The role of volcanic ash in the global dispersion of the aerosol cloud from major tropical eruptions

Explosive volcanic eruptions can inject huge quantities of ash and sulfur dioxide (SO2) into the stratosphere, which can significantly enhance the stratospheric aerosol layer leading to complex effects on the Earth’s climate. One key effect is the oxidation of SO2 into highly scattering sulfate aerosol, which can cool the Earth’s surface globally. The 1991 Mount Pinatubo eruption is the largest of the past century with many aspects of the aerosol cloud not understood. This eruption is an important case study for understanding the climate impacts of eruptions, especially in aerosol-climate models. Injection height, the erupted mass of SO2 and the phase of the quasi-biennial oscillation (QBO) are all important factors that impact the subsequent dispersion and climate impact of a volcanic aerosol cloud. However, the importance of these factors, as well as their associated values, is disputed across the aerosol modelling community. Ash particles are usually disregarded in climate modelling studies, assumed to fall out within days of the eruption, however recent studies have found that they may impact the global dispersion of major volcanic eruptions. The aim of this thesis is to investigate the initial dispersion of the Mount Pinatubo eruption cloud, focusing on the vertical extinction profiles from ground-based lidars to analyse the vertical dispersion of the volcanic aerosol cloud. Using an interactive stratospheric aerosol model, the impact of varying initial conditions is assessed, also differing injection heights and initial mass of SO2 for a simulated Mount Pinatubo eruption. Finally, this thesis aims to analyse the role of ultra-fine ash in the stratosphere following the Mount Pinatubo eruption. The results demonstrate the vertical profile of extinction is strongly dependent on the QBO-phase, with an injection height of 21-23 km producing the closest variation in extinction with time to observations. Injection height has a significant impact on the vertical dispersion of extinction, stratospheric aerosol optical depth (SAOD) and sulfate burden, with an injection height of 18-20 km preferentially removing aerosol from the cloud. An injection height of 21-23 agrees best with vertical extinction, sulfur burden and SAOD observations. An injection of 10-14 Tg SO2 is, found to produce the best comparison to observations, with modelled SAOD values for 10 Tg and sulfate burdens of 14 Tg aligning best with observations. Ultra-fine ash is found to decrease tropical sulfate burdens up to 0.6 Tg and decrease tropical SAOD up to 0.06, due to increased dispersion to the northern hemisphere from increased heating and then lofting of aerosol out of the tropics into the northern hemisphere.