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Why Measure Ultra-Low Levels of Mercury?
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Why Measure Ultra-Low Levels Of Mercury?
Despite the relatively large natural abundance of mercury (0.5 ppm), most of the mercury in the biosphere is believed to be anthropogenic, ie: to originate from human activity. Evidence such as ice and sediment core samples indicate that mercury levels in the environment have increased markedly over the last century.
Mercury from anthropogenic sources is released and dispersed throughout the atmosphere. Elemental mercury has a long atmospheric lifetime, estimated to be on the order of one year. As a result, background levels throughout the Northern Hemisphere are in the range of 1 to 2 ng/m3 (125 to 250 ppq. NB: One part per quadrillion is 1:10-15).
Bio-accumulation has been seen in many ecosystems throughout the world. In particular, fresh water fish in remote lakes routinely have mercury concentrations of one to ten million times higher than the mercury present in the atmosphere. Atmospheric wet and dry deposition are believed to be the main input pathways for mercury into these lakes. The levels in fish are high enough to be toxic when eaten by humans and hence many regions in Ontario, Michigan, Florida, Sweden, etc. have restrictions on allowable fish consumption. The direct health effects on human populations of exposure to low level atmospheric levels are unknown, partly due to the difficulties in establishing a control group with no exposure.
The ability of mercury to accumulate has resulted in a need to measure extremely low levels in a convenient and accurate manner. In ambient air, levels of less than 1 ng/m3 (equivalent to 125 parts per quadrillion by volume (125 x 10-15)) are significant.
The issue of mercury contamination in fish is global in scope and high levels are found in many, if not the majority of locations where testing is performed. Note that most jurisdictions where mercury contamination is not considered a problem have not yet looked for it in their ecosystems.
Species of Mercury in the Atmosphere
In the mid 1990's, researchers discovered that very high levels of mercury contamination in ecosystems could not always be traced to local geological abundances or to high levels of total mercury in the air. It was eventually determined that the form of the mercury was important in determining ecological impacts.
A surprising observation in the Arctic (made using Tekran equipment) revealed that during polar sunrise, virtually all of the elemental mercury in the atmosphere can be rapidly converted into other forms. This phenomenon has been observed in arctic and subarctic regions around the world including south polar regions. This has profound impacts on the global cycling of mercury and explains why arctic life forms have long been found to contain high mercury levels, even in the absence of local sources.
In most cases, virtually all of the mercury in the atmosphere is present in elemental form. However, a few percent may be present in the form of gaseous mercury chloride or other halogenated forms. These reactive gaseous mercury (RGM) species are water soluble and thus deposit into the ecosystem very quickly. Some of the mercury may also be present and bound up on particulates. Particulate bound mercury (HgP) is also quick to deposit.
It has become important to differentiate among these forms of mercury in ambient air. RGM and HgP can be ecologically significant even when present at concentrations in the low pg/m3 range. (A picogram is 1/1000 of a nanogram.)
It is also important to differentiate between the forms when measuring emissions from point sources such as power plants and incinerators.
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