Mercury is a naturally occurring substance that is present in very small quantities in all environmental media. Mercury is mobilized into air and water through natural processes such as volcanoes and forest fires and through human industrial activities. Evidence indicates that human activities have led to an increase in mercury mobilization following the onset of the industrial revolution. Mercury emitted to the air may enter directly into a global atmospheric pool of elemental mercury vapor or may deposit locally as particulate matter or reactive gaseous mercury. When mercury is deposited into waterways it may undergo a complicated cycle of biotransformation, bioconcentration and bioaccumulation. The magnitude of these phenomena are highly dependent on local ecological conditions as well as the rate of mercury delivery to waterbodies. Ultimately, atmospheric mercury deposition may give rise to elevated levels of methylmercury in fish, which in turn can lead to elevated levels in those organisms that ingest fish - including human beings. For these reasons, mercury has been highlighted as an area of concern by the Federal Environmental Protection Agency as well as many states across the U.S.

Atmospheric mercury is ubiquitous and exists in a variety of chemical and physical forms as a result of both natural and man-made releases. Elemental mercury vapor (Hg⁰) is present globally in ambient air at concentrations on the order of 1.5 -2.0 ng/m³. Hg⁰ is the least water-reactive atmospheric species of mercury, which allows it to persist in the ambient air and travel over long distances. It is currently thought that emissions of this form of mercury may contribute to this world-wide atmospheric pool of mercury, making it a truly global concern. Evidence indicates that the concentration of this form of mercury in the atmosphere has likely increased over the past century as a result of anthropogenic activities.

Mercury may also exist in ambient air in particulate or reactive gaseous (RGM) forms. Less is known about typical ambient levels of these forms of mercury, but these species are especially important because of their ability to deposit onto land and waterways. By mechanisms including adsorption into cloud droplets or precipitation "scrubbing," RGM and particulate mercury can be added to sensitive waterways during precipitation events. During dry conditions, it is believed that these species may also be able to "dry deposit" onto plants, land or other structures. While considerable uncertainty persists, it is generally believed that emissions of RGM or particulate mercury are more capable of having local or regional scale impacts on inputs of mercury to land and water.

The North Carolina Division of Air Quality (DAQ) has been actively monitoring atmospheric trends for mercury since 1996 with two wet deposition monitoring sites and several elemental mercury vapor monitors. The DAQ collects wet deposition data from two sites in eastern North Carolina: Pettigrew State Park at Phelps Lake in Washington County and Waccamaw State Park in Columbus County. Both sites are rural locations characterized by elevated levels of methylmercury in fish, but they differ somewhat in their relative proximity to significant known anthropogenic emissions sources of mercury. Wet deposition rates have been measured in rainfall at both sites, on a weekly basis, since 1996. These data may be used by the Division of Water Quality and other scientific or regulatory agencies to determine the relationship between atmospheric mercury and the inputs to sensitive waterways in the eastern part of the State. [Mercury Deposition Data]

Ambient elemental mercury vapor data has been collected on a semi-continuous basis since 1996 at the Waccamaw State Park location, and is available for a shorter time period during 1996 for Phelps Lake. By using co-located meteorological data, the DAQ has been able to establish a relationship between wind direction and elevated mercury readings at the Waccamaw State Park sampling site. Additional mercury vapor sampling efforts are currently underway in Riegelwood, NC area. The intent of these efforts is to more fully characterize trends in ambient air mercury levels in this sensitive area. In addition to a complement of two elemental mercury vapor monitors the Toxics Protection Branch will be introducing a cutting-edge technique for monitoring RGM species in the air using a Tekran Model 1130 Mercury Speciation Unit. It is hoped that during the course of this study we will be able to detect an improvement in trends for ambient air mercury resulting from a significant reduction in mercury use by local emissions sources. We also hope to continue simultaneous monitoring for elemental and RGM to determine the relationship between ambient air trends for these species in this area of the State. 

Please contact Todd Crawford with the DAQ's Toxics Protection Branch at 733-7199 for any additional information.

Report: Long-Term Atmospheric Mercury Trends for Eastern N.C. [PDF]  [Text]

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