of air pollution can be invisible, and while they may not be harmful to breathe, they can have an unacceptable effect on our global atmosphere. Chlorofluorocarbons (CFCs), for instance, released from refrigerants and cleaning agents, are associated with the destruction of ozone in the stratosphere, and carbon dioxide contributes to global warming.

To keep Earth safe and habitable, we need to improve our ability to monitor and control human-induced pollution and to understand how various pollutants interact among themselves and how they affect different parts of the atmosphere (see box “Earth’s Atmosphere”). This chapter describes how AMO science provides the essential data for atmospheric models and a variety of innovative techniques that are used to monitor pollution quantitatively.

Understanding the Atmosphere and Climate

Atmospheric science (and other environmental sciences) is different from many other areas of science in that controlled large-scale experiments are often difficult if not impossible. Nevertheless, microscopic physical and chemical processes that affect the climate occur constantly throughout the atmosphere, the oceans, on land, and in the biosphere. Atmospheric scientists strive to understand these processes in conjunction with sunlight and human activities such as transportation, combustion, industry, and agriculture and to determine how their interaction globally influences the atmosphere and climate.

The complexity of the atmospheric system makes it difficult to describe all of its behavior using a small number of theoretical expressions. Therefore, comprehensive numerical models play an essential role (see box “Scientific Models”). Atmospheric scientists must also make extensive use of data provided by


A model is a collection of mathematical expressions that describe what we know about a system. Models allow scientists to carry out controlled virtual experiments on computers to determine how a system will behave under various circumstances.

The power of a theoretical model lies in the predictions it makes and its ability to identify key relationships between physical and chemical processes. Differences between the model’s predictions and the measured parameters are used as a guide to further our understanding of the system and to improve the model.

Atmospheric models have had a critical impact on national and international policy. The implication of chlorofluorocarbons (CFCs) in the destruction of the ozone layer—which led to the Montreal Protocol, an international treaty banning the manufacturing and use of CFCs—is a success story to which AMO instrumentation and research made a significant contribution. Three chemists were awarded the 1995 Nobel Prize in Chemistry for their work on the formation and decomposition of ozone.

laboratory experiments as well as information and parameters obtained from monitoring the atmosphere from the ground, balloons, aircraft, and satellites. AMO science makes an important contribution to instrumentation, data collection, and analysis in atmospheric science.

To be a reliable, predictive tool, a model must include as completely as possible the phenomena affecting the system, and it must be based on accurate data. Models of the upper atmosphere (the mesosphere and thermosphere) require knowledge of the intensity of the Sun’s radiation as a function of wavelength, densities of constituent molecules and atoms (primarily N2,O2,

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