Ozone Spiked Across Northern Hemisphere in Last 20 Years

The Space Shuttle Endeavour silhouetted against Earth’s atmosphere. The orange layer is the troposphere, the white layer is the stratosphere, and the blue layer is the mesosphere. The shuttle is actually orbiting at an altitude of more than 200 miles, far above all three layers. (Photo courtesy NASA)

(CN) — The ozone layer has been growing in multiple locations across the Northern Hemisphere since its recorded low in the mid-1990s, according to recent observations taken from commercial aircraft.

After methane and carbon dioxide, ozone is the atmosphere’s leading anthropogenic climate forcer — a chemical agent that exerts change on the Earth’s protective bubble. Itself a greenhouse as, ozone is composed of three oxygen atoms and helps protect the Earth from harmful radiation emitted by the sun, yet can also prove damaging to animal and plant life on the ground.

Scientists from the University of Colorado Boulder studied these increasing ozone levels using a fleet of specially equipped aircraft in a study published Thursday in the journal Science Advances. They determined that ozone levels have been steadily rising over the Northern Hemisphere for the past 20 years, especially over the tropics, even as stricter emission controls have lowered ground levels throughout much of North America and Europe.

“That’s a big deal because it means that as we try to limit our pollution locally, it might not work as well as we thought,” said lead author Audrey Gaudel, a CIRES scientist working in the NOAA Chemical Sciences Laboratory, in a statement.

The greatest increases in ozone levels occurred above Malaysia, Indonesia, Southeast Asia and India, followed by Northeast China and Korea, and the Persian Gulf. The warm season from April to September exhibited the highest levels recorded in these regions. Increases above the United States and Germany were found to be relatively weak compared with those above Asia in recent years, which surprised researchers since much of Asia recorded very low ozone values between 1994 and 2004.

The researchers employed global atmospheric chemistry models to quantify long-term changes in the troposphere, the layer of atmosphere nearest the Earth’s surface at an altitude of about 11 miles except at the poles. They predicted a 40% increase over preindustrial tropospheric ozone levels, along with a 30-70% increase on the ground between 1990 and 1994 at mid to high latitudes — admittedly with a roughly 50% degree of uncertainty owing to the lack of preindustrial data.

The In-Service Aircraft Global Observing System, or IAGOS, provides scientists with data from over 60,000 commercial aircraft flights, offering them a treasure trove of atmospheric information to detangle and compare with satellite data. Between 1994 and 2016 commercial aircraft equipped with specialized atmospheric sensors have compiled roughly four ozone profiles per day.

“Since 1994, IAGOS has measured ozone worldwide using the same instrument on every plane, giving us consistent measurements over time and space from Earth’s surface to the upper troposphere,” Gaudel said.

Recent reductions of ozone precursors across North America such as nitrous oxide have been most effective at lowering surface ozone levels, especially during the warmer months. Meanwhile, increased fossil fuel emissions in the tropics are likely behind rising ozone levels observed throughout that region.

Radiative forcing, the difference between incoming energy from the sun and energy expelled by the planet, is an important measure of greenhouse gas emissions because the balance leftover represents the amount of thermal energy building up in the atmosphere. Researchers found radiative forcing in the Northern Hemisphere to be 50% greater than the worldwide mean, with the tropics recording the highest levels due to the blanket-like effect imparted by the increased ozone concentration there.

The next step for Gaudel and her team is to look closer at the rising ozone levels above the tropics, particularly in Africa, where air pollution precursor emissions have been increasing rapidly in recent years. Along with the IAGOS data, the team plans to analyze measurements from NASA’s Atmospheric Tomography campaign, and compare that with European satellite data on atmospheric composition.

“The main takeaway is that these last 20 years ozone increased above all 11 regions that we sampled,” Gaudel said in an email interview. “We now know for sure that ozone is increasing across the Northern Hemisphere. Also, regions showing low ozone below 10-20 parts per billion (Indonesia/Malaysia, India, South East Asia), aren’t showing these low values anymore. The entire distribution of ozone has shifted towards higher values.”

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