29
Properties of Pesticides and Contaminants
accomplished using techniques discussed in Section 3.2.4. Another useful
relationship for fux is the one developed by Hartley:
1/2
1/2
˛
ˆ
Flux (cmpd) = P cmpd ( M cmpd ) / P water (M w ater ) Flux (water) (3.6)
˝ ˙
ˇ ˘
If the fux for a reference compound (e.g., water) is known at a given set of
conditions, then, with molecular weights and vapor pressures, it is possible
to calculate fux for the test compound. A recent study (Mackay and Van
Wesenbeeck, 2014) correlated fux (µg/m 2 ·hr) with the product of vapor pressure and molecular weight (VP [Pa] × M [g/mol]):
×
(3.7)
Flux = 1,464 (VP M)
This applies only to liquid surfaces that are unaffected by the underlying
solid substrate, as occurs in the standard ASTM evaporation rate test and to
quiescent liquid pools. The inclusion of M increased the slope of previous Ln
fux vs. Ln VP regressions to a value close to 1.0. This correlation can be used
for screening level assessment and ranking of liquid chemicals for evaporation rate, such as pesticides, fumigants, and hydrocarbon carrier fuids used
in pesticide formulations, liquid consumer products used indoors, and accidental spills of liquids. In addition to vapor pressure, other factors that infuence volatilization include movement of air over chemical deposits exposed
to the open environment and the thickness of the deposit. The direct effect
of wind fow rate on the volatilization of weed oil mixtures (e.g., Beacon oil,
Chevron oil) was demonstrated in an earlier study (Woodrow et al., 1986).
This study also showed that the weed oils (mixtures of hydrocarbons of
varying molecular weight and vapor pressure) volatilized differentially from
deposits on inert Tefon and glass surfaces (Figure 3.2). That is, components
with higher vapor pressures and lower molecular weights volatilized early
on, eventually leaving the original deposit enriched in the components of
higher molecular weight and lower vapor pressure. This phenomenon was
demonstrated both in the laboratory and in the feld. In the feld, weed oils
applied to seed alfalfa led to signifcant volatilization, which is thought to
have contributed to photochemical smog formation (see Table 2.2).
Another example of differential volatilization is the fux of hydrocarbons
to the vapor above a residual jet fuel mixture in an airliner fuel tank, leading
to concentrations of fammable volatile components in the headspace that
could cause an explosion of considerable power (Woodrow, 2003). Figure 3.3
compares the chromatograms of jet fuel vapor and liquid. The liquid composition spans a carbon number range of about C5 to C16-C17, with the bulk of
the hydrocarbon mass centered at about C12-C13 (hydrocarbon standards are
shown superimposed on the gas chromatogram). By contrast, the vapor, in
equilibrium with a pool of liquid fuel, spans the range C5 to about C11, with
the bulk of the vapor below about C9.
Properties of Pesticides and Contaminants
accomplished using techniques discussed in Section 3.2.4. Another useful
relationship for fux is the one developed by Hartley:
1/2
1/2
˛
ˆ
Flux (cmpd) = P cmpd ( M cmpd ) / P water (M w ater ) Flux (water) (3.6)
˝ ˙
ˇ ˘
If the fux for a reference compound (e.g., water) is known at a given set of
conditions, then, with molecular weights and vapor pressures, it is possible
to calculate fux for the test compound. A recent study (Mackay and Van
Wesenbeeck, 2014) correlated fux (µg/m 2 ·hr) with the product of vapor pressure and molecular weight (VP [Pa] × M [g/mol]):
×
(3.7)
Flux = 1,464 (VP M)
This applies only to liquid surfaces that are unaffected by the underlying
solid substrate, as occurs in the standard ASTM evaporation rate test and to
quiescent liquid pools. The inclusion of M increased the slope of previous Ln
fux vs. Ln VP regressions to a value close to 1.0. This correlation can be used
for screening level assessment and ranking of liquid chemicals for evaporation rate, such as pesticides, fumigants, and hydrocarbon carrier fuids used
in pesticide formulations, liquid consumer products used indoors, and accidental spills of liquids. In addition to vapor pressure, other factors that infuence volatilization include movement of air over chemical deposits exposed
to the open environment and the thickness of the deposit. The direct effect
of wind fow rate on the volatilization of weed oil mixtures (e.g., Beacon oil,
Chevron oil) was demonstrated in an earlier study (Woodrow et al., 1986).
This study also showed that the weed oils (mixtures of hydrocarbons of
varying molecular weight and vapor pressure) volatilized differentially from
deposits on inert Tefon and glass surfaces (Figure 3.2). That is, components
with higher vapor pressures and lower molecular weights volatilized early
on, eventually leaving the original deposit enriched in the components of
higher molecular weight and lower vapor pressure. This phenomenon was
demonstrated both in the laboratory and in the feld. In the feld, weed oils
applied to seed alfalfa led to signifcant volatilization, which is thought to
have contributed to photochemical smog formation (see Table 2.2).
Another example of differential volatilization is the fux of hydrocarbons
to the vapor above a residual jet fuel mixture in an airliner fuel tank, leading
to concentrations of fammable volatile components in the headspace that
could cause an explosion of considerable power (Woodrow, 2003). Figure 3.3
compares the chromatograms of jet fuel vapor and liquid. The liquid composition spans a carbon number range of about C5 to C16-C17, with the bulk of
the hydrocarbon mass centered at about C12-C13 (hydrocarbon standards are
shown superimposed on the gas chromatogram). By contrast, the vapor, in
equilibrium with a pool of liquid fuel, spans the range C5 to about C11, with
the bulk of the vapor below about C9.