Forest fires as mercury-hurling

Fire clearings are more burdensome than anthropogenic emissions

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Mercury is extremely detrimental to health, but by no means merely a product of human activity. Nature, too, releases the toxin: scientists have now shown that forest fires and other biomass burns also release mercury, thereby contributing significantly to air pollution.

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Mercury is an environmental pollutant on the priority list of almost all international agreements and conventions. For example, in 2005, the United Nations Environment Program (UNEP) called for more research into the global mercury cycle. Researchers at the Mainz Max Planck Institute for Chemistry have now investigated the origin of mercury in the atmosphere in collaboration with colleagues from the Institute for Coastal Research at the GKSS Research Center Geesthacht.

On scheduled flights from Frankfurt to Santiago de Chile via São Paulo, the scientists used the flying air monitoring system CARIBIC on board a Lufthansa Airbus 340-600 for the first time to observe exhaust air plumes of large-scale forest fires and to detect increased mercury concentrations. They report in the current issue of the journal Geophysical Research Letters that the release of mercury by biomass combustion in the southern hemisphere in some cases significantly exceeds the previously known anthropogenic emissions.

From mercury vapor to contaminated fish

Mercury is predominantly in the atmosphere as steam. Since the gaseous element is very poorly water-soluble, it is only slowly washed out of the atmosphere. Therefore, the elemental mercury can be widely transported and deposited. Part of the mercury deposited in lakes and oceans is converted into the extremely toxic methylmercury, which accumulates in the food chain. In some species of fish, such as tuna and pike, concentrations of methylmercury quickly reach levels of harmfulness. In Canada and Scandinavia, thousands of lakes are so polluted that their fish are no longer suitable for human consumption. Even ocean fish like tuna should not be consumed frequently. display

A quarter of natural sources

Currently, around three-quarters of atmospheric mercury is emitted by human activities such as coal burning, waste incineration and smelting of ores. Only a quarter come from natural sources such as volcanoes. However, about five years ago, it has been observed that biomass burning, mainly forest and savanna fires, emits larger amounts of mercury. However, these measurements were limited to the mid-latitudes of the Northern Hemisphere, although about 90 percent of all forest fires and deforestation take place in the tropics. A statement about the worldwide importance of this source in relation to other emissions was thus not possible.

Strong emissions from slash-and-burn

With the sensitive analyzers in the CARIBIC measuring container, it was possible for the first time on the flights from Frankfurt to Santiago de Chile via São Paulo during the burning season in 2005 to measure significantly elevated mercury and carbon monoxide concentrations in huge exhaust air plumes. These flues were due to proven combustion products clearly from large-scale forest fires. Evaluation of satellite images and the calculated backward trajectories indicate fires in southern, central and eastern Brazil.

Analyzes of the many CARIBIC measurements and the mid-latitude data to date show that the global mercury emissions from biomass incineration from 210 to 750 tonnes per annum amounts to between three and eleven percent of all mercury emissions. These emissions vary from year to year and are highly seasonal. In the southern hemisphere, mercury emissions from biomass combustion exceed the total anthropogenic emissions in the combustion season (August to October). Combustion of biomass is thus an important source of the environmental pollutant mercury, which must be taken into account when modeling the atmospheric mercury balance.

(Max Planck Institute for Chemistry, 03.07.2007 - NPO)