Validity of global lifetime estimates by a simple general limiting law for the decay of organic compounds with long-range pollution potential

In order to assess the persistence of organic compounds with global pollution potential on the basis of a minimal set of compound-specific parameters, Muller-Herold has proposed a simple general law giving a lower bound tau(infinity) for the overall lifetime tau of a chemical (Environ. Sci Technol. 1996, 30, 586-591). tau(infinity) is the limiting value of tau in case of infinitely rapid exchange between the compartments of the environment. This paper investigates the accuracy of tau(infinity) as an estimate for the overall lifetime tau if degradation is much slower than transport. First, a criterion is given for degradation to be slower than transport. Then tau and tau(infinity) are calculated from a three-box unit-world model for a group of 29 relevant chemicals. It turns out that, for the chemicals considered, (i) tau(infinity) is indeed a perfect estimate if degradation is slower than transport, i.e., under the conditions the method has been designed for; (ii) tau(infinity) should not be used as an approximation if the pollutant is short-lived and its geographical range is only local; (iii) in all other cases, tau(infinity) has at least the same order of magnitude as tau.