Hexachlorocyclohexanes in the North American Atmosphere

Annually integrated air concentrations of α- and γ-hexachlorocyclohexane (HCH) were determined in 2000/2001 at 40 stations across North America using XAD-based passive air samplers to understand atmospheric distribution processes on a continental scale. Elevated levels of γ-HCH in the atmosphere of the Canadian Prairies are consistent with the ongoing use of lindane as a seed treatment on canola and confirm the feasibility of detecting the agricultural use of a pesticide using long-term integrated passive air sampling. In contrast to γ-HCH, the atmospheric concentrations of α-HCH show a rather uniform distribution across Canada and the United States, which is expected for a chemical with no current use on the continent. Higher levels in the atmosphere over Atlantic Canada can be explained by α-HCH evaporating from the waters of the Labrador Current, which is supported by the chiral composition of α-HCH and the temperature dependence of its atmospheric concentrations along the east coast of Canada. Similarly, α-HCH is volatilizing from Lake Superior. Atmospheric HCH levels increase with elevation in the Canadian Rocky Mountains. The results suggest that evaporation, in particular from cold water bodies, is an important source of α-HCH to the North American atmosphere. Low levels of HCHs in Central America hint at efficient degradation under tropical conditions. Chiral analysis shows that (+)-α-HCH is often enriched in air over continental areas and at the Pacific Coast, which is opposite to the enantiomeric enrichment in the proximity to the Great Lakes and the Atlantic Ocean. Passive air sampling is a powerful tool to discern the large-scale variability of semivolatile and persistent organic chemicals in the atmosphere.