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Old ice and snow provide traces of pre-industrial ozone

  Old ice and snow provide traces of pre-industrial ozone
Researchers and employees from Rice University used ice cores, such as those shown here from Antarctica, in combination with atmospheric chemistry models to determine an upper limit for the increase in ozone content in the lower atmosphere since 1850. Credit: Jeff Fitlow / Rice University

Using rare oxygen molecules trapped in air bubbles in old ice and snow, American and French scientists have responded to a long-standing question: How much has the ozone levels increased since the beginning of the industrial revolution?

"We have been able to track how much ozone there was in the ancient atmosphere," says Rice University geochemist Laurence Yeung, leading author of a study published online today in Nature . " is remarkable that we can do it at all. "

Researchers used the new data in combination with state-of-the-art atmospheric chemistry models to determine that the ozone level in the lower atmosphere or troposphere has increased by an upper limit of 40% since 1850.

"These results show that today's best models simulate ancient tropospheric ozone levels well," said Yeung. "It strengthens our confidence in its ability to predict how tropospheric ozone levels will change in the future."

The rice-led research team includes University investigators of Rochester in New York, French National Center for Scientific Research (CNRS) Institute of Environmental Geosciences at Un iversité Grenoble Alpes (UGA), CNRS Grenoble Images Speech Signal and Control Laboratory at the UGA and the French Climate and Environmental Science Laboratory for both the CNRS and the French Alternative Energy and Atomic Energy Commission (CEA) at the Université Versailles-St Quentin.

"These measurements limit the amount of warming caused by anthropogenic ozone," Yeung said. He said, for example, that the latest report by the Intergovernmental Panel on Climate Change (IPCC) estimates that ozone in the lower atmosphere today contributes 0.4 watts per square meter of radiation to the planet's climate, but the margin of error for that prediction was 50% or 0.2%. watt per square meter.

"It's a really big bug bar," said Yeung. "Having better preindustrial ozone estimates can significantly reduce these uncertainties."

"It's like guessing how heavy your suitcase is when there is a charge for bags over 50 pounds," he said. "With the old bug bars you would say," I think my bag is between 20 and 60 pounds. "It's not good enough if you can't afford to pay the penalty."

Ozone is a molecule that contains three oxygen atoms. Produced in chemical reactions with sunlight, it is highly reactive, partly because of its tendency to give up one of its atoms to form a more stable oxygen molecule. The majority of the earth's ozone lies in the stratosphere, which is more than five miles above the planet's surface. Stratospheric ozone is sometimes called "good" ozone because it blocks most of the sun's ultraviolet

  Old ice and snow provide traces of preindustrial ozone
Rice University geochemists Laurence Yeung and Asmita Banerjee studied the rise of tropospheric ozone from preindustrial time to present through to build a record of oxygen-18 "clumsy isotopes" pairs from small bubbles of atmospheric gas trapped in ice and snow in Antarctica and Greenland. Credit: Jeff Fitlow / Rice University

The rest of the earth's ozone lies in the troposphere, closer to the surface. Here, the reactivity of the ozone can be harmful to plants, animals and humans. Therefore, tropospheric ozone is sometimes called "bad" ozone. For example, ozone is a primary component of urban smog, which forms near ground level in sunlit reactions between oxygen and impurities from motor vehicle exhaust. The Swedish Environmental Protection Agency considers that exposure to ozone levels above 70 parts per million for eight hours or longer is unhealthy.

"The case of ozone is that the researchers have only studied it in detail for a few decades," said Yeung, an assistant professor of soil, environmental and planetary sciences. "We did not know why ozone was so rich in air pollution until the 1970s. It was then that we began to recognize how air pollution changed the atmosphere. Cars drove up ground-level ozone."

While the earliest measurements of tropospheric ozone date to the end of the 19th century, Yeung said these data conflicts with the best estimates from today's state of the art atmospheric chemistry models.

"Most of the older data comes from starch samples where the paper changes colors after reacting with ozone," he said. "The tests are not the most reliable. For example, the color change is due to relative humidity, but they still suggest that ground level ozone may have increased up to 300% over the past century. However, today's best computer models suggest a more moderate 25-50% increase "There is a big difference.

" There are just no other data out there, so it is difficult to know which one is right, or if both are right and the particular measurements are not good benchmark for the entire troposphere, " Yeung. " Society has long struggled with this issue. We wanted to find new data that could speed up this unresolved problem. "

However, it is not easy to find new data." Ozone is too reactive, in itself, to be preserved in ice or snow, "he said." So we are looking for the ozone, the traces it leaves in oxygen molecules.

"When the sun shines, ozone and oxygen molecules are constantly created and broken in the atmosphere of the same chemistry," says Yeung. "Our work in recent years has found a naturally occurring" tag "for that chemistry: the number of rare isotopes clumped together. "

Yeung's lab specializes in both measuring and explaining the presence of these clumsy isotopes in atmosphere. They are molecules that have the usual number of atoms-two for molecular oxygen, but they have rare isotopes of those atoms which are substituted instead. For example, more than 99.5% of all oxygen atoms in nature have eight protons and eight neutrons, for a total atomic number of 16. Only two of every 1,000 oxygen atoms are the heavier isotope-oxygen-18 containing two additional neutrons A couple of these oxygen-18 atoms are called an isotope lump.

The vast majority of oxygen molecules in any air sample will contain two oxygen-16s. The case will contain one of the rare oxygen-18 atoms, and rarely will it be the pair of oxygen-18s.

Yeung's lab is one of the few in the world that can measure exactly how many of these oxygen pairs are in a given sample of air. He said these molecular isotopes in molecular oxygen vary in abundance depending on where ozone and oxygen chemistry occur. Since the lower stratosphere is very cold, the odds increase that an oxygen-18 pair comes from ozone / oxygen chemistry to increase somewhat and predictably compared to the same reaction in the troposphere. In the troposphere, where it is warmer, ozone / oxygen chemistry yields fewer oxygen-18 pairs.

With the beginning of industrialization and the burning of fossil fuels around 1850, people began to add more ozone to the lower atmosphere. Yeung and colleagues justified that this increase in the proportion of tropospheric ozone should have left a recognized trace – a decrease in the number of oxygen-18 pairs in the troposphere.

The use of ice cores and firn (compressed snow that has not yet formed ice) from Antarctica and Greenland designed the researchers a task of oxygen-18 pairs in molecular oxygen from pre-industrial times to the present. The evidence confirmed both the increase of tropospheric ozone and the magnitude of the increase predicted by the latest atmospheric models.

"We limit the increase to less than 40% and the most comprehensive chemical model estimates about 30%," Yeung said.

"One of the most exciting aspects was how well the ice core record matched model predictions," he said. "This was a case where we made a measurement and independently produced a model that was very close to experimental evidence. I think it shows how far atmospheric and climate scientists have come to be able to accurately predict how people are changing the Earth's atmosphere – especially its chemistry. "

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More information:
Isotopic limitation of the increase of tropospheric ozone in the 20th century Nature (2019). DOI: 10,1038 / s41586-019-1277-1, https://www.nature.com/articles/s41586-019-1277-1

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Old ice and snow provide traces of preindustrial ozone (2019, June 12)
June 13, 2019
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