Mercury levels in the atmosphere have decreased throughout the 21st century

"By tracking mercury pollution over four decades at the top of the world, we show that global efforts to reduce pollution are working -- mercury levels in the air around Mount Everest have dropped significantly in the last two decades," explains Yindong Tong, the corresponding author on the study.

Humans contribute mercury to the air by burning fossil fuels, incinerating waste and mining. Environmental protections, such as the Minamata Convention on Mercury, aim to target those sources of pollution. The convention's effectiveness is judged partly on the amount of mercury in the atmosphere. Gaseous elemental mercury is also released from its largest natural reservoir, the soil, which might be accumulating even more mercury because of a changing climate. To distinguish between new, human-caused emissions and re-emissions of terrestrial mercury stored in the soil, researchers can look at patterns of mercury isotopes in the atmosphere. But regular atmospheric mercury isotope measurements have been collected for only about a decade. So, Tong, Ruoyu Sun and colleagues wanted to reconstruct information on past atmospheric mercury levels to understand how they've changed.

To look back in time, the researchers turned to the leaves of a tiny, low-to-the-ground perennial plant (Androsace tapete) growing at high elevations on Mount Everest. Much like the rings in a tree trunk, this plant grows a new layer of outer leaves every year, and they reflect what the plant's surrounding environment was like. So, by sampling older leaves from the center of two plants on Everest, the team gained a sense of atmospheric mercury levels as far back as 1982. They found that between 2000 and 2020, the total atmospheric concentration of elemental mercury decreased by 70%, with terrestrial mercury emissions making up a larger fraction of total emissions year over year. Currently, the soil emits significantly more mercury (62%) than human-related sources (28%).

Researchers attribute this overall decrease, based on patterns observed in mercury isotope data in plant leaves, to reduced human-caused mercury emissions led by efforts like the Minamata Convention. These observed trends match the reductions in atmospheric mercury seen from areas across the northern hemisphere that have been reported in previous studies. The researchers conclude that though recent efforts focused on human-related emissions appear to be successful, future efforts should be aimed at curbing re-emissions from soil.

The authors acknowledge funding from the Tianjin Natural Science Foundation and the National Natural Science Foundation of China.