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Ozone Layer Depletion

Ozone Layer Depletion. Ozone in the Stratosphere. 平流層的臭氧 (O 3 ) 阻檔有害的紫外線 (UV) 保護地球上的生物是好的臭氧 對流層 ( 地面附近 ) 的臭氧是光化學煙霧的重要成份是壞的臭氧. Figure source: Synthesis and Assessment Product 2.4, Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research.

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Ozone Layer Depletion

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  1. Ozone Layer Depletion

  2. Ozone in the Stratosphere 平流層的臭氧(O3)阻檔有害的紫外線(UV)保護地球上的生物是好的臭氧 對流層(地面附近)的臭氧是光化學煙霧的重要成份是壞的臭氧 Figure source: Synthesis and Assessment Product 2.4, Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research

  3. Ozone is naturally produced in the stratosphere in a two-step process. In the first step, ultraviolet sunlight breaks apart an oxygen molecule to form two separate oxygen atoms. In the second step, each atom then undergoes a binding collision with another oxygen molecule to form an ozone molecule. In the overall process, three oxygen molecules plus sunlight react to form two ozone molecules.

  4. 臭氧柱(ozone column)的濃度單位Dobson Unit (D.U.) • 如果我們將大氣層內的臭氧集中到地表，並讓其覆蓋整個地表，在標準狀態下臭氧的厚度平均只有3mm，這樣的臭氧量定義為300D.U. (Dobson Unit)，D.U.代表臭氧柱(ozone column)的厚度。

  5. 平流層臭氧可以阻擋紫外線(UV) • 紫外線依其波長可以分為：UV-a (315-400nm)、UV-b (280-315nm)、及UV-c (200-280nm)三種。 • UV-c在35km的高空中就幾乎完全被臭氧吸收，所以不會到達地表； • UV-a可以到達地面，但它不會對基因產生危害，所以比較不令人擔心； • 如果曝曬UV-b過久會造成曬傷，也可能危害DNA，造成皮膚癌，所以要加以注意。 • 平流層中的臭氧會吸收大部份UV-b，但仍有一部份會到達地面，但如果平流層中的臭氧減少，到達地面的 UV-b就會變多，產生不利影響， • 如大氣層中臭氧總量減少1%，地表UV-b輻射強度將增加約2%，這必然會給人類和生物帶來災難。

  6. 臭氧層破壞的風險 • 造成嚴重曬傷 • 增加眼睛白內障(eye cataracts) • 增加皮膚癌(skin cancer) • 降低免疫系統(Immune system) • 增加酸沉降 • 降低作物的產量

  7. Ozone layer depletion process

  8. 氟氯碳化物(Chlorofluorocarbons, CFCs) • 科學家發現破壞臭氧層的元凶為氟氯碳化物，氟氯碳化物非常穩定，不自燃、不助燃、不易起化學變化、對人體傷害較小，因而使用遍及各種工業及日常生活用品。其中又以CFC-11(CCl3F) 、 CFC-l2 (CCl2F2) 及CFC-113 (C2Cl3F3)三種原料佔最大使用量。 • CFC使用範圍包括： • 發泡劑： 硬質PU發泡、軟質PU發泡、聚苯乙烯(PS)發泡及PE發泡等。 • 冷媒 ： 冷凍機、冰箱、汽車、空調用冷媒。 • 清洗劑 ： 印刷基板、半導體材料等電子零件及光學零件清洗劑。 • 噴霧劑： 化粧品、醫藥品、清潔用品等需要推進之噴霧裝置。 • 消毒醫療器材 • 貨輪和穀倉的煙燻 • 此外，海龍(Halon)是含溴的全鹵化碳氫化合物，因具有特別的防火效果，常作為滅火劑。然而，由於海龍破壞臭氧的能力更甚於CFC，所以在使用上更值得關切。

  9. 南極臭氧洞(ozone hole) • 南極冬天非常冷 • 產生極地渦漩(polar vortex),和外界隔絕 • 產生極地平流層雲(PSC) • 在PSC表面產生化學反應 • 在南極春天(九月)產生臭氧洞

