Ozone Hole
Science
The “ozone hole” describes the reduced amount of ozone in the localised area above Antarctica that appears annually from August to December. This leads to a higher percentage of the Sun’s harmful ultraviolet (UV) radiation reaching the Earth’s surface over this region, adversely affecting human health and the environment.
Most satellites measure ozone levels by subtracting the UV reflected by Earth, from the incoming UV from the sun. This difference in energy is representative of the energy absorbed by the ozone. Therefore, the greater the energy difference, the greater the ozone concentration.
What is ozone?
Ozone (O3) is a highly reactive gas composed of three oxygen atoms, found in our lower atmosphere (troposphere) and our upper atmosphere (stratosphere). Ozone is also used harmlessly in industry to disinfect water, clean electronics, and bleach textiles. 1)
10% of atmospheric ozone is found near the ground in our troposphere which ranges from the Earth’s surface to an altitude of approximately 15 km (varies between latitudes and times of the year). The remaining 90% is found in our stratosphere which ranges from the top of the troposphere to an altitude of approximately 50 km. The lower part of the stratosphere where the majority of the ozone resides is commonly referred to as the ozone layer. 2)
Tropospheric ozone is formed mainly from the chemical reactions of two major air pollutants, hydrocarbons and nitrogen oxides, which are largely produced by fossil fuel combustion and deforestation. The reactions depend on the energy of sunlight to break up the chemical bonds, which initiates a chain of chemical reactions resulting in ozone creation. Due to the increased sunlight in summer, it is during this time that ozone concentration is at its highest. Tropospheric ozone is “bad” ozone as it has adverse health effects when inhaled, and is a significant greenhouse gas in the troposphere. 3)
Stratospheric ozone which includes the ozone layer, is formed through the chemical reactions of sunlight and oxygen gas (O2). Sunlight strikes and breaks apart one molecule of oxygen gas into two oxygen atoms (O). The highly reactive oxygen atoms then combine with a molecule of oxygen gas to create ozone. This reaction is balanced by the destruction of ozone when it reacts with sunlight and anthropogenic chemicals in the atmosphere. Stratospheric ozone is “good” ozone as it protects us from the most dangerous UV radiation from the sun.
How does the ozone layer protect us?
Whilst we can still get burnt from the Sun’s UV rays, the ozone layer protects us from the most harmful types of UV radiation. There are three categories of UV radiation: UV-C (100 - 280 nm), UV-B (280 - 315 nm), and UV-A (315 - 400 nm). UV-C radiation is the most damaging but is completely absorbed by the ozone layer. Excessive exposure to UV-B is associated with an increased risk of skin cancer, cataracts, and a suppressed immune system. Most, but not all of UV-B is absorbed by the ozone layer. UV-A is unimpeded by the ozone layer but is the least harmful.
The more concentrated the ozone layer is, the more UV-B absorbed. That is the type of UV radiation we are concerned with when discussing stratospheric ozone depletion including the ozone hole above Antarctica.
How and why has the amount of stratospheric ozone changed over time?
Stratospheric ozone began rapidly decreasing in the 1970s with the anthropogenic release of ozone-depleting substances (ODSs) into the atmosphere. These substances interact with ozone and strip oxygen atoms from it, creating oxygen gas. 5)
One example of ODS is chlorofluorocarbons (CFCs), which had applications for refrigeration, air conditioning, and spray can propellants. CFC emissions have significantly decreased since the Montreal Protocol was agreed in 1987. Hydrochlorofluorocarbons (HCFCs) are less harmful ODSs, which were developed as a substitute for CFCs. Due to the long lifetime of HCFCs, and the tardiness of global restrictions being placed, the abundance of HCFCs is expected to peak between 2023 and 2030. Another category of ODSs is those that contain bromine such as methyl bromide, which is used for pest control, the disinfection of shipping goods, and is also released by many marine organisms.
As a result of the Montreal Protocol in 1987, ODS emissions have decreased by more than 99%.
Why does the ozone hole only appear above Antarctica from August to December?
The ozone hole is present above Antarctica due to the meteorological and chemical conditions unique to Antarctica.
Strong winds circle the poles in winter which prevent air mixing into or out of the polar stratosphere. These polar vortexes are stronger in the Southern Hemisphere because the Northern Hemisphere has more ocean and land of relatively warm temperatures compared to the Southern Hemisphere. This results in colder and less varied temperatures in Antarctica compared to the Arctic. These very stable and very cold temperatures lead to the creation of long-lasting clouds in the stratosphere above Antarctica. These special clouds are called Polar Stratospheric Clouds (PSCs).
