Radiative and chemical response to interactive stratospheric sulfate aerosols in fully coupled CESM1(WACCM)

We present new insights into the evolution and interactions of stratospheric aerosol using an updated version of the Whole Atmosphere Community Climate Model (WACCM). Improved horizontal resolution, dynamics, and chemistry now produce an internally generated quasi-biennial oscillation and significant improvements to stratospheric temperatures and ozone compared to observations. We present a validation of WACCM column ozone and climate calculations against observations. The prognostic treatment of stratospheric sulfate aerosols accurately represents the evolution of stratospheric aerosol optical depth and perturbations to solar and longwave radiation following the June 1991 eruption of Mount Pinatubo. We confirm the inclusion of interactive OH chemistry as an important factor in the formation and initial distribution of aerosol following large inputs of sulfur dioxide (SO2) to the stratosphere. We calculate that depletion of OH levels within the dense SO2 cloud in the first weeks following the Pinatubo eruption significantly prolonged the average initial e-folding decay time for SO2 oxidation to 47days. Previous observational and model studies showing a 30day decay time have not accounted for the large (30-55%) losses of SO2 on ash and ice within 7-9days posteruption and have not correctly accounted for OH depletion. We examine the variability of aerosol evolution in free-running climate simulations due to meteorology, with comparison to simulations nudged with specified dynamics. We assess calculated impacts of volcanic aerosols on ozone loss with comparisons to observations. The completeness of the chemistry, dynamics, and aerosol microphysics in WACCM qualify it for studies of stratospheric sulfate aerosol geoengineering. Plain Language Summary Stratospheric aerosols form after volcanoes inject SO2 into the stratosphere, and can cool global surface temperatures. A new capability for simulating stratospheric aerosols from SO2 injections in the Whole Atmosphere Community Climate Model is shown to reproduce well observed climate and chemistry responses. The ability of the model to calculate accurately the reductions in sunlight and losses of ozone that have been observed following historical eruptions in the satellite era gives strong confidence in the model's ability to simulate such responses to potential future deliberate injections of SO2 to offset global warming. Such responses to geoengineering are presented in a series of companion papers.