Catherine D. Clark

A time-resolved fluorescence study of dissolved organic matter in a riverine to marine transition zone

Abstract Time-resolved and steady-state fluorescence measurements were used to characterize dissolved organic matter (DOM) in bulk water samples from a fresh to marine transition zone. The region studied was the Shark River and Florida Bay in Everglades National Park in Southwestern Florida. Samples were taken from the fresh waters at the head of the Shark River, the mouth of the river, and in the fresh-water river plume as it mixed into the saline waters of Florida Bay. The salinity varied from 0.47 at the head of the river to 36.0 for this series of 11 samples. Steady-state fluorescence intensity decreased with increasing salinity from the head of the Shark River into Florida Bay. This is due to dilution by mixing, as changes in the fluorescence intensity also correlated with changes in the levels of total organic carbon (TOC). Fluorescence lifetime measurements were performed on the five most concentrated samples (salinity from 0.47 to 30.5). The decay of the time-resolved fluorescence could be reasonably fit to a multi-exponential function, with three lifetime components on the order of 0.5–0.8, 2–3, and 6–9 ns. These ranges are in agreement with previous literature results for humic acids, fulvic acids, and marine CDOM concentrated by ultrafiltration. This is the first study of CDOM fluorescence lifetimes in coastal waters in a fresh to marine transition zone. Although photochemical and biological transformations of CDOM occur in these zones, no statistically significant difference in lifetimes was observed with increasing salinity. However, irradiating fresh water samples from the head of the Shark River at 280 and 334 nm resulted in a significant decrease in the two shorter lifetime components (by a factor of ∼4), but only a slight decrease in the longest lifetime component.

EFFECT OF SOLUTION MEDIUM ON THE RATE CONSTANTS OF EXCITED-STATE ELECTRON-TRANSFER QUENCHING REACTIONS OF RUTHENIUM(II)-DIIMINE PHOTOSENSITIZERS

Abstract In this paper, we review the effects of solution medium (pH, solvent, temperature, ionic strength, specific electrolytes) on the oxidative and reductive quenching rate constants k q of the excited states of Ru(II)-diimine photosensitizers. Diffusion of the donor and acceptor species together to form the precursor complex ( k et ) and electron transfer within the complex ( k et ) contribute to the value of k q . Values of k d vary with bulk solution properties; variations of k et can be described within the context of Marcus theory, wherein dynamic solvent effects influence the nuclear frequency factor and electronic coupling, and static properties cause changes in the driving force of electron transfer ΔG et o and the reorganization energy λ. The pH can affect the state of protonation of the excited photosensitizer and/or the quencher, thereby altering k et through changes in ΔG ct o and k d through changes in the charges of the reactants. Ionic species are ion-paired by the dominant counterion; the Olson-Simonson treatment allows the electron transfer components of quenching for ion-paired ( k ip ) and nonion-paired ( k mp ) species to be extracted. The quenching of ∗ Ru(bpy) 3 2+ by methylviologen is used to demonstrate specific salt effects, which result in variations in λ; λ is lowest, and k q highest, for the anions with the most weakly-held hydration spheres and the strongest structure-breaking abilities (e.g. ClO 4 − , I − ). Quenching rate constants can be fine-tuned through the variation of solvent, pH, electrolyte, ionic strength, and temperature.

Photochemical degradation of phenanthrene as a function of natural water variables modeling freshwater to marine environments.

Photolysis rates of phenanthrene as a function of ionic strength (salinity), oxygen levels and humic acid concentrations were measured in aqueous solution over the range of conditions found in fresh to marine waters. Photolysis followed first order kinetics, with an estimated photodegradation half-life in sunlight in pure water of 10.3±0.7h, in the mid-range of published results. Photolysis rate constants decreased by a factor of 5 in solutions with humic acid concentrations from 0 to 10 mg C L(-1). This decrease could be modeled entirely based on competitive light absorption effects due to the added humics. No significant ionic strength or oxygen effects were observed, consistent with a direct photolysis mechanism. In the absence of significant solution medium effects, the photodegradation lifetime of phenanthrene will depend only on solar fluxes (i.e. temporal and seasonal changes in sunlight) and not vary with a freshwater to marine environment.

