Diffuse attenuation coefficients

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

Note that S(A) is a function both of ctD(A)(the diffuse attenuation coefficient near the surface) and of aD(k) (the average diffuse attenuation coefficient for the whole water column of depth zmix). Using 5(A), we may express kf k) as ... 

Why can one not just use the decadic light absorption coefficient, a(A), to describe light attenuation in a given water body How is the diffuse attenuation coefficient, aD(A), related to a(A) ... 

Which water constituents primarily determine the magnitude of the diffuse attenuation coefficient D(A) ... 

Light attenuation effects were computed by using equation 6, assuming typical diffuse attenuation coefficients at 350 nm for the different oceanic types (20, 24). 

UV transparency of natural waters can be described empirically by two measures that are wavelength-specific and inter-related the downwelling diffuse attenuation coefficient, K, and the percent attenuation depth, Z o/ . A downwelling diffuse attenuation coefficient is nominally proportional to the concentration of substances in the water that absorb or scatter UVR [17,42]. It is typically calculated for specific wavelengths (A) from measurements of downwelling irradiance ( d,A) by fitting the following equation (in units of m ) [8] to irradiance versus depth data ... 

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. 

A similar exponential treatment has also been applied to spectral modeling of UV diffuse attenuation coefficients for natural waters [57,61,76] but this seems ill-advised unless the absorption spectrum of phytoplankton or other particles is insignificant or has been observed to follow the same exponential pattern as CDOM that may be present,... 

H.R. Gordon (1989). Can the Beer-Lambert law be applied to the diffuse attenuation coefficient of ocean water Limnol. Oceanogr., 34,1389-1409. 

K.S. Baker, R.C. Smith (1979). Quasi-inherent characteristics of the diffuse attenuation coefficient for irradiance, Ocean Optics VI, SPIE, 208, 60-63. 

Figure 1. Percent of surface irradiance present at depth in the clearest ocean waters. Percent surface irradiance was determined using diffuse attenuation coefficients derived from Smith and Baker [22]. Measurements of irradiance were taken with a submersible spectoradiometer in the Sargasso Sea and the Central Equatorial Pacific.

K, diffuse attenuation coefficient for downwelling irradiance organic-water partitioning coefficient Ku, diffuse attenuation coefficient for upwelling irradiance LMW, low molecular weight LUMO, lowest unoccupied molecular orbital MAA, mycosporine-like amino acid MCH, melanin-concentration hormone MDR, mean damage ratio MPB, microphytobenthos... 

Edo X) is the downwelling irradiance just below the sea surface and Kj X) is the vertical diffuse attenuation coefficient of downwelling irradiance. Kj X) can be approximated by eqn [5]. 

The physical and chemical properties of the Skidaway River measured during different times of the year are listed in Table IV. There were few changes during the year in pH, 8330 (absorbance of the particulate-free estuarine water at 330 nm) and the screening factor. Other properties including concentration of suspended particulates, K330 (diffuse attenuation coefficient at 330 nm) and Kg (specific attenuation coefficient due to suspended particulates)... 

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