Over the past decade or so, stratospheric ozone has been a subject of intense research that has vastly improved our understanding of the subject. And although it cannot be said that a scientific subject is ever completely "solved", there is little argument that our understanding of stratospheric ozone is far better now than it was even 5 years ago. Further, as our knowledge and understanding have matured, there has been a natural slowing in the rate of discoveries and in the development of new theoretical insights. The field has progressed to the stage that the theories are adequate not only to explain past experiments but also of accounting for most new experimental findings. As a result, i feel that it is now possible to write a textbook detailing our knowledge of stratospheric ozone that will (hopefully ) remain relevant for some years.

In this book I present an account of what I have learned in the past 1 I years about the physical processes that regulate stratospheric ozone. The presentations assume knowledge of the atmosphere at the level of an advanced undergraduate in atmospheric sciences, but it requires no specialized knowledge of chemistry or the stratosphere. To help those not familiar with stratospheric chemistry, a glossary is provided at the end of the book that defines commonly used terms in the field.

Deciding what material to include and exclude was a difficult task, and I regret that there are several subject areas that receive only fleeting coverage. In particular, 1 would have liked to include more about stratospheric dynamics, radiative transfer, and kinetics. 1 take some solace, however, in the knowledge that no book is ever truly-complete—any attempt on my part at writing a book that included everything about a subject would have meant the book would never have been written.


I am indebted to the scores of researchers that have worked on stratospheric ozone and defined our knowledge of the field. This book could of course not have been written without their hard work. 1 would also like to thank my colleagues in the Earth System Science Interdisciplinary Center (ESSIC) and the Department of Meteorology, both at the University of Maryland, and in the Atmospheric Chemistry and Dynamics Branch at NASA Goddard Space Flight Center. They provided an intellectually rich environment in which to produce this book.

Many of my colleagues provided comments and advice on the text and data for the figures. Among them are (in alphabetical order): Jon Abbatt, David Considine, Anne Douglass, Eric Fleming, Tom Hanisco, David Hanson, Stacey Hollandsworth, Charley Jackman, Ken Jucks, Randy Kawa, Jerry Lumpe, Gloria Manney, Ken Minschwaner, Paul Newman, Eric Nielsen, Greg Osterman, Bill Randel, Mark Schoeberl, Bhaswar Sen. and Darryn Waugh. I thank you for your help in producing this book.

Finally, I'd like to acknowledge my father, Alex Dessler. During the production of the book he was an almost infinite source of encouragement and advice. I don't think I would have or could have written the book without him.

Chapter 1

The Ozone Problem

The atmosphere is traditionally divided into layers on the basis of temperature (Figure 1.1). The lowest layer of the atmosphere, the troposphere, is characterized by decreasing temperature with increasing altitude. It contains -90% of the mass of the atmosphere, as well as most of the Earth's clouds and weather. The temperature minimum located between 10 and 15 km altitude (-100 and 200 hPa pressure) is called the tropopause, and it demarcates the boundary between the troposphere and the layer above it, the stratosphere. In the stratosphere, the temperature increases with altitude, primarily because of heating from the absorption of ultraviolet radiation by ozone. The stratosphere contains -9.9% of the mass of the atmosphere. The temperature maximum near 50 km (-1 hPa) is called the stratopause, and marks the boundary between the stratosphere and the layer above it, the mesosphere. In the mesosphere, the temperature again decreases with altitude, primarily because of radiative cooling by carbon dioxide (CO,). The top of the mesosphere is bounded by a temperature minimum called the mesopause. Above the mesopause sits the thermosphere. In this book, we are concerned almost exclusively with the stratosphere.

Molecular nitrogen (N,) and molecular oxygen (02) make up -99% of the mass of the atmosphere. Because N2 is about four times more abundant than 02, the average molecular weight of the atmosphere is -29 g mol"1. Of the remaining ~l%, there are several gases whose abundance in the atmosphere is small but whose impact on the atmosphere is not. Many of these gases absorb photons with wavelengths in the thermal infrared (between a few and -20 pm), where O, and N, are essentially transparent. As a result, these trace gases, which include CO,, water vapor (H-,0), and ozone (O,), exert an important influence on the Earth's climate.

The atmosphere also filters the solar radiation reaching the surface. Most solar radiation incident on the top of the atmosphere is in the wavelength range between -100 nm and a few micrometers. Wavelengths less than 200 nm are absorbed fairly high in the atmosphere (50-150 km altitude) by O-, and O (see Chamberlain and Hunten [I], Section 1.5.1). while wavelengths greater than -300 nm can penetrate to the Earth's surface. For wavelengths between 200 and 300 nm, ozone (03) is the primary absorber, and without O, much of this

Pressure (hPa)

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