Polychlorinated dibenzo-p-dioxins and dibenzofurans in the remote North Atlantic marine atmosphere

We have developed a sampling strategy that allows us to determine femtogram/cubic meter concentrations of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in remote marine atmospheres. Using this sampling strategy, a total of 37 air samples were taken during two extended sampling periods at Bermuda between September 1993 and August 1997. During this time, the average total PCDD/F concentrations at Bermuda decreased from 105 +/- 30 to 35 +/- 10 fg/m(3), giving a half-life of about 2 years for these com pounds in the remote marine atmosphere. PCDD/F concentrations during both sampling periods were somewhat higher in the winter when air parcels originated from North America. A second air-sampling station was established at Barbados where 22 air samples were taken between March 1996 and August 1997; an average total PCDD/F concentration at Barbados of 15 +/- 7 fg/m(3) was found. This value was not significantly different than the 27 +/- 7 fg/m(3) found at Bermuda during this time when air arrived from the east. This indicated that the remote marine background concentration for these compounds is currently on the order of 20 fg/m(3). Using these background concentrations, the dry depositional rate of PCDD/F to the world's oceans was estimated to he 200 +/- 80 kg/year, and the wet depositional rate was estimated to be 900 +/- 300 kg/year. This is a total deposition rate of about 1 t/year to the oceans as compared to our previous estimate of 12 t/year PCDD/F deposition from the atmosphere to the land.