We collected air, water, sediment samples in the summer of 2019 from on board the CCGS Amundsen in the central and eastern Canadian Archipelago. We also collected air and water (grab and passive) from the CCGS Laurier in the Beaufort Sea and the R/V William Kennedy in the western Hudson Bay and Chesterfield Inlet. Air samples were continuously collected form the very front of the bow of the ship to reduce contamination from the smoke stack. The air sampler consisted of a filter to collect the particulate followed by a cartridge containing a sandwich of polyurethane foam and XAD resin to sorb the gas phase. Several types of water samples were collected: target compounds were concentrated from grab surface samples (200-L) by filtering through a column of XAD resin. Grab samples for flame retardants were collected in glass bottles and shipped back to the lab for processing. Grab samples for per-fluorinated compounds were collected at the surface and at two-three depths in plastic bottles and shipped back to the lab for processing. Sediment samples were collected from the box corer, the top 0-5cam was collected. Samples were frozen and shipped back to the lab for processing.

Sample collection took place during both legs of the 2005 CCGS Amundsen cruise, during Leg 1 in 2006 and during Leg 3 in 2007. Water samples were collected using Teflon-lined Niskin bottles mounted on the ship¿s rosette system, at sites ranging from Hudson Bay, the North Open Water Polynya, the Northwest Passage and the Beaufort Sea. The water column was sampled at the surface, middle and bottom in 2005 and 2006 and at the surface and chlorophyll maximum in 2007. The resulting water chemistry dataset includes concentrations of total Hg, MMHg, DMHg and GEM in water. In addition, samples were also analyzed for sulfate and dissolved organic carbon in 2007 and for sulfate only in 2006. To quantify rates of biogeochemical Hg transformations in the water column, some water samples were amended with Hg stable-isotope tracers, namely 198Hg(II) and MM199Hg, and incubated on-board the ship for up to 25 hours before being preserved by acidification or freezing. Samples were then analyzed for MMHg, DMHg and GEM, using various mass-spectroscopy techniques at either the University of Alberta or Trent University. Thus, the rate of transformation of the 198Hg(II) tracer into MM198Hg, for example, could be quantified. This dataset includes rate constants for the following water column processes: the methylation of Hg(II) to MMHg, the methylation of MMHg to DMHg, the demethylation of MMHg, the reduction of MMHg to Hg(0) and the reduction of Hg(II) to Hg(0).