Chemical Evolution of the Atmosphere
Chemical Evolution of the Atmosphere
复制标题
大气的化学演化
DOI：
10.1002/9780470999318.ch1
复制
发表时间：
2007
期刊：
影响因子：
--
通讯作者：
R. Wayne
中科院分区：
文献类型：
--
作者：
R. Wayne
本章关注的是地球如何形成大气层，以及大气层如何通过化学、物理和生物过程而改变，从而达到目前的组成。故事从创造宇宙的“大爆炸”开始，我们把它留在距现在几亿年之前。本书的其他章节将讨论最近的成分变化，特别是与气候有关的变化。当我们在一百万年左右的时间内接近我们自己的时代时，大气成分和气候的记录变得更加丰富和详细。事实证明，特别有效的信息来源是对深入海底钻探的岩芯以及从冰盖和冰川获得的岩芯的检查。最古老的冰芯来自南极洲东部：到2007年，其深度已达到3.2公里，相当于80万年的年龄。核心显示，在此期间发生了八个大气变化周期，二氧化碳和甲烷浓度达到峰值与气候变暖有关。地球及其邻居金星和火星一定在早期阶段就失去了它们可能诞生的原始大气层。相反，次生大气是由固体形成时捕获的挥发性物质形成的，或者是后来通过撞击太阳系碎片（彗星和流星）而带入的。地球上的生命对我们大气成分的后续变化产生了巨大影响，特别是在二氧化碳、氮气和氧气的相对丰度方面。二氧化碳在我们大气中的含量不到 0.04%，但在金星和火星的大气中却占到了 95% 以上。相反，构成大气大部分的氮气和氧气只是其他两种大气的次要成分。然而，这三颗行星最初很可能都获得了相似的次生大气：生物或生物介导的过程改变了我们的大气。因此，必须寻找生命进化与地球大气层进化之间的联系。更重要的是，我们稍后会看到，氧气在保护陆地生物免受太阳紫外线辐射方面起着至关重要的作用。分子氧及其大气产物臭氧（O3）是当代大气中唯一已知的此类辐射吸收剂。因此，生命与大气之间存在着进一步的联系。
This chapter is concerned with how the planet Earth comes to have an atmosphere, and how that atmosphere has been modified by chemical, physical and biological processes to move towards its present-day composition. The story begins with the ‘Big Bang’in which the universe was created, and we leave it some hundreds of millions of years before present. Other chapters of this book will discuss more recent changes in composition, especially in connection with climate. As we approach our own era within a million years or so, the record of atmospheric composition and climate becomes richer and more detailed. Particularly fruitful sources of information have proved to be the examination of cores of rock drilled deep into the ocean floors, and cores obtained from ice-sheets and glaciers. The oldest ice-core is that from East Antarctica: by 2007 its depth had reached 3.2 km, corresponding to an age of 800 000 years. The core shows that there have been eight cycles of atmospheric change over this period, with peaks in the concentrations of carbon dioxide and methane being linked to a warming of the climate. Earth and its neighbours Venus and Mars must have lost at an early stage any primordial atmosphere with which they might have been born. Instead, a secondary atmosphere was formed from volatile materials trapped within the solid body when it was formed, or brought in later by impacting solar-system debris (comets and meteors). Life on Earth has had an enormous effect in bringing about subsequent changes to the composition of our own atmosphere, especially in terms of the relative abundances of CO2, N2 and O2. Carbon dioxide, which is present at less than 0.04% in our atmosphere, makes up more than 95% of the atmospheres of Venus and Mars. Conversely, the N2 and O2 that make up the bulk of our atmosphere are only minor components of the other two atmospheres. Yet it is likely that all three planets acquired initially similar secondary atmospheres: biological or biologically mediated processes have modified our atmosphere. A link must therefore be sought between the evolution of life and the evolution of Earth’s atmosphere. What is more, we shall see later that oxygen has a critical role in protecting organisms on land from ultraviolet radiation from the Sun. Molecular oxygen and its atmospheric product ozone (O3) are the only known absorbers of such radiation in the contemporary atmosphere. There is thus a further link between life and the atmosphere.