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

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.

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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 research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology), the U.S. National Science Foundation, and from the TomKat Charitable Trust via the Harvard John A. Paulson School of Engineering and Applied Sciences.

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Frequently Asked Questions

1. Why is mercury a concern? Why is mercury important?

Mercury poses risks to both wildlife and humans when they are exposed to methylmercury in ecosystems. Methylation—the conversion of elemental mercury to organic methylmercury—is the most important step in the mercury cycle because it greatly increases toxicity and potential for accumulation in aquatic biota. As methylmercury is formed in an ecosystem, some portion of it is taken up or bioaccumulated by organisms such as in algae. Animals higher up in the food web accumulate mercury from their food. Methylmercury in aquatic food webs increases at every trophic level (biomagnifies), reaching highest levels at the top of the food web such as predatory fish (fish that eat mostly other fish). Methylmercury levels in predatory fish are typically more than one million times higher than methylmercury levels in water that the fish inhabit.

2. Is atmospheric mercury natural?

Although there has always been some mercury in the atmosphere from natural sources (volcanoes and degassing of elemental mercury from the oceans), human activities have increased the amount of mercury emitted to, and deposited from the atmosphere. Anthropogenic (human-caused) sources of mercury to the atmosphere are largely from combustion of materials that contain mercury, with coal-combustion (electric utility boilers and commercial/industrial boilers) being the largest source in the U.S. See Mercury in the Nation's Streams—Levels, Trends, and Implications: USGS Circular 1395.

3. What's the most important thing people should know about your findings?

For the past two decades, mercury (Hg) researchers have noted slow and steady declines in gaseous elemental mercury (Hg⁰) concentrations in North America, Europe, and over the open oceans. Initial attempts to rationalize these observations from a scientific perspective were confounded by a commonly held assumption among researchers that global mercury emission trends from anthropogenic sources were steady or increasing over this same time period. For this study, we paused and reevaluated global Hg⁰ emission trends in the light of new information and found that the perceived inconsistency between the trends in Hg emissions and atmospheric Hg⁰ concentration are in fact not at odds. The new, more harmonious set of data and observations supports the notion that spatial and temporal trends in atmospheric Hg⁰ concentrations are much more influenced by local and regional actions than we ever believed previously.

4. Which areas saw decreases and increases?

Western Europe and the US have realized the greatest declines, which correspond to the locations where the most aggressive mercury emission reduction actions (intentional and unintentional) have occurred. Hg⁰ concentrations in Asia have continued to increase at the same time that decreases in the US and Western Europe has occurred.

5. How were the decreases achieved?

Several factors have contributed to the observed declines in atmospheric Hg⁰ concentrations. First are reductions in mercury emissions that have come about from three actions:

1. Reductions Hg in commercial products
2. Reductions in emissions from implementation of control technologies on coal-fired utilities; and
3. Switching to natural gas as an alternative fuel at coal-burning plants.

In addition to the reduced emissions, significant and unintended co-benefits from nitrous oxides (NO_x) and sulfur dioxide (SO₂) emission controls have also resulted in reduced mercury emissions.