Mystery in the Ozone Layer
 
Sept. 5, 2014:  High above Earth, more than 20 miles above sea level, a
diaphanous layer of ozone surrounds our planet, absorbing energetic UV rays
from the sun.  It is, essentially, sunscreen for planet Earth. Without the
ozone layer, we would be bathed in dangerous radiation on a daily basis, with
side effects ranging from cataracts to cancer.
 
People were understandably alarmed, then, in the 1980s when scientists noticed
that manmade chemicals in the atmosphere were destroying this layer.
Governments quickly enacted an international treaty, called the Montreal
Protocol, to ban ozone-destroying gases such as CFCs then found in aerosol
cans and air conditioners.  On September 16, 1987, the first 24 nations signed
the treaty; 173 more have signed on in the years since.
 
Fast forward 27 years.  Ozone-depleting chemicals have declined and the ozone
hole appears to be on the mend. The United Nations has called the Montreal
Protocol "the most successful treaty in UN history." Yet, despite Montreal's
success, something is not . quite . right.
 
http://youtu.be/rq9EAHyHtu8
 
A new ScienceCast video looks into the surprising abundance of carbon
tetrachloride in the ozone layer.  Where is it coming from?
A new study by NASA researchers shows that a key ozone-depleting compound
named carbon tetrachloride (CCl4) is surprisingly abundant in the ozone layer.
 
"We are not supposed to be seeing this at all," says NASA atmospheric
scientist Qing Liang.
 
Between 2007 and 2012, countries around the world reported zero emissions of
CCl4, yet measurements by satellites, weather balloons, aircraft, and
surface-based sensors tell a different story.  A study led by Liang shows
worldwide emissions of CCl4 average 39 kilotons per year, approximately 30
percent of peak emissions prior to the international treaty going into effect.
 
In the 1980s, chlorofluorocarbons became well-known to the general public. As
the ozone hole widened, "CFC" became a household word.  Fewer people, however,
have heard of CCl4, once used in applications such as dry cleaning and
fire-extinguishers.
 
"Nevertheless," says Liang, "CCl4 is a major ozone-depleting substance. It is
the 3rd most important anthropogenic ozone-depleting compound behind CFC-11
and CFC-12."
 
http://www.epa.gov/ozone/science/process.html  
Click to learn about the chemistry of ozone depletion from the US
Environmental Protection Agency. Levels of CCl4 have been declining since the
Montreal Protocol was signed, just not as rapidly as expected. With zero
emissions, abundances should have dropped by 4% per year.  Instead, the
decline has been closer to 1% per year.
 
To investigate the discrepancy, Liang and colleagues took CCl4 data gathered
by NOAA and NASA and plugged it into a NASA computer program, the 3-D GEOS
Chemistry Climate Model.  This sophisticated program takes into account the
way CCl4 is broken apart by solar radiation in the stratosphere as well as how
the compound can be absorbed and degraded by contact with soil and ocean
waters.  Model simulations pointed to an unidentified ongoing current source
of CCl4.
 
"It is now apparent there are either unidentified industrial leakages, large
emissions from contaminated sites, or unknown CCl4 sources," says Liang.
 
Another possibility is that the chemistry of CCl4 might not be fully
understood. Tellingly, the model showed that CCl4 is lingering in the
atmosphere 40% longer than previously thought. "Is there something about the
physical CCl4 loss process that we don't understand?" she wonders.
 
It all adds up to a mystery in the ozone layer.
 
Liang's research was published online in the Aug. 18th issue of Geophysical
Research Letters. More information about the extra CCl4 may be found there.
 
Credits:
Author: Dr. Tony Phillips |  Production editor: Dr. Tony Phillips | Credit:
Science@NASA
 
 
Regards,
 
Roger

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