Total- and monomethyl-mercury and major ions in coastal California fog water: Results from two years of sampling on land and at sea

Abstract

Marine fog water samples were collected over two summers (2014)(2015) with active strand collectors (CASCC) at eight coastal sites from Humboldt to Monterey counties in California, USA, and on four ocean cruises along the California coastline in order to investigate mercury (Hg) cycling at the ocean-atmosphere-land interface. The mean concentration of monomethylmercury (MMHg) in fog water across terrestrial sites for both years was 1.6 ± 1.9 ng L -1 (<0.01-10.4 ng L -1 , N = 149), which corresponds to 5.7% (2.0-10.8%) of total Hg (HgT) in fog. Rain water samples from three sites had mean MMHg concentrations of 0.20 ± 0.12 ng L -1 (N = 5) corresponding to 1.4% of HgT. Fog water samples collected at sea had MMHg concentrations of 0.08 ± 0.15 ng L -1 (N = 14) corresponding to 0.4% of HgT. Significantly higher MMHg concentrations in fog were observed at terrestrial sites next to the ocean relative to a site 40 kilometers inland, and the mean difference was 1.6 ng L -1 . Using a rate constant for photo-demethylation of MMHg of -0.022 h -1 based on previous demethylation experiments and a coastal-inland fog transport time of 12 hours, a mean difference of only 0.5 ng L -1 of MMHg was predicted between coastal and inland sites, indicating other unknown source and/or sink pathways are important for MMHg in fog. Fog water deposition to a standard passive 1.00 m 2 fog collector at six terrestrial sites averaged 0.10 ± 0.07 L m -2 d -1, which was ∼2% of typical rainwater deposition in this area. Mean air-surface fog water fluxes of MMHg and HgT were then calculated to be 34 ± 40 ng m -2 y -1 and 546 ± 581 ng m -2 y -1 , respectively. These correspond to 33% and 13% of the rain fluxes, respectively.

Marine fog inputs appear to increase methylmercury bioaccumulation in a coastal terrestrial food web

“…Fog and rain data are from Weiss-Penzias et al . 16 . Bioconcentration factors (defined in the text) are shown for the Hg species and trophic relationship given by the horizontal line.…”

“…Organic Hg compounds (primarily monomethylmercury (MMHg)) are generally present in only trace amounts in the atmosphere 11–13 , as they are formed primarily in anaerobic soils, sediments, and waters, through the metabolic processes of a diverse array of microbes 14 . However, new research on atmospheric marine fog water deposition indicates a novel pathway through which coastal terrestrial food webs may be exposed to mercury 15,16 . Understanding this connection is particularly important since it is recognized that global environmental change can modulate the mercury risk to wildlife and humans through increased emissions, deposition and subsequent uptake by biota 4,5 .…”

“…Once MMHg is in the aqueous phase it may be stable for several days especially in clouds where sunlight is attenuated 29 . This marine based process results in elevated concentrations of MMHg in coastal fog drip 15,16 . The presence of MMHg in coastal cloud water is indicative of the air mass having had a recent source of ocean-derived methylated mercury 28 .…”

Atmospheric mercury deposition over the land surfaces and the associated uncertainties in observations and simulations: a critical review

“…Europe has the lowest Hg wet deposition level (on average 3.4 µg m −2 yr −1 ) according to the available observation and simulation data (Connan et al, 2013;Bieser et al, 2014;Siudek et al, 2016). Observation data for the tropics and the Southern Hemisphere are scarce with large uncertainties (Wetang'ula, 2011;Gichuki and Manson, 2013;Sprovieri et al, 2017). The one exceptional tropical site with a wet deposition flux of 16.8 µg m −2 yr −1 is in Kenya, while the other sites in the tropics are all in Mexico (Wetang'ula, 2011;Hansen and Gay, 2013).…”

Distribution of mercury species across a zonal section of the eastern tropical South Pacific Ocean (U.S. GEOTRACES GP16)

“…Subsurface concentration maxima were observed in the OMZ where microbial processes may reduce Hg during organic matter remineralization (Lehnherr et al, 2011;Monperrus et al, 2007;Whalin et al, 2007). Concentrations of Hg 0 in the thermocline (150-1000 m) were variable, but on average were 2.5× greater in the eastern suboxic thermocline (0.065 ± 0.075 pM, n = 116) than in the more oxygenated thermocline to the west (Stations 14-36; 0.026 ± 0.036 pM, n = (Conaway et al, 2009;Weiss-Penzias et al, 2016).…”