Photochemical production of hydrogen peroxide in size-fractionated Southern California coastal waters

Hydrogen peroxide (H(2)O(2)) photochemical production was measured in bulk and size-fractionated surf zone and source waters (Orange County, California, USA). Post-irradiation (60 min; 300 W ozone-free xenon lamp), maximum H(2)O(2) concentrations were approximately 10000 nM (source) and approximately 1500 nM (surf zone). Average initial hydrogen peroxide production rates (HPPR) were higher in bulk source waters (11+/-7.0 nM s(-1)) than the surf zone (2.5+/-1 nM s(-1)). A linear relationship was observed between non-purgeable dissolved organic carbon and absorbance coefficient (m(-1) (300 nm)). HPPR increased with increasing absorbance coefficient for bulk and size-fractionated source waters, consistent with photochemical production from CDOM. However, HPPR varied significantly (5x) for surf zone samples with the same absorbance coefficients, even though optical properties suggested CDOM from salt marsh source waters dominates the surf zone. To compare samples with varying CDOM levels, apparent quantum yields (Phi) for H(2)O(2) photochemical production were calculated. Source waters showed no significant difference in Phi between bulk, large (>1000 Da (>1 kDa)) and small (<1 kDa) size fractions, suggesting H(2)O(2) production efficiency is homogeneously distributed across CDOM size. However, surf zone waters had significantly higher Phi than source (bulk 0.086+/-0.04 vs. 0.034+/-0.013; <1 kDa 0.183+/-0.012 vs. 0.027+/-0.018; >1 kDa 0.151+/-0.090 vs. 0.016+/-0.009), suggesting additional production from non-CDOM sources. H(2)O(2) photochemical production was significant for intertidal beach sand and senescent kelp (sunlight; approximately 42 nM h(-1) vs. approximately 5 nM h(-1)), on the order of CDOM production rates previously measured in coastal and oceanic waters. This is the first study of H(2)O(2) photochemical production in size-fractionated coastal waters showing significant production from non-CDOM sources in the surf zone.

Marine Organic Photochemistry: From the Sea Surface to Marine Aerosols

Photochemistry in the sunlit surface waters of the ocean is dominated by colored dissolved organic material (CDOM) which lies at the center of a photochemical cycle that critically impacts the marine environment. Sunlight-irradiated CDOM undergoes a complex series of reactions to produce biologically available photodegraded DOM, volatile organic carbon compounds, and reactive species (e.g., excited triplet states, OH, superoxide). These react with each other, trace metals (e.g., iron), and other substances in the ocean in a complex series of reactions that affect marine biota and influence the composition of the surface ocean and marine atmosphere. Although DOM has been the focus of several decades of active research in marine chemistry, fundamental questions about its sources, composition, and reactivity remain. Sea salt aerosols produced at the ocean surface will incorporate some of the photochemically-active organic materials concentrated in the sea surface microlayer, including CDOM. There have been very limited studies on photochemistry in aerosol particles, but likely reactions and yields can be conjectured. The photochemistry of the sea surface micro-layer and marine aerosol particles constitutes an important new area of research for marine photochemists.

Hydrogen peroxide measurements in recreational marine bathing waters in Southern California, USA

Hydrogen peroxide (H(2)O(2)) was measured in the surf zone at 13 bathing beaches in Southern California, USA. Summer dry season concentrations averaged 122 +/- 38 nM with beaches with tide pools having lower levels (50-90 nM). No significant differences were observed for ebb waters at a salt marsh outlet vs. a beach (179 +/- 20 vs. 163 +/- 26 nM), and between ebb and flood tides at one site (171 +/- 24 vs. 146 +/- 42 nM). H(2)O(2) levels showed little annual variation. Diel cycling was followed over short (30 min; 24 h study) and long (d) time scales, with maximum afternoon concentration = 370 nM and estimated photochemical production rate of 44 nM h(-1). There was no correlation between the absorbance coefficient at 300 nm (used as a measure of chromophoric dissolved organic matter (CDOM) levels) and H(2)O(2). H(2)O(2) concentrations measured in this study are likely sufficient to inhibit fecal indicator bacteria in marine recreational waters through indirect photoinactivation.

