New study reveals how mercury pollution reaches Earth's most remote regions

New findings reveal that the stratosphere is acting as a global conveyor belt, transporting atmospheric mercury from polluted areas to the planet's most isolated areas.

Mercury is toxic to human health, posing a particular threat to the development of the child in utero and early in life. Its toxic health impacts are numerous, including kidney and nervous system damage and skin problems. The World Health Organization considers it one of the top ten chemicals of major public health concern.

People are mainly exposed to mercury when they eat fish and shellfish that contain it. However, the element cannot enter the food chain unless it has been oxidised. Human activity is the main cause of mercury release, with primary sources including coal combustion and small-scale mining for mercury, gold, and other metals.

The results of a new study, “Role of the Stratosphere in the global mercury cycle,” led by Professor Alfonso Saiz-Lopez at the Spanish CSIC in Madrid, were published last week in Science Advances. According to the study, air circulation in the stratosphere works as a global conveyor belt in which large quantities of gaseous mercury are transported from intensely contaminated areas to the most isolated and remote places, such as the polar regions.

It found the layer of the Earth’s atmosphere, around 12 to 40 km above its surface, also functions as a reactor with atmospheric mercury absorbing ultraviolet radiation, oxidizing and producing soluble compounds quickly deposited on the Earth’s surface, in seas and continents far from the areas in which the original metal was emitted. From there, the mercury is absorbed into ecosystems. The team has created, for the first time, a complete geochemical model of the mercury cycle. The study also predicts that climate change will increase the stratospheric transport of mercury.

For this study, Professor John Plane from the School of Chemistry developed the chemistry network for mercury in the stratosphere. He determined the role of ultra-violet light once the mercury atoms ascend above the ozone layer, where they are no longer protected from UV radiation and, therefore, absorb it. His task required identifying new reactions and estimating their rates using quantum chemistry calculations.

Professor Plane said: “This study shows that much of the oxidation process is taking place in the stratosphere before oxidised mercury is spread globally and then deposited at the Earth’s surface, often in the most remote places such as the Arctic and Antarctica."