R8 AM Contamination Source Identification and Apportionment
Thursday, 17 November 2005: 8:00 AM - 11:40 AM in 331-332

(BID-1117-839609) DDT/DDE Ratios as Indicators of "Old" and "New" Residues in the Atmosphere.

Bidleman, T.¹, Wong, F.¹, Alegria, H.², ¹ Meteorological Service of Canada, Egbert, ON, Canada² California Lutheran University, Thousand Oaks, CA, USA

ABSTRACT- The proportion of p,p'-DDT/p,p'-DDE (DDT/DDE) has long been used as a rough indicator of the age of DDT residues in the environment, higher ratios suggesting relatively recent DDT application. However, there are no guidelines on "high" and "low" with respect to atmospheric samples. Recent work by ourselves and others in southern Mexico and Central America has revealed concentrations of airborne DDT residues that exceed ambient air levels in Canada. These measurements were made between 2000-2004, coincident and just after Mexico officially stopped using DDT for disease vector control. The availability of air data from a region where DDT has recently been used provides an opportunity for comparisons with the U.S. and Canada, where DDT has been banned for several decades. Ratios of DDT/DDE were compared in the following media: a) air samples from southern Mexico and Central America, b) literature reports for air and soil samples from the U.S. and Canada, c) experiments in which air samples were collected 40 cm above agricultural soils containing DDT residues. Mean DDT/DDE ratios in soils from 11 different regions of the U.S. and Canada ranged from 0.5 - 7.3, with a median of 1.3. The wide range reflects a high variability in degradation rates, as has been reported in many studies. DDT is less volatile than DDE, and vapour-phase emissions from soil reflect this fractionation. Soil-air exchange experiments at four sites indicated that the DDT/DDE ratios in overlying air could be accurately predicted from the DDT/DDE ratio in soil multiplied by the liquid-phase vapour pressures of DDT/DDE. Predicted DDT/DDE ratios in air emissions from the 11 soil survey sites ranged from 0.07 - 1.12 (median 0.20). Reported DDT/DDE ratios in regional ambient air from the southern U.S., eastern and western Canada, the Great Lakes and the Canadian Arctic ranged from 0.33 - 0.79 (median 0.45), while the range and median from 10 air sampling surveys in Mexico - Central America were 0.83 - 6.0 (1.8). Results suggest that the DDT residues in ambient air of the U.S. and Canada are largely due to emission of old residues in soils, whereas continued DDT usage and/or emission of undegraded residues plays a greater role in Mexico - Central America.

Key words: DDT, atmospheric transport, soils