A Laboratory Study of the Photolysis of the ClO Dimer

平流层臭氧层位于大约15至40公里的高度之间，在地球大气层中发挥着一些关键作用：它保护生物圈免受有害紫外线辐射的影响，决定温度结构，从而影响平流层的循环，是一种辐射活性气体，也就是说，它在我们的大气层中充当温室气体。在20世纪80年代初发现南极洲上空的“臭氧空洞”后，大量的科学努力集中在了解臭氧耗尽的原因上。人为排放增加了平流层的卤素负荷，而极地平流层极地夜间的气象条件有利于特定的化学反应周期：ClO自由基发生自我反应形成二聚体Cl2O2，该二聚体光解释放组成氯原子，然后与臭氧重整ClO反应。这一循环的速度是极地平流层臭氧破坏的主要途径，取决于Cl2O2的光解率(吸收截面)。以前已经对Cl2O2的吸收截面进行了一些实验室研究，但研究之间存在一些不一致，特别是在300 nm以上的波长，那里的信号很小，因此很难测量，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，只有大于300 nm的波长对大气中Cl2O2的光解有显著贡献。最近，各种遥感和现场技术对大气中ClOx物种的测量被用来限制Cl2O2的光化学，结果表明截面积应该比评估(NASA-JPL，IUPAC)所建议的要高。然而，2007年3月，一个备受尊敬的实验室动力学小组发表了一项关于Cl2O2横截面的新研究，发现Cl2O2的光解速率比以前的测量结果低6*倍。这一结果意味着我们没有对极地平流层臭氧损失有一个定量的了解，这是一个具有重大科学和社会意义的发现。这个项目的目的是应用一种新的方法来研究Cl2O2的光化学，使用一系列新的仪器来明确地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，使用激光在选定的波长光解Cl2O2，并测量产生的氯原子。我们将使用共振荧光技术来检测氯原子，提供比以前使用的吸收方法更高数量级的灵敏度，并将使用化学电离质谱仪(CIMS)来量化Cl2O2以及Cl2和Cl2O等干扰物种，这些物种的存在可能会导致以前的研究之间的差异。同样，CIMS系统的检测下限比以前使用的吸收方法好几个数量级。我们的重点将放在对平流层至关重要的300-350纳米区域。实验将在伯明翰进行，由威廉·布洛斯博士领导，使用曼彻斯特大学的卡尔·珀西瓦尔博士为大气场测量开发的新的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循环的臭氧破坏速率，比以前实现的准确度和精密度要高得多，并将解决先前测量之间的差异。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输送的模型中，以确定修正的臭氧损失率，以便与观测结果进行比较

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