Despite most ODSs being produced in the Northern Hemisphere, they are more and less evenly distributed throughout the stratosphere in both hemispheres. The liquid and solid particles of PSCs act as a surface for the ODSs to undergo chemical reactions and create chlorine gas (Cl2) and other byproducts.
When sunlight returns to Antarctica towards the end of winter, the energy of the sunlight breaks up the chlorine gas into two highly reactive chlorine atoms. Each chlorine atom will then steal an oxygen atom from ozone, converting the ozone into oxygen gas.
Furthermore, the ozone destruction rate increases when the heavy PSC particles slowly fall due to gravity taking “good chemicals” with them that would have otherwise slowed down ozone destruction. This process is called denitrification.
Will the ozone hole recover?
As a result of restrictions (such as the Montreal Protocol) placed on ODSs, their emissions have decreased by more than 99%. Stratospheric ozone began increasing in concentration in 2000. Non-polar stratospheric ozone levels are predicted to return to 1980 values by 2040, and Antarctica’s levels are predicted to return by 2066. 6)
Example Products
ESA Ozone Project
ESA's Ozone Project provides data suitable for assessing changes in the total ozone column, the tropospheric ozone column, as well as its vertical distribution across the upper troposphere and stratosphere. This data has been collected from the EO satellites ERS-2, Envisat, Metop-A, Metop-B, Metop-C, Aura, and Sentinel-5P.
NASA Ozone Watch
NASA Ozone Watch provides data and data visualisations concerning the ozone hole. NASA Ozone Watch provides users with global ozone maps from January 1979 to today. It also provides plots describing the changes in the ozone hole area, ozone minimum, ozone mass deficit, and polar cap ozone. For all cases listed above, NASA Ozone Watch provides raw data and links to where you can access more data. The satellite source of this data has changed over the years: Nimbus-7 (1979 - 1992), Meteor-3 (1993 - 1994), Earth Probe (1996 - 2004), Aura (2004 - 2016), and Suomi NPP (2016 - present).
Related Missions
Sentinel-5 Precursor (S5P)
Launched in October 2017, S5P is jointly operated by ESA and the Netherlands Space Office (NSO), as part of ESAs Global Monitoring for Environment and Security (GMES) programme. S5Ps Tropospheric Monitoring Instrument (TROPOMI) is a passive grating imaging spectrometer. This will be used to monitor the vertical distribution of trace gases such as ozone, methane, and carbon monoxide.
Stratospheric Aerosol and Gas Experiment (SAGE)
SAGE is a series of instruments that have been operating since 1979 under the National Aeronautics and Space Administration (NASA) to determine atmospheric composition. The SAGE-I instrument was launched aboard the Applications Explorer Mission-B (AEM-B) satellite in 1979. The SAGE-II instrument was launched aboard the Earth Radiation Budget Satellite (ERBS) in 1984. The SAGE-III instrument was first launched in 1984 aboard the Метеор-3М satellite, and again in 2017 to the International Space Station (ISS). SAGE-IV is currently in a prototype state and is looking to be installed on a series of 6U cubesats in the future.
Read more (ISS: SAGE-III)
Read more (SAGE-IV)
Polar Orbiting Environmental Satellites (POES)
The POES program is a joint effort by NASA, the National Oceanic and Atmospheric Administration (NOAA), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), and a group of space organisations across France, Canada, and the United Kingdom. These 15 satellites were and are equipped with a variety of instruments allowing them to observe several surface and atmospheric parameters. The first POES was TIROS-N launched in 1978, and the last to be launched was NOAA-19 in 2009. The two instruments currently used by POES to monitor ozone levels in the atmosphere are the High Resolution Radiation Sounder (HIRS), and the Solar Backscatter Ultraviolet Spectral Radiometer (SBUV). The only POES satellites still operational are NOAA-15, -18, and -19.
Atmospheric Limb Tracker for Investigation of the Upcoming Stratosphere (ALTIUS)
The ALTIUS satellite is developed by ESA, and built by Redwire Space. It will use its three-channel hyperspectral imaging spectrometer to monitor the distribution of stratospheric ozone over time. This will be done, not by observing the nadir, but rather by observing Earth’s limb.
Science Satellite-1 (SciSat-1)
Launched in 2003, SciSat-1 is an atmosphere-observing satellite operated by the Canadian Space Agency (CSA). Both of its instruments are used to monitor ozone levels. The primary instrument is the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), whilst the secondary instrument is the Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO).
