INTERHEMISPHERE EXCHANGE OF HEXACHLOROCYCLOHEXANES, HEXACHLOROBENZENE, POLYCHLOROBIPHENYLS, AND 1,1,1-TRICHLORO-2,2-BIS(PARA-CHLOROPHENYL)ETHANE IN THE LOWER TROPOSPHERE

Two nearly independent hemispheric compartments exist in the atmosphere for hexachlorobenzene [HCB; south/north (S/N) ratio of 0.06] and alpha-hexachlorocyclohexane (alpha-HCH; S/N = 0.09), as base-line measurements in both hemispheres indicate. The interhemispheric differences in the profile of the alpha and gamma isomers of hexachlorocyclohexane can be explained best by the different HCH products applied worldwide. Both isomers occur in technical HCH, the so-called benzene hexachloride (BHC), which is preferably used in the Northern Hemisphere, and the purified HCH "lindane", consisting of > 99% gamma-HCH, which is used worldwide. The distribution of the PCB congeners between gas phase and aerosol particles in dependence of the degree of chlorination is examined in urban air at a temperature of -8-degrees-C, which is close to the global medium temperature of -11-degrees-C at 4000 m. In clean air the interhemispheric contrast, the south/north ratio, is approximately 0.7 or even close to unity for the PCB. 4,4'-DDT is detected at 8 times higher levels in the southern as compared to the Northern Hemisphere. 4,4'-DDE, the transformation product of 4,4'-DDT, follows the same pattern (S/N = 3). The quotient of Henry constant and octanol-water distribution coefficient H/K(ow) correlates to the S/N ratio of semivolatiles in the troposphere. These results require the conclusion that the interhemispheric exchange of other compounds with similar physicochemical data as given by HCB and alpha-HCH must be very slow in the atmosphere if it occurs at all at a significant level. This can have far-reaching consequences in the understanding of both the interhemispheric and finally the global distribution of xenobiotics, as the oceanic system has to be discussed as the major long-range transport medium.