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.

Tropospheric ozone is a secondary pollutant, formed from the gaseous emissions from fossil fuel or biomass burning - nitrogen oxides (NOx) and volatile organic compounds (VOCs) - and sunlight. Some tropospheric ozone also finds its way to the troposphere via stratospheric-tropospheric exchange, where air from each layer mixes dynamically. This is particularly the case in the midlatitudes. Ozone in the troposphere is also a greenhouse gas. High levels of ozone have been linked to increased mortality/excess deaths when associated with localised heat wave events. Tropospheric ozone can be harmful to agriculture by increasing the failure rate of crops by damaging leaves. For these reasons, it is vitally important to monitor ozone in the troposphere as well as the stratosphere, but in situ surface observations and ozonesondes are sparse and heavily favour the Northern Hemisphere. With satellite data from GOME, SCIAMACHY and GOME-2, we are able to monitor tropospheric ozone globally every day.

The ozone layer's thickness is not solely controlled by chemical processes in the stratosphere. As an atmospheric trace gas, O₃ is also transported over large distances by stratospheric winds, which significantly affects global ozone distribution. Dynamic and chemical processes in the atmosphere interact in highly complex ways. Radiative processes also play a crucial role since O₃ is one of the most important radiatively active gases in the atmosphere. It absorbs both shortwave and longwave radiation and thus influences the vertical temperature distribution of the stratosphere. 

Chapter Editor

Diego Loyola (DLR)

Contributors

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

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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).

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A new perspective on the current state of the ozone layer derived using the merged total ozone data record (GTOECV) 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).

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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)

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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).

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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).

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• Coldewey-Egbers, M., D. Loyola, P. Braesicke, M. Dameris, M. van Roozendael, C. Lerot, and W. Zimmer, A new health check of the ozone layer at global and regional scales, Geophys. Res. Lett., 41, 4363-4372, doi 10.1002/2014GL060212, 2014.
• Dameris, M. and D. Loyola: Recent and future evolution of the stratospheric ozone layer, Chapter 45 in Atmospheric Physics, Background-Methods-Trends, Ed. U. Schumann, Springer, Heidelberg, ISBN 978-3-642-30182-7, doi: 10.1007/978-3-642-30183-4, pp.747-761, 2012
• Miles G. M., R. Siddans, B. J. Kerridge, B. G. Latter, and N. A. D. Richards, Tropospheric ozone and ozone profiles retrieved from GOME-2 and their validation, Atmos. Meas. Tech., 8, 385–398, 2015.
• Weber, M., S. Dikty, J. P. Burrows, H. Garny, M. Dameris, A. Kubin, J. Abalichin, and U. Langematz, The Brewer-Dobson circulation and total ozone from seasonal to decadal time scales, Atmos. Chem. Phys., 11, 11221-11235, doi:10.5194/acp-11-11221-2011, 2011.
• Weber, M., W. Steinbrecht, C. Roth, M. Coldewey-Egbers, R. J. van der A, D. Degenstein, V. E. Fioletov, S. M. Frith, L. Froidevaux, C. S. Long, D. Loyola, and J. D. Wild, [Global Climate] Stratospheric ozone [in "State of the Climate in 2014"], Bull. Amer. Meteor. Soc., 96, S44–S46, 2015.
• WMO (World Meteorological Organization), Scientific Assessment of Ozone Depletion: 2014, Global Ozone Research and Monitoring Project–Report No. 55, Geneva, Switzerland, url: http://www.wmo.int/pages/prog/arep/gaw/ozone_2014/ozone_asst_ report.html, 2014.