Atmospheric Mercury Measurement
Mercury is a neurotoxin that can have health impacts when humans consume contaminated fish. Most mercury pollution is emitted to the atmosphere, not the aquatic environment. Once mercury is emitted, complex chemical reactions and transport processes in the atmosphere determine when and where it will return (deposit) to the Earth. After deposition, mercury can enter aquatic ecosystems, transform into methylmercury via bacterial processes, and accumulate in fish.
Unfortunately, many aspects of the mercury cycle are uncertain. We know that most emissions are of elemental mercury, which is relatively long-lived in the atmosphere, allowing it to be transported around the globe. We also know that elemental mercury can react with oxidants in the atmosphere, transforming into oxidized mercury compounds that deposit much more quickly and are more bioavailable (i.e., more readily converted to methylmercury, a critical step before the mercury can accumulate in the food chain).
The challenge is that we know neither the chemical reactions involved in the oxidation of elemental mercury, nor the chemical nature of the oxidized mercury compounds that are generated. Further, new evidence shows that past measurements of oxidized mercury are likely biased low. These gaps in scientific understanding of mercury limit our ability to predict how changes in emissions will affect mercury contamination in aquatic ecosystems.
New instruments are needed to accurately quantify and chemically identify atmospheric oxidized mercury. Successful instrument design will allow us to understand the chemical reactions involved in mercury oxidation and to improve predictions of oxidized mercury formation and deposition.
We are developing methods to quantify and identify oxidized mercury compounds via gas chromatography and mass spectrometry. Gas chromatography isolates and quantifies the constituent compounds of a sample, and mass spectrometry measures each compound’s molecular mass, allowing accurate identification. These exceptionally reactive compounds are not easy to keep in the gas phase, so we are creating an ultra-inert handling system to provide maximum sample transmission. In the ambient atmosphere, these compounds typically exist at concentrations of a few tens of parts-per-quadrillion or less (one molecule of oxidized mercury per one quadrillion molecules of air = one part-per-quadrillion), so capturing enough sample to overcome the instrument’s inherent noise level is another challenge.
We are also developing a field-deployable calibrator that will allow automatic verification of ambient oxidized mercury measurements. Currently, no calibration system is well accepted for oxidized mercury, and very few field calibrations have been performed. Our calibrator uses multiple permeation tubes to allow calibration with a variety of compounds, and it has a built-in pyrolyzer to convert oxidized mercury compounds to elemental mercury on the fly, so oxidized mercury permeation rates can be checked with an elemental mercury detector.