Overview of the Chemistry of Polluted and Remote Atmospheres

Atmospheric chemistry is an exciting, relatively new field. It encompasses the chemistry of the globe, from polluted to "clean," remote regions and from the region closest to the earth's surface, the troposphere (< 10-15 km), through the tropopause ( ~ 10-15 km) into the upper atmosphere—which, for the purposes of this book, we restrict to the stratosphere ( ~ 10-50 km). Chemical and physical processes occurring at the earth's surface—emissions, transport, lifetimes, and fates of certain anthropogenic and biogenic/geogenic chemicals—can impact the stratosphere—and vice versa. Thus, even though in some early studies the tropopause was perceived as being a "barrier" between the lower and upper atmospheres, it has become increasingly clear that the troposphere and stratosphere are intimately connected. Witness the vertical transport of long-lived ozone-destroying anthropogenic emissions of chlorofluorocarbons (CFCs) and conversely the downward transport of stratospheric ozone into the troposphere. Hence, even though we devote separate chapters to the chemistry of the troposphere and stratosphere, the emphasis throughout is on one integrated "system " of global atmospheric chemistry.

In this regard, there are several topics that fall outside the scope of this book, including the evolution of the earth's atmosphere. For reviews, the reader is referred to articles by Kasting (1993) and Allègre and Schneider (1994). Although we point out throughout this book the interconnectedness of the lower and upper atmospheres, practicality and length preclude extension to the obvious interrelationships and feedbacks with other components of the earth system, including the controversial Gaia Hypothesis (named after the Greek goddess of the earth). The reader is referred to articles by Lovelock (1989), Kirchner (1989), Schneider (1990), and Lenton (1998) for discussions of the latter.

Although atmospheric chemistry is sometimes viewed as an "applied" science, its foundations rest on funda mental research in diverse areas of chemistry. These include theoretical and experimental aspects of spectroscopy, photochemistry, and the kinetics and mechanisms of homogeneous and heterogeneous organic and inorganic reactions. We believe it is useful for today's students, researchers, and educators to be aware that major resources for Leighton's masterful 1961 treatment of the newly emerging field of atmospheric chemistry, Photochemistry of Air Pollution, were, in fact, pioneering, basic research monographs published years earlier. They include Photochemistry of Gases by W. A. Noyes, Jr., and P. A. Leighton (1939), two editions of Atomic and Free Radical Reactions by E. W. R. Steacie (1946, 1954), and G. Herzberg's classics Atomic Spectra and Atomic Structure (1944); Molecular Spectra and Molecular Structure I: Spectra of Diatomic Molecules (1950); and Infrared and Raman Spectra of Polyatomic Molecules (1945).

Throughout the body of this book, we address the basic chemistry driving key atmospheric processes in the natural and polluted troposphere and stratosphere and illustrate their critical interactions on local, regional, and global scales. In so doing, our treatment overall reflects the message that Sam sings so eloquently to Bogart and Bergman in the classic movie Casablanca,... "The fundamental things apply, as time goes by."

In this chapter we provide an overview of the chemistry of the lower and upper atmospheres. In Chapter 2, we illustrate how this chemistry plays a critical role in the concept of an integrated "atmospheric chemistry system"—a loop that starts with emissions (anthropogenic and natural) and ultimately closes with scientific health and environmental risk assessments and associated risk management decisions for the control of air pollutants.

Chapters 3-12 present a detailed examination and explanation of how one applies the theoretical and experimental fundamentals of photochemistry, spectroscopy, and kinetics and mechanisms (structure and reactivity) to the most important homogeneous and heterogeneous processes that take place in our natural and polluted atmosphere.

We conclude by illustrating how our understanding of these chemical processes in our clean and polluted troposphere and stratosphere plays a crucial role in generating the "exposure" portions of scientific health risk assessments. Such assessments provide the foundation for sound, health-protective and cost-effective strategies for the control of tropospheric ozone, particles, acids, and a spectrum of "hazardous air pollutants" (including carcinogens and pesticides)—as well as for the mitigation of stratospheric ozone depletion.

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