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Title: Detection and effects of volcanic sulphur in the stratosphere
Author: Miles, Georgina
ISNI:       0000 0004 2724 809X
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2011
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Explosive volcanic eruptions perturb the atmosphere, and their main impact arises from the large quantities of 802 emitted. Within this thesis this effect is characterised from a modelling perspective and the detection and measure- ment of 802 by a satellite are improved. A stratospheric aerosol box model is developed that parameterises the aerosol evolution from an emitted mass of volcanic 802. Using aerosol optical depth measurements and a simple energy balance model, the 802 mass is directly related to a global average tempera- ture change. Combining this model with historical eruption records yields a measure of the significance of eruptions based on frequency and magnitude. The model predicts that eruptions of 0.1-1 Mt of 802 are the most significant for perturbing the climate. This is consistent with recent satellite observations of stratospheric optical depth. The model establishes that the radiative forcing from a large volcanic eruption is determined by the mass of 802 erupted, sug- gesting accurate measurement of volcanic 802 is paramount for quantitative monitoring of its atmospheric impact. The brightness temperature difference method developed by Prata et al. (2003) demonstrated the potential for mon- itoring volcanic 802 using the High Resolution Infrared Radiation Sounder 2 (HIR8/2) instruments on the NOAA and MetOp platforms. The Prata method is fast but of limited accuracy. This thesis improves upon this by using an optimal estimation retrieval approach yielding increased accuracy for only moderate computational cost. This is principally achieved by fitting the column water vapour and accounting for its interference in the retrieval of 802. A cloud and aerosol model is used to evaluate the sensitivity of the scheme to the presence of ash and water/ice cloud. This identifies that the cloud or ash above 6 km limits the accuracy of the water vapour fit, increasing the error in the 802 estimate. Cloud top height is also retrieved and this finding quantifies a cloud screening limit that can be imposed. The scheme is applied to a case study event, the 1991 eruption of Cerro Hudson in Chile. A new total erupted mass estimate was found to be 2200 kT ± 600 kT. This fit method yields a minimum mass per unit area detection limit of 3 DU, which is comparable with that for the Total Ozone Mapping 8pectrometer (TOM8), the only other instrument capable of monitoring 802 from 1979-1996. v.
Supervisor: Grainger, R. G. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available