Estimated pseudo-first-order loss rates

Table VII. Estimated Pseudo-First-Order Loss Rates for Hydrolytic and Selected Light-Induced Processes Affecting Methylchloroform in the...

Now we have an easy way of describing the indirect photolysis of a pollutant by a pseudo-first-order rate law, provided that the compound considered does not significantly affect [Ox]°, and that we are able to measure or estimate [Ox]°. The nearsurface rate of loss of the pollutant is then given by ... 

Fourthly, the starting point for lifetime estimations is often laboratorygenerated kinetic data for reaction of the compound of interest with OH radicals. The bimolecular rate constants measured in laboratory kinetic experiments need to be converted into a pseudo first order rate constant for loss of the compound, k . In principal this conversion is simple, i.e., the bimolecular rate constant merely has to be multiplied by the OH concentration ([OH]). In practice there are difficulties associated with the choice of an appropriate value of [OH], At present we cannot measure the global OH concentration field directly. The OH radical concentration varies widely with location, season, and meteorological conditions. To account for such variations requires use of sophisticated 3D computer models of the atmosphere. 

In the estimation of rate constants through the fitting of degradation data to a kinetic model, the validity of the model and the reliability of the estimated rate constant should be evaluated, taking into account experimental errors. Additional data are sometimes required to obtain accurate estimates. For example, in the case of consecutive reactions, the time courses for both the parent drug and the intermediate are required to estimate the pseudo-first-order rate constant for the formation and loss of the intermediate (see Section 2.2.3.7.f), especially when k/k, is larger than O.5.297... 

Figure 6 Illustrates pseudo-first-order system halflives, estimated by the EXAMS model ( 9), for photolysis of a series of generic organic substrates with differing K s (FaK ) and sedlment/water partition coefficients, (K ) out with the same photolysis rate constants in the water column of a freshwater pond. As the partition coefficients Increase, the fraction of substrate In the illuminated water column, and thus the system loss rate, decreases due to Increased sorption Into bottom sediments. It Is noteworthy that this simulation, when compared with Figure 5, Indicates that sorption Into bottom sediments has little or no effect on the total rate of photochemical loss for a preponderance of Industrial chemicals.

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