Modest ozone hole despite optimum conditions of depletion of the ozone layer

The hole in the ozone layer that forms in the upper atmosphere over Antarctica each September was slightly above average in 2018, scientists today said. NOAA and NASA.

Colder than average temperatures in the Antarctic stratosphere have created the ideal conditions for the destruction of ozone this year, but declining concentrations of ozone – depleting chemicals have prevented the to become as vast as it would have been 20 years ago.

"Chlorine levels in the Antarctic stratosphere have dropped about 11% from the 2000 peak," said Paul A. Newman, Chief Scientist for Earth Sciences at NASA's Goddard Space Flight Center. in Greenbelt, Maryland. "The colder temperatures this year would have left us with a much bigger hole in the ozone layer if chlorine had reached the levels we had seen in the year 2000.

According to NASA, the annual hole in the ozone layer reached an average area of ​​22.9 square kilometers (8.83 million square miles) in 2018, almost three times the size of the contiguous United States. It ranks 13th out of 40 years of NASA satellite observations. The nations of the world began phasing out the use of substances that deplete the ozone layer in 1987 under an international treaty known as the Montreal Protocol.

The hole in the 2018 ozone layer was strongly influenced by a stable and cold Antarctic vortex, the stratospheric low pressure system that circulates clockwise in the atmosphere above the ground. above Antarctica. These colder conditions – among the coldest since 1979 – have contributed to the formation of more polar stratospheric clouds, whose particles activate the forms of chlorinated and brominated compounds destroying ozone.

In 2016 and 2017, warmer September temperatures limited the formation of polar stratospheric clouds and slowed the growth of the ozone hole. In 2017, the hole in the ozone layer reached an area of ​​19.7 square kilometers (7.6 million square miles) before starting to recover. In 2016, the hole reached 8 million square miles (20.7 square kilometers).

However, the hole area in the ozone layer is still significant compared to the 1980s, when depletion of the ozone layer over Antarctica had been detected for the first time. Atmospheric concentrations of ozone – depleting substances have increased until the year 2000. Since then, they have slowly decreased, but remain high enough to cause a significant loss of ozone – depleting substances. ozone.

NOAA scientists said the colder temperatures of 2018 allowed the near-complete elimination of ozone in a deep layer 5 km above the South Pole. This layer is where the active chemical depletion of ozone occurs on polar stratospheric clouds. The amount of ozone on the South Pole reached a minimum of 104 Dobson units on October 12, making it the 12th lowest year of the 33-year NOAA ozononde layer at the South Pole. , according to NOAA scientist Bryan Johnson.

"Even with the optimal conditions this year, the ozone loss was less severe in the upper layers of the altitude, which we expected given the decline in chlorine concentrations in the stratosphere" Johnson said.

A Dobson unit is the standard measure of the total amount of ozone in the atmosphere above a point on the surface of the Earth. It represents the number of ozone molecules required to create a pure ozone layer 0.01 millimeter thick at a temperature of 32 degrees Fahrenheit (0 degrees Celsius) at an atmospheric pressure equivalent to the surface of the Earth. A value of 104 Dobson units corresponds to a layer with a thickness of 1.04 millimeters on the surface, less than the thickness of a penny.

Prior to the emergence of the Antarctic ozone hole in the 1970s, the average amount of ozone over the South Pole in September and October ranged from 250 to 350 Dobson units.

What is ozone and why is it important?

Ozone includes three atoms of oxygen and is highly reactive with other chemicals. In the stratosphere, about 11 to 40 kilometers above the Earth's surface, an ozone layer acts as a sunscreen, shielding the planet from ultraviolet rays that can cause skin cancer and cataracts, suppress the immune system and damage the plants. Ozone can also be created by photochemical reactions between the sun and pollution from emissions from vehicles and other sources, forming a harmful smog in the lower atmosphere.

NASA and NOAA use three complementary instrumental methods each year to monitor the growth and dissolution of the hole in the ozone layer. Satellite instruments such as the ozone monitoring instrument on NASA's Aura satellite and the suite of ozone mapping mappers on the National Polar Partnership satellite in orbit between NASA and NOAA measure ozone in large areas of space. The Aura microwave microwave sounder also measures some chlorine-containing gases, providing estimates of total chlorine levels.

The total amount of ozone in the atmosphere is extremely small. The totality of ozone in a column of the atmosphere extending from the ground to the space would constitute 300 Dobson units, or about the thickness of two sub piled up l & # 39; one on the other.

The NOAA scientists are monitoring the thickness of the ozone layer and its vertical distribution over the South Pole by regularly releasing weather balloons fitted with ozone "probes" at a altitude up to about 34 km, and by means of a ground-based instrument called Dobson Spectrophotometer.

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