Light Attenuation

A non-intrusive optical probe has been developed for the identification of flow patterns during liquid–liquid two-phase flow through a conduit. It is based on the difference in optical properties of the respective phases and works on the basis of the proportion of light attenuated and scattered by the two-phase mixture. The measuring system consists of a laser source, a light-dependent

The photocatalytic bleaching of dyes using TiO2 has appeared promising in laboratory studies, but little attention has been focused on whether other species such as might be found in wastewater have a deleterious effect on the photobleaching. This study describes the effects that result from the presence of ionic species, organic solvents and humic substances on the photobleaching of several dyes. The manner in which the photobleaching was affected by the presence of adventitious materials was carefully examined with an eye toward elucidating the mechanistic origin of the loss of the photochemical efficiency in the reaction. The overall retardation effects can be attributed to the combination of light attenuation, inhibition and competition effects.

Estuarine eutrophication has led to numerous ecological changes, including loss of seagrass beds. One potential cause of these losses is a reduction in light availability due to increased attenuation by phytoplankton. Future sea level rise will also tend to reduce light penetration and modify seagrass habitat. In the present study, we integrate a spectral irradiance model into a biogeochemical model coupled to the Regional Ocean Model System (ROMS). It is linked to a bio-optical seagrass model to assess potential seagrass habitat in a eutrophic estuary under future nitrate loading and sea-level rise scenarios. The model was applied to West Falmouth Harbor, a shallow estuary located on Cape Cod (Massachusetts) where nitrate from groundwater has led to eutrophication and seagrass loss in landward portions of the estuary. Measurements of chlorophyll, turbidity, light attenuation, and seagrass coverage were used to assess the model accuracy. Mean chlorophyll based on uncalibrated in-situ fluorometry varied from 28 microg L-1 at the landward-most site to 6.5 microg L-1 at the seaward site, while light attenuation ranged from 0.86 to 0.45 m-1. The model reproduced the spatial variability in chlorophyll and light attenuation with RMS errors of 3.72 microg L-1 and 0.07 m-1 respectively. Scenarios of future nitrate reduction and sea-level rise suggest an improvement in light climate in the landward basin with a 75% reduction in nitrate loading. This coupled model may be useful to assess habitat availability changes due to eutrophication and sediment resuspension and fully considers spatial variability on the tidal timescale.

As a part of the IGAC Mega-cities program, aerosol optical and chemical properties were continuously measured from April 2007 to March 2008 at an urban site (37.57°N, 126.94°E) in Seoul, Korea. Aerosol light scattering and column integrated aerosol optical thickness (AOT) were measured by a nephelometer and a sky-radiometer, respectively. Particulate organic carbon (OC) and elemental carbon (EC) were also measured by a semi-continuous OC/EC monitor. Black carbon (BC) was also measured by a 7-wavelength aethalometer. Multiple scattering correction factor for aethalometer instrument was determined for each measurement date. Mixing state of EC was indirectly determined based on the BC/EC ratio in order to retrieve absorption coefficient of EC, babs from the aethalometer BC data. Mass absorption efficiency of EC at 550nm wavelength has been determined to be 9.1, 9.6, 10.0, and 10.0 m2/g during spring, summer, fall, and winter, respectively. Since the measurement site is located in the downwind area of Asian continental outflow, long-range transport Asian dust (AD) and pollution (LTP) particles frequently affect the local air quality. Ångström exponent of aerosol light extinction coefficient between 550 ~ 1020nm was determined to be <0.2 and >1.4 during AD and LTP periods, respectively. Based on the spectral optical scattering and absorption coefficients obtained, single scattering albedo (SSA) was determined as a function of wavelength for different seasons. This measured spectral signature of SSA can provide information about the chemical characteristics of atmospheric aerosol.

