Ozone Levels Have Been Rising Over Northern Hemisphere

Researchers used aircraft data to document the changes.

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Polluting clouds of exhaust fumes rise in the air
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Using aircraft data, scientists have been able to understand the bigger picture on ozone levels. Their new research shows that the pollution levels in the lower part of the Earth's atmosphere have increased over the last two decades. Called tropospheric ozone, this greenhouse gas and air pollutant can harm lungs and damage plants at high levels. The increases have occurred even as tighter restrictions have lowered ground-level ozone in some places such as North America and Europe.

This is not the upper layer of ozone or "good" ozone that protects the Earth from harmful UV light.

In the past, researchers turned to satellite data to capture ozone information, but researchers were not able to draw firm conclusions because results often offered conflicting results.

"We were not able to say if ozone was increasing or decreasing with time globally. That is a real issue, knowing the impacts that ozone has on climate, health and vegetation," lead researcher Audrey Gaudel, a scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, told Treehugger.

Turning to the Skies

Frustrated with satellite data, researchers opted to analyze tropospheric ozone changes using commercial aircraft data.

"They give rather regional information but if enough regions are covered, we can get a global picture," Gaudel said. "That is what this study is about. We were able to cover the Northern Hemisphere and that is significant because it represents 88% of human lives on Earth that potentially affect or is impacted by the quality of the air we breathe."

Gaudel and her team analyzed 34,600 ozone profiles captured between 1994 and 2016 by commercial aircraft. They published their results in a study in Science Advances.

"The main takeaway is that these last 20 years ozone increased above all 11 regions that we sampled. We now know for sure that ozone is increasing across the Northern Hemisphere. Also, regions showing low ozone below 10-20 ppb (Indonesia/Malaysia, India, South East Asia), are not showing these low values anymore. The entire distribution of ozone has shifted towards higher values," Gaudel said.

"This ozone increase is a big deal, especially above regions that are already actively trying to mitigate air pollution, because it shows that local efforts to reduce emissions of this pollutant are not enough. The problem, thought to be local, becomes global."

Researchers found that ozone decreased in the "lower troposphere" closer to Earth's surface in some areas, including parts of Europe and North America, where emissions from ozone-forming chemicals have dropped. But the researchers found that those decreases were offset by increases higher in the troposphere.

The study findings point to the importance of areas like tropical regions where ozone isn't regulated. Gaudel plans to focus on those areas next.

"In the tropics, regulations of emissions are often poor or not followed, and many of these regions do not always monitor ozone and its precursors," she said. "I would like to make a difference and promote in situ measurement and long-term monitoring of ozone at all places we can."