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technical guidance documents - Institute for Health and Consumer ...

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2.3.5.1 Adsorption to aerosol particles<br />

ENVIRONMENTAL EXPOSURE ASSESSMENT<br />

The fraction of the substance associated with aerosol particles can be estimated on the basis of<br />

the substance's vapour pressure, according to Junge (1977). In this equation, the sub-cooled<br />

liquid vapour pressure should be used.<br />

Explanation of symbols<br />

Fass aer =<br />

CONjunge • SURFaer<br />

VP + CONjunge • SURF<br />

CONjunge constant of Junge equation [Pa . m] *<br />

SURFaer surface area of aerosol particles [m 2. m -3 ] *<br />

VP vapour pressure [Pa] data set<br />

Fassaer fraction of the substance associated with aerosol particles [-]<br />

* as a default the product of CONjunge <strong>and</strong> SURFaer is set to 10 -4 Pa (Van de Meent, 1993; Heijna-Merkus <strong>and</strong> Hof, 1993).<br />

Alternatively the octanol-air partition coefficient could be used as described by Finizio et al.<br />

(1997).<br />

For solids, a correction of the vapour pressure is required to derive the sub-cooled liquid vapour<br />

pressure (Mackay, 1991):<br />

VPL =<br />

Explanation of symbols<br />

e<br />

VP<br />

6.79 ( -TEMP<br />

TEMP )<br />

melt<br />

• 1<br />

TEMP environmental temperature [K] 285<br />

TEMPmelt melting point of substance [K] data set<br />

VPL sub-cooled liquid vapour pressure [Pa]<br />

VP vapour pressure [Pa] data set<br />

2.3.5.2 Volatilisation<br />

The transfer of a substance from the aqueous phase to the gas phase (e.g. stripping in the aeration<br />

tank of a STP, volatilisation from surface water) is estimated by means of its Henry's Law<br />

constant. If the value is not available in the input data set, the required Henry's Law constant <strong>and</strong><br />

the Kair-water (also known as the “dimensionless” Henry's Law constant) can be estimated from<br />

the ratio of the vapour pressure to the water solubility. For water miscible compounds direct<br />

measurement of the Henry’s Law constant is recommended.<br />

HENRY =<br />

VP • MOLW<br />

SOL<br />

aer<br />

(19)<br />

(20)<br />

(21)<br />

45

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