[Environmental Science] Precise Measurements of Ozone-Destroying Molecule Calm Debate on How the Ozone Layer is Depleted by Human Activities

[Environmental Science] Precise Measurements of Ozone-Destroying Molecule Calm Debate on How the Ozone Layer is Depleted by Human Activities (Chinese version)

According to Academia Sinica's Newsletter (2009/05/08), Joint Appointment Associate Research Fellow of the Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, and the Department of Applied Chemistry, National Chiao Tung University, Dr. Jim J. LIN and his colleagues including Dr. Yuan-Tseh LEE, former President of Academia Sinica, have made precise measurements of chlorine peroxide (ClOOCl), calming recent international debate on exactly how humans are depleting the ozone layer. Their results were published in Science on 8th-May.

According to the newsletter, the ozone in the stratosphere (commonly known as the Ozone Layer) absorbs ultraviolet radiation from sunlight and thus protects the earth. Over the last 25 years there has been increasing concern over the disappearance of ozone and its effects on the earth. In 1989 an international treaty, the Montreal Protocol, was enacted to phase out the production of substances thought to be depleting ozone. In 1995, the Nobel Prize in Chemistry was awarded to three scientists for their contributions to understanding the formation and decomposition of ozone further highlighting the international importance of the issue.

According to the newsletter, since the discovery of the ozone hole back in 1985, intense attention has been paid to the issue of ozone depletion. So far, scientists believed that the chemical processes responsible for the formation of the ozone hole were reasonably well understood. However, laboratory data on the ultraviolet absorption spectrum of chlorine peroxide published in 2007 by Dr. F. D. POPE and coworkers from the Jet Propulsion Laboratory at the California Institute of Technology cast doubt on that understanding. POPE et al. measured chlorine peroxide absorption cross sections that were about ten times smaller than previously accepted values, meaning that it would be impossible to produce enough chlorine atoms to fully explain the observed ozone loss via any known chemical mechanisms. As a result of Dr. POPE’s data, in 2007, top scientific journals Nature [Nature, 449, 382 (Sep. 2007)] and Science [Science, 318, 1878 (Dec. 2007)] reported concerns respectively with discrepant commentary attitude about the validity of current scientific models for the ozone hole formation.

In the present issue of the Science (8th-May), Dr. Jim J. LIN and his colleagues have published data suggesting the observed ozone depletion can after all be well explained by traditional scientific models. The chemical under debate, ClOOCl, is formed in the atmosphere due to human emissions of chlorofluorocarbons, major ingredients in refrigerants and propellants. In the presence of sunlight, Cl atoms are formed from photolysis of ClOOCl; Cl atoms react with ozone (O3) to form O2 and ClO, and ClO can then dimerize to form ClOOCl again, thus catalytically converting O3 to O2. The larger the absorption cross section of chlorine peroxide is, the faster chlorine peroxide absorbs sunlight, then the faster the chlorine atoms are generated, and thus the faster the ozone is destroyed. Dr. LIN and his colleagues at IAMS designed an elegant experiment using a chlorine peroxide molecular beam, and determined the photodissociation probability (the probability of the chlorine peroxide being split into chlorine atoms and oxygen by light) by measuring the decrease in beam intensity after laser irradiation. By precisely measuring the ratio of the molecules before and after laser irradiation, they were able to quantify the absorption cross section without having to know the absolute concentration. Using currently accepted kinetic models, their new results can well explain the ozone-hole formation as well as the partition of ClO/ClOOCl from field measurements, indicating that the currently challenged atmospheric models are in fact still valid.

With prior experiences with molecular beams, the IAMS group was able to get exciting results within a short period of time. The results set new and stringent constraints for atmospheric chemistry allowing scientists in the future to investigate more deeply and more accurately on the Ozone Hole Chemistry.


Related Website:
http://www.sciencemag.org/current.dtl

Media Contacts:
Dr. Jim J. Lin,
Joint Appointment Associate Research Fellow of the Institute of Atomic and Molecular Sciences, Academia Sinica (IAMS) and the Department of Applied Chemistry, National Chiao Tung University
(Tel) +886-2-2366-8258
Mei-Hui Lin, Public Relations Office, Central Office of Administration, Academia Sinica
mhlin313@gate.sinica.edu.tw (Tel) +886-2-2789-8821, (Fax) +886-2-2782-1551, (M) 0921-845-234


Reference:
Academia Sinica Newsletter 2009/05/08



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