Date of Award

2015

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

Sarah Green

Committee Member 3

Shiliang Wu

Abstract

Soot/ black carbon particles are believed to be the second largest anthropogenic contributor to the Earth’s radiative forcing, and are emitted from combustion processes. Freshly emitted soot has a fractal-like structure in which monomers are arranged into branched chain-like configuration. In the atmosphere, soot mixes with and is processed by interacting with other co-existing particle and vapors. The processing that soot undergo after emission alters its morphology; for example, condensation of vapors results into coated, mixed or compacted soot depending upon the environmental conditions. Changes in soot morphology have a strong and direct influence on its optical properties.

The unique and complex structure of soot and its behavior and evolution in the atmosphere are difficult to quantify and to model accurately. Radiative models use simplified assumptions for soot morphology, which results in large uncertainties on the forcing and therefore on the effect that soot has on climate. The temporal and spatial variability of soot morphology and mixing state may also add to already severe biases in absorption measurements in filter-based methods traditionally used to estimate aerosol absorption in the atmosphere. Since optical properties (absorption and scattering) are key input parameters to climate models, biases associated with their measurements add to the uncertainty in the forcing estimates.

During my graduate research, I worked on the development of a new multi-wavelength instrument, capable of measuring aerosol absorption and scattering over the entire visible range of the solar spectrum. The instrument combines photoacoustic spectroscopy and nephelometry with a supercontinuum laser to measure aerosol absorption and scattering over a broad wavelength range (~400 nm to ~700 nm). Since the instrument measures the aerosol optical properties while the particles are suspended in air, this instrument is free from biases common in filter based instruments due to changes in morphology of the particles when they are deposited on the filter or due to multiple scattering of light from the filter.

To complement the instrument development discussed above, my research also included an in depth study of the evolution of soot morphology and mixing state upon its interaction with organics in the atmosphere. We investigated the samples collected from a forested site during the Carbonaceous Aerosols Radiative Effects Study (CARES), conducted in June 2010 in the Sacramento area, CA. Using a scanning electron microscopy, we characterized the morphology and mixing state of soot due to its interaction with biogenic secondary organic aerosols. Based on our analysis we found that both condensation and coagulation were accountable for the mixing of soot with SOA during the episodes we studied in CARES. We found that during coagulation, the viscosity of secondary organic aerosol (SOA) plays a crucial role in determining the soot-SOA mixing state. The viscosity of SOA can be linked to environmental factors like relative humidity, therefore the mixing of soot particles with biogenic aerosol may be affected by factors controlling the viscosity of SOA.

The mixing of soot with biogenic SOA through condensation was further investigated under controlled laboratory conditions in a set of follow-up chamber experiments at the Pacific Northwest National Laboratory during the Soot Aerosol Aging Study (SAAS). In this study, the condensation of α-pinene SOA was examined on diesel soot at different relative humidity conditions. In this study, we find that soot in humid conditions becomes compact, which results into reduction of soot surface area for condensation. The reduction in surface area results in a much slower SOA condensational growth on the soot particles. Compacted soot particles in the atmosphere have been reported in several previous studies and their compaction is attributed to the condensation of organic vapors on soot in humid conditions, ice nucleation or water processing in clouds. Our findings are relevant to mixing scenarios where preprocessed, compacted soot competes with freshly emitted soot for SOA uptake. Based on the findings of our study we expect larger SOA growths on fresh soot particles as compared to compacted particles.

The research carried out during my PhD contributes to the field of atmospheric science on several aspects. The results from our study can be used to reduce the uncertainties in radiative forcing estimates.

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