This summer, Europe’s weather services and air quality experts were on high alert not just because of a predicted sequence of record-breaking heatwaves, but also because of potential heat-triggered pollution episodes. In July and August, many parts of Europe recorded extreme episodes of surface ozone pollution, with values above the safety level across Portugal, Spain, and Italy, and in many European capitals, including London, Paris and Brussels.
While emission reduction measures can bring gradual improvements for other pollutants, surface ozone pollution remains more difficult to tackle, as hot weather and high temperatures are predicted to become more frequent. This makes monitoring ozone pollution increasingly important for public health.
Why do we need to care about ozone pollution?
We've heard a lot already about the ozone layer. It is essential for us as it protects the Earth from dangerous UV radiation. This ozone states at very high altitudes. But when it comes to pollution that can affect our health, we are looking at nose-level ozone, the one that we can breathe.
This ozone needs a few factors to come together to get formed: sunlight, warm temperature, and the chemical compounds that react with the solar radiation to produce ozone itself. The latter are known as ozone precursors, such as nitrogen oxides and hydrocarbons emitted by human activities or vegetation.
“The ozone present in the troposphere is a secondary pollutant that is formed from photochemical reactions from other pollutants,” says Dr Patricia Tarín-Carrasco, a postdoctoral fellow at the Barcelona Institute for Global Health. She has been studying the combined effect of heatwaves and ozone episodes on people.
“Ozone has several effects on human health, it can cause irritation of the respiratory tract and mucous membranes, headaches, nausea, increase cases of asthma or bronchitis, cerebrovascular diseases or even cause premature death,” says Dr Tarín-Carrasco. The World Health Organisation recommends a maximum ozone concentration of 100µg/m3 on average over 8 hours, while the EU has a slightly higher value of 120µg/m3. This past summer, 16 EU states saw ozone concentrations above the EU threshold, while 33 countries across Europe jumped above the WHO safe standard, data from the European Environment Agency shows.
Ozone’s longer term health effects are less clear, according to Dr Cristina Guerreiro, research director at the Norwegian Institute for Air Research (NILU), and include an increase in asthma cases among children and worsening of already existing asthma sufferers. “In 2019, it was estimated that acute ozone pollution led to 16,800 premature deaths in Europe,” says Dr Guerreiro.
Heatwaves and their impact on ozone pollution
According to the Copernicus Atmospheric Monitoring Service (CAMS), high atmospheric pressure during a heatwave, which involves lower wind speeds and air stagnation can allow ozone precursors to concentrate, while clear skies and strong sunlight favour the chemical reactions leading to ozone formation. When heatwaves get worse, ozone pollution can follow too unless emissions of precursors are mitigated.
The link between ozone precursor emissions, weather and ozone production is complex. But scientists warn that the strong link between climate change and air quality means that reducing emissions in the long run to tackle global warming will play a key role in alleviating ozone pollution and improving the air we breathe.
“There are two main drivers of ozone pollution in Europe that we expect to change in the future,” says Dr Guerreiro. “The first is the expected increase in heatwave frequency due to climate change, which will exacerbate ozone pollution problems. The second is the expected reduction in ozone precursors’ concentrations, which should come from efforts to reduce carbon emissions.” Guerreiro explains that in the long-term emission reductions will have a bigger influence on ozone formation, leading to lower ozone concentrations even if heatwaves will become more intense and more frequent.
Scientists play with a lot of uncertainties. How heatwaves influence ozone is not yet fully understood. “Heatwaves could lead to extensive drought (as we’ve seen this year), that in turn leads to less biogenic uptake (i.e. by living organisms) of ozone, leaving more of it in the atmosphere. [This process] is difficult to model correctly,” says Dr Guerreiro at NILU.
Given the complex link between weather, ozone precursors and ozone production, there are still uncertainties with regard to ozone behaviour, which in turn makes its interaction with other phenomena trickier to decipher. “The main gaps in science lie in the complex chemistry leading to ozone formation, especially because of the myriad of volatile hydrocarbons involved,” says Dr Augustin Colette, head of the Air Quality Modelling Unit at the French National Institute for Industrial Environment and Risks (INERIS) and lead scientist for CAMS regional air quality activities. “The situation is even more complex because it is not only the variety of hydrocarbons related to human activities that we must understand, vegetation is also a strong source of such compounds but these emissions remain quite uncertain. Modelling atmospheric chemistry, therefore, requires a good understanding of the exchanges between the ground, the canopy, and the air.”
Keeping track of ozone
Predicting when ozone pollution could happen is critical for protecting human health, especially in the case of vulnerable people such as children, or the elderly. In some countries, such as France, Dr Colette says, air quality forecasts can trigger emergency measures for reducing polluting activities from industry and transport which produce ozone precursors. “Several types of predictive models have been developed over the years,” says Dr Colette, explaining those include machine learning models as well as physical ones, which consider air chemistry and air transport elements.
“We have models covering typically a country, a continent such as Europe, and even global models covering the whole earth. But such models are better constrained in the areas where we can benefit from in situ monitoring stations, although satellites also provide valuable information in remote locations,” says Dr Colette.
In Europe, CAMS provides daily ozone concentration forecasts, while also producing state-of-the-art maps for past conditions that help policymakers better understand exposure to air pollution, Dr Colette says. “In its policy Service, CAMS designs value-added products that go an extra mile compared to mapping ozone concentrations. It has become possible today to say how much of the ozone pollution is due to local air pollutants, and how much has been transported from a neighbouring country. This is an extremely important information for a policy maker who needs to know how efficient environmental policies would be when enforced only at the scale of a given city.”
Better air quality calls for strong climate policies
Reducing ozone concentrations is a regional, national and international matter, Dr Guerreiro explains, as ozone is formed over large distances. “Even if local authorities are often responsible for ensuring air quality standards are attained, they cannot manage this problem efficiently with local policies. In practice, awareness and action have been more at European and national authorities’ level, and is low at the local one,” Guerreiro says.
According to her, good policies for tackling ozone pollution need sound science, which means sufficient monitoring and study, via research and modelling (for climate and air quality. “In general, international abatement protocols need to take into account the combined effect on tropospheric (nb: ground-level) ozone and climate, whereas these have traditionally been seen as separate issues,” says Guerreiro. “Research on ozone levels during heatwaves also needs to be strengthened in order to build good policy actions. Although the knowledge of the general processes controlling ozone are fairly well established, the special conditions experienced during heat waves and the impact on ozone are not as well known.”
Addressing climate change is a key tool for addressing, among other dangerous phenomena, ozone pollution. “It’s incredibly important to have a high level of cooperation between authorities at all levels (from local to international) and across different sectors and policy areas. It is also very important to continue assessing the effects of the implemented policies and of climate change on ozone levels.”