Atmospheric ozone: Benefit and problems

The stratospheric ozone layer is situated between 10 and 40 km in altitude. The high ozone concentration almost completely absorb the sun’s harmful shortwave UV radiation and is therefore vital for protecting life on Earth. For example, excessive exposure of the skin to ultraviolet radiation (high UV index) can cause skin cancer in humans. In contrast, high ozone concentrations at ground level are harmful to humans and nature, because the gas is extremely reactive. This can lead to respiratory problems in humans and impaired growth in plants.

Is the problem of ozone layer depletion solved?

The answer to this question is still NO. The reasons are the following:

• The ozone layer has not yet fully recovered. It is not yet back to the level it was prior to 1970, and it is expected to take several more decades to reach this level.
• The depletion of the ozone layer at the South Pole (Antarctica) is a recurring yearly phenomenon.
• Ozone depletion was observed at the North Pole (Arctic) for the first time in 2011.
• The two issues of "climate change" and "the alteration of the ozone layer" are closely related.

The Montreal Protocol, which was enacted in 1986, is working brilliantly - it's considered the most successful environmental treaty in history. The ozone layer is healing but continues to be at risk also because new concerns have emerged.

Natural events: Volcano eruptions, such as the Hunga Tonga-Hunga Ha'apai in 2022, are susceptible to cause a reduction in ozone concentrations by injecting SO2 or water vapor in the stratosphere.

Wildfires: Smoke-charged vortex resulting from wildfires transport aerosols into the stratosphere, and this leads to both depleting and increasing the ozone layer.

Anthropogenic emissions:  emissions of anthropogenic very short-lived chlorine substances and of methyl bromide continue to grow and contribute to ozone depletion. Unexpected emissions of CFC-11 have been reported between 2012 and 2018.

Climate change interactions: Increasing GHGs cause stratospheric cooling and slow the Brewer-Dobson circulation, while ozone recovery tends to warm the stratosphere and strengthens the circulation. The stratospheric cooling and the acceleration of the circulation seems to dominate the opposing effect of O3 recovery.

Measuring and analysing the ozone development in the different layers of the atmosphere are therefore essential.

Ozone measurements are a Swiss tradition

Measurements of the ozone column

Switzerland has a long history of conducting ozone measurements in the upper atmosphere. As early as 1926, Prof. P. Götz started measuring the ozone levels in Arosa on the top of his house “Haus Firnelicht”, and these measurements have continued with almost no interruptions ever since. This worldwide unique series of measurements makes it possible to study the development of the ozone layer over a very long period of time.

The figure below clearly shows the ozone depletion at the measurement station of Arosa in the period between 1970 and 1990. The depletion is associated with the use of ozone-depleting substances (called CFCs). The depletion is followed by a period of stabilization and a starting return to pre-1970-values.

Ozone profile measurements

The ozone concentration changes with altitude and its evolution too. Monitoring the ozone profile is essential because it enables the early detection of any recovery or decline in ozone levels.

Outside of the polar regions, observations and models are in agreement that ozone in the upper stratosphere is recovering at a rate of 2-5%/decade. In contrast, ozone in the lower stratosphere has not shown signs of recovery. Models simulate a small recovery in mid-latitude lower-stratospheric ozone in both hemispheres that is not seen in observations.

MeteoSwiss measures the vertical ozone profile using different ground-based techniques. Balloon-borne ozone sondes allow the measurement of the ozone content in the atmosphere from ground up to 35 km altitude. They have been conducted in Payerne since 1966. This uninterrupted timeseries helps us tracking the evolution of the tropospheric and stratospheric ozone content.
Since 1956, ozone profiles have been measured twice daily using Dobson and Brewer spectrophotometers (Umkehr method). This is the longest Umkehr timeseries in the world.
Since 2000, the microwave radiometer SOMORA (Stratospheric Ozone Monitoring Radiometer) measures the ozone mixing ratio from the stratosphere to the lower mesosphere.