Mercury levels in rainfall are rising in parts of U.S.

and some sites in the western U.S. are even seeing increases, which we think is due to increasing mercury emissions from Asia,” Brigham said. “So the reductions in North American mercury emissions are primarily benefiting the eastern U.S., where there is a greater concentration of emissions.”

The researchers also looked at trends in sulfate concentrations in rainfall, which is an indicator of local sources of emissions from coal combustion. While mercury and sulfate concentrations were closely correlated at East Coast monitoring sites, other sites showed a stark contrast between rising mercury concentrations and falling sulfate concentrations.

“This indicates that mercury emissions in the U.S. and Canada are not the cause for the observed upward tendencies in mercury concentrations,” Weiss-Penzias said.

Emission controls
Mercury emissions have declined in North America and Europe for several reasons, including better emissions controls and increasing use of natural gas rather than coal as a fuel for power plants. The “scrubber” systems installed at coal-fired power plants in North America and Europe since the early 1990s to combat acid rain also reduce mercury emissions.

UCSC notes that emissions from Asia have been increasing, however, and are transported over long distances in the upper atmosphere. The influence of the Sierra Nevada and Rocky Mountains on weather systems results in mercury from the upper atmosphere being deposited in precipitation in western states such as Nevada and Idaho and in the central United States, Weiss-Penzias said.

The researchers also looked at trends for concentrations of gaseous mercury in the atmosphere, although less data are available than for mercury in rainfall. By merging several short-term datasets for air concentrations in North America, they found the trends were similar to the pattern for mercury in rainfall: a downward trend for the early time period (1998-2007), shifting to a flat slope for the recent period (2008-2013).

“A lot less mercury is being emitted to the atmosphere in the U.S. and Canada than 20 years ago as a result of regulations, efforts by industry, and the economic realities of cheap natural gas,” Weiss-Penzias said. “In spite of that, there are other factors, including emissions from other parts of the world, that are causing an increase in the amount of mercury being deposited in certain locations in North America.”

— Read more in Peter S. Weiss-Penziasa et al., “Trends in mercury wet deposition and mercury air concentrations across the U.S. and Canada,” Science of the Total Environment (21 January 2016) (doi:10.1016/j.scitotenv.2016.01.061)