Light attenuation model

The rates of photochemical production that were input into the model are taken from laboratory quantum yield measurements, discussed earlier (see section In situ formation"). These are Incorporated Into a light attenuation model that employs the extinction coefficient of light In water with the clarity of the Gulf of Mexico. The dark decay lifetime of 2 2 chosen to be 4 days, and constant with depth. The dark decay lifetime varies from hours In coastal environments to over a week In ollgotrophlc waters, but 4 days Is commonly observed for offshore surface water (7). It is clear from Figure 7 that. In the absence of any mixing processes, builds up to high concentrations with little dlel... 

The coupling of photochemistry and physical mixing in a model is a challenging task for three reasons. First, the photochemical production and decomposition rates, as well as the dark decay rates of Interest, are often poor estimates. This Is both because of the lack of good laboratory kinetic measurements and because the light attenuation models Into which such measurements are Incorporated are subject to additional uncertainties. Second, the one-dlmenslonal mixing models make certain assumptions and... 

In spite of these difficulties with DOM chemistry, environmental chemists are frequently asked what molecular structures within the mixture are responsible for contaminant binding, haloform production, light attenuation, protonation characteristics, and other problems of environmental relevance. The chemist usually hypothesizes that DOM features such as aromaticity, polarity, functional-group content and configuration, molecular interactions, and molecular size can explain the observed phenomena. However, models of DOM (or DOM-fraction) structures must be based on average-mixture analyses to support these hypotheses. Such models represent average properties of thousands to millions of mixed compounds. 

In a mixed water column (or mixed layer) with relatively high values of K, the model generates a uniform phytoplankton distribution throughout the column/layer, independent of production, light attenuation, or distribution of grazing organisms. 

Pottier L, Pmvost J, DeremetzJ, CometJF, LegrandJ, Dussap CG A fuUy predictive model for one-dimensional light attenuation by chlamydomonas reinhardtii in a torus photobioreactor, Biotechnol Bioeng 91(5) 569—582, 2005. 

Bricaud A, Morel A Light attenuation and scattering by phytoplanktonic cells a theoretical modeling, Appl Opf 25 4) 571—580, 1986. 

Whichever production mode (continuous or batch) is used, the control of light attenuation conditions, represented here by the illuminated fraction (with y < 1 for cyanobacteria and y = 1 =b 15% for microalgae), makes it possible to obtain the maximum biomass productivity of the cultivation system in light-limited conditions (volume and surface). If radiative transfer conditions are known (using a radiative transfer model, as already described), then the optimal biomass concentration can be determined theoretically, or else experimentally simply by varying the residence time and measuring corresponding biomass concentration and productivity (Takache et al, 2010). 

The culture system described earlier is based on cylindrical tubes, which makes it difficult to calculate radiative transfer in the culture volume, which has to be solved numerically (Lee et al, 2014). As already described, the one-dimensional hypothesis where light attenuation occurs along only one main direction serves to obtain analytical relations to represent the light attenuation field (as with the two-flux model, Eq. 12). This enables accurate and easy determination of light attenuation conditions for any operating conditions and thus greater system control. Based on this statement, researchers designed a specific PBR. Like the multimodule external-loop airlift PBR, this system is of industrial size (130 L), but the unit is a flat panel with front illumination so as to respond to the one-dimensional hypothesis. It is also illuminated on both sides to increase specific illuminated area... 

The shock tube and basic peripheral instrumentation have been described previously by Lester et al. (6,7). In their experiment a simple attenuation technique was used to monitor the soot formation. The primary light source, shown in Figure 1, was a Spectra-Physics model 120,... 

The 488.0 nm Coherent Radiation Model 520-B argon-ion laser of 300 mW power was focused on the center of the drop and the Raman light is detected perpendicular to both the laser beam and the sample tube. Qualitative polarization measurements were made by rotating the plane of polarization of the laser beam with a half-wave plate. In most of the spectra, it was not necessary to use a spike filter to attenuate the plasma lines. The spectra were recorded using a modified Cary 81 spectrophotometer employing a 9558A EMI photo-multiplier counting detection system. 

The diffuse attenuation coefficient (K ) is one of several apparent optical properties (AOPs) of natural waters described by Preisendorfer [25]. Unlike inherent optical properties (lOPs) described below, AOP s depend on the quality of incident light as well as the optical qualities of the water. In spite of this apparent limitation (and in part because the differences between AOP s and lOP s were said to be small in many instances [26]), a case was argued for the standard use of to characterize natural waters for purposes of optical comparisons and bio-optical models [27,28]. Gordon [17,29] provided a practical means to adjust measurements to remove much of its dependence on the ambient light field. In particular, Gordon [17] established that, after adjustment (described below), averaged from surface to Zio% is proportional to the summed concentrations of constituent optical compounds. 

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