Smog hovers over a city skyline.
Stratospheric ozone shields Earth’s surface from harmful ultraviolet radiation, but tropospheric ozone, a prominent component in smog, is harmful to human health and acts like a greenhouse gas. Credit: Fernando José Ignacio Gárate Parra, CC BY-NC-ND 2.0
Source: Geophysical Research Letters

Ozone is crucially important in Earth’s atmospheric chemistry. The planet’s protective ozone layer, which exists mainly in the stratosphere at altitudes between 15 and 35 kilometers, absorbs ultraviolet radiation from the Sun, shielding the surface from hazardous high-energy light. However, ozone is also found beneath the stratosphere in the troposphere, where the molecule behaves more like a greenhouse gas and contributes to global warming. Ozone concentrations in the troposphere result from an interplay between transport from the stratosphere above and ozone production driven by emissions at the surface.

In a new study, Griffiths et al. model the atmospheric chemistry of ozone in the troposphere and stratosphere between 1979 and 2010 using the U.K. Met Office’s Unified Model together with data on emissions, historical meteorological conditions, and sea surface temperatures. The scientists quantified effects on tropospheric ozone during that time from both increasing emissions of ozone precursors and losses of stratospheric ozone resulting from the prevalent human usage of chlorofluorocarbons (CFCs), atmospheric pollutants famously responsible for depleting the ozone layer.

Overall, their results show that ozone production in the troposphere from anthropogenic sources increased during the study period, with the added pollution almost counterbalancing a decrease in stratosphere-to-troposphere transfer of ozone: Only a slight decrease in tropospheric ozone was detectable, with the two competing factors nearly canceling each other out. More recently, however, between 1994 and 2006, as the effects of bans on CFCs began to influence stratospheric ozone levels, the group’s model shows a slight increase in tropospheric ozone.

With the ozone layer recovering, a strengthening of the stratosphere-to-troposphere transport of ozone expected under climate change, and ozone precursor emissions continuing to rise in many places, the team says the results highlight the importance of studying ozone transport from the stratosphere to the troposphere—especially in the midlatitudes in spring, when atmospheric conditions favor such a downward flow of ozone. (Geophysical Research Letters,, 2020)

—David Shultz, Science Writer


Shultz, D. (2020), Tracking tropospheric ozone since 1979, Eos, 101, Published on 19 May 2020.

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