Tracking bad ozone around Europe and the Mediterranean

In the 1980s, scientists discovered that the layer of ozone (O3), a protective layer that shelters all life on Earth and is located in the stratosphere, was thinning. The destruction of ozone led to what is called “the ozone hole” in the Southern Hemisphere.

The battle to save ozone meant engineering changes to products like refrigeration systems, air conditioners, and aerosol cans to remove the chlorofluorocarbons. These CFCs formerly used in such systems destroy ozone. Measurements of the stratospheric ozone in recent years have shown that it is slowly recovering.

For years “ozone friendly” was an important selling point on supermarket shelves and people concluded that ozone is positive to the environment. It is true that in the stratosphere, it protects us from ultraviolet radiation from the Sun, but ozone in the troposphere, the lowermost atmospheric layer, is also an efficient greenhouse gas – meaning it traps heat from the Sun and can contribute to climate change. It is the third most important greenhouse gas with anthropogenic contributions (contributions from human activities) after carbon dioxide and methane.

“Negative effects of tropospheric ozone on air quality, human health and vegetation are still not very familiar to the public”, says Sarah Safieddine, a PhD student at the Pierre and Marie Curie University, in Paris.

With her colleagues, Safieddine has been studying how ozone behaves in the atmosphere. Although this “bad” ozone often results from human activities, such as from cars, power plants, and various industries, it is not emitted directly into the atmosphere.

Ozone is a by-product of polluting human activities and results from an interaction in the atmosphere. Other pollutants and compounds in the atmosphere react with each other in the presence of sunlight to make ozone, which can then travel a long distance from where it was first formed.

The scientists found that ozone levels over the eastern part of the Mediterranean Sea are particularly high during the summer months due to these transport effects. Several factors combine to make the area prone to a particularly large ozone build-up.

“High temperatures, clear sky conditions, and the mega cities in the east of the basin contribute to the local high production of near-ground ozone. Going up in altitude, the transport in the troposphere from Europe as well as from the stratosphere contributes to the ozone peaks seen in summer over this region,” says Safieddine.

IASI before launch

IASI before launch

The team used a computer chemistry model to assess the behaviour of gases, and in particular ozone, in the atmosphere. They compared the model with measurements from the infrared atmospheric sounding interferometer (IASI) on board the Metop satellites, and with measurements from the ground taken from the European Monitoring and Evaluation Programme (EMEP) monitoring stations.

“For health protection, the daily maximum eight hours average threshold is 60 parts per billion per hour as specified by the European Environment Agency. The EMEP network shows that in Europe this threshold is exceededed the most in the Mediterranean region.” says Safieddine

The exact way in which ozone is produced and transported around the globe is still under investigation and being studied with measurements from space, global and regional models, and ground observations. After the “good ozone” of recent decades, “bad ozone” will continue to be a hot topic for the twenty-first century.

You can read the full scientific paper by Sarah Safieddine and her colleagues in Atmospheric Chemistry and Physics online.

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Ruth McAvinia

Ruth McAvinia

Learning Zone Writer

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