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: Fourteen bulk aerosol deployments were made over periods of three to five days using high-volume aerosol samplers following methods in Morton et al. (2013). During each deployment, aerosol samples were collected in triplicate on pre-combusted glass-fiber (GFF) or quartz fiber (QMA) filters. Unused filters were set aside for blank analysis. Sampling duration lasted an average of 31.0 h (11.0 to 80.0 h) with an average volume filtered of 172.5 m3 (60.6 to 451.9 m3). Aerosol filters were stored in acid cleaned polystyrene petri dishes. All samples were kept frozen at -20 degrees 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.

Aerosol filters were digested in acid-cleaned 15-mL centrifuge tubes with 10 mL of 4.57 M trace metal grade HNO3, placed in a covered 60 degrees C water bath for 12 hours (Hammerschmidt and Fitzgerald, 2006). A subsample of this digest was taken for CH3Hg analysis, and the remainder was further digested with BrCl overnight at room temperature.

Hg_Aerosol.csv