Research on hydrogen production by photofermentation has gained renewed interest in recent times because of its potential to generate hydrogen from renewable sources in a sustainable manner for use as an alternate energy carrier. Photofermentative bacteria use nitrogenase enzyme in the presence of adenosine triphosphate (ATP) and reducing power for hydrogen production in photobioreactors (PBRs). Depending on the pigments present in the bacteria, an optimal combination of photoparameters such as light source, intensity, duration, and wavelength have to be maintained in PBRs for efficient light-to-hydrogen conversion. In this article, over 130 literature reports on photoparameters are reviewed to aid in optimal design and operation of photobioreactors. This review includes a discussion of mathematical models reported in the literature to predict light attenuation and photochemical efficiencies of photobioreactors. As part of this study, models for predicting hydrogen evolution and predicting photochemical efficiency as a function of light wavelength and quantum requirement were developed and validated using a range of experimental data compiled from the literature. A case study is presented to illustrate how literature data could be used to size solar-based PBRs for hydrogen production. Based on this case study, it is concluded that major technological breakthroughs are required to reduce the current cost for biohydrogen production by solar-powered PBRs.

The spatial extent and timing of the impact of fish farms on the distribution and performance of a Posidonia oceanica meadow were examined in an embayment of the south-eastern coast of Spain (Hornillo Bay, Murcia). Changes in seagrass distribution were determined using available seagrass mapping (from 1988, i.e., before the onset of aquaculture activities and 1998) and by successive sampling in 1994 and 1998. Environmental variables (light attenuation coefficient, water-column dissolved nutrients and organic content of sediments) together with plant performance (shoot biomass, leaf growth rate, photosynthetic activity, carbohydrate reserves, the number of leaves per shoot, epiphyte loads and herbivore pressure) were measured in plants affected by organic discharges, and were compared with those found in reference healthy plants over an annual growth cycle. Since the onset of fish farm activity, 11.29 ha of P. oceanica meadow has been completely lost and 9.86 ha significantly degraded, thus resulting in a total affected area which accounts for about 53% of the former meadow, or 7-fold the fish farming area. Unequal propagation of seagrass die-off or degradation reflects the relevance of local factors such as depth and hydrodynamism on the true extent of fish farm impact. Water transparency decreases and dissolved nutrient and organic content of sediments increases in the vicinity of cages compared to distant reference stations, thus supporting the notion of environmental gradients caused by the organic release from cages, which spreads outwards. Shoot size, leaf growth rate and the number of leaves per shoot in plants close to the fish farm decreased. Moreover, low leaf growth and low rhizome carbohydrate concentration (always relative to that found in an undisturbed area) indicated carbon budget imbalances. Since light reduction in the affected area was only modest (31% of light reaching the sea surface, while at the same depth this figure was 39% at the reference site), and light availability was well above the minimum requirement estimated for this species, neither this factor nor epiphyte overgrowth (epiphyte load was lower in the affected area) seem to explain such carbon imbalances or the observed meadow regression. Alternatively, the high herbivore pressure found in the affected zone suggests that overgrazing is one of the main causes of decreasing shoot sizes and hence of carbon imbalance, reduced growth and shoot mortality. The impact of fish farms on seagrasses, therefore, seems to be highly variable and depends on complex interactions between a large number of processes.

In California, much of the remaining vernal pool habitat is used for cattle grazing. Some studies suggest that grazing helps promote native plant diversity on grasslands, but the impact of grazing on plants that reside in pool basins is largely unknown. We investigated how one aspect of cattle grazing, the deposition of waste, affects these plant species by adding dung

