Award: OCE-1333738

This project was primarily focused on making measurements of atmospheric mercury (Hg) in cloud and fog water. The main idea was that volatile dimethyl Hg was being emitted to the atmosphere by the ocean when seasonal wind-driven upwelling was occurring. This time of year between April and October is  when low-level stratus clouds can form over the Pacific Ocean and these clouds often advect onto land resulting in fog and associated water deposition to the land environment. This project examined the monomethyl Hg and total Hg concentrations in fog and cloud water collected on land, sea, and aircraft in coastal Califoronia, with the goal on determining patterns in space and time of these different Hg species. We wanted to know what were the likely sources of the different Hg compounds that we find in cloud and fog water, and what are their principle atmospheric removal processeses. Shipboard measurements of Hg compounds in seawater as a function of depth were determined during the times when fog was being collected on land at eight sites. Ocean conditions that produced upwelling (as measured by satellite observations) yielded the greatest flux of dimethyl Hg from the ocean to the atmosphere. Fog water methyl Hg concentrations on land were higher the closer the site was to the ocean and in the early part of the upwelling/fog season compared to later in the summer when upwelling is less. Sixteen aircraft flights in the marine stratus cloud along coastal California yielded over 20 cloud water samples that were analyzed for Hg compounds. Spatial patterns of monomethyl Hg in cloud water revealed a source region associated with colder ocean temperatures, a signature of upwelling. Based on these results we can anticipate that ocean emissions of methylated Hg compounds will increase if wind speeds and upwelling were to increase due to potentially stronger pressure gradients between land and sea as a function of global warming.

This project also examined total Hg and monomethyl Hg concentrations in biotic samples taken from foggy and non-foggy environments. Samples taken were plants (tree and woody shrub leaves), lichen (Ramalina menziesii), and the fur or whiskers of grass-fed cows, deer (Odocoileus hemionus), and mountain lions (Puma concolor). The goal was to evaluate the potential for an atmospheric source of Hg from marine fog to bioaccumulate in the food web of the coastal terrestrial environment.  We found that all sample types from the Central California coastal region showed statistically significantly higher total Hg and monomethyl Hg concentrations than samples from inland California and mountain regions. We hypothesize that monomethyl Hg carried to land via fog water deposition contributes Hg in a form that is quickly taken up by plants and microbes in topsoil and results in elevated Hg concentrations in the tissue of top-level predators to levels that pose a risk to wildlife. 

The broader impacts of the research are numerous. Several publications and media reports were generated on this work. The PIs have been interviewed on radio and podcasts, been the subject of video documentaries, given press conferences and were guest lecturers at community colleges, high schools and for the general public. Two master's students were recruited and at least 20 undergraduates contributed. Some undergraduate students were named coauthors for the contributions they made with fog water collection method development. Upper division undergradiate cirriculum has been developed based on the findings of this research.  This research has prompted other Hg scientists to look at how Hg compounds move from the surface ocean to the atmosphere and back to the ocean in an attempt to understand what are the factors controlling its accumulation in animal tissue where it can ultimately represent a human health concern. 

Last Modified: 10/29/2017
Modified by: Peter S Weiss-Penzias