115
Pesticides in Fog
FIGURE 7.6
EF of pesticides and wood smoke markers plotted against octanol–water partition coeffcient
(a) and water solubility (b). (Reprinted with permission from Sagebiel, J.C., and Seiber, J.N.
(1993). Studies on the occurrence and distribution of wood smoke marker compounds in foggy
atmospheres. Environ. Toxicol. Chem. Int. J. 12, 813–822. Copyright (1993), Wiley.)
third phase in the air/fog system which might be made up of fne particulate
matter, colloids, dissolved organic matter, or a surface flm (frst hypothesis).
A theoretical treatment showed that this unknown sorptive phase could
be described with the existing data for pesticides and wood smoke marker
chemicals. The authors pointed out that they made no supposition regarding
the composition of this third phase, but the concept of surface-active organics
(frst hypothesis) was one which was considered.
Goss (1994) summarized arguments in favor of the second hypothesis. He
argued that the existence of an inseparable organic phase in which nonpolar
chemicals accumulate provides only a partial explanation for the observations. Perona (1992), Valsaraj (1988), and Valsaraj et al. (1993) pointed to the
importance of adsorption at the air–water interface as the missing process
which had been overlooked. The very large specifc area of small fog droplets
provides an adsorptive phase which contributes to enrichment. Adsorption
at this interface can be described by a partition coeffcient relating the amount
adsorbed to the equilibrium concentration in the air (K ia ) or to the equilibrium concentration in the water (K iw ). Valsaraj et al. (1993) found a correlation
existed between K iw and K ow (octanol–water partition coeffcient) for selected
hydrophobic organics: K iw =3 × l0 −7 K
0.68
ow . They used this equation to predict
the adsorption of pesticides at the air–water interface and predict the resulting enrichment in fog droplets. Goss (1994) developed a more complex equation, relating K ia and vapor pressure, which could be used in a similar way
to that of Valsaraj et al. (1993). Applying this equation, and some temperature
corrections to adjust the vapor pressure and fogwater collection, produced
the results in Table 7.4. The calculated adsorption on the water surface led to
a signifcant enrichment of chemicals in fogwater of the same magnitude as
observed experimentally.
Pesticides in Fog
FIGURE 7.6
EF of pesticides and wood smoke markers plotted against octanol–water partition coeffcient
(a) and water solubility (b). (Reprinted with permission from Sagebiel, J.C., and Seiber, J.N.
(1993). Studies on the occurrence and distribution of wood smoke marker compounds in foggy
atmospheres. Environ. Toxicol. Chem. Int. J. 12, 813–822. Copyright (1993), Wiley.)
third phase in the air/fog system which might be made up of fne particulate
matter, colloids, dissolved organic matter, or a surface flm (frst hypothesis).
A theoretical treatment showed that this unknown sorptive phase could
be described with the existing data for pesticides and wood smoke marker
chemicals. The authors pointed out that they made no supposition regarding
the composition of this third phase, but the concept of surface-active organics
(frst hypothesis) was one which was considered.
Goss (1994) summarized arguments in favor of the second hypothesis. He
argued that the existence of an inseparable organic phase in which nonpolar
chemicals accumulate provides only a partial explanation for the observations. Perona (1992), Valsaraj (1988), and Valsaraj et al. (1993) pointed to the
importance of adsorption at the air–water interface as the missing process
which had been overlooked. The very large specifc area of small fog droplets
provides an adsorptive phase which contributes to enrichment. Adsorption
at this interface can be described by a partition coeffcient relating the amount
adsorbed to the equilibrium concentration in the air (K ia ) or to the equilibrium concentration in the water (K iw ). Valsaraj et al. (1993) found a correlation
existed between K iw and K ow (octanol–water partition coeffcient) for selected
hydrophobic organics: K iw =3 × l0 −7 K
0.68
ow . They used this equation to predict
the adsorption of pesticides at the air–water interface and predict the resulting enrichment in fog droplets. Goss (1994) developed a more complex equation, relating K ia and vapor pressure, which could be used in a similar way
to that of Valsaraj et al. (1993). Applying this equation, and some temperature
corrections to adjust the vapor pressure and fogwater collection, produced
the results in Table 7.4. The calculated adsorption on the water surface led to
a signifcant enrichment of chemicals in fogwater of the same magnitude as
observed experimentally.