Audrey Gaudel, Owen R. Cooper, Kai-Lan Chang, Ilann Bourgeois, Jerry R. Ziemke, Sarah A. Strode, Luke D. Oman, Pasquale Sellitto, Philippe Nédélec, Romain Blot, Valérie Thouret, and Claire Granier

Science Advances

Published date August 21, 2020

Aircraft observations since the 1990s reveal increases of tropospheric ozone at multiple locations across the Northern Hemisphere

Study key findings & significance

  • The study finds that tropospheric ozone levels have increased considerably above the Northern Hemisphere.
  • The researchers looked at 34,600 ozone profiles captured by commercial aircraft from 1994 to 2016.
  • Earlier studies could not reach a solid conclusion regarding ozone trends in the Northern Hemisphere. This was because there are not many long-term monitoring sites and the latest satellites with near-global coverage have given contradictory outcomes relating to ozone trends.
  • The team observed an overall increase in ozone levels in all the regions where they investigated, including two in the tropics, two in the subtropics, three equatorial areas, and four in the mid-latitudes. There was a 5% increase in median ozone values on average for every 10 years.
  • The highest increase in ozone levels occurred in the tropics and ozone exported from the tropics could be driving increased ozone levels above other regions of the Northern Hemisphere.
  • The researchers also observed that in the lower troposphere—that is, the atmospheric layer closest to the surface of the Earth—ozone levels had reduced above certain mid-latitude areas, such as the United States and Europe, where emissions of ozone precursors have reduced. These reductions were compensated by increased ozone trends higher in the troposphere—with the net outcome being an overall increase in ozone levels from the surface to 12 km.

Author quotes

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.

Audrey Gaudel, Study Lead Author and Scientist, NOAA Chemical Sciences Laboratory, Cooperative Institute for Research in Environmental Sciences


Tropospheric ozone is an important greenhouse gas, is detrimental to human health and crop and ecosystem productivity, and controls the oxidizing capacity of the troposphere. Because of its high spatial and temporal variability and limited observations, quantifying net tropospheric ozone changes across the Northern Hemisphere on time scales of two decades had not been possible. Here, we show, using newly available observations from an extensive commercial aircraft monitoring network, that tropospheric ozone has increased above 11 regions of the Northern Hemisphere since the mid-1990s, consistent with the OMI/MLS satellite product. The net result of shifting anthropogenic ozone precursor emissions has led to an increase of ozone and its radiative forcing above all 11 study regions of the Northern Hemisphere, despite NOx emission reductions at midlatitudes.