Open ocean precipitation was collected during the summer cruise using a manual collection method. The samples from the Island of Hawaii were collected in 2005 at the Hakalau Wildlife Refuge and at the Nature Conservancy Ka’u Preserve. Procedural field blanks were collected periodically during the sampling campaign using 1L of de-ionized water.

After collection all the samples were oxidized with 1% BrCl (wt/v) and allowed to react with the water sample in dark, refrigerated storage for a minimum of one month. Then, the samples were analyzed for THg concentrations using cold vapor - atomic fluorescence spectrophotometry.

For THg isotope determination the oxidized precipitation samples were subsequently reduced, purged and trapped into 1% KMnO4 solution for isotope analysis. The 1% KMnO4 solution was analyzed for Hg stable isotope composition using a multiple collector inductively coupled plasma mass spectrometer.

All the methods are detailed in Motta et al., (2019).