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。我们周围的这层主要是气体的外壳具有许多重要的功能，例如，它保护我们免受来自太阳的有害紫外线（高能量光）穿过平流层臭氧层。通过水循环，云在大气中形成，重新分配了地球系统中的水。这些云还像镜子一样冷却地球，将来自太阳的一些能量反射回外层空间。这种冷却机制对地球系统来说是必不可少的，它能让地球表面的温度保持平衡，让地球上所有不同的生命形式得以生存。除了这种自然冷却机制外，大气中所谓的温室气体（如二氧化碳、二氧化碳和甲烷CH4）吸收地球释放的红外能量，并将其中一些能量封存起来，这与盖上毯子使地球表面变暖的做法非常相似。这两种自然的冷却和升温机制对于一个宜居的表面都是必不可少的，只要它们保持平衡，表面温度就会保持相当稳定。然而，在过去的200年里，人类通过燃烧化石燃料增加了大气中温室气体的含量，有证据表明，这正在导致地球表面的整体变暖。即使是全球平均地表温度略有上升，对人类、动物和植物的影响也是重大的。众所周知，植物自然释放的化学物质（烯烃等不饱和有机分子）可与大气中的氧化剂发生反应，生成反应极快的中间体，即克里吉中间体（Criegee intermediates，简称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许多物种（如二氧化硫（SO2））迅速反应。最终，这些反应导致硫酸的形成，硫酸非常有利于促进气溶胶（云前体）的形成。在受污染的环境下，气溶胶的形成可能对健康产生有害影响，但在背景大气中，促进云的形成导致地球表面降温。我们利用大气的计算机模型评估了这些自然排放的化学物质可能产生的影响，结果表明，这一过程在产生云前体方面可能非常重要，因此对地球气候（使其降温）有重要影响。然而，我们只能研究两种可能的克里吉中间体的反应，而可能有数千种不同的中间体。虽然研究它们是不可能的，而且确实不是一个明智的努力，但研究不同类型的克里吉中间体是很重要的。如果它们都具有相似的反应性，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是很重要的。为了研究这些Criegee中间体与SO2等物质反应的速度有多快，我们利用光学的最新发展在实验室设计了一个实验。使用激光产生这些Criegee中间体，我们将能够使用一种称为腔衰荡光谱（CRDS）的高灵敏度技术来探测它们。在实验中，Criegee中间体是在一个封闭的系统中产生的，在这个系统中，光被困在两个高反射镜之间。当光在镜子之间来回反射时，它可能被克里吉中间体吸收，因此留下的光就少了。Criegee中间体的含量越高，来回反射的光就越少，所以我们有了一种测量这种物质的方法。通过这种方式，我们将能够研究这些克里吉中间体与地球大气中许多重要气体发生反应的速度。

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.