Sources and occurrence of dacthal in the Canadian atmosphere.

Dacthal is a herbicide that can undergo long-range atmospheric transport. Due to limited use in Canada, its occurrence in the Canadian environment is likely associated with transboundary flow from the United States where 100 times more dacthal is used. To investigate its atmospheric distribution and possible sources, two sampling strategies were applied. First, polyurethane foam-disk passive air samplers were deployed across the country from the spring to summer of 2004 and 2005. Results showed highest dacthal concentrations at two sites in southern Ontario (max: 50 pg m(-3)) and much lower concentrations in other agricultural regions across Canada. Second, daily high-volume air samples were collected at a field site in north Toronto (Downsview). Sampling at this site was triggered by real-time meteorological forecast models that indicated when air flow to the site originated in the United States. Two such events, one in late June and one in late September 2005, were sampled. In both cases, elevated dacthal concentrations were captured (e.g., up to 319 pg m(-3) in the June event) and they were well correlated with transboundary flow from the United States. Finally, the octanol--air partition coefficient (K(OA)) of dacthal and other current-use pesticides was measured. K(OA) was used to derive a particle--gas partitioning coefficient (K(p)) for dacthal (Log K(p) = -4.1), indicating that this compound should exist mainly in the gas phase.