Document Type

Dissertation - Open Access

Award Date

2020

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

First Advisor

Jihong Cole-Dai

Keywords

Chlorine, Diffusion, Ice Core, Perchlorate, Stratosphere, Volcano

Abstract

Perchlorate, suspected to be chemically formed in both the troposphere and stratosphere, has been recently measured in Arctic snow and ice cores. These comprise both discontinuous snow and ice cores from the Canadian Arctic and a continuous record of perchlorate was compiled from an analysis of Greenland ice cores. While the background perchlorate concentration typically is very low, a few spikes in concentration coinciding with deposition of volcanic sulfate were observed in the Greenland record, suggesting that perchlorate levels in the atmosphere may be impacted by volcanic eruptions. As of yet, no work has been done to investigate the connection between volcanic eruptions and perchlorate formation. It was not known 1) whether the volcanic perchlorate response is limited to samples collected in Greenland, 2) if the volcanic perchlorate response is just limited to certain eruptions, 3) what factors influence the magnitude of the response, and 4) what chemistry drives the volcanic perchlorate response. In this work, detailed analysis and careful examination of the data collected from Antarctic, Alaskan, and Greenland ice cores show no seasonal oscillation during nonvolcanic periods prior to the 1980s, indicating a relatively small role for stratospheric photochemical production pathways in the Arctic during these times. The development of a strong seasonal oscillation in perchlorate concentration in snow since 1980 corresponds with a drastic increase in stratospheric organic chlorine throughout the late 20th century, indicating that the relative contribution of stratospheric photochemical processes to the perchlorate deposition in the Arctic has increased as a result of increased stratospheric organic chlorine. Correlation of annual perchlorate flux with mean annual ozone abundance in recent decades suggests that the abundance of both ozone and perchlorate are influenced by stratospheric chlorine. The analysis of ice cores from the South Pole containing the sulfate fallout from several large volcanoes revealed that volcanic perchlorate response is not restricted to the Arctic and occurs in the Antarctic as well. The large flux of perchlorate deposition during periods perturbed by volcanic eruptions relative to background levels in snow during nonvolcanic periods indicate volcanism is a significant source of perchlorate in the environment. Perchlorate formation and deposition in response to some eruptions easily exceed decades of perchlorate deposition during nonvolcanic periods. The flux of sulfate is highly correlated with the flux of perchlorate during volcanically perturbed periods in the stratosphere, indicating that the impact of volcanic aerosol on stratospheric chlorine chemistry results in increased formation of perchlorate. Examination of proposed perchlorate chemistry and formation mechanisms leads to the conclusion that chlorine activation is likely the key process. Volcanic eruptions enhance perchlorate formation through injections of aerosol-forming sulfur, promoting formation of chlorine radicals and other important perchlorate intermediates. Unlike sulfate, perchlorate in snow experiences post-depositional change. The main characteristic of that change is probably diffusion in the firn column. The perchlorate response in ice cores exhibits what appears to be high effective diffusivity relative to sulfate. The diffusivity for perchlorate is modeled based upon the perchlorate response of the 1912 eruption of Katmai and estimated to be about 10-5 m2 yr-1. Comparison of volcanic perchlorate response signals in ice cores from multiple time periods and sampling locations support diffusion in the firn layer.

Library of Congress Subject Headings

Ice cores -- Analysis.
Perchlorates.
Stratospheric chemistry.
Chlorine.
Volcanic eruptions.

Format

application/pdf

Number of Pages

138

Publisher

South Dakota State University

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Rights Statement

In Copyright