  10. 在1985年，英國南極觀測站的科學家法曼(Joseph C. Farman)等人發現，從1977~1984年，每年南半球的春季時（約9~12月）南極上空的大氣臭氧含量約減少了40%以上，其他研究機構也證實這項發現，並指出臭氧量急遽減少的區域面積甚至大於南極大陸，高度則是介於12~24公里之間的平流層，這就是所謂的「臭氧洞」(Ozone Hole)。其實臭氣洞並不是真正有個「洞」，而只是表示臭氧含量反常稀少的區域。南極臭氧層厚度變化極大，從100至400 Dobson Unit，而厚度若在220 Dobson Unit以下，即稱為臭氧層破洞。 造成CFCs在南極上空較易產生光化學反應而形成臭氧破洞的主要因素是南北極圈海路的差異性。南極大陸塊的平均溫度因海洋調節的緣故比北極圈的平均溫度低，南極冰凍大地的上空平流層溫度非常的低，而較易形成所謂的極區平流層雲(PSCs, Polar Stratospheric Clouds)。CFCs經過了大氣中化學反應會形成ClONO2及HCl等化合物（稱為氯貯存物質），並被吸附在PSCs表面。而PSCs中所含的冰粒，不僅會使氯貯存物質釋放出氯，更會進一步妨礙氯貯存物質的生成，加速臭氧與CFCs光化學反應，因此南極圈臭氧層的破壞速度會較北極為高。 Source: http://www.saveoursky.org.tw/2_science/ozonehole.asp

  11. 除PSCs外，另一種與南極臭氧洞形成有關的氣候特徵，是所謂的「極地渦旋」(polar vortex)。極地渦旋形成的時間大約是在每年的5、6月間，也就是在南極冬季開始時，由強烈的冷氣團環流所形成的渦旋，這種現象會一直持續到大約11月，當溫度回升時，極地渦旋才會消解。由於形成極地渦旋的冷氣團風速強勁，因此渦旋內部的空氣會與周圍的大氣完全隔離，而從低緯度地區所吹來溫暖而富含臭氧的空氣，便無法進入渦旋，使內部溫度無法上升，而有助於生成PSCs，造成臭氧分解；同時，能吸收紫外線輻射，使大氣溫暖的臭氧被分解，氣溫亦愈下降，又促進了PSCs的生成，也使低溫的極地渦旋更為穩定。這種渦旋和PSCs互相回饋的機制，使南極臭氧含量在每年大約10月間達到最低點，之後，隨著溫度回升，渦旋瓦解，PSCs也隨之消融，南極臭氧量方逐漸回升。 雖然北極不會形成如南極一般的強大渦旋，而且北極渦旋存在的時間較短，因此目前臭氧分解情形並不嚴重。但是科學家已在北極上空發現有與南極相同濃度的氯，若再加上渦旋和PSCs之間的回饋作用，近年來在北極也發現有在冬季減少，形成類似臭氧洞的現象。 Source: http://www.saveoursky.org.tw/2_science/ozonehole.asp

  12. Solutions: Protecting the Ozone Layer • CFC substitutes • Montreal Protocol 1987 • London(1990), Copenhagen(1992), Beijing(1999) Amendments • CFCs take 10-20 years to get to the stratosphere • CFCs take 65-385 years to break down

  13. Figure source: Synthesis and Assessment Product 2.4, Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research

  14. Depletion of the stratospheric ozone layer by human-produced ozone-depleting substances has been recognized as a global environmental issue for more than three decades, and the international effort to address the issue via the United Nations Montreal Protocol marked its 20-year anniversary in 2007. • Scientific understanding underpinned the Protocol at its inception and ever since. As scientific knowledge advanced and evolved, the Protocol evolved through amendment and adjustment. Policy-relevant science has documented the rise, and now the beginning decline, of the atmospheric abundances of many ozone-depleting substances in response to actions taken by the nations of the world. • Projections are for a return of ozone-depleting chemicals (compounds containing chlorine and bromine) to their “pre-ozone-depletion” (pre-1980) levels by the middle of this century for the mid-latitudes; the polar regions are expected to follow suit within 20 years after that. • Since the 1980s, global ozone sustained a depletion of about 5 percent in the mid-latitudes of both the Northern Hemisphere and Southern Hemisphere, where most of the Earth’s population resides; it is now showing signs of turning the corner towards increasing ozone. The large seasonal depletions in the polar regions are likely to continue over the next decade but are expected to subside over the next few decades. Ozone-depleting substances should have a negligible effect on ozone in all regions beyond 2070, assuming continued compliance with the Montreal Protocol. source: Synthesis and Assessment Product 2.4, Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research

