atmospheric chemistry: cool mercury
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462 nature geoscience | VOL 2 | JULY 2009 | www.nature.com/naturegeoscience
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of many measurement constraints. For example, a high degree of thermal contrast (that is, the temperature difference between the Earth’s surface and lower atmosphere) is essential for the determination of ammonia, and thus only spectral data from morning orbits were considered suitable. However, peak biomass burning occurs in the afternoon. Furthermore, cloudy spectra were not included in the analysis as cloud cover impedes thermal contrast, and resolution was poor in regions with low levels of ammonia (~0.1 mg m–2), due to instrumental noise. Thus when Clarisse and colleagues compared their satellite measurements of ammonia concentrations with modelled results it came as no surprise that the satellite data underestimated ammonia concentrations in the majority of hotspots. However, in certain regions of the
Northern Hemisphere above 30° N, satellite measurements actually exceeded modelled estimates. The discrepancy was particularly great in central Asia and over agricultural valleys, suggesting that current inventories underestimate ammonia emissions in northerly regions of the globe.
Clarisse et al.3 present a promising technique for investigating global ammonia emissions. Of course, future work is needed to minimize instrumental noise to enable the detection of low-level ammonia emissions. Direct comparison of infrared spectral data with in situ measurements will help to validate this method. Once fine-tuned, satellite measurements could contribute to a growing body of evidence that highlights the need for regulatory controls on anthropogenic ammonia emissions. Most countries do
not regulate ammonia emissions, despite detrimental health and ecosystem effects. But international efforts are beginning to focus on global best practices for ammonia management, and satellite measurements could provide essential information for future policy development. ❐
LaToya Myles is at the National Oceanic and Atmospheric Administration, Air Resources Lab, PO Box 2456 Oak Ridge, Tennessee 37831, USA. e‑mail: [email protected]
references1. Galloway, J. N. et al. Bioscience 53, 341–356 (2003).2. Aneja, V. P. et al. Nature Geosci. 1, 409–411 (2008).3. Clarisse, L. et al. Nature Geosci. 2, 479–483 (2009).4. Aneja, V. P. et al. J. Geophys. Res. 108, 4152–4162 (2003).5. Sutton, M. A. et al. (eds) Atmospheric Ammonia (Springer, 2009).6. Beusen, A. H. W. et al. Atmos. Environ. 42, 6067–6077 (2008).7. http://smsc.cnes.fr/IASI/
today, atmospheric mercury is closely tied to man-made pollution. Indeed, the chemical industry,
metallurgy and waste incineration are the most important mercury sources, with only small natural contributions from volcanic and marine emissions and biomass burning. The dominant atmospheric mercury component — gaseous elemental mercury (Hg0) — is chemically quite inert. Consequently, its atmospheric lifetime is 1 to 2 years — long enough to allow the global distribution of this toxic element. However, an ice-core study from Dome C, East Antarctica showed a surprisingly high mercury deposition during the Last Glacial Maximum, about 18,000 years ago1. Now on page 505 of this issue, Jitaru and colleagues reconstruct the deposition of atmospheric mercury over Antarctica for the past 670,000 years, and suggest that atmospheric interactions between sea salt, mercury and mineral dust led to periods of enhanced deposition over this continent2.
Once atmospheric mercury is deposited, it can accumulate in the tissues of wildlife, primarily affecting marine and aquatic organisms. It can also be re-mobilized by biogeochemical processes, which tend
to turn the mercury into volatile and even more toxic organic species such as methylmercury. In the Arctic environment, the atmospheric mercury burden is now at least three times higher than before industrialization3.
Atmospheric mercury depletion events, in which mercury is rapidly removed from the atmosphere and deposited on the underlying snowpack, were first detected in the Canadian Arctic in 1982 (refs 4, 5). Similar events were later detected in Antarctica6. These mercury depletion events occur sporadically during the polar springtime, following the return of daylight, and are closely linked with the depletion of surface ozone7. This interrelation between two seemingly disparate processes points to a role for reactive halogen species, primarily bromine and bromine oxide. Both these species break down ozone, and also oxidize gaseous mercury to a form that is easily removed from the atmosphere. In the polar regions, photochemical reactions on ice-crystal surfaces transform the inert bromide found in sea salts into a reactive species through a process that only occurs after the springtime polar sunrise8.
Jitaru and colleagues2 used ice samples from the Dome C ice core in Antarctica to assess whether similar depositional events had occurred in the past. They faced a formidable challenge in analysing mercury concentrations on the order of picograms per gram of ice. By combining mass spectrometry and gas chromatography techniques, the group was able to not only detect the mercury, but distinguish between methylmercury (MeHg+) and mercury (Hg2+) cations.
