Annual cycles of organochlorine pesticide enantiomers in Arctic air sug- gest changing sources and pathways

Air samples collected during 1994–2000 at the Canadian Arctic air monitoring station Alert (8230 N, 6220W) were analysed by enantiospecific gas chromatography–mass spectrometry for αhexachlorocyclohexane (α-HCH), trans-chlordane (TC) and cis-chlordane (CC). Results were expressed as enantiomer fractions (EF = peak areas of (+)/[(+)+ (−)] enantiomers), where EFs = 0.5, < 0.5 and > 0.5 indicate racemic composition, and preferential depletion of (+) and (−) enantiomers, respectively. Long-term average EFs were close to racemic values for α-HCH (0.504± 0.004, n= 197) and CC (0.505± 0.004, n= 162), and deviated farther from racemic for TC (0.470± 0.013, n= 165). Digital filtration analysis revealed annual cycles of lower α-HCH EFs in summer–fall and higher EFs in winter–spring. These cycles suggest volatilization of partially degraded α-HCH with EF < 0.5 from open water and advection to Alert during the warm season, and background transport of α-HCH with EF > 0.5 during the cold season. The contribution of sea-volatilized α-HCH was only 11 % at Alert, vs. 32 % at Resolute Bay (74.68 N, 94.90W) in 1999. EFs of TC also followed annual cycles of lower and higher values in the warm and cold seasons. These were in phase with low and high cycles of the TC /CC ratio (expressed as FTC = TC/(TC+CC)), which suggests greater contribution of microbially “weathered” TC in summer–fall versus winter–spring. CC was closer to racemic than TC and displayed seasonal cycles only in 1997–1998. EF profiles are likely to change with rising contribution of secondary emission sources, weathering of residues in the environment, and loss of ice cover in the Arctic. Enantiomer-specific analysis could provide added forensic capability to air monitoring programs.