Reductions in emissions in Europe and North America have not translated into a comparable fall in ground-level ozone, according to new research that points to rising pollution transported from overseas.
A study by scientists at the Research Institute for Sustainability (RIFS) at GFZ finds that increasing amounts of ozone formed outside Europe and North America are offsetting the benefits of local emissions controls, leaving overall exposure levels largely unchanged.
Between 2000 and 2018, emissions of key ozone precursors – nitrogen oxides (NOx) and volatile organic compounds (VOCs) – fell by around 50% in both regions, largely as a result of tighter vehicle and industrial standards, cleaner technologies and stricter air quality regulations. While summer ozone concentrations declined as expected, levels during winter and spring rose, meaning annual exposure for the population did not decrease significantly.
First author Tabish Ansari. said: ‘The observed seasonal and annual changes have been discussed in studies since the early 2000s. These refer to a variety of possible causes, for example increased intrusion of ozone from the stratosphere due to climate change and higher ozone imports from regions with rapidly rising emissions, particularly in East Asia.
‘In addition, there is a reduced removal of ozone in winter due to decreases in local NOx emissions. It is important to understand that the same NOx which produces ozone in the presence of sunlight also removes it in the absence of sunlight. However, these explanations have largely remained within the realm of statistical conjecture.’
The new study set out to provide direct evidence by using a sophisticated atmospheric chemistry transport model combined with a novel emissions-tagging system developed at RIFS. This approach allowed researchers to track ozone around the globe and attribute it to emissions from specific regions and sectors.
The results show that an increasing share of ozone measured in Europe and North America is produced abroad and transported over long distances through the atmosphere, particularly from rapidly industrialising regions in Asia. This imported ozone is now strong enough to cancel out much of the progress made through local pollution controls.
The study also highlights the growing role of natural sources of NOx, such as vegetation, forest fires and lightning, in ozone formation. While emissions from these sources have not increased substantially, they are now generating more ozone because of reduced competition from human-made NOx emissions. With fewer anthropogenic pollutants present, naturally emitted NOx becomes more chemically active, a well-known effect in atmospheric chemistry.
Researchers say the findings underscore the limits of regional air quality policies when dealing with pollutants like ozone, which can persist in the atmosphere for weeks and travel thousands of kilometres across continents.
The authors argue that effective ozone mitigation will require coordinated international action rather than isolated national or regional measures. They emphasise the need for sustained cooperation and trust between regions, supported by reliable scientific estimates of how pollution is transported around the globe.
The full research can be read here.
Photo: Carl Wang / unsplash
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