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Support for Mercury Regulations and Policy
Abstract:
Environmental contamination from mercury has been recognized for decades as a growing problem to humans and wildlife. It is released from a variety of sources, exhibits a complicated chemistry, and proceeds via several different pathways to humans and wildlife. According to the 1997 Mercury Study Report to Congress, mercury deposition has increased by a factor of two to five over preindustrial levels. The most significant releases of mercury in the U.S. are emissions to the atmosphere as a result of human activities (i.e., anthropogenic), particularly the combustion of fuel containing trace amounts of mercury. Other sources of anthropogenic mercury include industrial processes and the disposal of products containing mercury.
While a plausible link has been made between mercury emissions and the presence of mercury in humans, a number of uncertainties related to both the risk assessment and risk management of mercury remains. The EPA ORD has developed a mercury research strategy which identifies seven key scientific questions that need to be addressed. EPA NERL/HEASD is presently focused on improving information on the transport, transformation and fate of mercury in the environment.
Objective:
The overall research objective of this task is to improve our understanding of the emission, transport, transformation, and deposition of atmospheric mercury. Information garnered from this research is used to improve and evaluate EPA deterministic models that are used to investigate the (i) relative impact to local, regional, and global sources to atmospheric mercury deposition, and (ii) benefits of various emission reduction scenarios.
Relevance/Significance/Impact:
Research being conducted under this task is being performed to address several of the research questions identified in ORD’s Mercury Research Strategy. EPA NERL/HEASD is presently able to contribute in answering two of these key questions, mainly to quantify speciated mercury emissions (elemental, ionic, and particulate phase) from combustion and non-combustion sources and improve information on the transport, transformation and fate of mercury in the environment.
The research is divided into the following four subtasks.
1. Monitoring Mercury Speciation and Reactions in Utility Emissions Plumes
Principal Investigator: Dr. Matthew Landis
Abstract: Mercury is emitted to the atmosphere from coal-fired power plants in multiple forms. The chemical and physical characteristics of mercury affect the way it travels and transforms in the atmosphere. Some recently published studies have highlighted scientific uncertainties about the chemical and physical changes that mercury undergoes in the atmosphere which, in turn, has the potential to impact local and regional deposition. Findings from this EPA research study will further the Agency’s understanding about the local, regional, and global deposition of the various forms of mercury and improve the models used by EPA to estimate local mercury deposition rates.
Objective: This study seeks to determine the existence of a reaction in coal-fired utility boiler emissions plumes that reduces RGM to Hg0, and if found quantify the extent of conversion.
2. Dry Deposition Methodology
Principal Investigator: Dr. Matthew Landis
Abstract: Excessive levels of mercury in the nations waters is a significant source of water quality impairment in the US. Atmospheric emissions and deposition processes drive mercury accumulation in soils and sediments, and are now recognized as the major route of mercury contamination to most aquatic ecosystems. In pioneering studies conducted in South Florida and the Great Lakes Region, rainfall was observed to be the primary pathway for delivery of mercury loads to the Everglades and Lake Michigan, respectively. However, the Everglades and Lake Michigan studies were not able to adequately resolve the relative contribution of gas phase mercury dry-deposition (particularly the impact of divalent reactive gaseous mercury (RGM)) because of limitations in contemporary measurement methods. It was estimated that RGM dry deposition was perhaps equally as important as wet deposition. Atmospheric constituents are scavenged by rainfall for only a small fraction of the year while dry deposition, much less well understood and less tractable to model, is omnipresent. Thus, field studies of mercury dry deposition of any sort are few because of the
minuscule but near-continuous nature of dry deposition processes. This lack of basic scientific understanding and appropriate measurement tools has led to widespread and systematic underestimates of the importance of gas phase mercury; and thus, skewed results of the few efforts devoted to understanding of comprehensive accounting
Objective: The objective of this research is to improve our understanding of gas phase mercury dry deposition. Specifically the objectives will be to:
- Develop/evaluate a direct measurement method(s) for gas phase mercury dry deposition.
- Conduct a laboratory evaluation of methods thought to be most promising and improve performance where possible.
- Conduct field evaluation of methods that preformed well in laboratory evaluation and improve performance where possible.
