1IntroductionThe atmosphere is a thin layer of gases separating the earth’s surface fromspace. It protects the surface from radiation, provides a blanket that elevatesthe surface temperature, is responsible for the redistribution of water and heat,provides us with oxygen, and removes harmful gaseous compounds. Theseproperties are essential to the earth’s ability to support life. For example, itis believed that only the ozone layer formed 0.5–1 billion years ago reducedharmful solar UV radiation sufficiently to allow life to leave the oceans andfurther develop on land. While the atmosphere has undergone natural vari-ations during the past 4.5 billion years, the arrival and expansion of humansocieties over the recent millennia have led to changes in the chemistry andphysics of the atmosphere. Industrialisation, which began approximately 200years ago, has had a marked impact on the composition of the atmosphere,for example by increasing levels of carbon dioxide and tropospheric ozone.During the same period scientific interest in the chemistry and physics of theatmosphere has expanded. While at the beginning this interest was driven byscientific curiosity, over the past few decades research has been increasinglymotivated by the urgency to understand the response of the atmosphere toanthropogenic influence.As the atmosphere is the medium in which most plants, animals, and hu-mans live, the ‘health’ of the atmosphere is of great societal concern today.Numerous examples have shown the impact that emission and transformationof gases have on the atmosphere. In particular, urban air quality has beena concern for many years. Beginning with observation of increased numbersof deaths in industrialised areas in 19th century Europe, air pollution wasfirst recognised as a serious threat after the London ‘killer-smog’ in December1952. A new kind of air pollution was simultaneously recognised in Los Ange-les, which lead to serious plant damage, and lung and eye irritation in humans.Although recognised for half a century, urban and regional air quality prob-lems remain among the most challenging issues of the 21st century. After fivedecades of air pollution research, political efforts to improve air quality, andan increasing public awareness, more than 70 million people in the UnitedU. Platt and J. Stutz, Introduction. In: U. Platt and J. Stutz, Differential Optical AbsorptionSpectroscopy, Physics of Earth and Space Environments, pp. 1–4 (2008)DOI 10.1007/978-3-540-75776-41c Springer-Verlag Berlin Heidelberg 20082 1 IntroductionStates still live in or near areas where ozone levels exceed national air qual-ity standards (NRC, 2000). K¨unzli et al. (2000) found that between 19,000and 44,000 people per year die prematurely from the effects of air pollutionin three European countries. Air pollution is also an increasing problem indeveloping countries that strive to reach western living standards throughrapid industrialisation. Secondary pollutants, such as ozone, particulate mat-ter, and most likely nitrogen dioxide are known to have detrimental effects onhuman health (Brunekreef and Holgate, 2002; Fenger, 1999) The World HealthOrganization estimates that 1.4–6 million people die each year because of airpollution (WHO, 1999). Pollution can also be transported over long distances,for example between Asia and the United States, and the United States andEurope, therefore influencing the composition of the atmosphere on a globalscale (IPCC, 2002).Another well-known example of human impact on the composition of theatmosphere is stratospheric ozone depletion events over Antarctica. Whilescientists had anticipated a thinning of the ozone layer due to the release ofman-made chemicals summarised under the term chlorofluorocarbons (CFCs),discovery of the extent and severity of this ‘ozone hole’ in 1985 was a surprise.The urgent need to protect the ozone layer, which was supported by a soundscientific assessment of the depletion mechanism, led to a number of inter-national treaties to curb the release of CFCs. Although the ozone hole willprobably not ‘close’ until the year 2050, the ban of CFCs is the first successfulinternational effort to abate a severe change in the atmospheric compositionon a global scale.The most publicly discussed topic of atmospheric change in the beginningof the 21st century is the anthropogenic impact on the climate system of ourearth. In this context, climate change refers not only to temperature but to allaspects of the atmospheric system, including weather and rain patterns, thefate of global ice sheets, the frequency of severe weather events, etc. The factthat the earth’s climate is altered by a change in atmospheric composition iswell known (Arrhenius, 1896). The analyses of various paleorecords, such as icecores, have shown that atmospheric composition and climate have been corre-lated over the past 100,000 years. Major volcanic eruptions also perturbed theclimate on shorter time scales. For example, weather pattern changes and aglobal temperature decrease of up to 1.2◦C were observed for 5 years after theeruption of Krakatau in 1883. These natural variations in the earth climatehave been ongoing throughout the entire history of our atmosphere.It is, however, becoming increasingly clear that human activities havechanged the climate of our planet in an unprecedented way during the past twocenturies. Observations show a 0.6◦C increase in surface temperature over thepast 100 years, a shrinking of Arctic sea ice volume, a retreat of glaciers, andmany other examples that all support this conclusion (IPCC, 2002). The maininstigators of these changes are trace gases that trap infrared radiation. Thesetrace gases are released from human activities or formed as a consequence ofthese emissions. Emission and formation of small particles suspended in air is1 Introduction 3also likely to play an important role. Although many uncertainties about theprocesses influencing our climate system remain, it is clear that the possibleconsequences of these changes are severe, and that strategies to mediate thesechanges are needed.Efforts to understand the fundamental physical and chemical processesthat control the atmosphere are thus not only motivated by scientific curios-ity, but also more importantly by the desire to assess and mitigate anthro-pogenic impact. Abatement strategies for air pollution, climate change, andother environmental problems