An aerosol consists of a dispersion of liquid particles in a gas. Coalescence occurs when Brownian motion, differential sedimentation or turbulent gas flow drive two drops into contact and the drops form a larger drop. Coalescence modifies the size distribution of an aerosol which in turn affects its main properties. For instance, raindrop coalescence play an important role in cloud dynamics, precipitation and the scavenging of pollutants by precipitation. The rate of coalescence depends on the forces driving the relative motion of the particles. A good fundamental understanding of the mechanisms of coalescence leading to quantitative predictions of coalescence rates has been achieved for colloidal particles suspended in liquids. This achievement is largely attributable to the ability of researchers to isolate each of the driving forces in turn by judicious use of the density matching of the fluid and particles and adjustment of the fluid viscosity. In contrast, the current understanding of aerosol coalescence is rudimentary. Moreover, in experimental conditions on Earth, one is likely to observe mixed effects of Brownian motion, sedimentation and turbulence on coalescence making a critical test of theory difficult. We present an experimental ground-based study of aerosol coalescence due to turbulence. It will provide the necessary background to plan a microgravity experiment on aerosol coalescence. An initially nearly monodisperse aerosol with high particle number density (typically 10 particles/cu cm) is produced using a Condensation Monodisperse Aerosol Generator (TSI 3475). It flows continuously through a plexiglass cell where it experiences a turbulent flow field generated by an oscillating grid. The initial size distribution is then modified due to turbulence-induced coalescence. Using droplets with an initial mean diameter of 3 m, we maximize the importance of turbulence on coalescence relative to Brownian motion (important for smaller particles) and sedimentation (important for larger particles). To be able to compare experimental data on turbulence-induced coagulation with theory, a precise description of the turbulent flow field is necessary. It has been characterized by measuring the velocity fluctuations using a laser Doppler velocimetry technique. The turbulent kinetic energy is found to be constant in the central region of the cell (within a distance away from the grid mean position equal to half the stroke of the grid displacement) and then to decay away from the grid. Our results are similar to that which one expects, based on previous experiments in liquids. In order to characterize the coalescence, the particle size distribution is measured in situ using a Particle Dynamics Analyzer. This technique is based on the analysis of the light scattered by the particles as they cross a volume of measurement defined by the intersection of two laser beams. The size distribution being known, the particle number density can be measured using a light attenuation technique. As already mentioned, the extent to which we can isolate turbulence from the other mechanisms that may lead to coagulation is limited and this forms the motivation for the future microgravity experiments. Under microgravity, we will be able to use substantially larger particles and eliminate Brownian motion without introducing effects of sedimentation. However, this ground-based experiment will demonstrate the validity of our experimental methods to characterize the coalescence. Additional information is contained within the original extended abstract.

The aim of this study was to check the suitability of some trophic models developed for temperate regions to classify the non-perennial reservoirs of Sri Lanka in order to manage culture-based fisheries of those reservoirs. A limnological study was carried out in 45 non-perennial reservoirs, which have been randomly selected for stocking of fish fingerlings for the development of culture-based fisheries. High total phosphorous (TP) content in relation to algal biomass indicates high non-algal turbidity in all reservoirs. Carlson's trophic state indices (TSI) measured on the basis of Secchi disc depth [TSI (SDD)], TP [TSI (TP)] and chlorophyll a [TSI (Chl-a)] show that the 45 reservoirs studied are characterized by TSI (TP) = TSI (SDD) > TSI (Chl-a), indicating that non-algal particulate matter or colour dominates underwater light attenuation. As TSI (Chl-a) is positively correlated to culture-based fisheries yield, it is useful for planning culture-based fisheries development strategies in non-perennial reservoirs of Sri Lanka, and has the potential to be used elsewhere in the tropics for comparable developments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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A model was developed to examine the influence of phytoplankton community structure on the relationship between diffuse attenuation and ratios of upwelling radiance. Shifts in phytoplankton communities were represented by changing mean optical properties as a function of chlorophyll (C, mg m-3), consistent with large data sets from the field and laboratory. The product of cell size and internal pigment concentration, dci, governs pigment packaging, which alters the specific absorption coefficients of phytoplankton (aph*, m2 mgChl-1). Pigment packaging was parameterized as a function of C by combining the relationship between dci and aph* from phytoplankton cultures with that between aph* and C from the field, using data for 675 nm, where absorption by accessory pigments is low. Changes in accessory pigmentation were approximated by quantifying residual variability in aph* at other wavelengths, as functions of C, once the variability with dci was taken into account. Absorption by colored dissolved organic matter (CDOM), detrital absorption, and scattering by particles were also parameterized as functions of C, so that bio-optical relationships could be modeled as functions of trophic status. The model thus reconciled recognized relationships between optical properties and C with ecologically interpretable shifts in phytoplankton communities. Empirical relationships between diffuse attenuation and ocean color were well reproduced at low (0.5 mg m-3) to medium (10 mg m-3) C. Analysis of variability imposed by a range of dci suggests that it may be possible to recognize phytoplankton communities with cell sizes and intracellular pigment concentration different from the central tendency, given a set of wavelengths which minimizes the influence of CDOM and detrital absorption.