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FORMATION OF REACTIVE GASEOUS MERCURY IN THE ARCTIC: EVIDENCE OF OXIDATION OF HG0 TO GAS-PHASE HG-II COMPOUNDS AFTER ARCTIC SUNRISE
Citation:
Lindberg, S. E., S. Brooks, C. J. Lin, K. Scott, T. Meyers, M S. Landis, AND R. K. STEVENS. FORMATION OF REACTIVE GASEOUS MERCURY IN THE ARCTIC: EVIDENCE OF OXIDATION OF HG0 TO GAS-PHASE HG-II COMPOUNDS AFTER ARCTIC SUNRISE. WATER, AIR AND SOIL POLLUTION:FOCUS 1(5-6):295-302, (2001).
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:
We have measured total gaseous mercury concentrations (Hgo) at Point Barrow, Alaska since September 1998 in an effort to determine the geographic extent and reaction mechanism of the so-called mercury depletion events (MDE) previously reported in the high Arctic at Alert, Canada. Hgo has been sampled now for nearly 2 years at Barrow. In September, 1999, we began making the first automated measurements of reactive gaseous mercury (RGM) attempted in the Arctic, along with measurements of Hg accumulation in snowpack to determine the fate of the "depleted" Hgo. During the fall and early winter, Hgo and RGM exhibit only minor variation, Hgo remaining within ~10% of global background, near 1.6-1.8 ng/m3. The MDE periods are quite different, however; within days of Arctic sunrise in January, Hgo exhibits major variations from the mean, rapidly dropping as low as 0.05 ng/m3 and then cycling back to typical levels, sometimes exceeding global background. These events continue throughout Arctic spring, then end abruptly following snowmelt, in early June. Prior to Arctic sunrise, RGM remains near detection (<2 pg/m3), but after sunrise increases dramatically (to levels as high as 900 pg/m3) in synchrony with the "depletion" of Hgo. Both phenomena exhibit a strong diel cycle, in parallel with UV-B. We conclude that MDE's involve rapid in-air oxidation of Hgo to a species of RGM by photochemically-driven reactions, probably involving the same reactive bromine and chlorine compounds involved in ozone destruction. Sharp increases in Hg in the surface snowpack after sunrise coincident with periods of peak RGM suggest surface accumulation of the RGM by dry deposition.
This article has been subjected to Agency review and approved for publication.