Mining, coal-fired power plants responsible for sevenfold increase in atmospheric mercury – study

Jharia coal mine in India. (Reference image by TripodStories- AB, Wikimedia Commons)

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences estimated that emissions from coal-fired power plants, mining and waste incineration have increased the concentration of potentially toxic mercury in the atmosphere sevenfold since the beginning of the modern era around 1500 C.E.

To reach this conclusion, the scientists developed a new method to accurately estimate how much mercury is emitted annually from volcanos, the largest single natural emitter of mercury. The team used that estimate—along with a computer model—to reconstruct pre-anthropogenic atmospheric mercury levels.

Their calculations show that before humans started pumping mercury into the atmosphere, it contained on average about 580 megagrams of mercury. However, in 2015, independent research that looked at all available atmospheric measurements estimated the atmospheric mercury reservoir was about 4,000 Mg—nearly seven times larger than the natural condition estimated in this study.

“Methylmercury is a potent neurotoxicant that bioaccumulates in fish and other organisms—including us,” Elsie M. Sunderland, senior author of the Geophysical Research Letters paper that presents these findings, said. “Understanding the natural mercury cycle driven by volcanic emissions sets a baseline goal for policies aimed at reducing mercury emissions and allows us to understand the full impact of human activities on the environment.”

Satellites to the rescue

Sunderland explained that the challenge with measuring mercury in the atmosphere is that there is not much of it despite its outsized impact on human health. In a cubic meter of air, there may be only a nanogram of mercury, making it virtually impossible to detect via satellite. Instead, the researchers needed to use another chemical emitted in tandem with mercury as a proxy. In this case, the team used sulphur dioxide, a major component of volcanic emissions.

“The nice thing about sulphur dioxide is that it’s really easy to see using satellites,” Benjamin Geyman, first author of the article, said. “Using sulphur dioxide as a proxy for mercury allows us to understand where and when volcanic mercury emissions are occurring.”

Using a compilation of mercury to sulphur dioxide ratios measured in volcanic gas plumes, the researchers reverse-engineered how much mercury could be attributed to volcanic eruptions. Then, using the GEOS-Chem atmospheric model, they modelled how mercury from volcanic eruptions moved across the globe.

The team found that while mercury mixes into the atmosphere and can travel long distances from its injection site, volcanic emissions are directly responsible for only a few percent of ground-level concentrations in most areas on the planet. However, there are areas— such as in South America, the Mediterranean and the Ring of Fire in the Pacific—where levels of volcanic emissions of mercury make it harder to track human emissions.

“In Boston, we can do our local monitoring and we don’t have to think about whether it was a big volcano year or a small volcano year,” said Geyman. “But in a place like Hawaii, you’ve got a big source of natural mercury that is highly variable over time. This map helps us understand where volcanos are important and where they aren’t, which is really useful for understanding the impact of humans on long-term mercury trends in fish, in the air and the ocean. It’s important to be able to correct for natural variability in the volcanic influence in places where we think that influence may not be negligible.”