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INVESTIGATING OXIDATION MECHANISMS OF HG0 IN THE FREE TROPOSPHERE AND ITS INFLUENCE ON LONG RANGE MERCURY TRANSPORT
Citation:
Landis, M S. INVESTIGATING OXIDATION MECHANISMS OF HG0 IN THE FREE TROPOSPHERE AND ITS INFLUENCE ON LONG RANGE MERCURY TRANSPORT. Presented at Fourth SETAC World Congress, Portland, OR, November 14-18, 2004.
Impact/Purpose:
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.
Specifically, individual research project objectives are listed below:
(1) Evaluate the ability of speciated mercury (Hg0, Hg2+, HgP) measurements to aid source apportionment models in identifying anthropogenic source contributions to atmospheric mercury deposition
(2) Elucidate the contribution of coal combustion sources to observed mercury wet deposition in the Ohio River Valley
(3) Obtain atmospheric profiles (200 - 12,000 ft) of speciated ambient mercury off the south Florida Coast
- Evaluate the role of long range transport of RGM to Florida in the marine free troposphere.
- Identify any vertical mercury gradients that might indicate the presence of rapid mercury chemistry in air or in cloud water.
(4) Conduct research at Mauna Loa Observatory to elucidate elemental mercury oxidation in the remote marine free troposphere.
(5) Conduct laboratory kinetics experiments to determine the rate constants of elemental mercury oxidation to gaseous inorganic divalent mercury species from atmospheric halide species (e.g. BrO, ClO).
Description:
In 2000, the US EPA Office of Research and Development (ORD) initiated a study to evaluate the magnitude of long-range transport of mercury through the marine free troposphere to South Florida via aircraft measurements (200 to 12,000 feet). ORD funded the National Oceanic and Atmospheric Administration (NOAA) to provide a DeHavilland Twin Otter aircraft and personnel to collaborate with the collection of aerosol and trace gas measurements. One study period was conducted from January 18, through February 1, 2000 when the tropospheric winds typically transport air to South Florida from the Gulf of Mexico and the North American continent. Flights for a second study period were conducted from June 3 through June 26, 2000 when South Florida is predominantly influenced from easterly marine transport. Preliminary results from the flights indicate that mercury concentrations were significantly higher within air masses with continental influence. The Hg0 concentrations were observed to be higher below 5000 feet (2.3 +/- 0.4 ng m-3 ) than above 5000 feet (1.5 +/- 0.4 ng m-3). Conversely, the concentrations of Hg2+ were typically higher above 5000 feet (49 +/- 28 pg m-3) than below (12 +/- 7 pg m-3).
ORD subsequently initiated speciated mercury measurements at the NOAA Mauna Loa Observatory (MLO), a high altitude research station (~11,500 feet) in 2001. Observations at MLO are consistent with the aircraft results suggesting that there is a significant Hg0 oxidation mechanism at altitude. These observations are not consistent with the contemporary conceptual atmospheric mercury model that estimates the Hg0 atmospheric lifetime to be approximately 1 year. Meteorological modeling and chemical tracer results will also be presented to investigate the importance of trans-Atlantic and trans-Pacific transport of mercury species.
This work has been funded wholly or in part by the United States Environmental Protection Agency Office of Research and Development. It has been subjected to peer review and approved for publication.