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Why Is Ground Level Ozone Bad, But Stratospheric Ozone Is Good?

Gene J. Puskar
Gas prices at a pump in West Mifflin, Pa. on Nov. 16, 2018.

Allegheny County residents are no longer required to fill their cars with a special blend of gasoline in the summer, the Environmental Protection Agency announced on Friday.

The rule was first introduced in 1995 in order to reduce the formation of ground-level ozone, a pollutant that has been linked to negative health outcomes such as asthma and chronic obstructive pulmonary disorder.

Ozone is oxygen, but it’s not the stable type of oxygen that humans depend on for survival. That kind of oxygen has two atoms, whereas ozone has three.

Carnegie Mellon University environmental engineering professor Peter Adams said ozone formation is one of the more complicated topics in atmospheric chemistry.

“You need something called volatile organic compounds, which you can think of gasoline vapors and other things like that. You need nitrogen oxides which comes from vehicles and power plants,” Adams said. “You need it to be sunny, and generally you need warmer temperatures.”

Ozone formation is a little different about 20 miles up in the stratosphere, where it protects us from the sun’s harmful ultraviolet radiation.

“When ozone is up high in the ozone layer … you don’t have to breathe it,” Adams said. “If you did, it would still be equally bad for you.”

Last May, the Allegheny County Health Department announced that the EPA had designated the county as being in compliance with national air quality standards for ozone pollution, but said “air quality in the county continues to pose a challenge.”

Air quality advocates question whether the exemption from summer blend gasoline is a good idea, said Matt Mehalik, executive director of the Breathe Project.

“If you believe every contribution makes a difference, by rolling that summer blend requirement [back] … it just makes it that much more difficult for our region to improve its air quality.”