Photochemical influences on the air-water exchange of mercury.

The formation of dissolved gaseous mercury (DGM) in natural waters is an important component in the biogeochemical cycle of mercury (Hg). The predominate form of DGM in natural waters, gaseous elemental Hg (Hg$\sp0),$ may be transferred from the water to the atmosphere. Gas exchange may reduce the amount of Hg available for methyl-Hg formation, the most toxic form of Hg that bioaccumulates in the food chain. Determining the mechanisms and rates of DGM formation is essential in understanding the fate and cycling of Hg in aquatic ecosystems. Field and laboratory experiments were conducted to evaluate the effect of light on DGM formation in surface waters containing different levels of dissolved organic carbon (DOC). Water samples collected from the Tahqwamenon River and Whitefish Bay on Lake Superior were amended with divalent Hg (Hg$\sp{2+})$ and irradiated under a variety of reaction conditions to determine rates of DGM formation. The water samples were also analyzed for various Hg species (total, filtered, easily reducible and dissolved gaseous Hg), DOC and light attenuation. Additional field studies were conducted on Lake Michigan to measure gaseous Hg in air and water. These data were used to develop a mechanistic model to estimate air-water exchange of gaseous Hg. This research found that photochemical formation of DGM was affected by penetration of UV$\sb{\rm A}$ radiation (320-400 nm). Formation of DGM was enhanced at higher DOC concentrations, indicating DOC photosensitized the reduction of Hg$\sp{2+}$ to Hg$\sp0.$ Wavelength studies determined that formation of DGM was significantly reduced in the absence of UV$\sb{\rm A}.$ Field studies showed DGM concentrations were highest near the water surface and peaked at mid-day, indicating a photo-induced source of DGM. The conversion of reducible Hg$\sp{2+}$ to Hg$\sp0$ was suppressed in high DOC waters where UV$\sb{\rm A}$ penetration was limited. The mechanistic model predicted similar DGM concentrations to the observed values and demonstrated that deposition and emission fluxes of gaseous Hg were similar in Lake Michigan. In addition, deposition and emission fluxes of gaseous Hg were similar to Hg loadings by precipitation. The formation and emission of DGM from surface waters represents a significant contribution to the Hg cycle in aquatic ecosystems.PhDBiogeochemistryEarth SciencesEnvironmental scienceHealth and Environmental SciencesPhysics, Atmospheric SciencePure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131563/2/9910015.pd