Atmospheric Mercury Depletion Events Research Articles

The mass balance of mercury in the springtime arctic environment

The load of mercury in the Arctic environment is to a large extent controlled by atmospheric mercury depletion events. At Barrow, Alaska, these depletion events have been linked with the near‐surface air formation of reactive gaseous mercury (Hg(II)) (RGM), to a much lesser extent in a particulate‐bound form, and the accumulation of total mercury in the snow pack (>100 ng/L in late Spring). This transport of Hg from atmospheric conversion, to deposition, to bio‐available forms is likely to be the predominant pathway for mercury into Arctic biota. For the first time we combine flux rate measurements, atmospheric chemistry measurements, and air mass trajectories to give a comprehensive two‐week window into the springtime dynamics and mass balance of Arctic mercury. We have conducted polar‐sunrise to snowmelt mercury monitoring at Barrow from 1998 to 2004, and the time period March 25th–April 7th (Julian days 84–97), 2003 appears typical for this time of year. A clear link was observed between air of marine origin, the build‐up of BrO together with removal of gaseous elemental mercury (GEM)), and the formation of RGM. This provides the most direct evidence so far for Br and Hg chemistry as the direct source of RGM. The fluxes of RGM and GEM were determined and the net flux calculated.

Mercury in the Atmosphere, Snow and Melt Water Ponds in the North Atlantic Ocean during Arctic Summer

Atmospheric mercury speciation measurements were performed during a 10 week Arctic summer expedition in the North Atlantic Ocean onboard the German research vessel RV Polarstern between June 15 and August 29, 2004. This expedition covered large areas of the North Atlantic and Arctic Oceans between latitudes 54 degrees N and 85 degrees N and longitudes 16 degrees W and 16 degrees E. Gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and mercury associated with particles (Hg-P) were measured during this study. In addition, total mercury in surface snow and meltwater ponds located on sea ice floes was measured. GEM showed a homogeneous distribution over the open North Atlantic Ocean (median 1.53 +/- 0.12 ng/m3), which is in contrast to the higher concentrations of GEM observed over sea ice (median 1.82 +/- 0.24 ng/m3). It is hypothesized that this results from either (re-) emission of mercury contained in snow and ice surfaces that was previously deposited during atmospheric mercury depletion events (AMDE) in the spring or evasion from the ocean due to increased reduction potential at high latitudes during Arctic summer. Measured concentrations of total mercury in surface snow and meltwater ponds were low (all samples <10 ng/L), indicating that marginal accumulation of mercury occurs in these environmental compartments. Results also reveal low concentrations of RGM and Hg-P without a significant diurnal variability. These results indicate that the production and deposition of these reactive mercury species do not significantly contribute to the atmospheric mercury cycle in the North Atlantic Ocean during the Arctic summer.

Climate Change, Risks and Contaminants: A Perspective from Studying the Arctic

ABSTRACT The Arctic faces threats from climate change and contaminants. Together, these two threats are likely to present surprises centered around the zero-degree isotherm because the phase change of water has enormous potential to affect contaminant transport and transfer, and biological distribution and stress. Particularly at risk are top aquatic predators, migratory species, and species narrowly adapted to ice. These species are most exposed to contaminants, are most likely to become stressed by climate change, or contain within their life cycles efficient vectors of contaminants and diseases. In the Arctic, mercury presents a special case where risks can be altered at many places in the biogeochemical cycle. Atmospheric mercury depletion events offer one such location; however, the methylation of mercury in aquatic systems appears a far more important and presently neglected component of risk from mercury to Arctic ecosystems. Climate variables alter transport, transfer, and capture of contaminants. Therefore, monitoring for contaminants must be conducted with a systems approach that includes climate-related factors. To ensure that the perception of risk is accurate and that priority risks are addressed first, a closer dialogue between scientists, the public, and public administers is urgently required.

Atmospheric mercury behavior at different altitudes at Ny Alesund during Spring 2003

Intensive field measurements of atmospheric mercury and related species were carried out in Ny Alesund, Spitsbergen, during the spring of 2003 at two altitudes. Measurements were made at the Italian research station Dirigibile Italia (12 m a.s.l.) and on the top of Zeppelin Mountain at the Norwegian Research Station (474 m a.s.l.). Ambient concentrations of gaseous elemental mercury, divalent reactive gaseous mercury and particulate phase mercury were semi-continuously measured at both sites using an integrated Tekran system. Atmospheric elemental gaseous mercury depletion events (AMDEs) were observed at both locations, the lowest observed Hg 0 concentration was 0.3 ng m −3. At sea level, mercury species concentrations following AMDEs were found to be higher and to exhibit larger variability in comparison to those observed at Zeppelin station.

