Monitoring mercury in the air

Mercury can enter Wisconsin's environment through atmospheric deposition, washed out of the air by rain or snow as "wet deposition" or deposited directly by particles as "dry deposition." Atmospheric deposition of mercury is a significant contributor to fish consumption advisories in our rivers and lakes.

There are three forms of mercury in the atmosphere: elemental mercury, oxidized mercury and particle-bound mercury. Both oxidized mercury and particle-bound mercury can be easily removed from the atmosphere while elemental mercury tends to stay in the atmosphere for a longer time (up to two years). Mercury is monitored by measuring its amount in collected rain or snow which is referred to as "deposition" monitoring, and measuring its concentration in air which is referred to as "ambient" monitoring.

Deposition monitoring

Since 1994, the DNR has monitored atmospheric deposition of mercury at a network of sites in Wisconsin. This work is conducted as part of the National Mercury Deposition Network [exit DNR]. The Wisconsin mercury deposition network currently consists of five monitoring stations - three operated by the department, one by the Forest County Potawatomi Community and one by the Lake Michigan Air Directors Consortium - designed to collect information on wet mercury deposition to the environment.

Ambient monitoring

More than 99 percent of the mercury in the ambient air is elemental mercury. Less than one percent of mercury in the ambient air is oxidized mercury and particle-bound mercury. The DNR has conducted several short-term studies in urban and rural areas around the state to better understand ambient mercury concentrations in Wisconsin. Many studies are intentionally short-term since the mercury monitors are equipped with mobile trailers. The reports from these studies are technical in nature and include mainly the monitored ambient air concentrations of mercury, monitor site locations, and meteorology during the monitoring events. The reports make no determinations regarding any public health or wildlife effects associated with the sources that were monitored for mercury. Currently, long-term ambient mercury concentration information is monitored and collected in Milwaukee and Horicon areas by DNR and in Forest County by Forest County Potawatomi Community.

The ambient concentrations of mercury monitored by DNR in these studies do not exceed the air regulatory threshold levels for mercury. The ambient air quality standard for mercury is 300 nanograms per cubic meter (or 0.3 micrograms per cubic meter) on an annual average basis, as established in ch. NR 446, Wis. Adm. Code [PDF exit DNR]. The ambient background concentration for mercury is about 1-2 nanograms per cubic meter, based on data collected throughout the state.

Interactive map - Ambient mercury concentration studies

The following interactive map shows information about both active and inactive ambient air mercury monitors.

• To view information on a site, click on a pinpoint. You may need to zoom in on the map to differentiate between pinpoints, especially in the southeast area of the state.
• Orange pins represent inactive monitor sites; green pins represent active monitors.
• Each pinpoint contains information about the monitor and links to special studies, where available.