Mercury emissions are falling

High up on the rocks and dry earth of Everest, a small plant is helping researchers monitor atmospheric mercury levels. About a decade ago, Ruoyu Sun from the Institute of Surface-Earth System Science, Tianjin University, started studying the concentration of mercury in that plant, Androsace tapete, which adds a layer of tiny leaves on top of the last every year. “We found that the mercury concentration in the first 10 years of this century declined,” Sun says. “But we did not know why.”

A. tapete traps signatures of mercury in its leaves linearly with the concentration of mercury in the air, so the team knew there must be less mercury in the air, but why? Around 2018, advancements in technology allowed Sun and his team to begin studying isotopes of mercury in the leaves. The researchers say their measurements show that anthropogenic mercury emissions have declined but that the proportion of legacy mercury—mercury produced in the past that has settled into soils and oceans and reemitted into the atmosphere over time—has increased (ACS ES&T Air 2025, DOI: 10.1021/acsestair.4c00296).

The researchers dissected the growth layers and analyzed two mercury isotope concentrations per year from 1982 to 2020. ¹⁹⁹Hg represents mercury from human activities such as coal combustion and artisanal gold mining; ²⁰²Hg is associated with legacy mercury.

The team found that overall mercury levels increased until 2002 and then began to fall as regulations to reduce emissions began to have an effect in the environment. During that time, the proportion of ¹⁹⁹Hg production fell, thus driving the decline in overall levels.

“One of the really unique parts of the study is that it looked at natural archives,” says Noelle Eckley Selin, an atmospheric chemist and associate professor at the Massachusetts Institute of Technology. Selin and her team published a similar finding last year using atmospheric data from 51 observatories across the northern hemisphere. But their timeline data were limited to between 2005 and 2020.

“The current levels of mercury are way, way more than the preindustrial levels probably by a factor of 5–10,” Selin says. “And human activities have mobilized a great deal of mercury. The relevant question is what's been happening in the last 10–15 years.”

Both Sun and Selin give credit to international government ambitions like the Minamata Convention on Mercury, which was signed in 2013 and came into force in 2017 for reducing emissions of the element. According to Sun and Selin, countries began implementing measures to reduce mercury concentration from the sources even before the convention came into force.

Both studies focus on the northern hemisphere, which has been the historical source of mercury production. “The southern hemisphere story is a little bit different,” Selin says, “because there aren't as many measurement stations in the southern hemisphere and there’s different things going in that region.”

Even though both papers report that mercury emissions from anthropogenic sources have declined, the researchers are concerned about the impact of climate change on legacy mercury trapped in the soil. Sun’s data from Mount Everest show that the percentage of mercury from legacy reemissions increased from ~44% in 2010 to ~62% in 2020—meaning that legacy sources now dominate atmospheric mercury at the site. “The temperature in the Tibet plateau is accelerating by two or three times compared to the global average. If the air temperature increases, the legacy mercury emission from the soil will increase,” Sun says.

Mercury in its gaseous form is not harmful to humans, but when it mixes with water it can reach us through bioaccumulation from food and impact human health by damaging the brain and nervous system. That’s why reducing atmospheric mercury levels is so important, the researchers say. “We've seen some attempts to roll back the mercury and air toxic standards, which is not the right direction,” Selin says.