Fugacity models multimedia model

Multimedia models can describe the distribution of a chemical between environmental compartments in a state of equilibrium. Equilibrium concentrations in different environmental compartments following the release of defined quantities of pollutant may be estimated by using distribution coefficients such as and H s (see Section 3.1). An alternative approach is to use fugacity (f) as a descriptor of chemical quantity (Mackay 1991). Fugacity has been defined as fhe fendency of a chemical to escape from one phase to another, and has the same units as pressure. When a chemical reaches equilibrium in a multimedia system, all phases should have the same fugacity. It is usually linearly related to concentration (C) as follows ... 

Besides the fugacity models, the environmental science literature reports the use of models based on Markov chain principle to evaluate the environmental fate of chemicals in multimedia environment. Markov chain is a random process, and its theory lies in using transition matrix to describe the transition of a substance among different states [39,40]. If the substance has all together n different kinds of states,... 

The current version of CalTOX (CalTOX4) is an eight-compartment regional and dynamic multimedia fugacity model. CalTOX comprises a multimedia transport and transformation model, multi-pathway exposure scenario models, and add-ins to quantify and evaluate variability and uncertainty. To conduct the sensitivity and uncertainty analyses, all input parameter values are given as distributions, described in terms of mean values and a coefficient of variation, instead of point estimates or plausible upper values. 

Mackay D, Paterson S (1991) Evaluating the multimedia fate of organic chemicals a level III fugacity model. Environ Sci Technol 25 427-436... 

While QWASI is an easy to use multimedia fate modeling tool, it has been originally designed as a fugacity model. Even though an adaptation to ionic substances exists and it has been applied to lead before, it needs to be recognized that it does not take speciation of metals into account. This adds to the overall uncertainty of results. 

In the multimedia models used in this series of volumes, an air-water partition coefficient KAW or Henry s law constant (H) is required and is calculated from the ratio of the pure substance vapor pressure and aqueous solubility. This method is widely used for hydrophobic chemicals but is inappropriate for water-miscible chemicals for which no solubility can be measured. Examples are the lower alcohols, acids, amines and ketones. There are reported calculated or pseudo-solubilities that have been derived from QSPR correlations with molecular descriptors for alcohols, aldehydes and amines (by Leahy 1986 Kamlet et al. 1987, 1988 and Nirmalakhandan and Speece 1988a,b). The obvious option is to input the H or KAW directly. If the chemical s activity coefficient y in water is known, then H can be estimated as vwyP[>where vw is the molar volume of water and Pf is the liquid vapor pressure. Since H can be regarded as P[IC[, where Cjs is the solubility, it is apparent that (l/vwy) is a pseudo-solubility. Correlations and measurements of y are available in the physical-chemical literature. For example, if y is 5.0, the pseudo-solubility is 11100 mol/m3 since the molar volume of water vw is 18 x 10-6 m3/mol or 18 cm3/mol. Chemicals with y less than about 20 are usually miscible in water. If the liquid vapor pressure in this case is 1000 Pa, H will be 1000/11100 or 0.090 Pa m3/mol and KAW will be H/RT or 3.6 x 10 5 at 25°C. Alternatively, if H or KAW is known, C[ can be calculated. It is possible to apply existing models to hydrophilic chemicals if this pseudo-solubility is calculated from the activity coefficient or from a known H (i.e., Cjs, P[/H or P[ or KAW RT). This approach is used here. In the fugacity model illustrations all pseudo-solubilities are so designated and should not be regarded as real, experimentally accessible quantities. 

The multimedia urban model (MUM) is a fugacity-based mass balance model that treats the movement of POPs in an urban environment and links emissions to ambient chemical concentrations, and thus outdoor exposure (Diamond et al., 2001). MUM considers longterm, average conditions of chemical transport and transformation among six environmental compartments in urban areas (air, soil, surface water, sediment, vegetation and surface film see Figure 6.1) shows a concepmal version of the model). The model does not estimate event-specihc processes as do meteorological-based air or stormwater models. 

Level I Model The components of a multimedia model are illustrated in the schematic (Fig. 10.10) with the four major compartments being, air, water, sediment, and soil. The equilibrium distribution of a fixed quantity of chemical among these compartments is defined by the Level 1 model, and by definition, the fugacities in each compartment would be equal... 

For a more elegant and rigorous explanation of fugacity, see Multimedia Emnronmental Models The Fugacity Approach [5]. 

Multimedia environmental models often incorporate the concept of fugac-ity into mass balance calculations. As pioneered by Dr. Donald Mackay and described in Table 2.3, fugacity models can reflect four levels of sophistication [64]. Level III fugacity models are commonly used to describe the fate and transport of a chemical released to the environment that undergoes degradation and advective transport between compartments. One such model is described below. 

LEV3EPI Level in multimedia fugacity model predicts partitioning of chemicals among air, soil, sediment, and water under steady-state conditions for a default model "environment"... 

In addition, two multimedia models were developed one which takes into account the flows between surface water, lower troposphere, and soil as they result from the three single-medium models EXWAT, EXAIR and EXSOL, and another one which is based on Mackay s fugacity approach for calculating the partitioning of a chemical between the three media (Mackay et al. 1983a 1983b). 

Mackay D., and S. Paterson, Evaluating the Multimedia Fate of Organic Chemicals A Eevel III Fugacity Model, Envzronmenta/ Science and Technology 25, no. 3 (1991) 427-436. 

In this chapter, we have explored the mechanism behind the high mobility of chemicals emitted into lu ban areas. Films on impervious siu faces, and of course the surfaces themselves, promote high export rates of emitted chemicals by air advection or transfer of atmospherically deposited chemicals into siu face waters via storm water runoff, rather than sequestration in the urban environment itself. We illustrate this quantitatively using the ftigacity-based multimedia lu ban model or MUM (Diamond et al., 2001 Priemer and Diamond, 2002 Jones-Otazo et al., 2005 Diamond et al., 2010). MUM is a seven-compartment fugacity Level HI model. The model is coded in Visual Basic and is run on a PC in a Windows environment. 

Mackay D (1991) Multimedia environmental models the fugacity approach. Lewis Publishers, Chelsea... 

Mackay D (2001) Multimedia environmental models - the fugacity approach, 2nd edn. CRC Press/Taylor Francis Group, Boca Raton... 

Lohmann U, Roeckner E (1996) Design and performance of a new cloud microphysics scheme developed for the ECHAM4 general circulation model. Clim Dyn 12 557-572 Mackay D (1991) Multimedia Environmental Models The Fugacity Approach. Lewis Publishers, Chelsea, MI, USA... 

Mackay, D. (2001) Multimedia Environmental Models The Fugacity Approach. 2nd edition, Lewis Publishers, CRC Press, Boca Raton, FL. 

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