On September 21, 2023, the ozone hole in Antarctica reached its maximum size, according to annual satellite and probe measurements from NASA and NOAA. The 10 million square mile, or 26 million square kilometer, hole is believed to be the 12th largest single-day ozone hole since 1979.

During peak ozone depletion season, from Sept. 7 to Oct. 13, the hole averaged 8.9 million square miles (23.1 million square kilometers) this year; which was roughly the size of North America, making it the 16th largest at the time.

“This is a very modest ozone hole,” said Paul Newman, NASA Ozone Research Group Leader and Chief Earth Science Scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Active stratospheric weather conditions in Antarctica, along with a decrease in man-made chlorine compounds, have slightly improved ozone levels this year.”

The ozone layer acts as Earth’s natural sunscreen, as this part of the stratosphere protects our planet from the sun’s harmful ultraviolet radiation. Depletion of the ozone layer means less protection against ultraviolet rays, which can cause sunburn, cataracts and skin cancer in humans.

Every September, the ozone layer thins and an “ozone hole” forms on the Antarctic continent. The hole is not a completely ozone-free hole; Scientists use the term “ozone hole” as a metaphor for the region in Antarctica where ozone concentrations have fallen well below the historical threshold of 220 Dobson units. Scientists first reported evidence of ozone depletion in 1985 and have monitored ozone levels in Antarctica every year since 1979.

The depletion of the ozone layer in Antarctica occurs when man-made chemicals containing chlorine and bromine first rise into the stratosphere. These chemicals break down and release chlorine and bromine, causing chemical reactions that destroy ozone molecules.

Ozone-depleting chemicals, including chlorofluorocarbons (CFCs), were once widely used in aerosol cans, foams, air conditioners, fire extinguishers and refrigerators. CFCs, the major ozone-depleting gases, have a lifespan in the atmosphere of 50 to 100 years.

The 1987 Montreal Protocol and subsequent amendments banned the production of CFCs and other ozone-depleting chemicals worldwide until 2010. The resulting reduction in emissions led to a decrease in the content of ozone-depleting chemicals in the atmosphere and signs of restoration of stratospheric ozone.

NASA and NOAA researchers monitor the ozone layer above the arctic and around the world using instruments on NASA’s Aura, NOAA-NASA Suomi NPP, and NOAA-20 satellites. The Aura microwave echo sounder also assesses ozone-depleting chlorine levels.

Scientists also monitor the average level of depletion by measuring the ozone concentration inside the hole. At NOAA’s Fundamental Atmospheric Observatory at the South Pole, scientists measure the thickness of the layer by launching meteozones with ozonesondes and taking ground-based measurements with a Dobsonian spectrophotometer.

NOAA measurements showed a low of 111 Dobson units (DU) over the South Pole on October 3. NASA measurements averaged over a larger area recorded at least 99 Dobson units on the same day. The average density in Antarctica in 1979 was 225 AU.

“Although the total column ozone level is never zero, most years we generally see zero ozone at some altitude in the stratosphere above the South Pole,” said NOAA research chemist Brian Johnson, project leader of the Global Monitoring Laboratory’s ozonesonde group. “We’ve seen about 95% ozone depletion this year, and we generally see almost 100% ozone loss in the stratosphere.”

The Hunga Tonga-Hunga Haapai volcano, which erupted violently in January 2022 and spewed large amounts of water vapor into the stratosphere, likely contributed to this year’s ozone depletion. This water vapor likely enhanced the response to ozone depletion in Antarctica earlier in the season.

“If Hunga Tonga had not erupted, the ozone hole would probably have been smaller this year,” Newman said. “We know that the explosion reached the Antarctic stratosphere, but we cannot yet measure its impact on the ozone hole.”

See the latest status of the ozone layer over Antarctica with NASA’s Ozone Clock. Source