Curbing sulfur and nitrogen emissions from coal also took out lots of mercury.

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Coal-fired plants and other human activities release mercury into the environment, where some of it ends up converted into methyl mercury, a potent neurotoxin. Because of this toxicity, the Environmental Protection Agency is currently in the process of tightening emissions rules at US power plants.

Global estimates of how much mercury we're emitting indicates that humanity is putting more of the substance into the atmosphere. But various direct measurements of the amount in the environment have been declining slowly over the past few decades. So where's the missing mercury? A new study suggests that we've cleaned it up while getting different pollutants under control.

Mercury emissions actually come in two forms. Some of it is released as neutral atoms, which are able to circulate widely in the atmosphere before being oxidized and falling to the surface. Another portion of the emissions are already oxidized and result in local contamination.

The amount of these two pollutants has been estimated simply by totalling up the human activities that cause emissions, such as mining operations and the burning of coal. These estimates suggest that, globally, the total mercury contamination should be experiencing a slight rise. And in areas where coal use is expanding, like China, this seems to be the case: mercury contamination is rising by about two percent per year.

But elsewhere, the data paints a different picture. In the northern hemisphere, mercury in the atmosphere is declining by up to two percent a year, depending on how far north you look. And higher in the atmosphere, the trends are flat. How do we make sense of all of this?

The key seems to be in data from the US. Here, mercury emissions have gone down by 75 percent over the last decade, even before the new rules had a chance to change anything. The drop has largely come about as a co-benefit from controls designed to limit sulfur and nitrogen emissions. The reaction that controls nitrogen oxide emissions also oxidizes mercury, while the controls that target sulfur emissions also remove oxidized mercury from the exhaust stream.

The sulfur controls appear to be key to the global trends. China went from having essentially none to having them installed on 86 percent of its coal plants in less than a decade. This meant that mercury emissions probably grew at half the rate of the increase in coal use. Japan more than doubled the plants with these controls over a 15-year period (to 70 percent); Europe's use of the technology increased at a similar pace.

Overall, the authors estimate that emissions of neutral mercury—the form that stays in the atmosphere and spreads globally—has declined by 30 percent over the 20 years ending in 2010. Globally, the emissions of oxidized mercury are up, but only by about nine percent. Since that doesn't spread widely, this increase will primarily impact the countries where coal use has gone up, such as India and China.

The results suggest that the benefits of existing pollution controls in the US may be underestimated, since these provide a substantial side benefit. And by primarily reducing the emission of the form of mercury that spreads globally, these pollution controls can have an impact well beyond the sites where they're implemented.