Mercury in our atmosphere

Mercury has fascinated humanity for centuries, for being a liquid metal that is slippery at room temperature, and for its resistance to reacting with its surroundings in the environment, which explains the characteristic shine that it retains for decades. For these reasons, our atmosphere contains traces of gaseous mercury, which come from natural sources (soil erosion, volcanic eruptions, etc.) and industrial processes (metallurgy, coal combustion, etc.).

This atmospheric metallic mercury is finally transformed through oxidation reactions that are not entirely well known, to finally deposit rapidly in seas and continents far removed from the areas where the original metal was emitted. The surface deposition process initiates new chemical reactions, in which very toxic products occasionally appear.

Until now, it has been considered that both the chemical reaction and the global transport of mercury occurred in the lower layer of our atmosphere in contact with the surface, the troposphere. However, the recent discovery by scientists shows that this is not the case, revealing that about a third of the mercury that enters the terrestrial ecosystem has reacted in the stratosphere (12 to 40 kilometers high), due to the unique characteristics of the stratospheric photochemical environment that accelerate mercury oxidation processes.

The study was carried out by an international team made up of, among others, experts from the Blas Cabrera Institute of Physical Chemistry (IQF), dependent on the Higher Council for Scientific Research (CSIC) in Spain.

Furthermore, the study shows for the first time that the circulation of air in this layer of the atmosphere functions as a global conveyor belt, in which large quantities of gaseous mercury are transported from intensely polluted areas to the most isolated and remote places, such as the polar regions. It also predicts that climate change will increase stratospheric transport of mercury.

Metallic mercury can’t stand the sun

This research explains that two factors intervene in the rapid oxidation of the resistant gaseous metallic mercury in the stratosphere. The first is the greater abundance at that high altitude of highly reactive chemical species, such as bromine, chlorine atoms and hydroxyl radicals, which intensely attack the metal.

The second factor “has an interesting relationship with the presence, around 30 or 35 kilometers high, of the well-known ozone layer, which protects the biosphere (and mercury) from lethal ultraviolet solar radiation,” explains Ulises Acuña, IQF emeritus researcher and co-author of the study.

When mercury particles rise above the ozone layer, they are suddenly unprotected and exposed to solar ultraviolet radiation. Atmospheric mercury absorbs this radiation and disappears quickly through photochemical processes to give way to soluble compounds that are easily deposited on the surface.

The Earth’s atmosphere above the clouds, seen from an airplane. (Photo: NASA)

Mercury’s long journey through stratospheric heights

Gaseous mercury reaches the highest layers of the atmosphere from the Earth’s surface through the movement of large air masses, mainly in the tropics. Although these physical mechanisms are remarkably complex, it is possible to simulate them in detail using climate models.

“When we include the described chemical reactions in the climate model, we obtain a numerical model of the so-called Geochemical Cycle of Mercury, which for the first time includes the new chemical and photochemical reactions of the metal that can take place in the stratosphere,” says Carlos Cuevas, researcher. of the IQF.

“The model has to go through several very demanding validation phases with real observations, so that the prediction results have a statistically reliable value,” explains Aryeh Feinberg, Marie Curie postdoctoral researcher on the IQF team.

On the other hand, Alfonso Saiz-López, head of the Department of Atmospheric Chemistry and Climate at the IQF, who leads this research, recalls that as a result of this study his research team has created for the first time a complete model of the mercury cycle on our planet. .

“This has allowed us to identify the complete process of transport and reaction to and from the stratosphere of gaseous mercury, which returns to the Earth’s surface in the form of soluble salts that are deposited in the most remote places, such as the Arctic and Antarctica, not not only today but also in the pre-industrial past and in a foreseeable future of altered climate.”

Saiz-López adds that “we finally have an interpretation of the difference observed in the atmospheric concentration of mercury between the northern and southern hemispheres of the planet, since until now it did not agree with the distribution of industrial emissions, which are much higher in the hemisphere.” north”.

John Plane, professor of atmospheric chemistry at the School of Chemistry at the University of Leeds in the United Kingdom, notes: “Mercury is a toxic pollutant with a substantial risk to human health. But it cannot enter the food chain unless that has been oxidized. The study shows that the oxidation process occurs in the stratosphere, before spreading globally, and then being deposited on the Earth’s surface. This means that the impact of localized emissions of mercury. It is noticeable globally: the main sources of mercury into the atmosphere (artisanal, small-scale mining and coal combustion) are damaging the health of ecosystems thousands of kilometers away.”

The study is titled “Role of the stratosphere in the global mercury cycle.” And it has been published in the academic journal Science Advances. (Source: CSIC)