temporal resolution to reveal the important chemical and physical processes in the production, transport, and removal of photochemical oxidants. A research strategy that is both evolutionary and revolutionary will be required.

Atmospheric Aerosols and UV Radiation Imperative

Atmospheric aerosols have important impacts on human health, quality of life, and materials degradation. Another NRC (1992) report summarizes key aspects of this issue. Despite recent advances in appreciating the importance of atmospheric aerosols, our understanding of this critical class of atmospheric species is in its infancy. One notable reason for this failure is the complexity of aerosols. Compared to gases, aerosols have additional dimensions: an infinite number of sizes and a variable mixed composition. We are not able to fully comprehend the impacts of aerosols now, and we cannot make predictions about how the impacts will change in the future through human activities.

The important questions about atmospheric aerosols in the next decade concern their effects on climate, atmospheric chemistry, and human health and well-being (see Box 5.4). To answer these questions we must go far beyond our current state of knowledge of atmospheric aerosols. The essential elements of the research strategy that will be needed are outlined in the section on research imperatives later in this chapter. A more detailed discussion of many aspects of this strategy can be found in A Plan for a Research Program in Aerosol Forcing and Climate (NRC, 1992).

Toxics and Nutrients Imperative

Many of the atmosphere's naturally occurring components can have toxic and/or nutritive effects on the biosphere.16 Myriad toxic and nutritive substances in the atmosphere are significantly influenced by anthropogenic activities. While

BOX 5.4 Atmospheric Aerosols: Key Scientific Questions

  1. What is the role of natural and anthropogenic aerosols in climate and how might future changes in levels of aerosol precursors affect this role?

  2. How are natural and anthropegenic aerosols likely to affect stratospheric and tropospheric ozone and the cleansing capacity of the atmosphere in the future? What are the sources and emission rates of natural aerosols?

  3. What is the role of atmospheric chemistry in changing the composition of aerosols that affect human health, the environment, visibility, and infrastructure materials?

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