  15. To facilitate a rapid phase-out of ODSs, the Montreal Protocol allowed the use of hydrochlorofluorocarbons (HCFCs) as interim substitutes for chlorofluorocarbons (CFCs). Temporary use of HCFCs was allowed because, even though HCFCs contain chlorine and are ODSs, they are much less efficient at causing stratospheric ozone depletion than the ODSs they replaced, and, therefore, have been considered as in-kind replacements to transition to a non-CFC world. Elimination of ODSs (including HCFCs) in nearly all applications is anticipated as the phase-out schedules run their course. Most uses of ODSs have been replaced with the non-ozone-depleting, non-chlorine-, and non-bromine-containing hydrofluorocarbons (HFCs) and other so-called “not-in-kind” alternatives (e.g., nonsolvent-based cleaning processes, and hydrocarbon-based refrigerants). These changes have had a measurable influence on the global atmospheric abundance of these gases, with the result that the overall abundance of chlorine and bromine reaching the stratosphere has declined in recent years.

  16. Large increases in surface ultraviolet (UVB; 280-315 nm) radiation and the associated impacts on human health and ecosystems would likely have occurred if atmospheric abundances of ozone-depleting substances had continued to grow. Scientific findings regarding the role of ozone-depleting chemicals, projected ozone losses, and the potential UV impacts galvanized international decision making in the 1980s. As a result of the worldwide adherence to the 1987 Montreal Protocol and its Amendments and Adjustments, the large impacts were avoided, and future trends in UVB and UVA (315-400 nm) at the surface are expected to be more influenced by factors other than stratospheric ozone depletion (such as changes in clouds, atmospheric fine particles, and air quality in the lower atmosphere). source: Synthesis and Assessment Product 2.4, Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research

  17. Comparison of the Ozone Depletion Potentials (ODPs) and Global Warming Potentials (GWPs) for principal ozone-depleting substances (ODSs) and hydrofluorocarbons (HFCs). The contributions of emissions to ozone depletion and climate change increase with the ODP and GWP values, respectively. HFCs are ODS substitute gases that do not destroy ozone (i.e., ODP = 0). The comparison is for emissions of equal mass. The GWPs are evaluated for a 100-year period after emission. The ODPs of CFC-11 and CFC-12, and the GWP of CO2 are defined to have values of 1.0 (Daniel et al., 1995; IPCC/TEAP, 2005; WMO, 2007).

  18. CFCs破壞臭氧層的重要歷史事件 • 1974年6月： Rowland 與 Molina 在 Nature 期刊上發表 CFCs 會破壞平流層的臭氧。 • 1984年10月：英國科學家 Joseph Farman 在南極的垂直觀測，發現近40%的臭氧已消失，並懷疑為CFCs所導致。 • 1985年8月：美國太空總署將衛星所拍攝之圖片公諸於世，確認了南極上空臭氧層已減少，形成一個臭氧洞。 • 1986年8月：美國十三位科學家組成國家臭氧層探險隊，展開遠征南極之旅。 • 1987年5月：美國科學家 Jim Anderson 以其發展的分析方法證實破壞臭氧的物質為ClO。 • 1994年11月：美國太空總署發布 UARS 全球衛星遙測結果，顯示平流層之氯與氟完全源自CFCs的分解產物。至此，有關CFCs的爭議終於塵埃落定，CFCs是破壞臭氧的物質。 • 1995年11月： Rowland 、 Molina 以及 Crutzen 三人因對臭氧平衡機制研究有重要貢獻而共同獲得諾貝爾化學獎。 • Source: http://www.saveoursky.org.tw/2_science/ozonehole.asp

  19. Links 臭氧層保護在台灣 http://www.saveoursky.org.tw/main.asp?cat=

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