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Minimize Stratospheric and tropospheric ozone

The largest amount of ozone molecules (O3) is found in the stratosphere with about 90 % of it at altitudes between 15 and 30 km. Stratospheric ozone filters out a large part of the ultraviolet (UV) radiation emitted by the sun, protecting life on Earth. Enhanced UV-B radiation (280 to 320 nm) can have a negative impact on photosynthesis, cause skin cancer and weaken the immune system. On the other hand, absorption of solar UV radiation by stratospheric O3 causes the temperature of the stratosphere to increase with height, creating a stable layer that limits strong vertical air movement. This plays a key role for Earth's climate system (Dameris and Loyola, 2012).

Following the discovery of the Antarctic ozone hole in 1985, an international agreement known as Montreal Protocol was reached in 1987 to progressively phase out the use of ozone-depleting chemicals. Conducted under the auspices of the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), the Scientific Assessment of Ozone Depletion periodically update governments on the latest scientific findings related to the ozone layer using satellite and ground-based measurements as well as model simulations. The last report released in 2014 indicates that stratospheric ozone depletion has leveled off and is showing telltale signs of recovery.


Chapter Editor

Diego Loyola (DLR)


Melanie  Coldewey-Egbers (DLR), Georgina Miles (STFC RAL), Mark Weber (U. Bremen)



Ozone is typically measured in Dobson Units (DU). The average amount of ozone in Earth's atmosphere is around 300 DU; a column ozone level of less than 220 DU (ozone hole) is generally a result of the ozone loss from chlorine and bromine compounds. Such chemical reactions cause ozone in the southern polar region to be severely destroyed during the spring. This depleted region is known as the "ozone hole". This image shows the 3-day mean ozone column densities in Antartic spring from 1995 to 2014 showing the evolution of the ozone hole as measured by the satellite instruments GOME, SCIAMACHY and GOME-2. The dark-blue and purple area indicates the regions affected by the ozone hole. Update from Dameris and Loyola (2012).


Annual mean total ozone from 1970-2014 in four zonal bands (panels a to d) and during polar spring (panel e). Data are from WOUDC ground-based measurements combining Brewer, Dobson, SAOZ, and filter spectrometer data; the BUV/SBUV/SBUV2 V8.6 merged products from NASA (MOD V8.6, dark blue) and NOAA (light blue), the GOME/SCIAMACHY/GOME-2 products GSG from University of Bremen (dark green) and GTO from ESA/DLR (light green) and the MSR V2 assimilated dataset complemented with very recent GOME-2/Metop-A data. Very recently record low ozone was observed in the polar northern hemisphere (March 2011) and in the southern hemisphere (October 2015) during the ozone hole period. Despite the large year-to-year variability, total ozone levels stopped their long-term decrease until the 1990s as a consequence of the phase-out of ozone depleting substances (ODS) due to the Montreal Protocol and its Amendments. The evolution of ozone recovery in future strongly depends on climate change. By the middle of this century most models predict that ozone will likely return to 1980 levels. Update from Weber et al. (2015).


A new perspective on the current state of the ozone layer derived using the merged total ozone data record (GTO-ECV) recently released in the framework of the ESA Climate Change Initiative. Based on a multivariate regression analysis covering the 1995-2013 period, various aspects of ozone change and variability are disentangled on global and regional scales. This enables the monitoring of the effectiveness of the Montreal Protocol. Given dominant natural variability the expected mid-latitude onset of ozone recovery is still not significant and it is estimated that 5 additional years of observations would be needed for an unequivocal detection. A regional increase identified in the tropics is a likely manifestation of a long-term change in El Niño-Southern Oscillation intensity over the last two decades induced by strong El Niño in 1997/1998 and strong La Niña in 2010/2011. From Coldewey-Egbers et al. (2014).


Spring-to-fall ratio of observed polar cap total ozone (>50°) as a function of the absolute extratropical winter mean eddy heat flux (September to March in the Northern Hemisphere and March to September in the Southern Hemisphere). Data from the Southern Hemisphere are shown as triangles (September over March ozone ratios) and from the Northern Hemisphere as solid circles (March over September ratios). Selected polar total ozone distributions for selected years are shown at the top. The eddy heat flux is a measure of the stratospheric circulation and the poleward transport of lower stratospheric ozone. High winter eddy heat flux values indicate strong winter transport from low latitudes (ozone production) into high latitudes and little polar chemical ozone loss while low values relate to weak transport and, due to lower polar stratospheric temperatures, enhanced chemical ozone losses. Update from Weber et al. (2011) and WMO (2014)


In the Northern Hemisphere summer months, the Mediterranean region typically experiences high tropospheric ozone amounts. This is caused by local emissions, but also long-range transport of anthropogenic pollution from Northern Europe, Asia and North America which flows into the Mediterranean basin via large scale atmospheric circulation. During the hot summer months, high pressure weather conditions cause the poor quality air to remain in the region. This figure shows 5 days (12-16th August 2008) of gridded and contoured ozone data for the lower troposphere, from profile measurements from the GOME-2 satellite. Significant and persistent enhancement of ozone is observed over the Mediterranean. Data has been cloud cleared. Courtesy of Georgina Miles (RAL).


Monthly mean ozone in the lower troposphere in 2008, measured in Dobson Units. The red and yellow areas in June to November over first the west of Southern Africa and then the Amazon region of South America are caused by biomass (forest and savannah) burning in the dry seasons. The African continent is the single largest source of biomass burning globally, where the seasonally dry conditions, large unpopulated areas of savannah and lightning strikes provide the prefect ingredients for wildfires. In the Amazon region, biomass burning is largely anthropogenic in origin but they can also occur naturally during the dry season. Courtesy of Georgina Miles (RAL).