Air-Terrestrial Surface Exchange

Similarly, the nonracemic signatures of heptachlor epoxide, cis- and traKi-chlordane, oxychlordane, and o,/i -DDT in soils of the US Midwestern Corn Belt [35, 143] were 

While agricultural soils now have nonracemic OC pesticide residues and emit these nonracemic compositions to the atmosphere, this was not necessarily always the case. Archived extracts of air samples collected in Sweden, Slovakia, and Iceland between in the early 1970s had racemic cis- and fra 5-chlordane [138]. This observation suggests that those residues were released either from fresh emissions, as these compounds were in active use at the time, and/or they were volatilized from racemic residues in soil. The former hypothesis is more likely, given the EFs of these compounds in sediment cores, which were racemic in the 1950s but less so after that point, including the 1970s [138, 139]. 

Since the production of PCBs ceased in the late 1970s in North America and Europe, current atmospheric sources of PCBs are believed to be predominantly from volatilization from contaminated environmental surfaces such as soil [312-314] and water [291, 315]. However, emission from existing PCB technical mixtures (i.e. capacitors, transformers, constmction materials) may also contribute to atmospheric contamination [316, 317]. The use of PCB atropisomers for source apportionment was first demonstrated by Robson and Harrad [182], who observed that nonracemic soil EFs for PCBs 95, 136, and 149 were statistically different from racemic EFs of all three congeners in the overlying air. In contrast to chiral OC pesticides, the source of atmospheric PCBs was thus not due to volatilization from the sod, but from emission of unweathered PCBs remaining in use. In subsequent work [318], PCB concentrations of indoor air in the West Midlands of the UK had not changed significantly over the past 10 years. Both indoor and outdoor air had 

Little work has been done to characterize air-surface exchange of chiral semivolatUe current-use pollutants. However, the observations of slightly enriched ( )-a-HBCDD and racemic fS- and y-HBCDD in Chinese urban air suggest that the atmospheric source of HBCDD may be a mixture of both local unweathered emissions and volatilization of weathered residues from soil [321]. Measurements of soil EF profiles for HBCDD have not yet been done to confirm or refute this hypothesis. 

Despite these insights from chirality in soil-air source apportionment, comphcations remain. As previously stated, few correlations have been observed between cis- and trans-chlordane soil EFs and soil concentrations, or labile versus persistent analyte concentration ratios [140,147]. In Costa Rica, no correlations were found between soil and air EFs of these 

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