A study by local scientists has confirmed that emissions of chlorofluorocarbons (CFC) — major ingredients in refrigerants and propellants — cause ozone layer depletion, a theory that was cast into doubt in the past two years.
Jim Lin (林志民), a joint appointment associate research fellow of Academia Sinica’s Institute of Atomic and Molecular Sciences and National Chiao Tung University’s Department of Applied Chemistry, was able to reach his conclusion by precisely measuring chlorine peroxide (ClOOCl) — generated when chlorofluorocarbons decompose — in a laboratory environment.
Chlorine peroxide has long been thought to trigger ozone destruction when the molecule absorbs sunlight and breaks into two chlorine atoms and an oxygen molecule, a process known as photolysis.
The larger the absorption cross section of chlorine peroxide, the faster chlorine peroxide absorbs sunlight and the faster chlorine atoms are generated, depleting the ozone layer at a rapid pace.
That fundamental theory was challenged, however, in 2007 when F.D. Pope and his co-workers from the Jet Propulsion Laboratory at the California Institute of Technology presented contradictory data.
An Academia Sinica statement said Pope measured absorption cross sections — a method to test the rate of photolysis — of C1OOCl that were about 10 times smaller than previously accepted values.
That meant “it would be impossible to produce enough chlorine atoms to explain the observed ozone loss via any known chemical mechanisms,” the statement said.
But Lin’s work, the results of which were published in Science on Friday, proved that traditional explanations of ozone layer depletion were valid.
Lin on Friday said the traditional way to test chlorine peroxide has been to test the amount of light that ClOOCl absorbs at various wavelengths using an optical spectrometer.
Lin said he suspected, however, that Pope’s results and his attempts to remove the optical method’s vulnerability to impurities in the sample were not successful.
To avoid the problem, Lin said he and his colleagues used a mass spectrometer to measure the molecular weight of the C1OOCl shot through the spectrometer in a chlorine peroxide molecular beam, impurities included.
They then determined the probability of the chlorine peroxide being split into chlorine atoms and oxygen by light by measuring the beam intensity after it was irradiated with a laser.
The experiment allowed his team to more accurately quantify the absorption cross section, and they obtained values consistent with previously calculated values, Lin said. By using the mass spectrometer, the scientists were able to factor out sample impurities from the experiment, impurities which Lin said explained Pope’s lower absorption readings.
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