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stand observed changes in global air quality and to predict future changes will depend strongly on answering two important questions:

  • How can global air quality change affect, and in turn be affected by, global climate change?

    Although air quality and climate are generally treated as separate issues, they are closely coupled through atmospheric chemical, radiative, and dynamical processes. The accumulation of pollutants in the atmosphere can affect climate through direct and indirect contributions to earth's radiative balance, and through chemical reactions that alter the lifetime of certain greenhouse gases. In turn, meteorological parameters such as temperature, humidity, and precipitation can affect the sources, chemical transformations, transport, and deposition of air pollutants. Our understanding of many of these climate-chemistry linkages is in its infancy. A better understanding is needed in order to make accurate estimates of future changes in climate and air quality and to evaluate options for mitigating harmful changes.

  • How is global air quality affected by the international and intercontinental transport of air pollutants?

    Total global emissions of species including NOx, VOCs, and CO may rise dramatically in the coming decades due to increasing population and industrialization, and in particular, the growth of “megacities” in many regions of the world. The transport of pollutants such as ozone and PM across national boundaries and between continents will increase in importance as total emissions rise. Such pollutant transport interconnects all the countries of the world to varying degrees and can raise “background” 1 pollution levels over large regions of the globe. Quantifying this long-range transport is essential in order to understand what future changes may occur in U.S. air quality, to assess how U.S. pollutant emissions affect global air quality, and to develop realistic and effective air quality management plans for the coming decades.

Answering the questions posed above requires a research strategy that integrates atmospheric observations covering a wide range of parameters and spatial and temporal scales together with diagnostic, global, and regional models (see Figure 1-1 ). In particular, a renewed and comprehensive observational strategy with global cooperation is required in order to fill the significant gaps and short-falls in our current observational capabilities. If we do not maintain existing

1“Background” is used here in a narrow sense as the concentration of a pollutant that would prevail in the absence of local anthropogenic emissions.

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