They found that during the coldest conditions of the past six glacial periods, mercury concentrations in the ice were higher than in the surrounding ice layers from warmer phases. These cold periods were also the dustiest, with high levels of mineral-derived manganese. However, based on the relative amounts of mercury and manganese, Jitaru and colleagues concluded that increased mineral dust alone could not explain the higher mercury concentrations. There is also no evidence that volcanic or marine emissions of mercury suddenly increased during these periods.
Instead, Jitaru and colleagues infer that, like modern atmospheric mercury depletion events, halogens had a role in
Atmospheric chemistry
cool mercuryIt is unclear whether the modern processes of mercury cycling — such as mercury deposition in polar regions — operated before anthropogenic emissions. Ice-core records from Antarctica now reveal strikingly high mercury concentrations during the coldest glacial periods.
rolf Weller
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ancient depletion events as well. Their results indicate that the Hg2+ species was an important component of the total mercury during these deposition periods. They suggest that during the coldest phases of past glacial periods, increased fallout of sea salts over Antarctica9 and subsequent photochemical reactions led
to an increased production of bromine radicals. Under cold enough conditions, the bromine–mercury reaction produces a stable intermediate, HgBr2. This intermediate molecule should be efficiently scavenged by the dust (also higher relative to interglacial periods), which would then carry the mercury to the Antarctic snow cover. Jitaru
and colleagues present a simple model that shows that the increased scavenging of mercury during the dustiest periods explains the high glacial mercury deposition during cold episodes, relative to the Holocene (~10,000 years ago to present day).
However, it is not yet possible to verify the operation of these processes over glacial Antarctica, as the reactive bromine species are not preserved in the glacial ice. Thus, although this is an elegant theory, the ice-core results alone are unable to provide a complete picture of ancient mercury-cycling in the atmosphere.
Nevertheless, Jitaru and colleagues2 provide interesting insights into the interplay between bromine–mercury chemistry and elevated mineral-dust load, which seems to favour polar regions as a global sink for atmospheric mercury, at least during glacial periods. ❐
Rolf Weller is at the Alfred Wegener Institute, Am Handelshafen 12, D‑27570 Bremerhaven, Germany. e‑mail: [email protected]
references1. Vandal, G. M., Fitzgerald, W. F., Boutron, C. F. & Candelone, J.-P.
Nature 362, 621–623 (1993).2. Jitaru, P. et al. Nature Geosci. 2, 505–508 (2009).3. Shotyk, W. et al. Geochim. Cosmsochim. Acta 67, 3991–4011 (2003).4. Schroeder, W. H. et al. Nature 394, 331–332 (1998).5. Lu, J. Y. et al. Geophys. Res. Lett. 28, 3219–3222 (2001).6. Ebinghaus, R. et al. Environ. Sci. Technol. 36, 1238–1244 (2002).7. Oltmans, S. J. J. Geophys. Res. 86, 1174–1180 (1981).8. Simpson, W. R. et al. Atmos. Chem. Phys. 7, 4375–4418 (2007).9. Wolff, E. et al. Nature 440, 491–496 (2006).
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Black-smoker hydrothermal venting, which occurs along mid-ocean ridges in the world’s oceans, results
in significant heat and chemical exchange between Earth’s crust and the overlying water. Black-smoker vents provide nutrients for extraordinary chemosynthetic biological communities, expanding our knowledge about how life may exist on Earth and elsewhere. Black smokers are generally associated with loss of heat from a magma body within the oceanic crust: as sea water percolating through the crust approaches the magma, it is heated and
laden with minerals, and subsequently returns to the sea floor. Venting sites are common along the global chain of seafloor volcanoes, but the factors influencing their location remain elusive. On page 509 of this issue, Wilcock and colleagues1 suggest that black smokers may occur at sites where the magma chamber is being actively recharged.
The discovery of high-temperature hydrothermal vents in 1977 (ref. 2) opened a new chapter in deep-sea exploration and sparked a search for more black smokers. So far, only about 20% of the global mid-
ocean-ridge system has been surveyed for indications of venting3, and only a fraction of these sites have been visually imaged. Global water-column mapping of hydrothermal plumes reveals a pattern of increased density of vent sites with increased rates of seafloor spreading at mid-ocean ridges3,4. Because magma supply varies in proportion with spreading rates, this pattern contributes to a growing body of evidence for the ‘magmatic budget hypothesis’ (for example, refs 5, 6), which proposes that magma supply is the primary control of global vent distribution.
mArine geophysics
Where there’s smoke there’s fireSeafloor vents spewing mineral-rich plumes of hydrothermal fluid — termed black smokers — can persist at mid-ocean ridges for decades or longer. Earthquake data indicate that ongoing magma injection may determine their locations.
maya tolstoy
Figure 1 | Where ice and water meet. Reconstructions of atmospheric mercury deposition over Antarctica during the past 670,000 years show episodes of enhanced deposition during the coldest periods of glacial climate. Jitaru and colleagues suggest that an increased production of bromine radicals triggered a series of reactions that led to mineral dust scavenging mercury from the atmosphere2.
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