A Laboratory Study of the Photolysis of the ClO Dimer

ClO二聚体光解的实验室研究

• 批准号: NE/F018045/1
• 负责人: Carl Percival
• 金额: $ 9.88万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF018045%2F1
• 关键词: Laboratory; Study; Photolysis; ClO; Dimer

The stratospheric ozone layer, located between altitudes of approximately 15 and 40 km, performs a number of critical roles in the Earth's atmosphere: It shields the biosphere from harmful UV radiation, determines the temperature structure and hence affects the circulation of the stratosphere, and is a radiatively active gas, that is, it acts as a greenhouse gas in our atmosphere. Following discovery of the 'ozone hole' over Antarctica in the early 1980s, considerable scientific effort has focussed upon understanding the causes of ozone depletion. Anthropogenic emissions have increased the stratospheric halogen loading, while the meteorological conditions of the polar stratosphere following the polar night favour a specific chemical reaction cycle: ClO radicals undergo self-reaction to form a dimer, Cl2O2, which photolyses releasing the constituent Cl atoms, which in turn react with ozone reforming ClO. The rate of this cycle, which is the major route for polar stratospheric ozone destruction, depends upon the photolysis rate (absorption cross sections) of Cl2O2. A number of laboratory studies of the absorption cross sections of Cl2O2 have been performed previously, with some disagreement between studies, particularly at wavelengths above 300 nm, where the signal is small and hence hard to measure, and interference effects from laboratory precursors may be significant. Unfortunately this is also the key region for the atmosphere / due to the spectral distribution of actinic flux, only wavelengths above 300 nm contribute significantly to the atmospheric photolysis of Cl2O2. Recently, measurements of ClOx species in the atmosphere from various remote sensing and in situ techniques have been used to constrain the photochemistry of Cl2O2, with results suggesting the cross sections should be *higher* than the evaluations (NASA-JPL, IUPAC) suggest. However, in March 2007 a new study of the Cl2O2 cross sections was published, from a highly respected laboratory kinetics group, which found the Cl2O2 photolysis rate to be a factor of 6 *lower* than earlier measurements indicated. This result implies that we do not have a quantitative understanding of polar stratospheric ozone loss, a finding of great scientific and societal importance. The aim of this project is to apply a new approach to the study of the photochemistry of Cl2O2, using a range of novel instrumentation to unequivocally constrain the various species present. In essence, we will generate Cl2O2 in a laboratory system under conditions representative of the polar stratosphere, photolyse the Cl2O2 at selected wavelengths using a laser, and measure the Cl atoms produced. We will use a resonance fluorescence technique to detect the Cl atoms, affording orders of magnitude greater sensitivity than the absorption approach employed previously, and will use Chemical Ionisation Mass Spectrometry (CIMS) to quantify both the Cl2O2, and interferant species such as Cl2 and Cl2O / the presence of which is likely to be responsible for discrepancies between previous studies. Again the detection limits for the CIMS system are orders of magnitude better than for the absorption approaches used previously. Our focus will be on the 300-350 nm region critical to the stratosphere. Experiments will be conducted at Birmingham, led by Dr William Bloss, using a new CIMS system developed for atmospheric field measurements by Dr Carl Percival from the University of Manchester. Our results will determine the photolysis rate for Cl2O2, and hence the rate of ozone destruction through the ClO + ClO cycle, with much greater accuracy and precision than has been achieved previously, and will address the discrepancies between previous measurements. Through our Project Partner, Prof. Martyn Chipperfield at the University of Leeds, our results will be incorporated in models of stratospheric chemistry and transport, to determine revised ozone loss rates for comparison with observations

平流层臭氧层位于大约 15 至 40 公里的高度之间，在地球大气中发挥着许多关键作用：它保护生物圈免受有害的紫外线辐射，决定温度结构，从而影响平流层的循环，并且一种辐射活性气体，也就是说，它在我们的大气中充当温室气体。 20 世纪 80 年代初发现南极洲上空的“臭氧空洞”后，大量科学工作集中在了解臭氧消耗的原因上。人为排放增加了平流层的卤素负荷，而极夜之后极地平流层的气象条件有利于特定的化学反应循环：ClO自由基进行自反应形成二聚体Cl2O2，其光解释放出组成的Cl原子，进而与臭氧反应重整ClO。该循环的速率是极地平流层臭氧破坏的主要途径，取决于 Cl2O2 的光解速率（吸收截面）。之前已经对 Cl2O2 的吸收截面进行了许多实验室研究，但研究之间存在一些分歧，特别是在 300 nm 以上的波长下，信号很小，因此难以测量，并且实验室前体的干扰效应可能很大。不幸的是，这也是大气的关键区域/由于光化通量的光谱分布，只有 300 nm 以上的波长对 Cl2O2 的大气光解作用有显着贡献。最近，通过各种遥感和原位技术对大气中 ClOx 物质的测量已被用于限制 Cl2O2 的光化学，结果表明横截面应“高于”评估（NASA-JPL、IUPAC）建议的值。然而，2007 年 3 月，一个备受推崇的实验室动力学小组发表了一项关于 Cl2O2 横截面的新研究，该研究发现 Cl2O2 光解速率比早期测量结果低 6 倍。这一结果意味着我们对极地平流层臭氧损失还没有定量的了解，这一发现具有重大的科学和社会重要性。该项目的目的是应用一种新方法来研究 Cl2O2 的光化学，使用一系列新颖的仪器来明确限制存在的各种物种。本质上，我们将在代表极地平流层的条件下在实验室系统中产生 Cl2O2，使用激光在选定波长下光解 Cl2O2，并测量产生的 Cl 原子。我们将使用共振荧光技术来检测 Cl 原子，其灵敏度比之前使用的吸收方法高出几个数量级，并将使用化学电离质谱 (CIMS) 来量化 Cl2O2 以及 Cl2 和 Cl2O 等干扰物质/ 其存在可能是造成先前研究之间差异的原因。同样，CIMS 系统的检测限比以前使用的吸收方法要好几个数量级。我们的重点将放在对平流层至关重要的 300-350 nm 区域。实验将在威廉·布洛斯 (William Bloss) 博士的领导下在伯明翰进行，使用曼彻斯特大学卡尔·珀西瓦尔 (Carl Percival) 博士为大气场测量开发的新型 CIMS 系统。我们的结果将确定 Cl2O2 的光解速率，从而确定通过 ClO + ClO 循环破坏臭氧的速率，其准确度和精度比以前实现的要高得多，并将解决以前测量之间的差异。通过我们的项目合作伙伴、利兹大学的 Martyn Chipperfield 教授，我们的结果将被纳入平流层化学和传输模型中，以确定修正后的臭氧损失率，以便与观测结果进行比较