No direct evidence is yet available for one of the most dangerous pollutants found in the upper air. This article authored primarily by Seth Lyman and Daniel A. Jaffe takes the best data from flight samples in order to illustrate the 2010 situation for oxidised mercury. Since the days of Lewis Carroll's mercury-poisoned hatters, we have struggled to restrict the organic mercury in our environment. We pump thousands of tons of volatile elemental mercury (from combustion processes) into our atmosphere every year.
Now oxidised mercury seems to be raining down on us, especially in certain parts of America and near the tropics. Recently, the Dead Sea was found to have one of the highest oxidised mercury levels outside the polar regions. The upper troposphere and the lower stratosphere present an ideal reaction site for the oxidation of mercury that finds its way there on microscopic particles. "The upper atmosphere is acting as a chemical reactor to make the mercury more able to be deposited to ecosystems," said Seth, who did the work as a research assistant professor in science and technology at the University of Washington Bothell.
It's possible that the ozone there could more easily oxidise the metal in a reaction. The model developed by him shows a rapid loss of elemental mercury from the stratosphere. The authors then suggest sedimentation of the mercury, along with the entrainment processes that occur by convection, causing the stratosphere to dump the HgO (oxidised mercury) in certain regions such as the SW United States. These pollutant areas have the climatic conditions to receive the oxidised mercury which will have originated thousands of miles away before its ascent and descent as described.
Chinese power stations burn a lot of the world's fossil fuels. This one is coal-powered, producing mercuric compounds The US produces around 48 tons of mercury from coal every year via Shutterstock
Coal burning areas throughout the world are key to this elemental mercury production, but we don't yet understand how to predict where the oxidised mercury will be deposited, usually in aquatic systems. Circling the earth several times, water facilitates its transfer into ecosystems where it could quickly enter our fish or shellfish food chains. Human levels using blood or hair samples and levels in other key species, such as fish, would need to be measured in order to find out where these depositions are.