Study of the origin of atmospheric mercury depletion events recorded in Ny-Ålesund, Svalbard, spring 2003

An international campaign involving six teams was organized in Ny-Ålesund, Svalbard, in order to understand better the origin of atmospheric mercury depletion events (AMDEs). Special emphasis was given to determining the source region of the observed events and the physical and chemical processes leading to AMDEs. Five AMDEs were recorded during a one-month field experiment (10 April–10 May, 2003). The different events presented various characteristics, especially in terms of mercury species formation, atmospheric particle variations and meteorological conditions. After careful examination of each event, we postulate that two were probably due to advection of already depleted air masses and three were a product of local or regional chemistry. The roles of different surfaces (frost flowers, snow, ice aerosol in clouds) involved in heterogeneous reactions leading to AMDEs are also discussed. We speculate that ice clouds may explain the particle variations observed during the three more local events.

Measurements of atmospheric mercury species during an international study of mercury depletion events at Ny-Ålesund, Svalbard, spring 2003. How reproducible are our present methods?

Six groups participated in an international study of springtime atmospheric mercury depletion events (AMDEs) at Ny-Ålesund in the Norwegian Arctic during April and May 2003 with the aim to compare analytical methods for measurements of atmospheric mercury species and study the physical and chemical processes leading to AMDEs. Five groups participated in the method comparison that was conducted at three different locations within Ny-Ålesund. Various automated and manual instrumentation were used to sample, measure and compare gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and mercury associated with particles (Hg-P). The concentration of GEM was reproducible during background conditions. For the first time using ambient air, the statistics associated with round robin test procedures were applied. This was found to be an appropriate tool to investigate the reproducibility of GEM measurements in ambient air. The precision for each group measuring GEM concentrations was found to be consistently good (within 5%). Five AMDEs were recorded during the study. Using four different methods, including single and replicate samples, all groups recorded higher values of RGM and Hg-P during AMDEs. The results show that measuring comparable atmospheric mercury species at both the same and different locations (within the Ny-Ålesund area) is difficult. Not only do site location and site characteristics create challenges when trying to intercompare results but there are difficulties, as well, in obtaining comparable results with similar sampling and analysis methods. Nevertheless, with our current procedures for atmospheric mercury identification we can differentiate with certainty between “high” and “low” concentration values of RGM and Hg-P.

Snow-to-air exchanges of mercury in an Arctic seasonal snow pack in Ny-Ålesund, Svalbard

The study of mercury (Hg) cycle in Arctic regions is a major subject of concern due to the dramatic increases of Hg concentrations in ecosystem in the last few decades. The causes of such increases are still in debate, and an important way to improve our knowledge on the subject is to study the exchanges of Hg between atmosphere and snow during springtime. We organized an international study from 10 April to 10 May 2003 in Ny-Ålesund, Svalbard, in order to assess these fluxes through measurements and derived calculations. Snow-to-air emission fluxes of Hg were measured using the flux chamber technique between ∼0 and 50 ng m −2 h −1. A peak in Gaseous Elemental Mercury (GEM) emission flux from the snow to the atmosphere has been measured just few hours after an Atmospheric Mercury Depletion Event (AMDE) recorded on 22 April 2004. Surprisingly, this peak in GEM emitted after this AMDE did not correspond to any increase in Hg concentration in snow surface. A peak in GEM flux after an AMDE was observed only for this single event but not for the four other AMDEs recorded during this spring period. In the snow pack which is seasonal and about 40 cm depth above permafrost, Hg is involved in both production and incorporation processes. The incorporation was evaluated to ∼5–40 pg m 2 h. Outside of AMDE periods, Hg flux from the snow surface to the atmosphere was the consequence of GEM production in the air of snow and was about ∼15–50 ng m −2 h −1, with a contribution of deeper snow layers evaluated to ∼0.3–6.5 ng m −2 h −1. The major part of GEM production is then mainly a surface phenomenon. The internal production of GEM was largely increasing when snow temperatures were close to melting, indicating a chemical process occurring in the quasi-liquid layer at the surface of snow grains.

Springtime accumulation of atmospheric mercury in polar ecosystems

Mercury and many of its compounds behave exceptionally in the environment due to their volatility, capability for methylation, and subsequent biomagnification in contrast with most of the other heavy metals. Long-range atmospheric transport of elemental mercury, its transformation to more toxic methylmercury compounds, the ability to undergo photochemical reactions and their bioaccumulation in the aquatic food chain have made it a subject of global research activities. Atmospheric Mercury Depletion Events (AMDEs) during polar springtime have been experimentally observed in the Arctic and in the Antarctic. During these events Hg 0 and ozone concentrations are significantly depleted and well correlated, whereas concentrations of reactive gaseous mercury species (RGM) simultaneously increase. The main reaction mechanism and corresponding chemical and physical properties of involved species in polar regions are summarized in this work. Hg 0 is removed from the atmosphere and deposited onto the underlying surface snow. This paper focused on the fast, photochemically driven, oxidation of boundary-layer Hg 0 , the influence of reactive halogen chemistry, and the resultant net input of mercury into the polar ecosystem during and after polar springtime. Several estimates of the size of the Arctic sink for newly deposited Hg range from 100 - 300 T/y, while estimates of the Antarctic sink are far more uncertain. The role of re-emission of elemental mercury from the snow surface is critically discussed.

