Near-surface total specific light absorption rate

Table 15.5 Calculation of the Near-Surface Total Specific Light Absorption Rate A of p-Nitroacetophenone (PNAP) at 40°N Latitude at Noon on a Clear Midsummer Day...

In Table 15.6, the 24 h total specific light absorption rate, (24 h), of PNAP is calculated (estimated) for a clear midsummer day in the well-mixed epilimnion of Greifensee in Switzerland (47°N latitude). For a latitude of 40°N the near-surface total specific light absorption rate k% (noon) of PNAP is calculated for a clear midsummer day in Table 15.5. Can you use this ka (noon) value to estimate kl (24 h) of PNAP in Greifensee If yes, perform the calculation and compare the resulting (24 h) value with the K(24 h) value obtained in Table 15.5. Assume that on a daily basis, a°(noon) is about 2.5 times larger than (24 h) [compare (24 h) given in Fig. 15.10 with (noon) (86400 sd 1)]. 

Estimate the 24 h averaged direct photolysis half-life of 4-nitrophenol (4NP) (a) near the surface, and (b) in the well-mixed epilimnion of Greifensee (pH = 7.5 zmix = 500 cm d(X) values given in Table 15.6) on a clear midsummer day. Furthermore, a friendly colleague has already calculated the 24 h averaged near-surface total specific light absorption rates of the nondissociated (HA) and dissociated (A") species ... 

P 15.3 Estimating the Near-Surface Total Specific Light Absorption Rates of Nitrobenzene... 

Figure 15.9 Graphical representation of the calculation of the near-surface specific light absorption rate, k°, for para-nitro-acetophe-none (PNAP) for a clear-sky midday, midsummer at 40°N latitude. The shaded area corresponds to the total rate. Note that the y axes are on logarithmic scales.

For PNAP, the calculation of k° (noon) for a midsummer day at 40°N latitude is shown in Table 15.5. The result k (noon) =14.5 x 10 3 einstein (mol PNAP) 1 s 1 indicates that, near the surface of a natural water body, a total of 14.5 millieinstein are absorbed per second per mole of PNAP present in dilute solution. This means that each PNAP molecule is excited once every 70 seconds. The corresponding calculated near-surface specific rate of light absorption averaged over one day is... 

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. 

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