The distribution and speciation of mercury in the California current: Implications for mercury transport via fog to land

Unfiltered seawater samples from vertical profiles collected at 60 stations within the California Current during four summer cruises spanning a two-year period from 2014 to 2015, were analyzed for elemental mercury (Hg°), monomethyl mercury (MMHg), dimethyl mercury (DMHg) and total mercury (THg). Fog water samples, taken at sea and throughout a network of land based stations were also analyzed for MMHg and THg. Vertical profiles indicate that midwater regions around 300 m are associated with concentration maxima in methylated species. Cyclonic mesoscale eddies were shown to be strong sources of the gaseous mercury species to the lower atmosphere and a likely source of these species to fog. Calculated evasive flux of Hg0 and DMHg were greatest in these regions (34 and 11 pmol m−2 d−1, respectively), whereas anticyclonic eddies support little or no sea-air evasion. Incubation experiments showed that DMHg is stable over short time scales at natural seawater pH (7.8–8.2) but degrades rapidly to MMHg at low pH. Demethylation of only a small percent of the evading DMHg, on acidic marine aerosols associated with fog condensation nuclei, can account for over 100% of the MMHg observed in fog. The surface microlayer, enriched in MMHg (by 30 ×), may also contribute to sea-air flux through aerosol production. Neither shelf sediments nor oxygen minimum zones appear to be a major source of methylated mercury in the California Current.