Stories of the Finnish Arctic Expertise
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Plants clear mercury from air

Finnish Meteorological Institute has participated in a new study that shows that plant leaf uptake of mercury lowers global air pollution. The researchers found that the biological mercury pump seizes half of all global mercury emissions caused by humans.

Study led by researchers from the CNRS, the University Grenoble Alpes, found out that atmospheric CO2 levels fluctuate seasonally as vegetation takes up the gas through leaves to produce biomass. Consequently, CO2 levels are lower during summer compared to winter. By comparing mercury observations at 50 forested, marine, and urban monitoring stations, the study published in Nature Geoscience finds that vegetation uptake of mercury is important at the global scale. The researchers estimate that the biological mercury pump annually sequesters half of all global anthropogenic mercury emissions.

Each year industrial activities emit between two and three thousand metric tons of mercury into the atmosphere. With a long atmospheric lifetime of about six months, mercury emissions spread across the globe. What goes up must ultimately come down and this applies to mercury as well. It has long been thought that atmospheric mercury deposition is predominantly by rainfall and snowfall, and monitoring networks measure mercury wet deposition worldwide.

A slowly increasing number of experimental, field and modeling studies has suggested that plant leaves can also directly take up gaseous elemental mercury from the atmosphere. In autumn, the falling leaves transfer leaf mercury to the underlying soil system. Yet, the importance of this alternative deposition pathway, at the global scale, has never been fully appreciated.

To understand if leaf uptake of atmospheric mercury is important on the global scale, Martin Jiskra and Jeroen Sonke, from the Géosciences Environnement Toulouse laboratory, teamed up with scientists who monitor atmospheric mercury and CO2 levels across our planet. CO2 has a well-known seasonality with concentration minima in late summer, at the end of the vegetation and leaf growth season, and higher levels during winter.

"To their surprise, the researchers found that mercury and CO2 show similar seasonal variations at five forested monitoring stations in the Northern hemisphere including Pallas", says researcher Katriina Kyllönen from FMI.

Observations of mercury and CO2 made at Amsterdam Island turned out to be key in identifying the role of vegetation. At the Amsterdam Island station, operated by the French polar institute Paul Emile Victor and surrounded by 3000km of Ocean in all directions, both mercury and CO2 show near-zero seasonal variations.

Next, the researchers turned to atmospheric monitoring databases examining seasonal mercury observations for another 43 sites globally, but for which CO2 observations were lacking. They found that the amplitude of seasonal atmospheric mercury variations is largest at inland monitoring sites away from the coast. At all of the terrestrial sites, they found strong inverse correlations between satellite-observed photosynthetic activity and mercury concentrations. At urban monitoring stations, the correlations were absent, and mercury seasonality was controlled by local anthropogenic mercury emissions.

The researchers conclude that vegetation acts as a biological pump for atmospheric mercury and plays a dominant role in the observed atmospheric mercury seasonality. By comparing the 20% amplitude of seasonal mercury variations to the known amount of mercury in the atmosphere (~5000 metric tons), they estimate that each year about 1000 tons of mercury is sequestrated in vegetation via leaf uptake. This amount is equal to half the annual global anthropogenic mercury emissions. They also suggest that the documented 30% increase in global primary productivity over the 20th century has likely enhanced uptake of atmospheric mercury, thereby practically offsetting increasing mercury emissions. Although leaf uptake removes mercury from air, autumn litterfall transfers the sequestered mercury to soils. Soil mercury ultimately runs off into aquatic ecosystems including lakes and Oceans where the mercury bioaccumulates to toxic levels in fish.

Observations at Pallas are part of the international measurements used in the study.

The Pallas research supersite consist of versatile research infrastructure for modern and versatile monitoring of the environment. Located 170 km north of the Arctic Circle, partly in the area of Pallas-Yllästunturi National Park, the site has various research stations and measurement sites established by the FMI. The main research themes include greenhouse gas concentrations and ecosystem–atmosphere fluxes, the climate effects of atmospheric aerosols, aerosol–cloud interactions and air quality.

Pallas is one of the sites of the Finnish network for monitoring the concentrations of mercury and other heavy metals, benzo(a)pyrene, ozone and other air pollutants, as required by the European legislation on ambient air quality. Mercury measurements have been conducted since 1996 in co-operation with Swedish Environmental Research Institute IVL. Pallas also serves as a platform for scientific collaboration with international as well as national research institutes (LUKE, SYKE and GTK).

Photo: Snow covers research equipment at the Pallas research site. © Joanna Saarinen

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