Microlayer was sampled by a hollow quartz tube dipped vertically into the water column, then slowly pulled vertically out of the water and held over a funnel attached to a receiving bottle for the microlayer sample to drip off. This process was repeated until the desired volume of sample was collected. Corresponding water column samples were collected about 30 cm from the surface where a closed bottle was submerged underwater, opened, then closed again underwater to prevent mixing with the microlayer. Microlayer and water column samples were filtered within 1 hour after collection using 47 mm 0.2 um pore sized polycarbonate track-etched membrane filters by vacuum filtration.

Dissolved trace elements were analyzed by a cation exchange column method described in Milne et al. (2010). Reactive particulate trace elements were leached from the filters using a weak acid (acetic acid) and reducing agent (hydroxylamine hydrochloride) solution described in Berger et al. (2008). Refractory particulate trace elements were digested using a microwave digestion technique described in Ebling and Landing (2015). All samples were analyzed on the Thermo Scientific Element 2 HR-ICP-MS.