Development and Application of a New Thermal Gradient Diffusion Chamber to Study the Ice

Development and Application of a new Thermal Gradient Diffusion Chamber to study the Ice Nucleation Properties of Inhomogeneous

Dust Aerosols

by

Gourihar Ramakant Kulkarni

Submitted in accordance with the requirements for the degree of

Doctor of Philosophy

University of Leeds

School of Earth and Environment

September 2007

The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the work of others.

This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement.

Acknowledgements

First and foremost I wish to express my greatest appreciation to my supervisor,

Dr. Steven Dobbie for introducing me to the field of Atmospheric Science and providing precious suggestions and inspiration on my research. This thesis grew out of a series of dialogues with Steven and his comments on chapter draftsare themselves a course in critical thought upon which I will always draw.

I would also like to thank Dr. Jim McQuaid and Prof. Mike Smith for their support and guidance. Special thanks I owe to Dr. Justin Lingard, Dr. Sarah Walker and Dr. Barbara Brooks for all the support during the experiments in the aerosol laboratory.

The experiments reported in this thesis would not have been possible without the help fromthe mechanical workshop technicians of School of Earth and Environment, School of Physics and Astronomy, and School of Mechanical Engineering. Also I would like to thank computer support staff of School of Earth and Environment.

Current students of the research group and my friends to whom I’m also indebted are Clare Allen, Huiyi Yang, Gerard Devine, Paul Manktelow, Sarah Lock, David Woodhead, Lindsay Bennett, Jason Lowenstein, Paul Smith, Sarah Norris, Caroline Bain, Ashish Raval, Kiran Bhagate, Srinath Thirumaliarajan, Puroshattam Joshi and Kasim Sader.

Last and most importantly, I wish to express my genuine gratitude to my family; especially to myparents. Without their spiritual support and motivation, even if they were far away, my studies couldnot have been carried through. And a special thanks to my fiancé Kalpana, who have supported and helped over the past few years.

Abstract

The role of mineral dust as heterogeneous ice nuclei in both ice and mixed phase clouds is not well understood. Heterogeneous ice nucleation is an important process in the formation and ultimate evolution of such clouds, and uncertainties surrounding this not only impacts our ability to model such clouds, but also to understand their effect on the climate system.

This thesis presents laboratory experiments carried out using a newly developed Thermal Gradient Diffusion Chamber (TGDC) to investigate the ice nucleating properties of genuine mineral dust particles. The dust particles are ground-collected from various locations across the Saharan desert (during the recent AMMA campaign) and thedesert along the South East coast of Spain

The aim of this work was to develop a TGDC that would allow for individual nucleation events of particles to be evaluated over the nucleation period. After many conceptual designs and testing using numerical codes developed in-house using commercial fluid dynamics software FLUENT, the final TGDC geometry was determined. The final design of the TGDC allows dust particles to be mounted on a hydrophobic substrate (with the ability to be raised and lowered) to be exposed to a range of temperature and corresponding relative humidity with respect to ice (RHi) values. The ice formation is observed using anoptical microscope. The advantages of this technique are a reduction in temperature uncertainties and thatthe same dust particles can be exposed many times to different RHi values and the nucleation events of individual particles can be directly observed.

The study focuses on the role of mineral dust particles as heterogeneous ice nuclei

at temperatures between -10 and -34 ˚C, and at various humidities below water saturation. The TGDC experiments are performed inside an Environmental Room (or walk-in cold room), maintained at a temperature below -40 ˚C. It is found that the spatial variation of onset RHi for the Saharan and Spanish dust samples is small (varying from 104 to 110% RHi) and the variation for different regions within the Sahara are even smaller. This suggests that single values may well represent large spatial regions.The scanning electron microscope, equipped with energy dispersive detector (SEM-EDX),shows common elements present in all the dust samples, and importantly that the dust particles have a uniform elemental composition across the individual surfaces. SEM-EDX also shows the particles to have many cracks and steps. Given these findings, it is suggested that nucleation events were taking place at steps and crack, as opposed to being strongly influenced by spatial variation in elemental composition.

The TGDC was also used to evaluatethe active IN fraction, which was observed to be between 0.7 and 0.48 depending on RHi. An investigation into the time taken for nucleation to initiate, the nucleation time-lag, shows it varies between 70 and 170 seconds at RHi values of 116% and 110%. Also the time lag is sensitive to the temperature.

To express the ice nucleation results quantitatively, the heterogeneous ice nucleation rates are calculated as a function of RHi and temperature.The results also indicate that ice particle numbers predicted using these observations agree reasonably well with other findings and support the noted difference with the Meyers (1992) formulation.