Gas phase studies of the kinetics of Criegee Intermediates

地球的大气层是气体、液体甚至固体的复杂混合物。我们周围的这种主要是气体的包层发挥着许多重要的功能，例如，它保护我们免受太阳通过平流层臭氧层发出的有害紫外线(高能光)的伤害。通过水循环，云在大气中形成，在地球系统中重新分配水。这些云层也像镜子一样冷却地球，将太阳的部分能量反射回外层空间。这种冷却机制对于地球系统来说是至关重要的，以允许公平的表面温度存在，从而允许地球上所有不同的生命形式存在。除了这种自然冷却机制，大气中的所谓温室气体(如二氧化碳、二氧化碳和甲烷CH4)吸收地球释放的红外能量并捕获其中的一部分，非常类似于披上毯子使地球表面升温。这两种自然冷却和变暖机制对宜居表面都是必不可少的，只要它们保持平衡，表面温度就会保持合理的恒定。然而，在过去的200年里，人类一直在通过燃烧化石燃料来增加大气中的温室气体水平，有证据表明，这正在导致地球表面的全面变暖。即使全球平均地表温度略有升高，对人类、动物和植物的影响也是巨大的。众所周知，植物自然释放的化学物质(不饱和有机分子，如烯烃)可以与大气中的氧化剂反应，产生极快反应的中间体，即所谓的Criegee中间体(CI)。然而，我们最近的研究表明，这些Criegee中间体与大气中存在的一些物种反应迅速，例如二氧化硫(SO2)。最终，这些反应导致硫酸的形成，这对促进气溶胶的形成(云的前体)非常有利。在污染环境下，气溶胶的形成可能会对健康造成不利影响，但在背景大气中，促进云的形成会导致地球表面的降温。我们已经使用大气的计算机模型评估了这些化学物质自然排放的可能影响，似乎这一过程在产生云前驱物方面可能非常重要，因此对地球气候(使其降温)产生重要影响。然而，我们只能调查两种可能的Criegee中间体的反应，而且可能有数千种不同的中间体。虽然不可能研究所有的中间体，而且确实不是一个明智的努力，但研究不同类型的Criegee中间体是很重要的。如果它们都有相似的反应性，那么对大气的影响很可能是真实的，因此将其纳入气候模型将是重要的。为了调查这些Criegee中间体与二氧化硫等物种的反应速度，我们在实验室里设计了一个实验，利用光学的最新发展。使用激光产生这些Criegee中间体，我们将能够使用一种名为腔振铃光谱(CRDS)的高灵敏度技术来检测它们。在实验中，Criegee中间体是在一个封闭的系统中产生的，光被困在两个高反射镜之间。当光线在两面镜子之间来回反弹时，它可能会被克里吉中间体吸收，因此留下的光就少了。Criegee中间体的水平越高，来回反射的光线就越少，所以我们有办法测量这个物种。通过这种方式，我们将能够调查这些Criegee中间体与地球大气层中的一些重要气体反应的速度。

The Earth's atmosphere is a complex mixture of gases, liquids and even solids. This mainly gaseous envelope around us performs many vital functions e.g. it protects us from harmful ultraviolet light (high energy light) from the Sun through the stratospheric ozone layer. Through the water cycle, clouds are formed in the atmosphere that redistributes water in the Earth systems. These clouds also cool the planet by acting like mirrors, reflecting some of the energy from the Sun back to outer space. This cooling mechanism is essential to the Earth system to permit an equitable surface temperature to exist that allows all the diverse life forms on it to exist. As well as this natural cooling mechanism, so called greenhouse gases in the atmosphere (e.g. carbon dioxide, CO2 and methane CH4) absorb infrared energy released by the Earth and trap some of it, very similar to the analogy of putting on a blanket and warm the Earth's surface up. Both these natural cooling and warming mechanisms are essential to a habitable surface and as long as they stay in balance, the surface temperature will remain reasonably constant. However, over the last 200 years humans have been increasing the level of greenhouse gases in the atmosphere by burning fossil fuels and evidence shows that this is leading to an overall warming of the surface of the Earth. The consequences of even a modest increase in average global surface temperature are significant for human, animal and plant life.It is known that chemicals released naturally by plants (unsaturated organic molecules such as alkenes) can react with oxidants in the atmosphere to produce extremely fast reacting intermediates, so called Criegee intermediates (CI). However, recent studies by us have shown that these Criegee intermediates react rapidly with a number of species present in the atmosphere such as sulphur dioxide (SO2). Ultimately, these reactions lead to the formation of sulphuric acid, which is very good at promoting aerosol formation (cloud precursors). Under polluted environments, aerosol formation may have detrimental effects on health but in the background atmosphere, promotion of cloud formation leads to a cooling of the Earth's surface. We have assessed the possible impact of these natural emissions of chemicals using computer models of the atmosphere and it appears that this process may be very important in producing cloud precursors and therefore be having an important impact on the Earth's climate (cooling it). However, we have only been able to investigate the reactions of two possible Criegee intermediates and there are potentially thousands of different ones. Whilst it would be impossible to study them all and indeed not a sensible endeavour, it is important to study different types of Criegee intermediates. If they all have a similar reactivity then the impact on the atmosphere is likely to be true and would then be important to include in climate models. In order to investigate how quickly these Criegee intermediates react with species such as SO2 we have devised an experiment in the laboratory that takes advantage of recent developments in optics. Using laser light to generate these Criegee intermediates we will be able to detect them using a highly sensitive technique called cavity ringdown spectroscopy (CRDS). In the experiment the Criegee intermediate is generated in a closed system where light is trapped between two highly reflective mirrors. As the light bounces backwards and forwards between the mirrors it may be absorbed by the Criegee intermediate and so less light is left. The greater the level of Criegee intermediate made the less light is reflected back and forth and so we have a way to measure this species. In this way we will be able to investigate how fast these Criegee intermediates react with a number of important gases in the Earth's atmosphere.