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Title: The behaviour of trace gases and aerosol particles in a cumulonimbus environment
Author: Devine, Gerard Majella
ISNI:       0000 0001 3422 9741
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2007
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Cumulonimbus clouds perturb their local environment in such a way that subsequent clouds may experience dramatically different fields of gas and aerosol species. A full understanding of the behaviour of these species in a convective environment therefore requires examination beyond single-cloud studies. This study uses a cloud-resolving model to examine the evolution of species typical of the remote marine atmosphere over a period of several days and across a domain size that encompasses several cloud systems· at once. Using a series of gas and aerosol sub-models that have been developed in the cloud-resolving model, the full effect of an evolving field of convective clouds on gas and aerosol formation, transport, and eventual removal is examined. The knowledge gained from these simulations will ultimately aid representation in global models where such processes are not explicitly resolved. Sea salt aerosol and dimethyl sulphide gas, or DMS, both exhibit large temporal and spatial variability in the convective environment. This variability is inextricably linked to the 'cold pool' outflow from convective systems. Resolving the interaction between the cold pool and the chemical species is shown to be important for various aspects of the species behaviour. An inability to resolve such features, such as in a global model, leads to inaccuracies in both species concentration and vertical transport. In the case of DMS, the abundance as well as the amount vented to the free troposphere is enhanced significantly in comparison to a global model estimate. Emissions into the domain are greater in the CRM because it resolves the localised intense winds embedded in cold pools. Additionally, secondary convection occurs through dynamical lifting at the leading edges of cold pools. Because DMS concentrations are high in these regions, the ability to resolve such features enhances the transport to the free troposphere by up to 50% in comparison to using a mean gas field, typical of a global model. In the case of sea salt aerosol, it is found that precipitation is co-located with regions of maximum and minimum sea salt. This feature arises due to a build up of sea salt inside the cold pool head, a region wh~re precipitation from secondary cono vection tends to occur. Aerosol is also found to be scavenged in localised regions faster than the precipitation lifecycle itself. An inability to resolve these features is shown to have a significant influence on the amount of aerosol scavenged from the domain. The examination of the marine sulphur cycle around convective clouds shows that an anti-correlation exists between 802 and its precursor gas, DMS. Although concentrations of DMS are high around convection due to enhanced surface fluxes, the efficient scavenging of S02 dominates over its formation. Also, it is found that the concentration of accumulation mode sulphate particles falls dramatically during convection. The reason for this is the dominance of nucleation scavenging events. Additionally, it is shown that although aqueous-phase oxidation of S02 is significant, a large majority of the sulphate generated in this way is precipitated to the ground in rain drops.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available