(À)-α-HCH (ER > 1.00) but, as with the water, the selectivity reversed at higher
latitudes. ERs in air over portions of the Arctic Ocean and the northern Atlantic
Ocean were < 1.00, from depletion of (+)-α-HCH. The similarities in air and water
enantiomeric profiles shown in Fig. 8.28 for the Bering and the Greenland Sea
suggest that sea-to-air gas exchange is an important source of α-HCH to the marine
boundary layer.
Jantunen and Bidleman stress that air–water gas exchange is a “two-way street”.
At equilibrium, the net flux is zero, but volatilisation and deposition still occur at
equal rates. This concept has been applied to the exchange of enantiomers (which
have the same Henry’s law constant) when the α-HCH in the bulk air is racemic and
the α-HCH in the surface water is non-racemic (Jantunen and Bidleman 1997;
Bidleman et al. 1998a, b, c, d). The concentration of α-HCH is about 10
4 greater
than in air. Because of this buffering capacity, the ER of α-HCH in the air will tend
toward the seawater value regardless of whether air–water equilibrium is approached
from the deposition or volatilisation side. Thus, a non-racemic ER value for air does
not itself indicate net volatilisation of α-HCH, although this is implied from the
fugacity ratios (Jantunen and Bidleman 1996). The proportion of α-HCH in the
atmosphere that has volatilised from the ocean can be estimated from the ER values
in boundary-layer air (ER bl ) and surface water (ER sw ) assuming that the ER of
volatilised α-HCH is equal to ER sw (6 ¼ 1.00) and the α-HCH in bulk air is racemic
(ER ¼ 1.00). The fraction of α-HCH in the air column arising from outgassing is
Thus for ER bl ¼ 0.90 and ER sw ¼ 0.80, f ¼ 0.5.
Fig. 8.28 Enantiomeric ratios (ERs) of α-HCH in air and water at different latitudes (ER ¼
(+)-α-HCH/(À)-α-HCH
204
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
latitudes. ERs in air over portions of the Arctic Ocean and the northern Atlantic
Ocean were < 1.00, from depletion of (+)-α-HCH. The similarities in air and water
enantiomeric profiles shown in Fig. 8.28 for the Bering and the Greenland Sea
suggest that sea-to-air gas exchange is an important source of α-HCH to the marine
boundary layer.
Jantunen and Bidleman stress that air–water gas exchange is a “two-way street”.
At equilibrium, the net flux is zero, but volatilisation and deposition still occur at
equal rates. This concept has been applied to the exchange of enantiomers (which
have the same Henry’s law constant) when the α-HCH in the bulk air is racemic and
the α-HCH in the surface water is non-racemic (Jantunen and Bidleman 1997;
Bidleman et al. 1998a, b, c, d). The concentration of α-HCH is about 10
4 greater
than in air. Because of this buffering capacity, the ER of α-HCH in the air will tend
toward the seawater value regardless of whether air–water equilibrium is approached
from the deposition or volatilisation side. Thus, a non-racemic ER value for air does
not itself indicate net volatilisation of α-HCH, although this is implied from the
fugacity ratios (Jantunen and Bidleman 1996). The proportion of α-HCH in the
atmosphere that has volatilised from the ocean can be estimated from the ER values
in boundary-layer air (ER bl ) and surface water (ER sw ) assuming that the ER of
volatilised α-HCH is equal to ER sw (6 ¼ 1.00) and the α-HCH in bulk air is racemic
(ER ¼ 1.00). The fraction of α-HCH in the air column arising from outgassing is
Thus for ER bl ¼ 0.90 and ER sw ¼ 0.80, f ¼ 0.5.
Fig. 8.28 Enantiomeric ratios (ERs) of α-HCH in air and water at different latitudes (ER ¼
(+)-α-HCH/(À)-α-HCH
204
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants