sulfate aerosol from volcanically injected sulfur dioxide (SO₂) from both major and minor volcanic eruptions from 1991 to 1993. Size distribution samples at 18 km over Laramie, WY (41.3˚ N, 105˚ W) from in-situ optical particle counter (OPC) data and E3SMv2-PA have similar background effective diameters in model and observations. By May 1992, OPC consistently observes larger effective diameters than simulated in E3SMv2-PA, driven by a larger accumulation and coarse mode D_g. Effective radii from the models and OPC are used to calculate scattering and absorption efficiencies. E3SMv2-PA maintains a maximum scattering efficiency at the wavelengths of peak solar irradiance (~0.5 µm) from the Pinatubo eruption through 1993, whereas the slightly larger OPC aerosols have 50-75% the scattering efficiency at this wavelength. Lastly, a lack of stratospheric water vapor – and therefore aerosol water – in the sampled region (Laramie, WY) in E3SMv2-PA leads to no absorption of outgoing longwave by the stratospheric aerosol due to an assumption of purely scattering sulfate. These findings indicate that E3SMv2-PA may overestimate the scattering of incoming shortwave radiation and underestimate the absorption of outgoing longwave radiation by volcanic aerosols, potentially overestimating the cooling effect of these events.