Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Atmospheric Sciences (PhD)

Administrative Home Department

Department of Physics

Advisor 1

Claudio Mazzoleni

Committee Member 1

Will Cantrell

Committee Member 2

Jacek Borysow

Committee Member 3

Paul Doskey

Abstract

Soot particles form during incomplete combustion of carbonaceous materials. These particles strongly absorb light and directly affect Earth’s climate by warming our atmosphere. When freshly emitted, soot particles have a fractal-like morphology consisting of aggregates of carbon spherules. During atmospheric processing, soot aggregates interact with other materials present in our atmosphere (i.e., other aerosol or condensable vapors) and these interactions can result in the formation of coated, mixed or compacted soot particles with different morphologies. Any process that alters the morphology (shape, size and internal structure) and mixing state of soot also affects its optical properties, which in turn affect the soot radiative forcing in the atmosphere. The complex morphology and internal mixing state of soot makes it difficult to estimate the soot’s radiative properties. A detailed investigation of soot at the single particle level using electron microscopy, thus, becomes essential to provide accurate information for climate models, which generally assumes simple spherical morphologies. Tar balls are another type of carbonaceous aerosol, in the brown carbon family, commonly formed during smoldering combustion of biomass materials. Like soot, tar balls can also form aggregates. Tar balls aggregates have different optical properties from those of individual tar balls. During my doctorate studies, I made extensive use of electron microscopy and image analysis tools to investigate the morphology and mixing state of soot and tar balls collected during different laboratory and field studies. In one of my research projects, I explored the morphology of cloud processed soot. For this, I investigated the morphology of soot particles collected from the Po Valley in Italy where fog often forms, especially in winter. Our investigation showed that soot particles became compacted after fog processing. The compaction of soot was further corroborated by a laboratory study, in which cloud processing was carried out within the Michigan Technological University cloud chamber. In another research project, I studied the effects of thermodenuding on the morphology of soot. I investigated the morphology of five sets of soot samples of different sizes before xiii and after themodenuding. Our investigation showed no significant change in the morphology of soot by thermodenuding, a result that is important for those who attempt to measure the optical properties of internally mixed coated particles. In a third study, I used T-Matrix and Lorenz-Mie models to calculate the optical properties and then estimate the radiative forcing of tar ball aggregates and individual tar balls. In fact, in a recent publication, we reported a significant fraction of tar ball aggregates from different locations on Earth. My numerical calculations showed that the optical properties of tar ball aggregates are different from those of individual tar balls and are not always well approximated by Lorentz-Mie calculations. These findings highlight the necessity to account for the aggregation of tar balls in global models. My doctorate research provides detailed information on the morphology and mixing state of soot and tar ball aggregates. This information can be used to improve global climate models and reduce uncertainties in the radiative forcing of these aerosol particles.

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