Factors influencing mercury photoreactions in fresh waters: An experimental approach manipulating light and DOC

The goal of this experiment is to explore the relationship between discrete wavebands of UVA and UVB light and dissolved gaseous mercury (DGM) formation using optical bandpass filters along with different concentrations of dissolved organic carbon (DOC). A DGM formation rate can be calculated by measuring emitted mercury as a function of time for each wavelength compared to the measured photon flux in the bandpass region. DOC can serve as electron donor or photoreactive intermediate and drive Hg transformations. Different DOC concentrations will be used in the exposure experiments to explore the relationship between DOC concentration and DGM formation rate. Our UVA and UVB light intensities are 10.01mW cm^-2 and 5.87 mW cm^-2. Bandpass filter transmittance measured by spectrophotometer shows that the filters allow 100% transmittance within a 10nm bandwidth. Light transmittance decays sharply to 0% approximately 5 nm outside the bandpass region. The UVA light intensity with 330nm filter is 0.22 mW cm^-2 and the UVB light intensity with 289nm filter is 0.18 mW cm^-2. We assume the light will only pass the solution one time and all the energy is absorbed by the solution. For a one hour exposure of Hg 1 ng L^-1 with DOC 2.68 mg L^-1 solution, DGM formation rates were highest with the unfiltered wavelengths, with UVB always stimulating more emission than UVA. UVB appears to contribute more to the DGM formation even though the total light intensity is smaller. We hypothesize that the cause is the higher absorbance by DOC at shorter wavelengths. Using the filters to select narrow bands of the UV spectrum, we can identify the most important wavelength(s) of light controlling DGM formation.