ER values in Alabama soils were also non-racemic, but the difference is smaller than
in the Corn Belt soils.
Enantiomeric ratios for ambient air samples taken at rural, non-agricultural
locations in the Corn Belt show the same general trend (although less pronounced)
as was seen in soil and air-above-soil samples. A comparison of the enantiomeric
ratios for Alabama and in Columbia, SC, was closer to racemic. In contrast, air
samples over the Great Lakes show a depletion of (+)-trans-chlordane and a slight
depletion of (À)-cis-chlordane enantiomers, similar to the ERs observed for Corn
Belt soils. ER values of chlordanes in air from homes in the Corn Belt states were all
very close to racemic. Racemic chlordane was also found in indoor air for several
homes in Columbia, SC and Alabama (Wiberg et al. 2001a, b).
The data set available until 1998 allowed the following conclusions with regard to
potential sources for chlordanes in the air (Falconer et al. 1998): High concentrations
of chlordanes (compared to average Great Lakes values) in indoor and ambient air
from Columbia, South Carolina, and intermediate levels near the town of Muscle
Shoals, Alabama, go along with racemic mixtures in the air in both locations, while
non-racemic chlordanes were reported in southern soils. These findings suggest that
termiticide treated houses, rather than soil emissions, are the main source of chlordane to southern U.S. air. Chlordane concentrations in home air from the Corn Belt
region are much greater than average ambient air concentrations from the Corn Belt
region or the Great Lakes. However, enantiomeric ratios of chlordane in ambient air
from the Corn Belt and the Great Lakes lie between the ER values in air-above-soil
(non-racemic) and home air (racemic). Lower concentrations coupled with
non-racemic ER values imply a greater import of agricultural sources in these
regions, that is, a mixture of termiticide-treated home air and emissions from
regional soils can be assumed.
The interpretation of the results for air/soil exchange of heptachlor included its
main transformation product heptachlor exoepoxide, also a chiral compound and
thus accessible to enantioselective gas chromatography. Concentrations of heptachlor exoepoxide in ambient air in Alabama ranged from 4–9 pg/m
3 in January–
February to 29–51 pg/m
3 in May–June, similar to the seasonality reported in
southern Ontario concentrations and enantiomers of pesticides in soil from the
U.S. Corn Belt (Bidleman et al. 1998a, b, c, d). This is consistent with
temperature-driven volatilisation from soil. The enantiomeric composition of this
compound in air samples from all locations was distinctly non-racemic (Table 8.18)
Average ER values ((+)-/(À)-enantiomer) ranged from 1.51 in Columbia to 2.02
over Lake Superior.
The enantiomeric ratios were remarkably consistent in each location, exhibiting
relative standard deviations of only 2.7–10.6% and showing no seasonal
dependence.
Heptachlor was quantified only in Muscle Shoals air samples and, unlike heptachlor exoepoxide, showed no seasonality, ranging from 27–45 pg/m
3 in January–
February to 31–49 pg/m
3 in May–June. The enantiomers of heptachlor were only
partially resolved on the BGB-172 column used by Bidleman et al. (1998a, b, c, d),
although well enough to determine the enantiomeric ratios in air samples. The
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8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
in the Corn Belt soils.
Enantiomeric ratios for ambient air samples taken at rural, non-agricultural
locations in the Corn Belt show the same general trend (although less pronounced)
as was seen in soil and air-above-soil samples. A comparison of the enantiomeric
ratios for Alabama and in Columbia, SC, was closer to racemic. In contrast, air
samples over the Great Lakes show a depletion of (+)-trans-chlordane and a slight
depletion of (À)-cis-chlordane enantiomers, similar to the ERs observed for Corn
Belt soils. ER values of chlordanes in air from homes in the Corn Belt states were all
very close to racemic. Racemic chlordane was also found in indoor air for several
homes in Columbia, SC and Alabama (Wiberg et al. 2001a, b).
The data set available until 1998 allowed the following conclusions with regard to
potential sources for chlordanes in the air (Falconer et al. 1998): High concentrations
of chlordanes (compared to average Great Lakes values) in indoor and ambient air
from Columbia, South Carolina, and intermediate levels near the town of Muscle
Shoals, Alabama, go along with racemic mixtures in the air in both locations, while
non-racemic chlordanes were reported in southern soils. These findings suggest that
termiticide treated houses, rather than soil emissions, are the main source of chlordane to southern U.S. air. Chlordane concentrations in home air from the Corn Belt
region are much greater than average ambient air concentrations from the Corn Belt
region or the Great Lakes. However, enantiomeric ratios of chlordane in ambient air
from the Corn Belt and the Great Lakes lie between the ER values in air-above-soil
(non-racemic) and home air (racemic). Lower concentrations coupled with
non-racemic ER values imply a greater import of agricultural sources in these
regions, that is, a mixture of termiticide-treated home air and emissions from
regional soils can be assumed.
The interpretation of the results for air/soil exchange of heptachlor included its
main transformation product heptachlor exoepoxide, also a chiral compound and
thus accessible to enantioselective gas chromatography. Concentrations of heptachlor exoepoxide in ambient air in Alabama ranged from 4–9 pg/m
3 in January–
February to 29–51 pg/m
3 in May–June, similar to the seasonality reported in
southern Ontario concentrations and enantiomers of pesticides in soil from the
U.S. Corn Belt (Bidleman et al. 1998a, b, c, d). This is consistent with
temperature-driven volatilisation from soil. The enantiomeric composition of this
compound in air samples from all locations was distinctly non-racemic (Table 8.18)
Average ER values ((+)-/(À)-enantiomer) ranged from 1.51 in Columbia to 2.02
over Lake Superior.
The enantiomeric ratios were remarkably consistent in each location, exhibiting
relative standard deviations of only 2.7–10.6% and showing no seasonal
dependence.
Heptachlor was quantified only in Muscle Shoals air samples and, unlike heptachlor exoepoxide, showed no seasonality, ranging from 27–45 pg/m
3 in January–
February to 31–49 pg/m
3 in May–June. The enantiomers of heptachlor were only
partially resolved on the BGB-172 column used by Bidleman et al. (1998a, b, c, d),
although well enough to determine the enantiomeric ratios in air samples. The
190
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants