A recent study by the Blas Cabrera Institute of Physical Chemistry (IQF-CSIC) has unveiled a previously unexplored role of the stratosphere in the transport and deposition of mercury from high-emission areas in the Northern Hemisphere to remote locations, including the poles.
Published in Science Advances, the research indicates that a significant portion of mercury entering terrestrial ecosystems is chemically activated and transported globally within the stratosphere. This finding has substantial implications for ecosystem exposure to mercury pollution, affecting both human health and wildlife.
Mercury, a liquid metal at room temperature, exists in trace gaseous forms in the atmosphere, originating from natural sources such as soil erosion and volcanic eruptions, as well as industrial processes. Atmospheric metallic mercury undergoes oxidation reactions, not fully understood, leading to its rapid deposition in distant seas and continents.
Contrary to previous beliefs that mercury reactions and transport occurred solely in the troposphere, the study reveals that nearly one-third of the mercury entering terrestrial ecosystems reacts in the stratosphere, between 12 to 40 kilometers above the surface. The unique photochemical environment of this layer accelerates the oxidation processes of mercury.
The stratospheric air circulation acts as a “global conveyor belt,” transporting large quantities of gaseous mercury from heavily polluted regions to isolated areas, including polar regions. The study also forecasts that climate change will enhance stratospheric mercury transport.
For the first time, the research has established a comprehensive model of the mercury cycle on Earth. This model identifies the complete process of mercury gas transport and reaction to and from the stratosphere, which returns to the Earth's surface as soluble salts deposited in remote locations like the Arctic and Antarctica.
According to Alfonso Sáiz-López, the study leader, this research provides an explanation for the observed differences in atmospheric mercury concentrations between the Northern and Southern Hemispheres, which previously did not align with industrial emission distributions.
Gaseous mercury reaches the upper atmosphere from the Earth's surface through complex physical mechanisms, primarily in the tropics, which can be detailed through climate models. Including the described chemical reactions in climate models results in a numerical representation of the geochemical mercury cycle, now incorporating new chemical and photochemical reactions that may occur in the stratosphere.