Wildfires recycle mercury pollution back to atmosphere

Mercury pollution that has become stored in foliage and ground leaf litter is being released by wildfires and sent back into the atmosphere, say US scientists monitoring mercury emissions from the nation’s current outbreak of burning.

Scientists from the National Centre for Atmospheric Research (NCAR) and the University of Washington are conducting flights over forest fires in the Pacific Northwest in order to measure mercury emissions in their smoke in an effort to add to international efforts to understand the causes and effects of mercury pollution.

According to the researchers, gaseous mercury in the atmosphere travels the globe for about a year before oxidation creates ionic mercury which is deposited on land and water as it falls or is washed by rain out of the atmosphere, with about 6,500 tonnes airborne at any one time. Approximately half of the mercury is from natural sources, such as soil and volcanoes, with the rest originating from human activities, including an estimated 41 tonnes per year from US coal-fired plants. “Wet deposition is [the] most important,” said NCAR scientist Hans Friedli. “Mercury is picked up by the surfaces – the leaves or needles – and it stays there.” The pollutant is only removed when the foliage is burnt, he says.

The research included setting alight vegetation samples from forests across the United States under laboratory conditions, which revealed that between 94 and 99% of the mercury stored in organic matter is released by fire. All coniferous and deciduous samples contained 14-71 nanograms of mercury per gram of fuel, one nanogram being one trillionth of a gram.

When extrapolated to global biomass burning – from both wildfires and human induced land clearance – this form of mercury emission contributes 800 tonnes per year. However, air samples taken above a burn in Prince Albert National Park in Saskatchewan, Canada, last year, found mercury emissions higher than the laboratory results had revealed. “[This was] presumably because mercury in real fires is also emitted from heated soil, a source not yet considered in our experiments,” said Friedli.

Earlier this year a US Government study revealed that an estimated 375,000 new born babies each year are at risk of mercury-induced neurological problems, primarily due to their mother’s consumption of fish (see related story).

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