Mercury Deposition Network
The MDN is the only network providing a longterm record of total mercury (Hg) concentration
and deposition in precipitation in the United States and Canada. All MDN sites
follow standard procedures and have uniform precipitation chemistry collectors and gages. The
automated collector has the same basic design as the NTN collector but is modified to preserve
mercury. Modifications include a glass funnel, connecting tube, bottle for collecting samples, and
an insulated enclosure to house this sampling train. The funnel and connecting tube reduce
sample exposure to the open atmosphere and limit loss of dissolved mercury. As an additional
sample preservation measure, the collection bottle is charged with 20 mL of a one percent
hydrochloric acid solution.
Site operators collect samples Tuesday morning or daily within 24 hours of the start of
precipitation. With each MDN sample, the entire sampling train is replaced with one that is
cleaned by the Mercury Analytical Laboratory (HAL) at Frontier Global Sciences, Inc., Seattle,
Washington. Rigorous cleaning ensures that each sampling train component is essentially
mercury-free. The HAL supplies the collection bottles already charged with the
hydrochloric-acid preservative. By following those procedures and stringent
sampling protocols, the MDN is able to report mercury concentrations below 1 part per trillion
(<1 nanogram/liter).
All MDN samples are sent to the HAL, which analyzes all forms of mercury in a single
measurement and reports this as total mercury concentrations. At the end of 2005, 23 MDN sites
also opted for methyl mercury analysis. The HAL reviews field and laboratory data for completeness
and accuracy, and flags samples that were mishandled, compromised by precipitation
collector failures, or grossly contaminated. The HAL delivers all data and information to the NADP
Program Office for final checks and resolution of remaining discrepancies. Data then are made
available on the NADP Web site.
