Dissertation Abstract
Investigations of Hydrogen Peroxide Photochemistry in Snow and Ice
Hullar, Ted 2013
Atmospheric Sciences Graduate Group, University of California at Davis (United States), 110 pp.
Hydrogen peroxide (HOOH) is an important trace constituent and significant oxidant in ice and snow; HOOH concentrations measured in ice cores have been used as a proxy for oxidative capacity of past atmospheres. HOOH is readily photolyzed in snow to form hydroxyl radical (•OH). Laboratory studies of this photolysis suggest summer snowpack HOOH should be completely destroyed; however, field measurements contradict this conclusion and show a continuous record of HOOH from the surface to deep below the photic zone. In this dissertation, we examine whether photochemical reactions involving HOOH in snow and ice can explain this discrepancy. Photolytically generated •OH could react with organics in aqueous phases through several steps to reform HOOH, but the efficiency of this process under natural conditions is not well understood and has not been studied in snow and ice. Aside from recycling reactions, HOOH can also be produced by illumination of organics in aqueous phases; again, these reactions have not been studied in snow and ice.
Here, we present the results of several laboratory studies of HOOH formation and recycling as well as a modeling study of HOOH concentrations in a hypothetical snowpack. First, we evaluated the yield of HOOH formation from reaction of •OH with different organic compounds representing a variety of chemical classes. We illuminated liquid and ice samples with and without nitrate, which was used as an •OH source, and measured the HOOH formation rate. We found that HOOH recycling through reaction with organics can occur in aqueous solution as well as laboratory ice samples, but at relatively low yields.
Our second set of experiments examined the production of HOOH from illumination of laboratory ice samples containing various organic compounds, as well as natural snow samples collected in polar regions. Of the laboratory ice samples, only half of the organic compounds produced appreciable HOOH when exposed to simulated sunlight. Illuminated natural samples also produced HOOH, but at low rates.
Neither HOOH recycling through organics nor direct illumination of organic-containing ices produced sufficient HOOH to resolve the discrepancy between estimated photolytic lifetimes and the continuous field record. To better understand the HOOH budget in polar snow, we developed a snow parcel model to study the time evolution of HOOH concentration in a hypothetical snowpack. We included relevant environmental factors (snowfall and variations in actinic flux), and evaluated the importance of three photochemical factors on HOOH concentrations: HOOH recycling through organic compounds, HOOH formation from illumination of organics in ice, and the quantum efficiency of HOOH photolysis in ice. Our results agreed well with field measurements of HOOH concentration, successfully explaining the preservation of HOOH in deeper snow and ice. HOOH preservation was sensitive to quantum efficiency and HOOH photoformation rates, while recycling efficiency was less important.
While most of this work focuses on the reactions of HOOH in snow and ice, we also present the design and construction of a low-cost cold stage for use in an X-ray micro-CT (computerized tomography) machine. The cold stage allows 3-dimensional visualization of a variety of temperature sensitive materials, including ice and biological samples.
Here, we present the results of several laboratory studies of HOOH formation and recycling as well as a modeling study of HOOH concentrations in a hypothetical snowpack. First, we evaluated the yield of HOOH formation from reaction of •OH with different organic compounds representing a variety of chemical classes. We illuminated liquid and ice samples with and without nitrate, which was used as an •OH source, and measured the HOOH formation rate. We found that HOOH recycling through reaction with organics can occur in aqueous solution as well as laboratory ice samples, but at relatively low yields.
Our second set of experiments examined the production of HOOH from illumination of laboratory ice samples containing various organic compounds, as well as natural snow samples collected in polar regions. Of the laboratory ice samples, only half of the organic compounds produced appreciable HOOH when exposed to simulated sunlight. Illuminated natural samples also produced HOOH, but at low rates.
Neither HOOH recycling through organics nor direct illumination of organic-containing ices produced sufficient HOOH to resolve the discrepancy between estimated photolytic lifetimes and the continuous field record. To better understand the HOOH budget in polar snow, we developed a snow parcel model to study the time evolution of HOOH concentration in a hypothetical snowpack. We included relevant environmental factors (snowfall and variations in actinic flux), and evaluated the importance of three photochemical factors on HOOH concentrations: HOOH recycling through organic compounds, HOOH formation from illumination of organics in ice, and the quantum efficiency of HOOH photolysis in ice. Our results agreed well with field measurements of HOOH concentration, successfully explaining the preservation of HOOH in deeper snow and ice. HOOH preservation was sensitive to quantum efficiency and HOOH photoformation rates, while recycling efficiency was less important.
While most of this work focuses on the reactions of HOOH in snow and ice, we also present the design and construction of a low-cost cold stage for use in an X-ray micro-CT (computerized tomography) machine. The cold stage allows 3-dimensional visualization of a variety of temperature sensitive materials, including ice and biological samples.