Ozone in the lower atmospheric layer is regarded as a serious pollutant that can cause respiratory problems, and even damage masonry and agricultural crops. The principal originating source is emissions from road vehicles, writes Jonathan Amos
Scientists think they have identified one key reason why ground-level ozone remains stubbornly high in Europe. They say it is the unfortunate but unintended consequence of what have otherwise been very successful efforts to improve air quality. It turns out the filters put on vehicle exhausts to remove fine particulate material have also unbalanced the chemistry behind ozone formation.
Chemical reactions that would normally remove ozone have been subdued. The insight comes from a study looking at London's air quality records. "Peak ozone levels have come down since the 1990s, but we haven't had the gains we expected on ozone," said Erika von Schneidemesser from the Institute for Advanced Sustainability Studies in Potsdam, Germany. "The data we've got from monitoring sites in London, and also the modeling work we have done, has helped us understand why ozone has behaved the way it has in London," she said.
Disturbed cycle
Ozone in the lower atmospheric layer called the troposphere is regarded as a serious pollutant that can cause respiratory problems, and even damage masonry and agricultural crops.
The principal originating source is the emissions from road vehicles. These include the exhaust gases such as nitrogen oxides (NOx), non-methane volatile organic compounds and carbon monoxide. Ozone is the product of these gases participation in a complex series of chemical reactions where sunlight and heat act as catalysts. Summer months are generally worse for ozone.
Von Schneidemesser and colleagues used the data from London's dense network of air quality monitoring sites to try to assess the performance of the ozone-producing reactions over the past 15 years. They found that although the ozone precursors have been falling, the ratio of two NOx gases in the atmosphere has changed.
In constant conditions, there is a neat cycle in which nitrogen dioxide helps to form ozone and nitric oxide helps to break it apart. This cycle appears to have been perturbed by control backmeasures that were actually intended to remove the fine particles and black carbon in vehicle exhausts. The measures achieved the desired outcome but also altered the relative emissions of the different NOx gases.
"There's this balance between nitrogen monoxide and nitrogen dioxide, and the diesel filters that we've been retrofitting onto things like buses mean that we now have a larger amount of primary nitrogen dioxide and so you get a reduction in nitrogen monoxide that is much greater than the reduction in nitrogen dioxide. This means that you are taking away some of the ozone suppression," said von Schneidemesser, who is also affiliated with the University of Leicester, UK.
While nitrogen monoxide in the atmosphere has been reducing by five to 20 per cent per year, nitrogen dioxide has been falling by just one to five per cent per year.
Further work is required, but the researchers suspicion is that London's experience is not unique. The big traffic-choked cities of Europe will suffer from similar emission inventories. However, the one rider here is that southern European cities will have more sunlight and heat to drive ozone producing reactions. But the London observations are unlikely to be the whole story.
Scientists say it's also that European ozone levels are being influenced by what is happening in other regions of the world. "There is an import of ozone and precursors from outside, and this influences what we call background ozone; and that's going up as global pollutants, particularly in Asia, go up. And that's affecting European ozone levels," explained professor Paul Monks at the University of Leicester.
"So, for something like ozone, we've probably got to move to a more global treaty like situation. We've got to look at control measures in other countries as well as our own.
"Peak ozone has gone down since the 1990s, but it has bottomed out now; and it's remaining fairly flat despite emissions reductions."