The ozone layer, a fragile shield of gas made up of three oxygen atoms (O3), a region of Earth’s stratosphere, absorbs most of the Sun’s ultraviolet (UV) radiation, thus helping to preserve life on the planet.
Location in Earth’s atmosphere
The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometres above Earth. However, its thickness varies due to atmospheric circulation patterns and solar intensity. The thickness of the ozone layer is generally thinner near the equator and thicker near the poles. Thickness refers to how much ozone is in a column over a given area and varies from season to season.
Discovery of the Ozone Layer
The ozone layer was first discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Measurements of the Sun showed that the radiation emitted from its surface and reaching the ground on Earth is usually consistent with the spectrum of a black body with a temperature in the range of 5,500–6,000 K, except that there was no radiation below a wavelength of about 310 nm at the ultraviolet end of the spectrum. It was deduced that the missing radiation was being absorbed by something in the atmosphere. Eventually, the spectrum of the missing radiation was matched to only one known chemical, ozone. Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer (the Dobson meter) that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958, Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The “Dobson unit”, a convenient measure of the amount of ozone overhead, is named in his honour.
The ozone layer absorbs 97% to 99% of the Sun’s medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which otherwise would potentially damage exposed life forms near the surface.
Depletion of the Ozone Layer
When scientists in the late 1970s discovered that humanity was creating a hole in this protective shield, they raised the alarm. The hole caused by ozone-depleting gases (ODSs) used in aerosols and cooling, such as refrigerators and air-conditioners, was threatening to increase cases of skin cancer and cataracts and damage plants, crops, ecosystems, etc. The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). While there are natural sources for all these species, the concentrations of chlorine and bromine increased markedly in recent decades because of the release of large quantities of man-made organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons. These highly stable compounds can survive the rise to the stratosphere, where the action of ultraviolet light liberates Cl and Br radicals. Each radical is free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. By 2009, nitrous oxide was the most significant ozone-depleting substance (ODS) emitted through human activities.
Levels of atmospheric ozone measured by satellite show apparent seasonal variations and appear to verify their decline over time. The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation can reach the Earth’s surface at a higher intensity. Ozone levels have dropped by a worldwide average of about 4 per cent since the late 1970s. For approximately 5 per cent of the Earth’s surface, around the north and south poles, much more significant seasonal declines have been seen and are described as “ozone holes”. The discovery of the annual depletion of ozone above the Antarctic was first announced by Joe Farman, Brian Gardiner, and Jonathan Shanklin in a paper that appeared in Nature on May 16, 1985.
Global Response
The global response was decisive. In 1985, the world’s governments adopted the Vienna Convention for the Protection of the Ozone Layer. Under the Convention’s Montreal Protocol, governments, scientists, and industry worked together to cut out 99% of all ozone-depleting substances. Thanks to the Montreal Protocol, the ozone layer is now healing and expected to return to pre-1980 values by mid-century.
The United Nations General Assembly has designated September 16 as the International Day for the Preservation of the Ozone Layer. This day shows that collective decisions and actions, guided by science, are the only way to solve major global crises.
The above image shows the latest false-colour view of total ozone over the Antarctic pole. The purple and blue colours are where there is the least ozone, and the yellows and reds are where there is more ozone.
In this year of the COVID-19 pandemic that has brought such social and economic hardship, the ozone treaties’ message of working together in harmony and for the collective good is more important than ever.
The phaseout of controlled uses of ozone-depleting substances and the related reductions have not only helped protect the ozone layer for this and future generations but have also contributed significantly to global efforts to address climate change. Furthermore, it has protected human health and ecosystems by limiting the harmful ultraviolet radiation from reaching the Earth.
“Let us take encouragement from how we have worked together to preserve the ozone layer and apply the same will to healing the planet and forging a brighter and more equitable future for all humanity.”
- António Guterres
References
https://www.britannica.com/science/ozone-layer
https://ozonewatch.gsfc.nasa.gov/
Author: Sulakna Isini
Editor — Praba Jalini
