Old ice and snow yields tracer of preindustrial ozone

Ancient air bubbles answer question about ozone levels after Industrial Revolution

Style Magazine Newswire | 6/12/2019, 7:21 p.m.
Using rare oxygen molecules trapped in air bubbles in old ice and snow, U.S. and French scientists have answered a ...

HOUSTON – (June 12, 2019) – Using rare oxygen molecules trapped in air bubbles in old ice and snow, U.S. and French scientists have answered a long-standing question: How much have "bad" ozone levels increased since the start of the Industrial Revolution?

"We've been able to track how much ozone there was in the ancient atmosphere," said Rice University geochemist Laurence Yeung, the lead author of a study published online today in Nature. "This hasn't been done before, and it's remarkable that we can do it at all."

Researchers used the new data in combination with state-of-the-art atmospheric chemistry models to establish that ozone levels in the lower atmosphere, or troposphere, have 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. "That bolsters our confidence in their ability to predict how tropospheric ozone levels will change in the future."

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

"These measurements constrain the amount of warming caused by anthropogenic ozone," Yeung said. For example, he said the most recent report from the Intergovernmental Panel on Climate Change (IPCC) estimated that ozone in Earth's lower atmosphere today is contributing 0.4 watts per square meter of radiative forcing to the planet's climate, but the margin of error for that prediction was 50%, or 0.2 watts per square meter.

"That's a really big error bar," Yeung said. "Having better preindustrial ozone estimates can significantly reduce those uncertainties.

"It's like guessing how heavy your suitcase is when there's a fee for bags over 50 pounds," he said. "With the old error bars, you'd be saying, 'I think my bag is between 20 and 60 pounds.' That'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 involving sunlight, it is highly reactive, in part because of its tendency to give up one of its atoms to form a more stable oxygen molecule. The majority of Earth's ozone is 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 radiation, and is thus essential for life on Earth.

The rest of Earth's ozone lies in the troposphere, closer to the surface. Here, ozone's reactivity can be harmful to plants, animals and people. That's why tropospheric ozone is sometimes called "bad" ozone. For example, ozone is a primary component of urban smog, which forms near ground level in sunlit-driven reactions between oxygen and pollutants from motor vehicle exhaust. The Environmental Protection Agency considers exposure to ozone levels greater than 70 parts per billion for eight hours or longer to be unhealthy.