Methodology: Details of the methods for the cruise are given in DiMento et al. (2019). Details of the overall method and approach for dissolved gaseous mercury and atmospheric mercury methods are given in Andersson et al. (2008), Mason et al. (2017), Soerensen et al. (2014), and Soerensen et al. (2013). Analytical methods are detailed in DiMento et al. (2019) with additional information in the papers listed above and in Munson et al. (2014), Morton et al. (2013), and Gichuki & Mason (2014). See "Related Publications" below for complete citations.

Sampling Procedures: An automated N-CON rain sampler (designated for Hg analysis) was used to collect rain (falling snow was not collected for Hg analysis) during the cruise. It was operated to avoid contamination from the ship and sea spray. Low rainfall warranted ultra high purity water rinses of the collection funnel in all but one sample, with the rinse and sample combined and recovered for analysis. Two deployment blanks were also collected. After collection, precipitation samples were stored in Teflon FEP bottles. All samples were kept frozen at -20 °C in the dark, and were transported back to the University of Connecticut for analysis.

Methylmercury concentrations were determined following the ascorbic acid-assisted direct ethylation method (Munson et al., 2014) using a Tekran 2700 instrument and autosampler to automate the purging, trapping, and detection via cold vapor atomic fluorescence spectroscopy (CVAFS). Samples were thawed then acidified to 1% (v/v) H2SO4 and left to digest overnight before neutralizing with 8N potassium hydroxide (KOH), buffering with 4M acetate, adding 2.5% (w/v) ascorbic acid and finally 1% (w/v) sodium tetraethyl borate (NaTEB) to ethylate the methylmercury. Total mercury concentrations were determined by dual gold-amalgamation CVAFS utilizing a Tekran 2600 instrument in accordance with U.S. EPA Method 1631. Briefly, waters were digested with bromine monochloride (BrCl) followed by a pre-reduction step with hydroxylamine hydrochloride (NH₂OH·HCl). Inorganic Hg(II) was then reduced to Hg⁰ using stannous chloride (SnCl₂) prior to automated analysis on the Tekran.