Light-screening factor

What does the light-screening factor, S(X), exactly describe For what can it be... 

Use Eq. 2 in Illustrative Example 16.1 to estimate the near-surface production rate of HO in the pond. To account for light attenuation in the water column, apply light-screening factors for the wavelength of maximum light absorption of NO (Amax = 320 nm) and NO (A = 360 nm) ... 

Illustrative Example 15.2 Using the Screening Factor S(X ) to Estimate the Total Specific Light Absorption Rate ofPNAP in the Epilimnion of... 

For practical applications, Eq. 15-33 is extremely useful, since in many cases experimental data are available only for the near-surface specific rate of light absorption or, even more frequently, for the total near-surface direct photolytic transformation rate of a given pollutant. An example demonstrating the use of screening factors is given in Illustrative Example 15.2. 

When the ratio r is much less than 1, that is, when the light signal travels across and back much faster than the length of time l/ that a fluctuation endures, then R = 1. There is no loss of signal between the two bodies then the finite velocity of light does not affect the van der Waals interaction from sampling frequency f . (For the "exact" screening factor see Level 2, Subsection L2.3.A.)... 

In the limit r = 0, at which there are no retardation effects because of the finite velocity of light, the screening factor Raj) rn) goes to 1 and the small-particle interaction becomes... 

The screening factor now has the velocity of light in water, c/ell2 at each frequency with... 

The relativistic screening factor at each frequency z (that is, ) is then (by the approximate equal-light-velocities formula) [Eq. (LI.16), Fig. LI.12, Eq. (L2.26)] ... 

Rates of direct photochemical reactions of aqueous species P as measured in small (thin-layered) samples and corrected for flat-layer exposure correspond to surface values or rates that would occur in the top few centimeters of a water column (Haag and Hoigne, 1986). For mixed water columns of greater depths (z), the mean rate of direct phototransformation must be corrected considering a screening factor (S ) that accounts for the light attenuation within the water column of depth z ... 

The rates of reaction of other compounds were also measured relative to this reference compound. Results of such comparisons are presented in Figure 6. Empirical correlations between the kinetic effects of fulvic acid derived peroxy radicals and those of unique reference peroxy radicals indicate that even for this case structure-reactivity relationships can be applied for estimations. However, correlations established for waters containing different types of DOM appear to be somewhat different (Faust and Hoigne, 1987). In addition, because the light screening effects depend on the wavelength, depth functions for different water bodies would require information also on the wavelength dependences of the formation of this environmental factor. 

If an intensifier, such as the 85 mm presented here, is now replacing the screen, a relative gain of the order of x50 is obtained which results in a conversion factor of 1 to 7.5 (1 incident X photon --> 7.5 electrons). This conversion efficiency not only resolves the quantum sink problem but also increases the light level significantly to compensate for the low gamma fluxes obtained from radioactive sources. 

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