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的紫外线辐射，决定温度结构，从而影响平流层的循环，并且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。自1980年代初在南极洲上空发现"臭氧洞"以来，大量的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自由基进行自我反应形成二聚体Cl2O2，Cl2O2光解释放出Cl原子，Cl原子又与臭氧反应形成ClO。这一循环是极地平流层臭氧破坏的主要途径，其速率取决于Cl2O2的光解速率（吸收截面）。一些实验室研究的吸收截面的Cl2O2之前已经进行了一些研究之间的分歧，特别是在300纳米以上的波长，信号很小，因此难以测量，和干扰效应从实验室前体可能是显着的。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的大气光解有显著贡献。最近，从各种遥感和原位技术中测量大气中的ClOx物种已被用于限制Cl2O2的光化学，结果表明横截面应该比评估（NASA-JPL，IUPAC）建议的要高。然而，在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，该研究发现Cl2O2的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没有定量的了解，这一发现具有重大的科学和社会意义。该项目的目的是应用一种新的方法来研究Cl2O2的光化学，使用一系列新颖的仪器来明确地限制存在的各种物种。从本质上讲，我们将在一个实验室系统中产生Cl2O2，在极地平流层的代表性条件下，使用激光在选定的波长下光解Cl2O2，并测量产生的Cl原子。我们将使用共振荧光技术来检测氯原子，提供数量级更大的灵敏度比以前采用的吸收方法，并将使用化学电离质谱法（CIMS）来量化的Cl2O2，和干扰物种，如Cl2和Cl2O/的存在，这可能是负责以前的研究之间的差异。CIMS系统的检测限也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 nm区域。实验将在伯明翰进行，由William Bloss博士领导，使用曼彻斯特大学Carl Percival博士为大气场测量开发的新CIMS系统。我们的研究结果将确定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更高，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，我们的研究结果将被纳入平流层化学和传输模型，以确定修订后的臭氧损失率，并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.