2003 Progress Report: Natural Mercury Isotopes as Tracers of Sources, Cycling, and Deposition of Atmospheric MercuryEPA Grant Number: R830603
Title: Natural Mercury Isotopes as Tracers of Sources, Cycling, and Deposition of Atmospheric Mercury
Investigators: Odom, A. Leroy , Landing, William , Salters, Vincent
Institution: Florida State University
EPA Project Officer: Chung, Serena
Project Period: October 2, 2002 through December 31, 2006
Project Period Covered by this Report: October 2, 2002 through December 31, 2003
Project Amount: $827,147
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text | Recipients Lists
Research Category: Mercury , Air Quality and Air Toxics , Safer Chemicals , Air
Previously we have demonstrated that the isotopic composition of mercury in ore minerals differs from refined mercury and that isotopic changes are induced by the evaporation of mercury. We propose to use variations in the isotopic composition of mercury as a new way of investigating natural and anthropogenic emissions of mercury into the atmosphere and of the atmospheric processes that affect transportation and deposition. We propose to determine the isotopic composition of atmospheric mercury: (1) thought to be dominated by far-field sources (Olympic Peninsula, WA; Barbados; New Zealand); (2) influenced by both far field and regional or local sources (Florida “Supersite”); and (3) influenced by known point sources (Southern Company’s coal burning generators as well as waste incinerators). These studies will include both elemental and oxidized mercury, collected total gaseous mercury (TGM), and rain. Near the Everglades, a Florida Supersite established by the U.S. Environmental Protection Agency, Florida Department of Environmental Protection (DEP), and Broward County receives mercury from both far-field and local sources. This site also is heavily instrumented for studying ambient atmospheric mercury speciation, transport, and deposition. We will collect for isotopic analyses rain samples and time integrated TGM samples for a 4-8 week interval during the summer “wet.” We shall perform stack sampling of those stacks that influence the super site and plan to include the use of Spanish moss. We shall conduct a detailed study of mercury isotope systematics of coal burning generators and local atmosphere. We will sample at several mercury monitoring “Supersites” in collaboration with the Southern Company. These sites will be heavily instrumented for a variety of high-frequency measurements, which can be used as a context for the interpretation of mercury isotopes. Southern Company will also provide samples of the fuel burned at the time of the stack sampling, as well as flyash. A variety of coal types and samples in addition to those used by Southern will be analyzed for mercury isotopic composition. In addition, we will determine the mercury isotope systematics around several waste incinerators to determine their contribution to the isotopic composition of atmospheric mercury. Another important area of study will be the measurement of the isotopic ratios of mercury emissions from naturally enriched soils in collaboration with the flux chamber measurements of Dr. Mae Gustin of the University of Nevada at Reno (a letter of collaboration is on file). Ore, soil, and plant also will be isotopically measured. We are joined in this effort by Frontier Geosciences Inc. Collaborators include Southern Company, Florida DEP, Mae Gustin (University of Nevada at Reno), and Keith Hunter (University of Otago, New Zealand).
Rain samples were collected on October 14-16, 2002 at Yorkville, Georgia, in collaboration with the Southeastern Aerosol Research and Characterization mercury emission experiment at the nearby Bowen Plant (Georgia Power) in Cartersville, Georgia. Nearly 7 liters of rain were collected on 2 consecutive days. These samples are intended to be analyzed for total mercury and mercury isotopes. Subsamples were obtained from the stack emission samples collected using the Ontario-Hydro sampling scheme, also intended to be analyzed for mercury isotopes. Samples of the fuel coal have also been collected for analysis of total mercury and mercury isotopes.
Much of our effort has been devoted to defining the operating conditions of the Isolab secondary-ionization heavy element mass spectrometer to obtain accurate mercury isotope ratios. For solid samples with high mercury content analysis using secondary ion sputtering yielded accurate, high-precision, isotope ratios. To achieve high precision isotope ratio measurements at the low-level mercury concentrations found in the environmental samples of interest, a way had to be found to concentrate all of the mercury in a sample within a 100-200 micron spot. Efforts were made to use electroplating as a means to deposit mercury from aqueous solutions onto the tips of gold-coated sample stubs. Brass sample mounting stubs were thinly coated with elemental gold and used as the anode in the electroplating setup. The recovery from the electroplating solutions was tested using a cold-vapor atomic fluorescence analytical system (CVAFS) and was found to be less than 90 percent and variable. Consistent recoveries greater than 98 percent are required to avoid inducing isotopic fractionation. At the end of calendar 2003, we were still investigating various means to quantitatively transfer dissolved mercury from aqueous samples onto a 100-200 micron spot.
One of our goals is to use Spanish moss as a passive collector and integrator of atmospheric mercury. So, the CVAFS system was used to quantify the mercury content of samples of Spanish moss collected from the Tallahassee area. New growth was found to contain lower total mercury concentrations (0.01-0.04 ng/g dry weight), but the sample replicate precision (samples from the same tree) was much better than for older growth (0.03-0.09 ng/g dry). The CVAFS system was also optimized for the analysis of total mercury in various solids including sediment, soils, fish tissue, and other organic materials. The analysis is calibrated with aqueous mercury standards and an elemental gaseous mercury standard, and the accuracy is assured via analysis of standard reference materials.
We will continue to optimize the sample preparation protocols to prepare samples that are suitable for isotopic analysis on the Isolab mass spectrometer. We cannot move forward with the analysis of field samples until the isotopic methods are consistently reproducible.