Plants and their leaves are believed to play a key role in intercepting human-made emissions of harmful mercury – according to a new study in the journal Nature Geoscience, in which scientists from the Helmholtz Center Geesthacht (HZG) are involved.
Each year industrial activities emit between two and three thousand metric tons of mercury into the atmosphere. With a long atmospheric lifetime of about 6 months, mercury emissions spread across the globe. What goes up must ultimately come down and this also applies to mercury. 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 fall, leaf mercury is then transferred to the underlying soil system by leaf senescence. Yet, the importance of this alternative deposition pathway, at the global scale, has never been fully appreciated.
„We estimate that vegetation can temporarily store half of all anthropogenic global mercury emissions annually,“ says co-author Dr Ralf Ebinghaus, environmental chemist at the HZG. Dr Johannes Bieser, environmental scientist at the HZG and also co-author adds: „This publication has now brought a process into focus that was previously underestimated.“ (source: HZG news)
Jiskra, M., Sonke, J.E., Obrist, D., Bieser, J., Ebinghaus, R., Lund Myhre, C., Aspmo Pfaffhuber, K., Wängberg, I., Kyllönen, K., Worthy, D., Martin, L.G., Labuschagne, C., Mkololo, T., Ramonet, M., Magand, O., & Dommergue, A. (2018): A vegetation control on seasonal variations in global atmospheric mercury concentrations. Nature Geoscience, Volume 11, pages 244–250 (2018), doi:10.1038/s41561-018-0078-8
Anthropogenic mercury emissions are transported through the atmosphere as gaseous elemental mercury (Hg(0)) before they are deposited to Earth’s surface. Strong seasonality in atmospheric Hg(0) concentrations in the Northern Hemisphere has been explained by two factors: anthropogenic Hg(0) emissions are thought to peak in winter due to higher energy consumption, and atmospheric oxidation rates of Hg(0) are faster in summer. Oxidation-driven Hg(0) seasonality should be equally pronounced in the Southern Hemisphere, which is inconsistent with observations of constant year-round Hg(0) levels. Here, we assess the role of Hg(0) uptake by vegetation as an alternative mechanism for driving Hg(0) seasonality. We find that at terrestrial sites in the Northern Hemisphere, Hg(0) co-varies with CO2, which is known to exhibit a minimum in summer when CO2 is assimilated by vegetation. The amplitude of seasonal oscillations in the atmospheric Hg(0) concentration increases with latitude and is larger at inland terrestrial sites than coastal sites. Using satellite data, we find that the photosynthetic activity of vegetation correlates with Hg(0) levels at individual sites and across continents. We suggest that terrestrial vegetation acts as a global Hg(0) pump, which can contribute to seasonal variations of atmospheric Hg(0), and that decreasing Hg(0) levels in the Northern Hemisphere over the past 20 years can be partly attributed to increased terrestrial net primary production.