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/F01791X/1
• 负责人: William Bloss
• 金额: $ 41.49万
• 依托单位: University of Birmingham
• 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%2FF01791X%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.

位于大约15至40 km之间的平流层臭氧层在地球大气中扮演许多关键作用：它屏蔽了生物圈免受有害紫外线辐射的影响，决定了温度结构，因此影响了平流层的循环，并且是一种辐射活性气体的循环，即在我们的大气层中起到辐射活性。在1980年代初发现南极洲的“臭氧孔”之后，大量的科学努力集中在理解臭氧耗竭的原因上。人为的排放增加了平流层卤素的负荷，而极性夜晚的极性平流层的气象条件有利于特定的化学反应循环：Clo自由基经历自我反应，形成二聚体Cl2O2，从而使该二聚体释放出释放组成型CL原子，从而又与焦油反应反应。该循环的速率，这是极地平流层臭氧破坏的主要途径，取决于Cl2O2的光解速率（吸收横截面）。先前已经对CL2O2的吸收横截面进行了许多实验室研究，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，因此很难测量，而实验室前体的干扰效应可能很重要。不幸的是，由于阳光通量的光谱分布，这也是大气 /大气的关键区域，仅超过300 nm的波长对CL2O2的大气光解会产生显着贡献。最近，通过各种遥感和原位技术的大气中的clox物种的测量已被用来限制Cl2O2的光化学，结果表明，横截面应 * *高于评估（NASA-JPL，IUPAC）建议。然而，2007年3月，来自备受推崇的实验室动力学组的CL2O2横截面的一项新研究，发现CL2O2光解速率比早期所指出的测量值低6 * *。这一结果意味着我们对极地平流层臭氧丧失的定量了解，这是对科学和社会重要性的发现。该项目的目的是将新方法应用于CL2O2光化学的研究，并使用一系列新型的仪器来明确限制存在的各种物种。从本质上讲，我们将在代表极性平流层的条件下在实验室系统中生成CL2O2，使用激光在选定的波长处将CL2O2照相，并测量产生的CL原子。我们将使用一种共振荧光技术来检测CL原子，比以前采用的吸收方法更高的敏感性，并将使用化学电离质谱法（CIM）来量化CL2O2的量化，以及对Cl2和Cl2O / Cl2O /可能对以前的研究负责的cl2O2和干扰物物种的量化。同样，CIMS系统的检测极限比以前使用的吸收方法要好。我们的重点将放在平流层至关重要的300-350 nm区域。通过威廉·布洛斯（William Bloss）博士领导的伯明翰，使用新的CIMS系统为曼彻斯特大学卡尔·珀西瓦尔（Carl Percival）博士开发的新CIMS系统进行实验。我们的结果将确定CL2O2的光解速率，从而确定通过CLO + CLO循环的臭氧破坏速率，其准确性和精度比以前的精度要高得多，并且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，我们的结果将纳入平流层化学和运输模型中，以确定修订后的臭氧损失率与观测值进行比较。