New photoreduction process decreases presence of oxidized mercury in atmosphere

The Excited State Quantic Chemistry group (QCEXVAL) of the Institute of Molecular Science (ICMol) of the Scientific Park of the University of Valencia, is taking part in a project that has identified a new photolysis process of atmospheric mercury, which modifies how it is transported and deposited in terrestrial ecosystems.

An international study which includes the participation of the ICMol and the Rocasolano Institute of Physical Chemistry (IQRF) of the Spanish National Research Council (CSIC) has identified the photoreduction of oxidized mercury as the main way to decrease the presence of oxidized mercury in the atmosphere. The new solar photolysis process reduces oxidized mercury to elemental mercury, which leads to a noticeable increase of the time that this metal exists in our atmosphere, and thus, an increase in the distances it can reach from the locations of origin.

Mercury originating from industrial, environmental and mining activities accumulates for a long period of time in our atmosphere in the shape of gas comprised of atoms of elemental mercury. In this shape it can live for around a year. But once in the atmosphere, in the presence of very reactive molecules, it turns into oxidized mercury compounds, and these new compounds, which are very toxic and polluting, are more soluble with rainwater and are therefore once again deposited on the ground when it rains. As it can reach locations that are far away from where it appeared, it is considered a "global pollutant".

The work, titled "Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition", and which has been published in the journal Nature Communications, shows new phenomena to transform oxidized mercury into elemental mercury, which would create changes in its life cycle in the terrestrial and maritime ecosystems. The chemical reactions of oxidation and reduction in atmospheric mercury are crucial to understand the dispersion and deposition processes of this metal.

As Alfonso Saiz-López, researcher for the IQRF of the CSIC, explains, "the oxidized mercury compounds that are created in the atmosphere can also be destroyed in the presence of solar radiation (photolysis), creating elemental mercury once again and extending the presence of the metal in the air. This photolysis process had not been considered as an option to destroy this metal until now."

In order to reach these conclusions, the research team has used advanced theoretical chemistry methods, photolysis experiments in the lab and complex numerical modeling methods of atmospheric chemistry, which have helped determine how the new photochemical reactions affect the distribution of mercury in our planet.

According to Daniel Roca-Sanjuán, researcher of the QCEXVAL group at the ICMol and head of the computational modelling conducted in the work, "quantic chemistry currently allows us to, without having to conduct complex or expensive experiments, predict how solar radiation affects the pollutants that are present in the atmosphere and thus better understand its consequences on the environment."