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MTU GE 4250 - Atmospheric Chemistry

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2Atmospheric ChemistryAir quality, the health of the ozone layer, and the earth’s global climate areclosely tied to the composition of the atmosphere and the chemical transfor-mation of natural and anthropogenic trace gases. Although the purpose ofthis book is not to cover the full breadth of atmospheric chemistry, this chap-ter provides a short introduction into this topic, with the intent to motivateand clarify the applications of the differential optical absorption spectroscopy(DOAS) technique. Key elements of the contemporary understanding of at-mospheric chemistry and ongoing research efforts are presented. This chapteris mostly restricted to chemical reactions occurring between gas molecules,although a few surface reactions are also presented. It should be noted thatmuch of our knowledge of atmospheric chemistry comes from studies employ-ing sophisticated instrumentation for the detection of atmospheric trace gases,including the DOAS technique.Much of the motivation to study atmospheric chemistry and the composi-tion of the atmosphere is based on various man-made environmental problemsthat have emerged over the past two centuries. Human activities have upsetthe natural balance of the atmosphere by influencing the trace gas and aerosolcomposition on local, regional, and even global scales. The following list namesthe most significant atmospheric environmental problems:• ‘London Type’ smog was first recognised as an environmental problemin the 19th century (Brimblecombe and Heymann, 1998). The emissionof soot particles and the formation of aerosols consisting of sulphuricacid, which were produced from photochemical oxidation of SO2emit-ted from combustion sources, had a severe impact on human health.During the so-called ‘London Killer Smog’, approximately 4000 excessdeaths were counted in London during a 4-day period. It should be notedthat the term ‘Smog’ was coined to describe the interaction of Smokeand Fog (Smog) that formed the strong haze observed in London duringwinter time.U. Platt and J. Stutz, Atmospheric Chemistry. In: U. Platt and J. Stutz, Differential OpticalAbsorption Spectroscopy, Physics of Earth and Space Environments, pp. 5–75 (2008)DOI 10.1007/978-3-540-75776-42c Springer-Verlag Berlin Heidelberg 20086 2 Atmospheric Chemistry• Crop damage and human discomfort were the first indication of high levelsof oxidants during sunny and hot days in Los Angeles (Haagen-Smit, 1952;Haagen-Smit and Fox, 1954; Finlayson-Pitts and Pitts, 2000). The forma-tion of this ‘Los Angeles Type’ smog was found to be primarily due tophotochemical formation of large amounts of ozone, carbonyl compounds,and organic aerosol from car exhaust and industrial emissions of nitrogenoxides, carbon monoxide, and volatile organic compounds (VOCs)(Haagen-Smit, 1952; Finlayson-Pitts and Pitts 2000). Despite manydecades of research and mitigation activities, Los Angeles type smog re-mains one of the most common air pollution problems in urban areastoday.• Forest decline and lake acidification were signs of another man-made en-vironmental problem. Increased emissions of sulphur dioxide and nitrogenoxides, followed by their gas and aqueous phase oxidation to sulphuric andnitric acid, lead to ‘Acid rain’ that lead to the deposition of these acidsto various ecosystems (Charlson and Rhode, 1982). While acid rain hassuccessfully been reduced in Europe and the United States, it remains aproblem in many developing countries.• The health of the atmospheric ozone layer has always been a concernof atmospheric scientists. In 1971, Johnston (1971) predicted the loss ofstratospheric ozone due to emission of oxides of nitrogen by a plannedfleet of supersonic passenger aircraft. Molina and Rowland (1974) warnedof a gradual loss of stratospheric ozone due to catalysed ozone destructionprocesses by halogen species transported to the stratosphere in the form ofextremely stable halogen-containing compounds, so called chlorofluorocar-bons (CFCs), used as coolants or spray can propellants. Their predictionsbecame reality in 1985, when Farman et al. (1985) discovered the strato-spheric ‘ozone hole’ over Antarctica. This recurring phenomenon reducesthe thickness of the stratospheric ozone layer over Antarctica to less thanone third of its normal level every Antarctic Spring (e.g. Farman et al.,1985; Solomon, 1999). While steps have been taken to stop the emissionsof CFCs, it is estimated that it will take another three decades before theozone hole will close again.• One of the most serious environmental problems is the impact of humanactivities on the climate of our planet. The potential of global warmingcaused by IR-active ‘greenhouse’ gases such as CO2,CH4,N2O, O3,CFCs(e.g. CFCl3and CF2Cl2), and the direct and indirect effects of chemicallygenerated aerosol, have been known for some time (e.g. Arrhenius, 1896;Rhode et al., 1997; IPCC, 2002). Human activities, and thus the emissionof the various greenhouse gases, over the past century have reached levelswhere an impact on global surface temperatures, the global water cycle,ocean levels, etc. has become very likely. Today, many signs point at thebeginning of a global climate change which will have a severe impact on ourearth. The global increase of tropospheric ozone, which apparently startedin the middle of the 19th century, also contributes to this phenomenon. Itis thought to be caused by catalytic ozone production due to wide-spread2.1 Atmospheric Structure and Composition 7emission of oxides of nitrogen and hydrocarbons (HC) (Volz and Kley,1988; Staehelin et al. 1994).Many of these phenomena are closely related to the atmospheric chemistrypresented in this chapter.Environmental conditions, such as temperature, pressure, and the solarspectrum, change with altitude and several altitude regimes of atmosphericchemistry can be distinguished. These properties of the atmosphere relevantfor atmospheric chemistry will be presented in Sect. 2.1. The composition ofthe atmosphere is influenced by emissions of natural and man-made species.A short overview over these emission and their sources is given in Sect. 2.2.Tropospheric chemistry is dominated by ozone and other oxidants, i.e. hy-droxyl/peroxy radicals, nitrogen oxides, and volatile organic carbons. Whiletheir chemistry is closely linked, we will discuss the chemistry of each class ofspecies in Sects. 2.4–2.7. Sulphur chemistry (Sect. 2.8) also plays an


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