Mystery in the Ozone Layer, presented by Science at NASA.
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 for example 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.
A new study by NASA researchers shows that a key
ozone-depleting compound named carbon tetrachloride (CCl4)
is surprisingly more abundant in the ozone layer than it should be.
"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." 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 a physical CCl4 loss process 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 August 18th
18th issue of Geophysical Research Letters.
More information about the extra CCl4 may be found there.
For more science mysteries, on Earth and beyond, stay tuned
to Science.nasa.gov.