Southeastern states in the US, such as Florida and Louisiana, receive higher deposits of mercury from the atmosphere compared to the rest of the North American continent. They also have the stormiest weather. Scientists at the University of Florida do not think this is a coincidence.
‘Tall thunderstorms are bringing mercury down to Earth,’ says atmospheric scientist Christopher Holmes. In a recent study, Holmes found thunderheads (the cumulonimbus clouds seen during a storm) to be carrying 50 per cent higher levels of the harmful pollutant than other cloud types, even when the rainfall itself was of equal amounts. ‘The highest concentrations of mercury occurred during thunderstorms,’ he says, ‘and the lowest during a regular rainstorm.’
“It seems the storms scour the mercury from the higher atmosphere and deposit it on the ground”
The link could be the thunderclouds’ height. Most mercury in the atmosphere sits at high altitudes, in the upper troposphere. While normal rain clouds are just a few kilometres thick, storms can form towering clouds that plume 15 kilometres in the air. ‘It seems the storms scour the mercury from the higher atmosphere and deposit it on the ground,’ says Holmes. The pollutant’s affinity for rough weather would explain why mercury levels are high in the southeast – often double those of northern states – despite the fact that Florida and Louisiana emit less mercury than many northern regions.
Mercury is a rogue element. Often emitted as an unwanted by-product of fossil burning and small-scale gold mining, it can remain in the upper atmosphere for decades, drifting thousands of miles from its original source. When it comes back down, it can accumulate in the food chain as methylmercury, which is highly toxic to animals and humans. In fact, the consumption of many fish species in southeastern states are banned due to excessive contamination.
This was published in the November 2016 edition of Geographical magazine.