After increased industrialization in the 1930s, atmospheric acidity levels rose up sharply to a peak in the 1970s, but 40 years after the US and Europe introduced legislation to combat air pollution, acidity in the air has now dropped back down to pre-1930 levels. These figures come out of research by the University of Copenhagen, which used a new technique to measure the pH balance of ice core samples from the Greenland ice sheet, and how it's changed year to year.

In an area as cold as Greenland, the snow that falls never has a chance to melt. Instead, every year a new layer forms over the top of the previous one, which eventually packs into a tightly compressed layer of ice. With each layer preserving a record of the climate and atmospheric conditions of the time, the Greenland ice sheet provides scientists with a time capsule dating back over 100,000 years.

The problem with looking at samples of the layers in the top 60 m (200 ft), representing the last 100 years, is that they are still relatively fresh and porous. Since it hasn't yet compressed into hard ice, it's more difficult to analyze, which is unfortunate because this past century is of particular interest to researchers looking into the human impact on the climate.

Building on a technique called Continuous Flow Analysis – where water from a piece of melting ice core is analyzed for chemical impurities – the research team at the University of Copenhagen's Niels Bohr Institute (NBI) has developed a new way to study the acid concentration of that upper portion using a spectrometer, providing near-instant analysis of the melting ice.

"We have an ice rod that is cut along the length of the ice core," says Helle Astrid Kjær, lead author of the study. "This ice core rod is slowly melted and the meltwater runs into a laboratory where they take a lot of chemical measurements. With our new method you can also measure the acidity, that is to say, we measure the pH value and this is seen when the water changes color after the addition of a pH dye. We can directly see the fluctuations from year to year."

The team's graph shows acid content fluctuations over the years, with a clear downward trend over the last few decades(Credit: Helle Astrid Kjær, NBI)

These annual figures can be influenced by natural emission spikes, from sources such as volcanic eruptions and large forest fires, but the system is able to distinguish between these and industrial pollution. The resulting data paints an optimistic portrait of environmental regulation.

"In the 1970s, both Europe and the United States adopted the Clean Air Act Amendments, which required filters in factories, thus reducing acid emissions and this is what we can now see the results of," says Kjær. "The pollution of acid in the atmosphere is now almost down to the level it was before the pollution really took off in the 1930s."

Teams from the US, New Zealand and Denmark have already put the method into practice on ice cores from other sites in Greenland and Antarctica.

The NBI research was published in the journal Environmental Science and Technology.

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