Continuous emission monitors for mercury would be more desirable. In direct measurements performed at a coal-fired power plant, Prestbo observed that mercury in the exhaust plume was dominated by its elemental form, which is not likely to be locally deposited from the atmosphere. It does not matter then whether an emission source is in Ohio or China. To further characterize the nature of mercury deposition and its connection to emissions sources, the transport of mercury once it exits the stack is being studied. There is controversy about whether mercury emissions from sources such as power plants have a local impact.
Few data are available about mercury concentrations in the vicinity of emissions point sources.
Christian Seigneur, a scientist with Atmospheric and Environmental Research, Inc., in San Ramon, Calif., thinks that mercury is more a global or regional problem than one of local concern. Some field measurements appear to contradict this conclusion. The effect of local or regional measures to limit mercury emissions could be minor if deposition is dominated by global atmospheric source contributions. In the United States, a program is under way to analyze regional deposition. The Mercury Deposition Network, a nationwide network for mercury sampling in precipitation, currently consists of about three dozen monitoring sites and is a cooperative effort among several groups, including state agricultural experiment stations, the U.S. Geological Survey, and the U.S. Department of Agriculture.