Soerensen, A. L. 2011: Gaseous Mercury in the Marine Boundary Layer: Measurements and Modeling. PhD thesis. Department of Atmospheric Environment, NERI. National Environmental Research Institute, Aarhus University, Denmark. 176 pp.
The atmosphere is the primary pathway from emission to marine ecosystems around the world. When atmospheric mercury deposits to marine systems, it can be transformed to the neurotoxin methyl mercury, which biomagnifies in the food web. Mercury is therefore found in harmful concentrations in some top predators including humans. Knowledge about mercury in the marine boundary layer (MBL) is important as the MBL is the interface between the atmosphere and the marine systems.
This thesis combines analyses of observational data with biogeochemical modeling to explore the environmental processes that control patterns and levels of gaseous mercury concentrations in the MBL. Measurements of gaseous elemental mercury (GEM) and gaseous oxidized mercury (RGM) from the Galathea 3 circumnavigation (August 2006 to April 2007) are analyzed. GEM and RGM were measured continuously with a Tekran 2537A mercury vapor analyzer equipped with a Tekran 1130 automated denuder unit and pump module. Measurements were available for 114 days, which make the Galathea 3 data the largest single data set on mercury in the MBL. In the 3-D global biogeochemical model GEOS-Chem the slab-ocean module is updated with a representation of the surface ocean. It includes redox processes and coupling between mercury cycling and organic carbon dynamics. After evaluation the model is used to explore spatial and temporal trends in the MBL discovered in the Galathea 3 data.
It is found that midday peaks in RGM concentrations are common in the MBL during cruise sections with mean midday insolation > 500 W m-2 and mean relative humidity < 85%. They indicate a photo-induced oxidation of GEM and a rapid uptake of RGM to water droplets in the air.
GEM enhancements of 0.1-0.5 ng m-3 are found in episodic plumes but RGM enhancements only in two cases. The low RGM in polluted air is suggested not to be due to a lack of RGM emissions but rater due to a rapid uptake into the water phase as a consequence of the high relative humidity in the MBL. The possible implications are that RGM emitted in coastal urban areas will deposit rapid and be a source for the coastal ocean. The GEM variability in the MBL within each hemisphere is found to be higher than has so far been implied by relevant studies. This could be due to variable influence of continental outflow and ocean evasion but also suggest that the lifetime of GEM in the MBL might be at the low end of previous estimates. The model results suggest that 80% of mercury deposited to the ocean will re-evade. The results show that ocean evasion drives the seasonal variability of GEM in the MBL.
The first estimate of seasonal variability in the MBL (Atlantic Ocean) based on cruise data is presented. Many of these cruise data have concentrations higher than at terrestrial sites. The model can explain these observations as seasonally dependent GEM evasion from the ocean driven by legacy mercury in subsurface waters. Also the Atlantic Ocean is found to currently be a net source of mercury to the atmosphere. This result combined with the result from the Galathea 3 cruise implies that the concept of background concentrations does not apply to the MBL.
Thesis in PDF format (11,7 MB)
