Aug. 21 (UPI) -- While emission levels for ozone-forming chemicals have fallen in some North American cities, new research suggests tropospheric ozone levels have increased across the Northern Hemisphere over the last 20 years.
In the upper layers of the atmosphere, the ozone layer helps protect Earth and its inhabitants from harmful ultraviolet radiation.
But closer to Earth's surface, ozone causes problems. In addition to accelerating climate change, the greenhouse gas can harm people's lungs and damage plants.
"Ozone is a secondary gas, which means that it is not directly emitted," lead researcher Audrey Gaudel, a scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, told UPI in an email.
"It is formed photochemically from primary gas such as nitrogen oxide, volatile organic compounds, carbon monoxide and methane, which are directly emitted," Gaudel said.
Chemicals that react to form ozone in the atmosphere are called ozone precursors.
Previous efforts to measure global ozone trends using satellite data have produce conflict results. Scientists weren't able to determine whether ozone concentrations in the troposphere were rising or falling.
"That is concerning, given the impacts that ozone has on climate, health and vegetation," Gaudel wrote.
Frustrated by the failures of the satellite data-derived Tropospheric Ozone Assessment Report, researchers turned to commercial aircraft data.
"They give rather regional information but if enough regions are covered, we can get a global picture," Gaudel wrote. "That is what this study is about.
"We were able to cover the Northern Hemisphere and that's significant because it represents 88 percent of the human population on earth that potentially affect or is impacted by the quality of the air we breathe."
Gaudel and her colleagues analyzed 34,600 ozone profiles captured by commercial aircraft between 1994 and 2016.
More specifically, researchers found ozone levels have dropped in the lower troposphere above North America and Europe, but higher up in the troposphere, increasing ozone levels from elsewhere in the Northern Hemisphere have offset those declines.
Even close to Earth's surface, polluted air from one place can travel and affect people and places far from the pollution source. Higher up, pollution spreads out even more.
"Ozone and its precursors are transported over long distances following the main transport pathways," Gaudel said.
The latest findings suggest that while regulations in North American and Europe have helped curb the release of ozone precursors, those gains are being washed out by emissions from places where ozone precursors aren't regulated.
Gaudel plans to study those regions in-depth in future ozone surveys.
"This study identified the important role of the tropical regions in the global ozone burden and changes," she told UPI in an email. "My research will focus on this part of the globe. In the tropics, regulations of emissions are poor or not followed."