Redox transformations of mercury in an Arctic snowpack at springtime

We investigated the springtime temporal dynamics of both total mercury (Hg) and gaseous Hg in snowpacks from the High Arctic. In situ incubation experiments of snow samples indicated that the production of volatile mercury in snow (VMS) was photo-mediated and occurred in the first 3 cm of snow. The newly produced VMS (consisting mainly of elemental Hg) was partly oxidized back to Hg(II) when light intensity declined or in the absence of UV radiation, probably through a chain of reactions involving photo-induced radicals and organic compounds in the surface snow. During a 2 week monitoring of total Hg in surface snow, we observed a sharp increase in total Hg concentrations, reaching levels 11 times higher than background concentrations, likely as a result of an atmospheric mercury depletion event. Stratigraphic depth profiles indicated that this increase was restricted to the first 2 cm of the snowpack. Total Hg levels subsequently decreased by 92%, reaching background concentrations within 2 days after this event. The photoproduction rate of VMS calculated on the basis of this episode could account for subsequent daily loss of total Hg from the surface of the snowpack.

Atmospheric particle evolution during a nighttime atmospheric mercury depletion event in sub-Arctic at Kuujjuarapik/Whapmagoostui, Québec, Canada

During a field experiment at Kuujjuarapik/Whapmagoostui (55.31°N, 77.75°W), Québec, we observed increases of concentrations of particles with diameters larger than 0.3 μm in the ambient air during a nighttime atmospheric mercury depletion event (AMDE). These increases were strongly correlated with decreases of ozone and atmospheric mercury, and we also observed a change in the particle size distribution during this AMDE. Assuming that these phenomena imply either a chemical link or an association through transport, we also studied the nature of this AMDE. We hypothesize that the observed AMDE was a result of an influx of already depleted air masses and that it was not a product of local chemical reactions.

Time series analysis of long-term data sets of atmospheric mercury concentrations.

Different aspects and techniques of time series analysis were used to investigate long-term data sets of atmospheric mercury in the Northern Hemisphere. Two perennial time series from different latitudes with different seasonal behaviour were chosen: first, Mace Head on the west coast of Ireland (53 degrees 20'N, 9 degrees 54'W), representing Northern Hemispherical background conditions in Europe with no indications for so-called atmospheric mercury depletion events (AMDEs); and second, Alert, Canada (82 degrees 28'N, 62 degrees 30'W), showing strong AMDEs during Arctic springtime. Possible trends were extracted and forecasts were performed by using seasonal decomposition procedures, autoregressive integrated moving average (ARIMA) methods and exponential smoothing (ES) techniques. The application of time series analysis to environmental data is shown in respect of atmospheric long-term data sets, and selected advantages are discussed. Both time series have not shown any statistically significant temporal trend in the gaseous elemental mercury (GEM) concentrations since 1995, representing low Northern Hemispherical background concentrations of 1.72+/-0.09 ng m(-3) (Mace Head) and 1.55+/-0.18 ng m(-3) (Alert), respectively. The annual forecasts for the GEM concentrations in 2001 at Alert by two different techniques were in good agreement with the measured concentrations for this year.

Experimental verification of Lamb waves propagation with a negative group velocity

Long-range atmospheric transport and deposition are important sources of mercury (Hg) to Arctic aquatic and terrestrial ecosystems. We review here recent progress made in the study of the transport, transformation, deposition and reemission of atmospheric Hg in the Canadian Arctic, focusing on field measurements (see Dastoor et al., this issue for a review of modeling studies on the same topics). Redox processes control the speciation of atmospheric Hg, and thus impart an important influence on Hg deposition, particularly during atmospheric mercury depletion events (AMDEs). Bromine radicals were identified as the primary oxidant of atmospheric Hg during AMDEs. Since the start of monitoring at Alert (NU) in 1995, the timing of peak AMDE occurrence has shifted to earlier times in the spring (from May to April) in recent years, and while AMDE frequency and GEM concentrations are correlated with local meteorological conditions, the reasons for this timing-shift are not understood. Mercury is subject to various post-depositional processes in snowpacks and a large portion of deposited oxidized Hg can be reemitted following photoreduction; how much Hg is deposited and reemitted depends on geographical location, meteorological, vegetative and sea-ice conditions, as well as snow chemistry. Halide anions in the snow can stabilize Hg, therefore it is expected that a smaller fraction of deposited Hg will be reemitted from coastal snowpacks. Atmospheric gaseous Hg concentrations have decreased in some parts of the Arctic (e.g., Alert) from 2000 to 2009 but at a rate that was less than that at lower latitudes. Despite numerous recent advances, a number of knowledge gaps remain, including uncertainties in the identification of oxidized Hg species in the air (and how this relates to dry vs. wet deposition), physical–chemical processes in air, snow and water—especially over sea ice—and the relationship between these processes and climate change.