Ozone across northern hemisphere increased over past 20 years

In a first-ever study using ozone data collected by commercial aircraft, researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder found that levels of the pollutant in the lowest part of Earth's atmosphere have increased across the Northern Hemisphere over the past 20 years. That's even as tighter controls on emissions of ozone precursors have lowered ground-level ozone in some places, including North America and Europe.

Tropospheric ozone—ozone between Earth's surface and 12 to 15 kilometers above Earth—is a greenhouse gas and air pollutant that, at high levels, can harm people's lungs and damage plants.

In a study published today in the journal Science Advances, the team found an overall increase in ozone levels above the Northern Hemisphere. "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 Audrey Gaudel, a CIRES scientist working in the NOAA Chemical Sciences Laboratory and the study's lead author. She and her colleagues documented the greatest ozone increases in the tropics, Gaudel said, noting that ozone exported from the tropics may be driving increases above other areas of the Northern Hemisphere.

Gaudel and her co-authors, CIRES scientists in NOAA and international colleagues, also found the most striking increases in areas where ozone levels were once lowest: Malaysia/Indonesia, Southeast Asia and India, for example. Those regions had very low ozone values between 1994-2004, and very high levels in recent years, between 2011-2016.

Previous studies could not draw firm conclusions on Northern Hemisphere ozone trends, according to Gaudel, because there are too few long-term monitoring locations and because new satellites with near-global coverage have provided conflicting results on ozone trends.

So the researchers turned to aircraft data from Europe's In-Service Aircraft for the Global Observing System (IAGOS) program. "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 troposphe