Sun, Aug 30, 2009 page7

• Laughing gas now biggest manmade risk to ozone layer

  • BLOOMBERG
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  Nitrous oxide, the laughing gas used by dentists to pacify anxious patients, is putting stress on the environment and is now the biggest man-made destroyer of the ozone layer. N2O, a gas produced by using fertilizers and burning fuels, accounts for about 40 percent of ozone-destruction after the use of gases such as chlorofluorocarbons, or CFCs, was banned because of their harmful effects, said A.R. Ravishankara, a US National Oceanic and Atmospheric Administration chemist.

  Laughing-gas emissions are rising by about 0.25 percent a year and may be responsible for up to 80 percent of human-caused ozone depletion by 2050, said the co-author of a study in the journal Science. Because N2O is also a greenhouse gas, reducing its output will result in a “win-win for both ozone and climate,” his team wrote.

  “Nitrous oxide right now is more than twice as large as the next largest ozone-depleting gas,” Ravishankara said in an Aug. 25 telephone interview from Boulder, Colorado. “Nitrous oxide is also a greenhouse gas so if you were to reduce emissions because of the ozone layer issue, you’d also be helping the climate issue.”

  The ozone layer, which protects humans from carcinogenic ultraviolet radiation, was depleted by man-made compounds such as halons and CFCs, whose use in aerosol cans and refrigerators was banned by the 1987 Montreal Protocol, an international treaty that didn’t regulate against N2O.

  While overall emissions of ozone-depleting gases have gone down, meaning less damage will be done to Earth’s protective layer going forward, N2O emissions haven’t declined so now account for a bigger proportion of the total The researchers calculated the so-called ozone-depletion potential of N2O and found that while proportionally it is only one-sixtieth as damaging as a gas called CFC-11, because so much more nitrous oxide is emitted, it now accounts for more than twice as much ozone destruction.

  While the sum total of gases regulated by the Montreal Protocol is still more damaging to the atmosphere than nitrous-oxide emissions, “things will change with time with CFCs declining and N2O growing,” Nobel Prize-winner Paul Crutzen, professor of atmospheric chemistry at the Max Planck Institute for Chemistry in Mainz, Germany, said in an e-mail. Crutzen, who wasn’t part of Friday’s study, shared the 1995 Nobel Prize for Chemistry for his research into ozone.

  The ozone layer is found mainly in the stratosphere about 25km above Earth. Ozone depletion above the poles is caused by reactions of ozone, or O3 molecules, with chlorine or nitrogen oxides, triggered by a combination of low temperatures and sunlight.

  Because temperatures are colder, Antarctica experiences more depletion than the Arctic and a “hole” forms annually. The ozone hole typically reaches a maximum extent in September or October and last year covered 27 million square kilometers, 2 million square kilometers more than in 2007, the World Meteorological Organization in Geneva said.

  N2O is also produced by natural processes, when bacteria convert compounds containing nitrogen in rivers and forests into the gas, Ravishankara said. Anthropogenic or man-made sources include sewage treatment, fuel combustion and fertilizer.
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