Near real-time measurement of sea‑salt aerosol during the SEAS Campaign: Comparison of emission-based sodium detection with an aerosol volatility technique

Abstract The first deployment of an emission-based aerosol sodium detector (ASD), designed to chemically characterize marine aerosols on a near-real-time basis, is reported. Deployment occurred as part of the Shoreline Environment Aerosol Study (SEAS) from 16 April to 1 May 2000 at Bellows Air Force Base on the east side of Oahu, where the University of Hawaiis Department of Oceanography maintains a tower for aerosol measurements. The instrument was operated in size-unsegregated mode and measurements were made that included two extended continuous sampling periods, each of which lasted for 24 h. During this time, the ASD was compared with measurements that used aerosol volatility coupled with optical particle counting to infer sea-salt size distributions. A reasonable agreement was obtained between the instruments when sampling in clean air, suggesting that under these conditions both approaches can provide reliable sea-salt distributions. The combination of these measurements suggested that sea salt was...

Protonation of the ground states of ruthenium(II) photosensitizers

Abstract Ru(II) complexes that possess 2,2′-bipyrazine, 2,2′-bipyrimidine, and 2-(2′-pyridyl)pyrimidine ligands in combination with 2,2′-bipyridine undergo protonation at the basic nitrogen heteroatoms on the periphery of the aromatic rings as the solution is made more acidic, resulting in changes in the absorption spectra. In this study, the acidity of the solution was controlled with concentrated H 2 SO 4 ; titration curves for the first protonations were obtained from plots of absorbance versus acidity, from which the p K a1 values for the monoprotonated species were extracted. As has been observed before for the acid-base properties of the excited states and the one-electron reduced forms of the complexes, the p K a1 values of the protonated forms of the ground states (−1.6 to −5.0) correlate with the reduction potentials, and demonstrate site selectivity of the protonation process.

Ion-pairing control of excited-state electron-transfer reactions Effect of cations on cationic reactants

Abstract The rate constants for the oxidative quenching of ∗Ru(bpy)32+ by MV2+ (kq) and the cage escape yields (ηce) of the redox products (Ru(bpy)33+ and MV+) were determined as a function of added electrolytes (Cl− salts of Li+, Na+, Cs+, Ca2+, La3+) and temperature (10–60°C) in aqueous solution. At 25°C and constant [Cl−], kq is independent of the cation. There is, however, a specific cation effect on ηce (La3+ > Ca2+ ∼ Li+ > Na+ > Cs+), which is attributed to differences in the rate constants of cage escape (kce) due to variations in the bulk properties of the solution (viscosity, dielectric constant); the rate constants of back electron transfer within the cage are essentially independent of the nature of the electrolyte cation. The reactant cations are extensively ion-paired by Cl− in bulk solution and within the quenching solvent cage. However, the electrolyte cations do not have any effect on the rates of electron transfer between the cationic species.

Photoproduction of hydrogen peroxide in aqueous solution from model compounds for chromophoric dissolved organic matter (CDOM)

To explore whether quinone moieties are important in chromophoric dissolved organic matter (CDOM) photochemistry in natural waters, hydrogen peroxide (H2O2) production and associated optical property changes were measured in aqueous solutions irradiated with a Xenon lamp for CDOM model compounds (dihydroquinone, benzoquinone, anthraquinone, napthoquinone, ubiquinone, humic acid HA, fulvic acid FA). All compounds produced H2O2 with concentrations ranging from 15 to 500 μM. Production rates were higher for HA vs. FA (1.32 vs. 0.176 mM h(-1)); values ranged from 6.99 to 0.137 mM h(-1) for quinones. Apparent quantum yields (Θ app; measure of photochemical production efficiency) were higher for HA vs. FA (0.113 vs. 0.016) and ranged from 0.0018 to 0.083 for quinones. Dihydroquinone, the reduced form of benzoquinone, had a higher production rate and efficiency than its oxidized form. Post-irradiation, quinone compounds had absorption spectra similar to HA and FA and 3D-excitation-emission matrix fluorescence spectra (EEMs) with fluorescent peaks in regions associated with CDOM.