October, 2010 from the National Ocean and Atmopheric Administration

Major success in reducing ozone-depleting substances may not pay off for several years.

August in Antarctica means the sun starts rising over the horizon again, following four months of darkness. For NOAA Corps Officer Lt. j.g. Nick Morgan, stationed at the South Pole, the month also marks the moment when he begins measuring ozone in earnest.

About once a week during most of the year, Morgan and his colleagues launch giant plastic balloons into the air. Tethered to the balloons are instruments that take ozone readings up to about 18 miles high. [Watch this cool video of an ozonesonde launch.]

Then, in the Antarctic spring (August through October), sunlight-sparked chemical reactions begin eating away at ozone. Scientists start making measurements more often, and by October, Morgan or his colleagues are outside in minus 80 F temperatures about every other day.

Morgan, who is with NOAA’s Earth System Research Laboratory (ESRL) in Boulder, Colo., and other scientists around the world are watching those data carefully, looking for evidence that the Antarctic ozone hole is beginning to heal after decades of hurt.

There’s scant evidence yet: At the end of September, total ozone levels had dropped to an annual low of 122 Dobson units. To put that into perspective, typical fall, winter and summertime levels are about 250–300 DU. During the last 24 years, the worst-of-the-year ozone level has averaged 108 DU.

It will be difficult to establish a clear-cut recovery trend in Antarctic ozone levels because seasonal cycles and other variable natural factors — from the temperature of the atmosphere to the stability of atmospheric layers — can make ozone levels dip and soar from one day to another, says NOAA ESRL scientist Bryan Johnson.

But, the time is coming, probably within a few more decades, when ozone depletion will no longer be observed each spring, Johnson predicts.

“And within the next decade, observations are anticipated to begin showing reduced severity of the ozone hole,” Johnson said.

What’s Eating the Ozone?

As soon as the sun crosses the horizon again during the Antarctic spring, sunlight-triggered chemical reactions involving air pollutants begin destroying ozone in a region of the atmosphere called the stratosphere. The stratospheric ozone layer protects Earth from some damaging ultraviolet radiation, so an ozone hole means more of that radiation can hit the surface and trigger elevated rates of skin cancer and crop damage.

In the Antarctic, the ingredients for ozone depletion line up perfectly around September: sunlight, low temperatures in the stratosphere, polar stratospheric clouds that help catalyze the destructive chemistry, and the continued presence of ozone-depleting chemicals, many of them released decades ago. Most years, those conditions ease by early December, and the hole closes back up again.

“The ozone hole has taken somewhat of a back seat in the public eye,” Morgan wrote in a recent blog post from the South Pole. “And, maybe that is a sign of success.”

Over Antarctica, Ozone ‘Wound’ Taking Longer to Heal
That international treaty initiated the phasing out of chemicals called chlorofluorocarbons (CFCs). Although levels of those ozone-depleting chemicals are dropping, many remain in the stratosphere for decades or more, and levels in the Antarctic are slower to dissipate than in other regions.

International scientists contributing to the quadrennial 2010 Ozone Assessment — including many NOAA scientists — have calculated that although global stratospheric ozone may recover by 2060, the ozone hole in the Antarctic will likely persist longer.

You can track the Antarctic ozone hole as it makes its annual recovery in late spring and summer (the region’s summer begins officially in December) by visiting: http://esrl.noaa.gov/gmd/dv/spo_oz/.

Also, check out Lt. j.g. Nick Morgan’s blog to learn more about his yearlong research at the South Pole.

Posted Oct. 19, 2010

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