Improving the Representation of Stratospheric Aerosol in E3SM and Its Impact on Ozone and Clouds

To investigate climate change under volcanic eruption and geoengineering injection scenarios, an Earth system model (ESM) needs to represent aerosols and chemistry in the troposphere and stratosphere appropriately. For this purpose, we developed a 5-mode version of the Modal Aerosol Model (MAM5), which is based on the previous MAM4 with one new aerosol mode (stratospheric coarse mode) dedicated to the stratospheric sulfate aerosol. Also, the MOZART full-chemistry mechanism has been improved, and the prognostic aerosol surface area has been implemented. With the above implementations, the Atmosphere Model (EAM) of the Energy Exascale Earth System Model (E3SMv1) can well capture the burden of the sulfate aerosol and the aerosol optical depth (AOD) introduced by the Mt. Pinatubo eruption (June 15, 1991), compared to satellite and in situ observations. Furthermore, the simulated results suggest that the correct OH-HO2 radical cycle is essential to appropriately reproduce the spatial distribution of the sulfate aerosol when dense sulfur dioxide plumes from volcanos are oxidized. Also, the generated sulfate aerosols can affect the aerosol surface area density, resulting in heterogeneous reaction rate changes, which modulate the stratospheric NOx and ozone concentrations. Importantly, the simulated results indicate that besides the sulfate aerosols direct cooling effect, the volcanic eruption caused sulfate aerosol number increment in both accumulation and stratospheric coarse modes significantly, resulting in enhanced homogeneous ice nucleation and thus boosted ice cloud coverage, which led to a positive cloud radiative forcing around 0.32 w/m2, globally. This perturbation lasted more than two years and is more remarkable over high latitudes than tropics.