Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s

Working on his doctoral dissertation in 2007, Xavier Faïn, now at DRI, and colleagues from LGGE (France), CREEL (NH), Bowdoin College (ME), University of California (CA), and University of Venice (Italy) investigated for the first time the polar firn as an archive of past atmospheric mercury. They show, in a paper recently published in the Proceedings of the National Academy of Sciences, that anthropogenic emissions had a large scale impact on the mercury atmospheric burden during the 1970s.

Polar ice sheets are unique archive to reconstruct the past atmospheric compositions. From the top surface to ~60 to 120 m depth is the firn, an openly porous and permeable consolidated snow through which air can diffuse. Consequently, changes in the composition of the atmosphere propagate downward through the firn, such that the mean age of the air increases monotonically with depth. Sampling of the firn air at Summit Station (72.6°N, 38.5°W) in central Greenland allowed Faïn and his colleagues to reconstruct the atmospheric history of gaseous elemental mercury (GEM) levels during the last ~70 years.

Mercury is an extremely toxic pollutant globally dispersed in the environment, and the gaseous form (GEM, Hg0) represents more than 95% of atmospheric mercury. However, direct monitoring of atmospheric GEM is only available since the 90s, and the influence of anthropogenic emissions on the mercury cycle is poorly characterized. Faïn and colleagues show that atmospheric GEM concentrations increased rapidly after World War II from ≈1.5 ng m−3 reaching a maximum of ≈3 ng m−3 around 1970 and decreased until stabilizing at ≈1.7 ng m−3 around 1995.

Faïn conclude that anthropogenic emissions of mercury largely impacted the atmospheric reservoir at the hemispheric scale before implementation of air regulations in both the United States and Europe (i.e., at the beginning of the 1970s). Thus, implementation of Hg air regulations had a fast and positive impact on the atmospheric Hg0 reservoir in the past.

“Increases in anthropogenic emissions are likely in the next few decades in Asia, however. They could cause a rise in atmospheric Hg concentrations similar to those observed in the 1970s and lead to higher deposition of this toxic element in both industrialized and remotes areas.” Faïn said.

Once deposited, mercury becomes available for methylation and can subsequently contaminate ecosystems. “Our research demonstrates that emission reductions and continued air regulations will be required during the next few years.” Faïn added. This linkage is of prime importance because many organizations (e.g., United Nations Environmental Program) and nations (e.g., the United States, Europe, and Canada) are presently debating the implementation and extension of mercury emission regulations and implementation of a global network to monitor mercury pollution.

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