Title

Investigations using time-resolved laser spectroscopy in marine photochemistry

Date of Award

1989

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine and Atmospheric Chemistry

First Committee Member

Rod G. Zika, Committee Chair

Abstract

Time resolved spectroscopic techniques, both emission and absorbance, were developed and applied to two problems in marine photochemistry. The first of these was an investigation into the nature of the fluorophores present in dissolved organic matter in seawater and the second was to measure the kinetics of the reaction of aqueous hydroxyl radicals with methanesulfonic acid.The fluorescence lifetimes of a number of samples of dissolved organic matter were measured. Their lifetimes were all closely similar ($\sim$2ns) despite their different origins and illumination histories. Steady state and time-resolved measurements of the luminescence quenching of samples of marine dissolved organic matter with excited state energy quenchers such as iodide, acrylamide and methyl viologen (1,1$\sp\prime$-dimethyl-4,4$\sp\prime$-bipyridinium) were compared. Quenching characteristics of these systems were analysed using Stern-Volmer plots for both intensity and lifetime quenching. The bimolecular quenching constant, $k\sb{\rm q}$, for these quenchers were found to decrease in the order MV$\sp{2+}$ ($k\sb{\rm q}\sim$1 $\times$ 10$\sp $ M$\sp{-1}$ s$\sp{-1}$) $>$ I$\sp-(k\sb{\rm q}\sim 2$ $\times$ 10$\sp9$M$\sp{-1}$ s$\sp{-1}$) $>$ CH$\sb2$CHCONH$\sb2$ ($k\sb{\rm q}\sim 2$ $\times$ 10$\sp8$M$\sp{-1}$ s$\sp{-1}$) for all of the samples measured. The results showed that different samples were quenched to differing extents by the quenchers studied, that ionic strength can affect the quenching constants and that both static and diffusional quenching mechanisms may operate. It is also shown that in seawater, quenching of the excited singlet states by dissolved oxygen is an unimportant deactivation path, and that these results support an unexpected picture of a single or predominant fluorophore in the dissolved organic matter. The rates of reaction of the hydroxyl radical formed by laser flash photolysis of dilute solutions of hydrogen peroxide with methanesulfonic acid, dimethyl sulfoxide or dimethyl sulfone were measured using competition kinetics with thiocyanate as the reference solute. The rate of reaction of OH. with SCN$\sp{-}$ was remeasured to be 9.60 $\pm$ 1.12 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$, in agreement with recent determinations. The rates of reaction of OH. with the sulfur compounds were found to decrease in the order DMSO (5.4 $\pm$ 0.3 $\times$ 10$\sp9$ M$\sp{-1}$ s$\sp{-1}$) $>$ MSA (4.7 $\pm$ 0.9 $\times$ 10$\sp7$ M$\sp{-1}$ s$\sp{-1}$) $>$ DMSO$\sb2$ (2.7 $\pm$ 0.5 $\times$ 10$\sp7$ M$\sp{-1}$ s$\sp{-1}$). Modeling studies confirmed the stability of methanesulfonic acid in sea-salt particles.

Keywords

Physical Oceanography; Chemistry, Physical

Link to Full Text

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