Advanced Earth Observing Satellite (ADEOS)/Midori Series
The objectives of the ADEOS series of satellites were to observe different parameters concerning Earth’s environment. Launched in 1996, ADEOS-I was the first Japanese satellite mission that involved international cooperation. It had seven different instruments with one being the Total Ozone Mapping Spectrometer (TOMS) that monitored Earth’s ozone layer by measuring the albedo of Earth’s atmosphere. Launched in 2002, ADEOS-II had the specific purpose of monitoring global change: carbon cycle, water cycle, and energy cycle. Two of its five instruments helped monitor ozone levels: the Global Imager (GLI), and the Improved Limb Atmospheric Spectrometer-II (ILAS-II). 8)
Read more (ADEOS)
Read more (ADEOS-II)
Odin
Launched in 2001, Odin is a minisatellite (< 500 kg) whose operation is led by the Swedish National Space Board (SNSB). The satellite’s purpose is to collect data concerning astronomy and aeronomy (the study of Earth’s upper atmosphere). One of its instruments is the Optical Spectrograph and InfraRed Imaging System (OSIRIS) which measures the concentration of ozone, aerosols, and nitrogen oxide.
Total Ozone Mapping Spectrometer - Earth Probe (TOMS-EP)
Launched in 1996, TOMS-EP was a NASA-operated satellite that carried a TOMS instrument. As TOMS was its sole instrument, the satellite’s sole purpose was to monitor ozone. It measured the total ozone by comparing the incoming solar energy and the backscattered energy. “Backscattered” radiation is solar radiation that is scattered by particles in the atmosphere back in the direction of the satellite.
Meteor-3 Series
The first of the Meteor-3 series, Meteor-3-1, was launched in 1985 with the last of the series, Meteor-3-7 launched in 1994. These satellites operated by the Russian Federal Space Agency (ROSCOSMOS) monitored Earth’s weather and environment. Most satellites in the series monitored ozone through a multichannel spectrometer known as “Device 174-K”, but one implemented the NASA-built TOMS.
Nimbus-7
Launched in 1978, Nimbus-7 was the final satellite of the Nimbus series operated by NASA. The Nimbus series were research and development satellites to test and collect data on different parameters of Earth’s atmosphere, ocean, and cryosphere. Nimbus-7 monitored ozone through the use of a TOMS and an SBUV instrument. It was the first satellite to detect the ozone hole.
References
1) Ozone | Gas Encyclopedia Air Liquide. (n.d.). https://encyclopedia.airliquide.com/ozone#applications
2) Salawitch, R. J., McBride, L. A., Thompson, C. R., Fleming, E. L., McKenzie, R. L., Rosenlof, K. H., Doherty, S. J., & Fahey, D. W. (2023). Twenty Questions and Answers About the Ozone Layer: 2022 Update, Scientific Assessment of Ozone Depletion: 2022. World Meteorological Organization. https://www.csl.noaa.gov/assessments/ozone/2022/
3) What is Ozone? | US EPA. (2023, July 11). US EPA. https://www.epa.gov/ozone-pollution-and-your-patients-health/what-ozone
4) Cooper, O. R., Parrish, D. D., Ziemke, J., Balashov, N. V., Cupeiro, M., Galbally, I. E., Gilge, S., Horowitz, L., Jensen, N. R., Lamarque, J.-F., Naik, V., Oltmans, S. J., Schwab, J., Shindell, D. T., Thompson, A. M., Thouret, V., Wang, Y., & Zbinden, R. M. (2014). Global distribution and trends of tropospheric ozone: An observation-based review. Elementa: Science of the Anthropocene, 2. https://doi.org/10.12952/journal.elementa.000029
5) World Meteorological Organization (WMO). Executive Summary. Scientific Assessment of Ozone Depletion: 2022, GAW Report No. 278, 56 pp.; WMO: Geneva, 2022.
6) National Aeronautics and Space Administration & National Oceanic and Atmospheric Administration. (2008). NOAA-N Prime. https://www.ospo.noaa.gov/assets/pdf/NOAA_NP_Booklet.pdf
7) WG, I. (2013). The physical science basis. Contribution of working group I to the fifth assessment report of the intergovernmental panel on climate change, 1535.
8) Total Ozone Mapping Spectrometer-Earth Probe (TOMS-EP) (n.d.) NASA. https://eospso.nasa.gov/missions/total-ozone-mapping-spectrometer-earth-probe.