Chiral signatures of chlordanes indicate changing sources to the atmosphere over the past 30 years

Chlordane was used as an insecticide for half a century until its withdrawal from the world market in 1997. Trans- and cis-chlordane (TC, CC) are racemic in the technical product, but undergo enantioselective degradation in soil to leave nonracemic residues. In most cases, the (+) enantiomer of TC and the (−) enantiomer of CC are depleted, leading to enantiomer fractions, EF=(+)/[(+)+(-)], that are <0.500 for TC and >0.500 for CC. Nonracemic EFs of TC and CC, showing the same degradation preference as soils, were found in 1998–2001 air samples from stations in Arctic Canada and Finland, and on the west coast of Sweden. Environmental samples representing different time periods of chlordane history were also examined in: (a) archived atmospheric deposition samples collected in Sweden, Slovakia and Iceland in 1971–1973, (b) soils from southern Sweden sampled in 2001 and (c) a laminated lake sediment core from the Canadian Arctic representing 50 years of accumulation. TC and CC were racemic or nearly so in the historical atmospheric deposition samples and nonracemic in the Swedish soils. The EF of TC in the dated lake sediment core decreased from nearly racemic in the past to nonracemic in recent times. These observations suggest that sources of chlordane to the atmosphere have changed over time and are now influenced to a greater extent by emission from soils.