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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.

Explosive volcanic eruptions can inject sulfur dioxide (SO2) into the stratosphere, which forms stratospheric sulfate aerosol that can significantly impact the climate. The radiative forcing of an eruption depends on several eruption source parameters: the mass of SO2 emitted, the latitude and the emission height. However, the combined effects of these parameters are not well understood. Reconstructions of historic volcanic radiative forcing, which are essential for understanding the role of volcanism on climate variability, are derived from ice core sulfate records, but rely on relationships between ice sheet deposition, stratospheric aerosol burdens and radiative forcing from limited observations. The aim of this thesis is to understand the impact of explosive volcanic eruptions on radiative forcing and volcanic sulfate deposition. The role of individual and combined eruption source parameters are comprehensively investigated using interactive stratospheric aerosol models, perturbed parameter ensembles and statistical emulation. The results demonstrate that radiative forcing is primarily determined by the SO2 emission magnitude, is stronger for tropical eruptions and has a greater dependency on the injection height if the eruption is tropical. The ice sheet deposition is relatively independent of the injection height. The results reveal complex combined effects of the eruption parameters and illustrate the importance of explicitly simulating aerosol microphysical processes to determine aerosol mass, size and lifetime. A wide range of eruption-realisations is found that produce ice sheet deposition that is consistent with ice-core-derived estimates for historic eruptions in the last 2500 years. These eruptions have a range in time-integrated radiative forcing (> ~300 MJ m-2) that is not considered in reconstructions. Sulfate deposition differs considerably between models for the 1815 eruption of Mt. Tambora. The results suggest there is a large uncertainty in radiative forcing derived from ice cores. Consequently, the efficacy of volcanic radiative forcing derived from previous reconstructions may be incorrect.

The European Centre for Medium-range Weather Forecasts’ (ECMWF) Composition - Integrated Forecast System (C-IFS) provides global operational forecasts and re-analyses of atmospheric composition at high spatial resolution (~80 km). In this thesis the C-IFS system, with Global Fire Assimilation System (GFAS) emissions, is used to assess the impact of fires on tropospheric ozone and aerosol concentrations. The skill of the model and the impact of data assimilation is evaluated using independent aircraft, ground and satellite observations. In particular, observations from the 2012 South American Biomass Burning Analysis (SAMBBA) field campaign is used as a case-study. This work represents the first evaluation of trace gas observations from SAMBBA. C-IFS simulations show that 4% of the total tropospheric column is produced by fire emissions, with significantly larger contributions closer to fire sources. Compared to both SAMBBA aircraft measurements and Ozone Monitoring Instrument (OMI) satellite data (0-6 km column) O3 concentrations are underestimated in the tropics. Evaluation of precursor emissions show that this is due to a combination of NOx flux underestimation in GFAS and the lack of a prescribed injection height, partly acting through a subsequent limited influence of PAN. Carbon monoxide, a tracer of biomass burning transport, is well captured by C-IFS implying that GFAS detects the majority of fires although the occurrence of small ones appears underestimated. The assimilation of satellite reactive gas measurements from OMI, Measurements Of Pollution In The Troposphere (MOPITT) and the Microwave Limb Sounder (MLS) generally improved the C-IFS comparison against SAMBBA and other independent data. The O3 tropospheric representation was improved by MLS assimilation but a significant negative bias against OMI (0-6 km column) remained. The chemical data assimilation impacts non-assimilated species such OH with increases up to 8% in the tropics. However, compared to observations this degraded the OH Northern/Southern hemisphere ratio and the impact of the OH change on other species was minimal. Fires in C-IFS are shown to dominate the global carbonaceous aerosol budget during the tropical fire season. Aerosol concentrations are slightly underestimated by C-IFS compared to SAMBBA. The relative contribution of black carbon (BC) and organic matter (OM) is similar to that observed in the eastern savannah but the contribution of BC is overestimated in the western deforestation region. The use of the suggested ×3.4 GFAS scaling factor leads to a large positive bias in the model aerosol compared to SAMBBA. The assimilation of aerosol optical depth (AOD) generally decreases concentrations near fire-sources, due to this scaling, but still increases concentrations in remote regions. This suggests an inaccurate model loss of aerosol is a cause of the bias. The analysis/model bias used to characterise a more realistic local scaling factor which is shown to be different for BC and OM, and for AOD metrics compared to aerosol mass.