Dataset: Suspended particle total mercury and monomethylmercury in the California Current Ecosystem (CCE) determined from samples collected on R/V Roger Revelle cruise RR2105 (P2107) in July to August 2021

Suspended particles were collected using McLane Research in-situ pumps (WTS-LV) using modified, two-layer filter holders. The top filter holder layer was loaded with a 51-micrometer (μm) polyester mesh filter to collect large-size fraction particles (LSF). The other filter holder layer was loaded with a pre-combusted GF/F glass microfiber filter (Whatman) to collect small-size fraction particles (SSF, 1-51 μm). Filter holders were acid-leached before use, and GF/F filters were additionally combusted at 450 degrees Celsius (°C) for 4 hours. Sinking particles were collected by sediment traps.

Total mercury (THg) and monomethylmercury (MMHg) in suspended and sinking particles were determined by digesting filter portions in nitric acid (2N, trace mental grade, Fisher) for 4 hours at 60°C constant water bath with intermittent sonication. For THg determination, digestates were oxidized with bromine monochloride for at least 2 hours and pre-reduced with hydroxylamine hydrochloride. Subsamples were then reduced with stannous chloride, and the evolved Hg0 concentrations were determined by dual gold amalgamation cold vapor atomic fluorescence spectrometry (CVAFS) with a Tekran 2600 against both gaseous Hg0 and aqueous Hg2+ standards. MMHg determination used a direct ethylation method. Digestates were treated with ascorbic acid, buffered with acetate, and neutralized with potassium hydroxide (KOH; 45%) to pH between 4 and 5. Sodium tetraethylborate (1%, in 2% KOH) was added in digestates reacting for 20 minutes prior to analysis by CVAFS. The method detection limits for THg and MMHg were 26.5 femtomolar (fM) and 3.3 fM in the SSF, and 0.75 fM and 0.24 fM in the LSF, respectively.

For carbon (C) mass determination in the SSF and LSF, filters were packed into a tin capsule and analyzed for particulate carbon mass and measured using Carlo Erba 1108 elemental analyzer coupled to a Thermo Finningan Delta Plus XP IRMS at University of California, Santa Cruz. The analytical precision of C % is 0.65% in the SSF and 1.1% in the LSF based on the organic standards IU Acetanilide and USGC41.