Scientists decipher the summer Arctic atmospheric mercury peak

Recently, professors Xie Zhouqing and Dr. Lefange of the University of Science and Technology of China cooperated with domestic and foreign scholars to carry out biogeochemical cycle research on mercury during the implementation of the International Arctic Climate Research Multidisciplinary Drifting Observation Program (MOSAiC), and found that the marginal ice area (that is, the transition area between the sea area completely covered by sea ice and the open sea) is an important source of atmospheric mercury, and proposed the mechanism of the peak phenomenon of atmospheric mercury in the Arctic in summer. The research results were recently published in Nature Communications.

Lefange debugs observation instruments on the German research ship Polar Star Photo courtesy of China University of Science and Technology

Mercury pollution prevention is urgent

Mercury is a highly toxic liquid metal with high volatility at room temperature and pressure. In the atmosphere, mercury exists mainly in three forms: gaseous zero-valent mercury, gaseous divalent mercury and granular bound divalent mercury. Among them, gaseous zero-valent mercury accounts for more than 90% of the total atmospheric mercury, so the content of gaseous zero-valent mercury is close to the total atmospheric mercury content. Moreover, gaseous zero-valent mercury stays in the atmosphere for a long time, about 0.5 to 1 year, and can be transported over long distances with the atmospheric circulation, becoming a global pollutant. Mercury entering the water body through atmospheric transport and sedimentation will enter the aquatic food chain after microbial methylation, and endanger the water ecological environment through enrichment and amplification.

Ingestion of aquatic products contaminated with mercury will affect human health, cause toxic effects on the gastrointestinal system, nervous system and even the fetus, resulting in hearing and sensory disorders, numbness, language disorders and even intellectual disability, cerebral palsy, blindness and so on in newborn babies.

In response to the severity of global mercury pollution, the United Nations Environment Programme adopted a legally binding international convention, the Minamata Convention on Mercury. This makes mercury pollution prevention and control not only a research hotspot in the field of the environment, but also a major demand for global environmental governance.

The Arctic plays an important role in the northern hemisphere mercury cycle and is sensitive to environmental mercury exposure. The latest Arctic Mercury Assessment Report shows that Arctic organisms face high levels of mercury exposure, and people living in the Arctic have one of the highest levels of mercury exposure in the world.

Scientists have proposed that the transport and transformation of mercury in the Arctic is related to its unique natural environment. Canadian scholars published a paper in Nature in 1998, saying that the study found that gaseous zero-valent mercury in the Arctic has a unique seasonal variation phenomenon of spring concentration loss and summer concentration peak, and its average summer concentration level exceeds the northern hemisphere background concentration. However, the sources and mechanisms of peak gaseous zero-valent mercury concentrations in summer remain controversial.

Map of local sea ice distribution in the Arctic Ocean Courtesy of China University of Science and Technology

Marginal ice is the dominant factor

From autumn 2019 to autumn 2020, Xie Zhouqing and other collaborators installed the Tekran 2537 atmospheric trace mercury on-line analyzer on the German research ship Polestar during the international Arctic climate research multidisciplinary drifting observation program, taking the lead in conducting one-year on-line atmospheric mercury observation in the Arctic Ocean waters. They observed gaseous zero-valent mercury concentrations showing summer peaks similar to those studied by previous studies.

Therefore, they further constructed a generalized additive model, established the relationship between gaseous zero-valent mercury concentration and anthropogenic emissions, regional transport and ocean release, and evaluated the contribution of these impact factors to the change of gaseous zero-valent mercury concentration.

The results show that more than 63% of the change in gaseous zero-valent mercury can be modelled. Of this, anthropogenic and land-based emissions contribute no more than 2%. Ocean emissions, which contribute nearly 52 per cent, are the dominant factor influencing changes in gaseous zero-valent mercury in the Arctic Ocean during summer.

Further, the analysis of potential source area contributions found that summer mercury emissions occurred mainly in marginal ice areas. It is estimated that mercury emission fluxes in this region are more than twice as high as in open waters.

Finally, the research team synthesized multidisciplinary observational data analysis and proposed the mechanism that drives the mercury release process in the marginal ice area: in spring, due to the photochemical reaction at the sea-ice-air interface, a large amount of gaseous zero-valent mercury is oxidized to divalent mercury and settles on the surface of the sea ice, that is, the concentration of gaseous zero-valent mercury is lost in spring. In summer, this divalent mercury accumulated on sea ice melts into the seawater, resulting in higher levels of divalent mercury in seawater in marginal ice areas.

Due to the high amount of phytoplankton in the marginal ice area, under the conditions of summer sea ice melting and enhanced sunlight entering seawater, the photosynthesis of phytoplankton is promoted, and a large amount of organic matter is produced, which in turn promotes the reduction of divalent mercury in water to zero-valent mercury, which is more volatile into the atmosphere. Combined with the melting of sea ice in marginal ice areas, the barrier of release of zero-valent mercury from seawater to the atmosphere has been reduced, making marginal ice areas an important source of gaseous zero-valent mercury in the Arctic atmosphere in summer, resulting in peak concentrations of gaseous zero-valent mercury in the Arctic Ocean in summer.

China’s icebreaker Xuelong investigates in the Arctic Ocean Courtesy of China University of Science and Technology

More mercury emissions from marginal ice are predicted to be in the future

Overall, the study reveals the impact of sea ice changes in the Arctic Ocean on mercury cycling processes, providing a scientific basis for properly assessing the risk of mercury exposure to polar organisms and populations.

In the context of global warming, the Arctic is one of the fastest warming regions in the world. So, what will happen to mercury emissions in marginal ice areas in the future? The research team further predicted this change scenario.

In the future, sea ice will melt faster as temperatures rise, and sea ice that has not melted all year round will gradually be replaced by seasonally melting sea ice. This condition promotes photochemical reactions at the sea-ice-air interface, leading to more gaseous zero-valent mercury deposition and increasing the divalent mercury input in marginal ice regions. On the other hand, further melting of sea ice promotes surface illumination and phytoplankton in the region, which facilitates the reduction of divalent mercury in seawater, resulting in more zero-valent mercury releases into the atmosphere. As the central Arctic Ocean margin ice area will be further expanded in the future, this area should be noted as a source area of mercury.

Xie Zhouqing said that in the future, the research team will further combine mercury observations and model simulation studies to quantitatively assess the contribution of mercury releases from marginal ice areas to the Arctic atmosphere, as well as changes in its contribution under future climate scenarios. (Source: Wang Min, China Science News)

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