The fog that hydrates coastal Californian vegetation during the hot summer months is bringing in more than just moisture. A new study by UC Santa Cruz researchers confirm that the fog is a carrier of toxic methylmercury.
Background:
Mercury is mainly emitted to the atmosphere from coal-fired power plants and other high temperature industrial processes, such as waste incineration, cement kilns and ore refining. Once emitted to the atmosphere it can be transformed in the environment by biotic and abiotic processes resulting in methylmercury. Methylmercury is a mercury species, easily taken up by organisms and accumulating in animals at higher levels in the food chain. Being present as a positively charged ion that readily binds with proteins, monomethyl mercury can be transported throughout the body, even across the blood-brain barrier into the brain and through the placenta into a developing fetus. Reaching the brain, the neurotoxin can impair neurological development in fetuses and young children, reducing memory, attention, and language and motor skills.
According to Peter Weiss-Penzias, an atmospheric chemist at the University of California, Santa Cruz, most scientists presumed mercury deposited from the atmosphere came either from raindrops that had absorbed the element from the air or via "dry deposition," in which the vaporized atoms stick to or chemically react with an exposed surface.
The new study:
Now, field experiments conducted by Weiss-Penzias and his colleagues have identified another atmospheric source of mercury that's likely to be substantial in some areas, especially along some coasts. The study that outlines the discovery, "Total and Monomethyl Mercury in fog water from the Central California Coast", was published in the Geophysical Research Letters in February of this year.
Fog water samples were gathered during the summer months of 2011 from four different locations in the Monterey Bay area for mercury speciation analysis. In these samples, methylmercury concentrations ranged from about 1.5 parts per trillion to 10 parts per trillion, averaging at 3.4 parts per trillion. The latter concentration, the first such measurement ever reported from fog water, "is about five times the highest concentration ever seen in rainwater," Weiss-Penzias notes.
It is not yet clear of how methylmercury gets into the fog. The study suggests that mercury is perhaps carried to the surface of the ocean through upwelling, a process by which wind drives denser and cooler nutrient-rich water upwards. Sediment-dwelling microbes consuming mercury-tainted organic matter then generate a compound called dimethyl mercury. When the contaminated waters reach the surface, the compound evaporates and is subsequently cleaved into several components, including monomethyl mercury, by chemical reactions driven by sunlight. It's not clear whether those reactions take place in the atmosphere or within fog droplets, Weiss-Penzias says, but in either case the monomethyl mercury ends up in the droplets, which can then be carried by winds to the coast.
Using data from previous studies, including those of other teams, the researchers estimated that between 61% and 99% of the monomethyl mercury deposited in coastal ecosystems around Monterey Bay over the course of a year comes from fog.
The original study:
Peter S. Weiss-Penzias, Cruz Ortiz Jr., R. Paul Acosta, Wesley Heim, John P. Ryan, Daniel Fernandez, Jeffrey L. Collett Jr., A. Russell Flegal,
Total and monomethyl mercury in fog water from the central California coast, Geophys. Res. Let., 39 (2012) L03804.
doi:10.1029/2011GL050324
Other related studies: D.R. Bittrich, S.P. Chadwick, C.L. Babiarz, H. Manolopoulos, A.P. Rutter, J.J. Schauer, D.E. Armstrong, J. Collett, P. Herckes,
Speciation of mercury (II) and methylmercury in cloud and fog water, Aerosol Air Qual. Res., 11 (2011) 161–169.
doi: 10.4209/aaqr.2010.08.0067
C.D. Ritchie, W. Richards, P.A. Arp,
Mercury in fog on the Bay of Fundy (Canada), Atmos. Environ., 40 (2006) 6321–6328.
doi:10.1016/j.atmosenv.2006.05.057.
E.G. Malcolm, G.J. Keeler, S.T. Lawson, T.D. Sherbatskoy,
Mercury and trace elements in cloud water and precipitation collected on Mt. Mansfield, Vermont, J. Environ. Monit., 5 (2003) 584–590.
doi:10.1039/b210124f
Related EVISA Resources EVISA Link database: Environmental mercury cycling EVISA Link database: Mercury pollution
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February 17, 2006: Study shows link between clear lakes and methylmercury contamination in fish
February 9, 2006: Study show high levels of mercury in women related to fish consumption September 13, 2005: Regulating Mercury Emissions from Power Plants: Will It Protect Our Health?last time modified: April 22, 2012
