According to US-based agencies NOAA and NASA, persistent cold temperatures and strong circumpolar winds (also known as the polar vortex) have supported the formation of a large and deep Antarctic ozone hole that should persist into November.
The annual Antarctic ozone hole reached its peak size at about 9.6 million square miles (24,800,000km2), on September 20. Observations revealed the nearly complete elimination of ozone in a four-mile-high column of the stratosphere over the South Pole. Researchers note that 2020 will have the 12th largest ozone hole by area in 40 years of satellite records, with the 14th lowest amount of ozone in 33 years of balloon-borne instrumental measurements.
“From the year 2000 peak, Antarctic stratosphere chlorine and bromine levels have fallen about 16% towards the natural level,” said Paul Newman, chief scientist for earth sciences at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We have a long way to go, but that improvement made a big difference this year. The hole would have been about a million square miles larger if there was still as much chlorine in the stratosphere as there was in 2000.”
NASA and NOAA use three complementary instrumental methods to monitor the growth and break-up of the ozone hole each year. Systems such as the Ozone Monitoring Instrument provided by the Netherlands and Finland on NASA’s Aura satellite, the Ozone Mapping Profiler Suites on the NASA-NOAA Suomi National Polar-orbiting Partnership satellite and the NOAA-20 polar satellite measure ozone across large areas. The Aura satellite’s Microwave Limb Sounder also measures both ozone and certain chlorine-containing gases, providing estimates of total chlorine levels in the stratosphere.
NOAA scientists also monitor the thickness of the ozone layer and the amount of ozone depletion inside of the hole. They regularly release weather balloons carrying ozone-measuring sondes above the South Pole that ascend up to 21 miles in altitude, and, once sunshine returns to the Antarctic after the long polar night, conduct measurements with a ground-based instrument called a Dobson spectrophotometer.
This year on October 1, ozone measurements taken by instruments carried aloft by weather balloons from NOAA’s South Pole atmospheric observatory recorded a low value of 104 Dobson units. NASA’s Ozone Watch reported the lowest daily value for 2020 from satellite measurements was 94 Dobson Units on October 6 over Antarctica.
Bryan Johnson, a scientist with NOAA’s Global Monitoring Lab, said scientists focus on the stratosphere between 8 miles and 13 miles in altitude, which is where major depletion occurs. In late October, ozone levels in the key altitude range remained close to record lows.
“It’s about as close to zero as we can measure,” Johnson said. Still, the rate at which ozone declined in September has slowed compared with 20 years ago, he said, which is consistent with there being less chlorine in the atmosphere.