Old ice and snow: pre-industrial ozone tracer

Using rare oxygen molecules trapped in air bubbles in old ice and old snow, US and French scientists have answered a long-standing question: how much "bad" ozone levels have they increased since the beginning of the industrial revolution?

"We were able to track the amount of ozone that existed in the ancient atmosphere," said Laurence Yeung, geochemist at Rice University, lead author of a study published online today. at Nature. "This has never been done before, and it is remarkable that we can do it at all."

Researchers used this new data in combination with advanced atmospheric chemistry models to establish that ozone levels in the lower atmosphere, or troposphere, had increased by 40% since 1850.

"These results show that the best current models are a good simulation of the old tropospheric ozone levels," Yeung said. "This reinforces our confidence in their ability to predict the evolution of tropospheric ozone levels in the future."

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

"These measures limit the magnitude of warming caused by anthropogenic ozone," Yeung said. For example, he stated that the latest report of the Intergovernmental Panel on Climate Change (IPCC) estimated that ozone in the Earth's lower atmosphere today contributes to 0, 4 watts per square meter of radiative forcing to the planet's climate, but the margin of error for this forecast was 50%, or 0.2 watts per square meter.

"It's a very big error bar," Yeung said. "Having better estimates of pre-industrial ozone can significantly reduce these uncertainties.

"It's like guessing the weight of your luggage when there is a charge for luggage over 50 pounds," he said. "With the old error bars, you would say:" I think my bag weighs between 20 and 60 pounds ". This is not enough if you can not afford to pay the fine. "

Ozone is a molecule containing three atoms of oxygen. Produced during chemical reactions involving sunlight, it is very reactive, partly because of its tendency to abandon one of its atoms to form a more stable oxygen molecule. The majority of terrestrial ozone is in the stratosphere, more than five miles above the surface of the planet. Stratospheric ozone is sometimes called "good" ozone because it blocks most of the sun's ultraviolet rays and is therefore essential to life on Earth.

The rest of Earth's ozone is in the troposphere, closer to the surface. Here, the reactivity of ozone can be harmful for plants, animals and humans. This is why tropospheric ozone is sometimes called "bad" ozone. For example, ozone is a major component of urban smog, which forms near the ground during sun-induced reactions between oxygen and pollutants from motor vehicle exhaust gases. . The Environmental Protection Agency estimates that exposure to ozone levels greater than 70 parts per billion for eight hours or more is unhealthy.

"The problem with ozone, is that scientists only study it in detail for a few decades," said Yeung, assistant professor of Earth Sciences, Environmental and planets. "We did not know why air pollution was so abundant in ozone until the 1970s. It was at that point that we began to recognize just how much pollution the planet was getting. air was changing the chemistry of the atmosphere, cars were driving ozone up to ground level. "

While the first measurements of tropospheric ozone date back to the late 19th century, Yeung said that these data were at odds with the best estimates from the most modern atmospheric chemistry models.

"Most of this old data comes from tests on starch paper where the paper changes color after reacting with ozone," he said. "The tests are not the most reliable – the color change depends on the relative humidity, for example – but they nonetheless suggest that ground-level ozone could have increased by 300% over the last century. On the other hand, the best current computer models suggest a more moderate increase of 25 to 50%, which is a huge difference.

"There is no other data available, so it is difficult to know who is correct or whether both are correct and that these particular measures are not a good reference for the whole troposphere", Yeung said. "The community has been fighting this issue for a long time, and we wanted to find new data that could advance this unresolved problem."

Finding new data, however, is not simple. "Ozone is too reactive, in itself, to be kept in ice or snow," he said. "So we are looking for the wake of ozone, the traces it leaves in the oxygen molecules.

"When the sun shines, the same chemistry makes it possible to continuously make and break up ozone and oxygen molecules," Yeung said. "Our work in recent years has found a natural" label "for this chemistry: the number of rare isotopes collected.

Yeung's laboratory specializes in the measurement and explanation of the presence of these isotopes grouped in the atmosphere. These are molecules that have the usual number of atoms – two for molecular oxygen – but they have rare isotopes of these substituted atoms instead of common atoms. For example, more than 99.5% of all oxygen atoms in nature have eight protons and eight neutrons, for a total atomic mass of 16. Only two out of every 1000 oxygen atoms are the heavier oxygen-18 isotope, which contains two additional neutrons. A pair of these 18-oxygen atoms is called a block of isotopes.

The vast majority of oxygen molecules in an air sample contains two oxygen 16. Some rare exceptions will contain one of the few oxygen-18 atoms and oxygen pairs. -18 will be even rarer.

Yeung's laboratory is one of the few in the world that accurately measures the number of oxygen-18 pairs in a given air sample. He added that the abundance of these clusters of isotopes in molecular oxygen varies depending on where the chemistry of ozone and oxygen is occurring. As the low stratosphere is very cold, the probability that an oxygen-18 couple will form from ozone / oxygen chemistry will increase slightly and predictably compared to the same reaction in the troposphere. In the troposphere, where it is warmer, ozone / oxygen chemistry gives slightly fewer oxygen-18 pairs.

With the onset of industrialization and the burning of fossil fuels around 1850, humans began to add ozone to the lower atmosphere. Yeung and his colleagues felt that this increase in the proportion of tropospheric ozone should have left a recognizable trace, namely a decrease in the number of oxygen-18 pairs in the troposphere.

By using ice cores and snowfields (compressed snow that has not yet formed ice) from Antarctica and Greenland, researchers have built a registry of oxygen-18 pairs in Molecular oxygen dating from the preindustrial period to the present day. Evidence has confirmed both the increase in tropospheric ozone and the magnitude of the increase predicted by recent atmospheric models.

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

"One of the most exciting aspects has been the fit of the ice record model with the model predictions," he said. "In this case, we did a measurement and, independently, a model came up with a result that was in perfect agreement with the experimental evidence, and I think it shows how much climate and atmospheric scientists are doing. able to accurately predict how humans are changing the Earth's atmosphere, especially its chemistry ".