Is There Still "New" DDT in North America? An Investigation Using Proportions of DDT Compounds

Abstract

Usage of DDT ceased over four decades ago in Canada and the United States, and since 2000 in Mexico. Potential sources in the North American atmosphere today include emissions of legacy residues from soils and long-range transport from other countries where DDT is still used or recently banned. Distinction of source types is investigated here using proportions of p,p'-DDT, o,p'-DDT, p,p'-DDE and p,p'-DDD. The relative volatilization of DDT compounds can be accurately described by their subcooled liquid vapor pressures (PO; e.g., (p,p'-DDT/p,p'-DDE)AIR = (p,p'-DDT/p,p'-DDE)son. x PL, (DDT)/P-L,P-DDE. Using this model, the expected proportions in air due to volatilization from technical DDT and from soils in Canada, the U.S.A. and Mexico were estimated and expressed as the fractions F-DDTE = p,p'-DDT/(p,p'-DDT + P,P'-DDE), F-DDTO = p,p1-DDT/(p,p'-DDT + o,p'-DDT), and FDDTD = p,p1-DDT/(p,p'-DDT + p,p1-DDD). FDDTE, FDDTO and FDDTD predicted from soil emissions were compared to compound fractions in ambient air sampled at the Integrated Atmospheric Deposition Network (IADN) of stations on the Great Lakes between Canada and the U.S.A., and at arctic monitoring stations. FDDTE in air at IADN stations on lakes Erie, Ontario, Michigan and Huron were lower than in technical DDT vapor. This is consistent with emissions of aged residues from agricultural land and urban centers near these lakes. By comparison, FDDTE values were higher at stations on Lake Superior where atmospheric DDT is likely due to long-range transport rather than regional soil emissions. FDDTE increased from the early 1990s to 2005 at the Lake Superior stations and at the Canadian arctic station Alert between 2002-2005, whereas a significant decline in FDDTE was observed at the Norwegian arctic station Zeppelin Mountain. The mean FDDTO in air at IADN stations were consistent with either soil emissions or technical DDT composition, but annual values showed significant downward trends at two Canadian stations, and also decreased with time at Zeppelin Mountain (but not at Alert). These trends might signify contribution from "dicofol-type" DDT sources, which have a lower FDDTO than technical DDT or soil emissions, or preferential degradation of p,p'-DDT vs. o,p'-DDT over time. FDDTD in air at IADN stations were lower than in technical DDT vapor, showing the influence of soil sources. The enantiomer proportions of the chiral compounds o,p'-DDT and o,p1-DDD were nonracemic in some soils and ambient air, but enantiospecific analysis has not been done for IADN air samples. It is suggested that isomer, parent/metabolite and enantiomer composition information be incorporated into air monitoring programs to help identify sources.