Ozone levels in the lowest part of the atmosphere have increased across the Northern Hemisphere over the past 20 years, according to the first study to use data collected by commercial aircraft.
And this is despite the fact tighter controls on emissions of ozone precursors have lowered ground-level ozone in areas such as North America and Europe, the researchers write in a paper in the journal Science Advances.
“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,” says lead author Audrey Gaudel from the University of Colorado at Boulder, US.
Tropospheric ozone – in the area stretching 12 to 15 kilometres above the Earth’s surface – is a greenhouse gas and air pollutant.
The study found the most striking increases have been in areas where levels were once lowest, including Malaysia, Indonesia and India. Significant increases also were noted in the tropics.
Gaudel says previous studies could not draw firm conclusions on trends because there are few long-term monitoring locations and new satellites with near-global coverage have provided conflicting results. She and her colleagues turned instead to 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 troposphere,” Gaudel says. Commercial aircraft captured 34,600 ozone profiles up to 2016 – about four a day.
The researchers used these measurements to calculate changes above 11 regions. They found an overall increase in all, including four in the mid-latitudes, two in the subtropics, two in the tropics and three equatorial regions.
On average, median ozone values increased by 5% per decade. In the lower troposphere they decreased above some mid-latitude regions where ozone precursor emissions have decreased, but these were offset by increases in the higher troposphere.
Gaudel now wants to look more closely at the tropics, comparing data from IAGOS, the NASA Atmospheric Tomography (ATom) field campaign – which measures trace gases and aerosol particles in less polluted regions – and TROPOMI, an instrument on board a European Space Agency satellite gathering information on atmospheric composition.