Paris, 4 April 2011
The ozone layer acts like a shield that protects life on Earth from harmful ultraviolet solar radiation. Ozone concentrations and total content have been continually monitored since the signing in 1987 of an international treaty, the Montreal Protocol, which regulates the production of halocarbons. These are chemical compounds that contain chlorine and bromine and cause the destruction of ozone in the stratosphere (the part of the atmosphere that extends from an altitude of around 10 to 50 km and where the concentration of ozone is at its highest). Since halocarbons remain in the atmosphere for tens of years, it will be several decades before their concentration falls back to its pre-1980 level.
Destruction of stratospheric ozone takes place in the polar regions when temperatures drop below -80°C. At these temperatures, clouds form in the lower stratosphere. Chemical reactions inside them transform compounds derived from halocarbons—which are harmless to ozone—into active compounds. These processes lead to the rapid destruction of ozone when sunlight returns over the pole. In the Antarctic, the 'ozone hole' (which results from the destruction of over half the total ozone content in spring) is a recurring phenomenon due to the extremely low temperatures in the stratosphere every winter. In the Arctic, on the other hand, wintertime temperatures are warmer on average than at the South Pole, and weather conditions vary considerably from one year to the next. This explains why ozone depletion can be less significant there. This year, the record ozone loss observed was caused by extreme weather.
Researchers at the Laboratoire atmosphère, milieux, observations spatiales (CNRS/UVSQ/UPMC) have access to a battery of measurement stations(1) and infrared(2) and UV-visible(3) monitoring systems that monitor ozone on a daily basis all around the planet. These observations are part of the World Meteorological Organization and United Nations Environment Program's Global Ozone Observing System.
This winter, observations as well as simulations carried out by these teams using the Reprobus(4) model revealed an extremely significant drop in total ozone content over a wide area (Figure 1). The persistence and extent of this loss, which has lasted for several weeks and reached 40% et the end of March, are absolutely unprecedented (Figure 2).
French(5) and European teams are currently working in the field north of the Arctic Circle (Kiruna, Sweden) to take detailed observations of these exceptional conditions using instruments on board stratospheric balloons operated by CNES(6). And instruments at the Haute Provence observation station (sounding balloons and LIDAR(7) system) will be used to detect the impact of this event on lower latitudes when springtime warming of the polar stratosphere pushes ozone-depleted air masses towards these regions.
Without the Montreal Protocol, this year's ozone destruction would have been considerably worse. As long as chlorine and bromine concentrations in the stratosphere remain high, significant ozone depletion similar to that observed this year may happen again during exceptionally cold Arctic winters. According to the latest international report assessing the state of the ozone layer, ozone should recover its pre-1980 levels around 2045-60 over the South Pole, and probably one or two decades earlier over the North Pole.
© M. George, LATMOS
Figure 1: Distribution of total ozone content measured by the IASI instrument on board the MetOp satellite, for the periods 18-20 March and 26-28 March 2011. Areas with high ozone concentrations are shown in red and yellow, while concentrations in blue-colored regions are twice as low. The grey shapes are clouds.
© F.Goutail LATMOS
Figure 2: Total ozone loss over the Arctic for every year since 1994, estimated from measurements by the SAOZ network.
1 - SAOZ monitoring network in the Arctic region: View web site (monitoring at LATMOS by Florence Goutail and Andrea Pazmino)
2 - IASI/MetOp satellite: View web site (monitoring at LATMOS by Cathy Clerbaux).
3 - GOMOS/ENVISAT satellite (monitoring at LATMOS by Alain Hauchecorne and Slimane Bekki).
4 - Reprobus model: View web site monitoring at LATMOS by Franck Lefevre.
5 - from the Laboratoire de physique moléculaire pour l'atmosphère et l'astrophysique (CNRS/UMPC) at the Institut Pierre Simon Laplace, the Laboratoire de physique et chimie de l'environnement et de l'Espace (CNRS/Université d'Orléans) and the Groupe de spectrométrie moléculaire et atmosphérique (CNRS/Université de Reims)
6 - ENRICHED project: View web site
7 - sounding balloon and LIDAR monitoring at the Observatoire de Haute-Provence: View web site (monitoring at LATMOS by Sophie Godin-Beekmann and Gérard Ancellet)
Latest WMO and UNEP report on the state of the ozone layer:
View web site
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