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North American and European Atmospheric Mercury Declines Explained by Local and Regional Emission Reductions

Looking up at at power plant smoke stack
Burning coal for energy production contributes mercury to the atmosphere. Photo Credit: David P .Krabbenhoft, USGS

Recent findings from a consortium of university, State, and U.S. Geological Survey (USGS) scientists indicate that declining atmospheric concentrations of mercury (Hg) can be explained by the phaseout of mercury in many commercial products and by reduced emissions from utilities over the past two decades.

Mercury is released to the atmosphere by human activities, including coal combustion, mining, and manufacturing and disposal of commercial products. Mercury is transported globally as elemental mercury (Hg0) in the atmosphere, eventually oxidizing to divalent mercury (HgII), which is deposited rapidly. Current mercury emissions inventories used in atmospheric deposition models indicate flat or increasing mercury emissions over the last two decades.

A longstanding conundrum has been the apparent lack of a connection between flat or increasing trends in global mercury emissions based on the current inventories and the measured declines in atmospheric elemental mercury in North America and Europe of approximately 1 to 2 percent per year from 1990 to 2015. Decreasing reemission of mercury from oceans and soils has been proposed as the reason for this finding.

In this study, the current global atmospheric emission inventory is updated for 1990-2010 with information that can explain the observed large-scale decline in atmospheric mercury in North America and Europe over the past two decades. The updated emission inventories include information that (1) accounts for the decline in atmospheric release of mercury from commercial products, (2) eliminates biases in artisanal and small-scale gold mining emissions, and (3) accounts for the changes in emission rate and mercury speciation from coal-fired utilities after implementation of emission controls targeted at sulfur dioxide and nitrous oxides.

Using the updated inventory, the consortium of scientists identified a 20- to 30-percent decrease in global mercury emissions, but much larger decreases in North America and Europe that offset the effects of increasing emissions in Asia. The scientists utilized the updated inventory in a three-dimensional model that globally simulates atmospheric mercury (GEOS-Chem [Goddard Earth Observing System-Chemistry]), along with mercury estimates for ocean reservoirs. They found that the updated emissions trends more closely matched observed large-scale trends in atmospheric Hg0 concentrations and in HgII wet deposition than models using the current emissions inventory.

The findings of this study reinforce the major benefits that have been derived from the phaseout of mercury in many products and emission controls on coal combustion. This finding also indicates that that locally deposited mercury close to coal-fired utilities has declined more rapidly than previously anticipated. The large trends observed in North America and Europe reflect the phaseout of mercury from commercial products as well as the benefit from sulfur dioxide and nitrous oxide emission controls on coal-fired utilities.

Exposures to Anthropogenic mercury poses health risks to wildlife and humans when it is converted to methylmercury in ecosystems. Findings of the current study imply that there are regional benefits from declines in local mercury emissions from coal-fired utilities and reductions in the number of consumer products containing mercury.

This work was funded by the U.S. National Science Foundation, the USGS Toxic Substances Hydrology Program, and from the TomKat Charitable Trust via the Harvard John A. Paulson School of Engineering and Applied Sciences


Zhang, Y., Jacob, D.J., Horowitz, H.M., Chen, L., Amos, H.M., Krabbenhoft, D.P., Slemr, F., St. Louis, V.L., and Sunderland, E.M., 2016, Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions: Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1516312113 (Advanced Web release).

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