Elevated concentrations of monomethyl mercury in Pacific marine fog water: investigation of sources and estimation of flux to the terrestrial landscape
Abstract
Fog deposition represents an important, and sometimes dominant input to the hydrologic cycle in coastal California yet its contribution to mercury (Hg) wet deposition is largely unknown. To address this data gap, ten fog water samples were taken during the summers 2011 and 2012 at two locations in Santa Cruz, California using an active Caltech strand collector (CASSC2). Two fog water samples from a 6 km inland site and one sample from a seaside location had MMHg concentrations that were 100 times higher than MMHg concentrations observed in rain. Concentrations of MMHg in fog water varied between 2.8 - 45 pM, which represented 2 - 100% (mean = 38%) of the total Hg (HgT) concentrations. In contrast, %MMHg in Santa Cruz rainwater was 3 - 5% of HgT in samples taken in 2007, 2008, and 2011. The resulting MMHg wet deposition flux, assuming a range of fog water depositions that have been observed in the region with standard passive mesh fog water collectors, is 65-7000 pmol m-2 yr-1 which represents 61-99% of the total MMHg flux due to wet deposition. Since the volatile dimethyl mercury (DMHg) is supersaturated in surface waters of the Monterey Bay during times of coastal upwelling, it was hypothesized that DMHg enters the atmosphere and converts to monomethyl mercury (MMHg) and is thus available for uptake by cloud and rain droplets. During the spring of 2012, we determined the concentrations of DMHg and MMHg in the center of the Monterey Bay at depths ranging from 0-1000 m. DMHg concentrations were about 0.1 pM at the surface and increased to about 0.5 pM at 200 m depth, similar to the pattern seen in previous work. MMHg concentrations were about a factor of 10 lower than DMHg and did not significantly vary with depth. DMHg concentrations below 200 m varied between 0.3 - 0.6 pM, but were not correlated with transmissivity (a proxy for suspended sediments), which suggests that DMHg was not associated with transport of detached particle plumes and resuspension events. These results suggest that other potential sources of DMHg in coastal waters should be examined, including formation within the water column, porewater advection/diffusion and submarine groundwater discharge.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B23K..02W
- Keywords:
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- 0322 ATMOSPHERIC COMPOSITION AND STRUCTURE / Constituent sources and sinks;
- 0365 ATMOSPHERIC COMPOSITION AND STRUCTURE / Troposphere: composition and chemistry;
- 0368 ATMOSPHERIC COMPOSITION AND STRUCTURE / Troposphere: constituent transport and chemistry;
- 0489 BIOGEOSCIENCES / Trace element cycling