Unusually chilly temperatures in part of the Arctic atmosphere and the
formation of iridescent ice clouds there triggered significant depletion of the protective ozone layer last spring.
"This was pretty sudden and unusual," said Bryan Johnson, a research chemist in the Global Monitoring Division of NOAA's Earth System Laboratory in Boulder, Colo., and a co-author of the new paper published in the October 2 issue of the journal, Nature.
The lowdown on ozone
The stratospheric ozone layer protects Earth from the sun’s damaging ultraviolet radiation.
But chemicals from air pollutants — such as chlorine and bromine — can eat away at ozone under the right conditions: sunlight combined with temperatures below -78 degrees C (-108 degrees F), which is cold enough to form nitric acid-containing ice crystals in the relatively dry atmosphere.
Scientists at the South Pole release a balloon (known as an ozonesonde) that carries ozone-measuring instruments and other sensors through the polar atmosphere 20 miles high.
High resolution (Credit: NOAA)
Earth’s best-known ozone hole forms with the return of sunlight in the Antarctic spring every year (that’s September and October), when total ozone levels can drop from 250-300 Dobson units to just above 100.
Springtime above the Arctic is usually far less eventful, but last spring, polar vortex winds — those circling strongly around the Arctic — kept the region very cold at high altitudes.
“The Arctic vortex held together, keeping stratospheric temperatures cold enough for these ice clouds to stay around,” said Johnson. “As sunlight returned, it all came together to trigger significant thinning of the ozone.”
The result? Total column ozone measurements in the Arctic, normally 400 Dobson units this time of year, were down to 310 Dobson units in late March. And although the thinner ozone meant more radiation could hit Earth’s surface, ozone levels in the Arctic remain higher than anywhere else, Johnson said.
Ozone levels are normally lower in regions near the equator. This means that spring travelers in Mexico, for example, were baring skin under an ozone blanket of only about 270 Dobson units.
What a thinning ozone means for researchers
The unusual Arctic ozone depletion this year prompted Johnson and his colleagues to log observations more frequently. NOAA scientists typically release ozonesondes — giant balloons carrying ozone-measuring instruments into the atmosphere — from the National Science Foundation’s Summit Station in Greenland weekly in the spring.
In fact, researchers sent up ozonesondes at least twice weekly in March and April to collect data, Johnson said. International scientists at other locations around the Arctic also took note of dropping levels and increased the frequency of their observations, too.
Stratospheric ozone generally varies from year-to-year, though satellite and radiosonde measurements show a long-term cooling trend. Climate models predict cooling of the stratosphere, along with warming at Earth’s surface.
To learn more about ozone-depletion research, read this interview with NOAA scientist Bryan Johnson. 
