From clear skies and test tubes to cloudy days and deep waters: Studies on the simplifying assumptions in aquatic photochemistry
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2023
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Doctoral Thesis
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Abstract
Photochemical transformations in global surface waters are important environmental removal processes for some classes of organic contaminants. These transformations can occur through either direct absorption of photons by the chemical, or through indirect processes whereby the chemical reacts with a photochemically produced reactive intermediate (PPRI). One important PPRI is singlet oxygen (¹O₂), which is generated when the ground state of the chromophoric fraction of dissolved organic matter (CDOM) absorbs a photon and is excited to its triplet state (³CDOM*). The reaction of ³CDOM* with molecular oxygen generates ¹O₂.
The photons that are absorbed either by an organic contaminant directly or by the CDOM ubiquitous in surface waters are transmitted through the atmosphere from the sun. The atmosphere and the water column can impact the light field in complex ways that are not commonly accounted for in photochemical models. In the laboratory, light sources are often used that are not representative of the light from the sun, but allow for specific parameters to be elucidated, such as those that are wavelength-dependent. It is relatively straightforward to account for the photons from a laboratory light source; it is much more complicated to account for the full range of variability in light from the sun over different latitudes, seasons, and meteorological conditions.
This thesis focuses on improving our ability to accurately describe photochemical transformations that occur in the environment. The following objectives were defined: 1) Improve estimates of singlet oxygen-induced oxidations in aquatic systems, 2) Provide a global, temporally resolved set of correction factors to incident irradiance spectra to account for non-clear-sky conditions, and 3) Determine photolysis half-lives for a set of common aquatic contaminants in a field setting.
Chapters 2 and 3 address the first objective. In Chapter 2 we used time-resolved ¹O₂ phosphorescence to determine the ¹O₂ quantum yields (ΦΔ) of a variety of natural waters and organic matter isolates over a wide range of environmentally relevant wavelengths. This approach to measuring ΦΔ uses a direct measurement technique rather than commonly used steady-state measurements, and as such, result in more accurate values. Wavelength-dependency is hardly ever assessed but is crucial to accurately describe the environmental behaviour of this PPRI. We used our wavelength-dependent ΦΔ values to estimate polychromatic ΦΔ values for DOM and suggest using a light source with emission centered at 400 nm for the most environmentally relevant single-wavelength measurements of ΦΔ. In Chapter 3 we focus on determining environmentally realistic ranges of ¹O₂ steady-state concentrations ([¹O₂]SS). Typical literature values of this parameter reflect optimum conditions, such as the near-surface of a water body and summer day or solar noon irradiance. We explored the influence of a variety of environmental and physical parameters on [¹O₂]SS, and found that the mixed layer (epilimnion) depth of a lake has the largest impact on ¹O₂ quantities, followed by the latitude. Interestingly, within the epilimnion the amount of dissolved organic carbon present has a limited impact, though it is important at the near-surface.
Chapter 4 addresses the second objective, and focusses on increasing the accuracy of the incident irradiance parameter by accounting for non-clear-skies in a systematic manner. Often clear sky conditions are assumed in the context of photochemical modelling, but the impact of this assumption is generally not well understood. Satellites can provide estimates of incident irradiance by using algorithms to convert reflected light from the surface of the earth to irradiance values. Satellite-derived incident irradiance accounts for non-clear skies and is globally and temporally well-resolved but is difficult to directly incorporate into photochemical modelling. We compared satellite-derived incident irradiance values to commonly used reference irradiance spectra to derive a set of correction factors that are straightforward for researchers, practitioners and regulators to use.
The third objective is addressed in Chapter 5, where we describe a field installation that is being used to study photochemical transformation in a realistic environmental setting. We present details of the experimental setup and some preliminary data that shows the photochemical transformation of two common aquatic pollutants, diclofenac and cimetidine. This type of whole-system study is crucial to understanding the environmental behaviour of aquatic contaminants and contributes to the growing body of knowledge on this topic.
This thesis advances our understanding of how photochemical transformations occur in the environment. It provides better alternatives to some common, but inaccurate, assumptions in photochemical modelling, and contributes important insights to our understanding of real-world photolysis.
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03850 - McNeill, Kristopher / McNeill, Kristopher