3. Ohio Mercury Monitoring and Receptor Modeling Study
Principal Investigator: Dr. Matthew Landis
Abstract: Mercury (Hg) is a toxic bioaccumulative substance in aquatic ecosystems. In its methylated form, Hg has been observed to bio-concentrate more than a million fold in the aquatic food chain. Consumption advisories are presently in effect for fish caught in the Great Lakes and a number of inland lakes in the surrounding states because of elevated Hg concentrations. Atmospheric deposition is widely recognized as an important link in the cycling of Hg in the environment and has been implicated as the primary pathway for inputs of Hg to the Great Lakes. Consequently, Hg has been identified as a critical pollutant for study and has specifically been targeted in the Great Lakes Water Quality Agreement Amendments of 1987 and Section 112(m) of the Clean Air Act Amendments of 1990 (“Great Waters Provision”). The U.S. EPA funded the Lake Michigan Mass Balance Study (LMMBS) from July 1, 1994 through October 31, 1995, in part to investigate the role of atmospheric Hg deposition to Lake Michigan. An important component of this investigation was to (i) determine the significance of local anthropogenic sources to atmospheric wet and dry deposition and (ii) identify the major source categories contributing to the deposition.
Source apportionment modeling (Principal Component Analysis/Multiple Linear Regression) conducted by researchers at the University of Michigan during the LMMBS found that a significant amount (>50%) of mercury wet deposition in Chicago could be attributed to anthropogenic sources, with coal combustion contributing approximately 20%. For this monitoring and receptor modeling study, a state-of-the-art environmental monitoring site was established in Steubenville, Ohio to investigate the impact of the high density of coal combustion sources in the Ohio River Valley to local and regional mercury wet and dry deposition.
Objective: Elucidate the contribution of coal combustion sources to observed mercury wet and dry deposition in the Ohio River Valley
4. Investigating the Transport and Transformation Mechanisms of Atmospheric Mercury in the Remote Central North Pacific
Principal Investigator: Dr. Matthew Landis
Abstract: While a plausible link has been made between mercury emissions and the presence of mercury in humans, a number of uncertainties related to both the risk assessment and risk management of mercury remains. One of the largest areas of uncertainty is the lack of information on atmospheric mercury chemistry. Atmospheric mercury exists in a variety of physical and chemical forms. Some gaseous mercury compounds are chemically reactive and/or water-soluble. Divalent or reactive gaseous mercury (RGM) is readily removed from the air by water surfaces, vegetation, or any other surface with which it may react. Conversely, elemental gas phase mercury (Hg0) is relatively inert and appears to stay in the atmosphere for many months, dispersing throughout the atmosphere before it is eventually removed. Although Hg0 is by far the most abundant form of mercury in air, its deposition is thought to be minor compared to that of RGM and particulate phase mercury. In fact, the slow removal of Hg0 from air may be largely due to its transformation to other forms of mercury, rather than its deposition. Obviously, an assessment of atmospheric mercury transport and fate requires an accurate accounting of the chemical and physical forms of the mercury emitted and an understanding of the chemical and physical transformations of mercury in air and cloud water are required.
NERL has been engaged in research efforts to improve information on the transport, transformation and fate of mercury in the environment for the last eight years. A research initiative in Barrow, Alaska found that Hg0 is converted to RGM during periods of surface ozone depletion at polar sunrise. The RGM formed during these periods was found to deposit efficiently to the snow pack, and may be a significant global sink of atmospheric mercury if extrapolated to the entire polar region. In addition, NERL conducted an aircraft study off the Atlantic coast of Florida. These flights utilized a NOAA Twin Otter aircraft outfitted with NERL’s mercury speciation instrumentation to conduct atmospheric profiling experiments from 200 - 12,000 feet. The Florida aircraft study found relatively high concentrations of RGM in the marine free troposphere, suggesting that either (i) anthropogenically produced RGM was being transported up to that altitude or (ii) homogeneous and/or heterogeneous formation of RGM aloft in the marine free troposphere. Additional high altitude research is necessary to elucidate the potential mechanisms for Hg0 oxidation to RGM and to evaluate the potential for long-range transport of anthropogenically emitted RGM. This Inter-Agency Agreement (IAG) Statement of Work outlines a five (5) year atmospheric mercury study that combine the unique capabilities of the CMDL’s Mauna Loa Observatory (MLO) with NERL’s mercury measurement expertise to investigate atmospheric mercury chemistry in the marine free troposphere.
Objective: Conduct research at Mauna Loa Observatory to elucidate elemental mercury oxidation in the remote marine free troposphere.