must not be forgotten in studying the complexities of climate, ecosystems, and the chemistry of the troposphere.

Today, we have far deeper knowledge about the chemistry of the atmosphere than we did just a decade ago. We also know more clearly what we do not know. These issues are also addressed in a recent National Research Council report (NRC, 1998) that is consistent with the perspective put forward in this chapter. Key challenges to atmospheric chemistry in the coming decade can be expressed in five Research Imperatives, where each Research Imperative combines one or more primary Scientific Questions with the need to know from a human dimensions perspective:

  • Stratospheric ozone and ultraviolet (UV) radiation. Define and predict secular trends in the intensity of UV exposure that the Earth receives. Document the concentrations and distributions of stratospheric ozone and the key chemical species that control its catalytic destruction and elucidate the coupling between chemistry, dynamics, and radiation in the stratosphere and upper troposphere.

  • Greenhouse gases. Determine the fluxes of greenhouse gases into and out of the Earth 's systems and the mechanisms responsible for the exchange and distribution between and within those systems. Expand global detection techniques to elucidate the processes that control the abundances and variability of atmospheric CO2, CH4, N2O, and upper-tropospheric/lowerstratospheric O3 and water vapor.

  • Photochemical oxidants. Develop the observational and computational tools and strategies that policy makers need to effectively manage ozone pollution, and elucidate the processes that control and the relationships that exist among ozone precursor species, tropospheric ozone, and the oxidizing capacity of the atmosphere.

  • Atmospheric aerosols and UV/visible radiation. Document the chemical and physical properties of atmospheric aerosols, and elucidate the chemical and physical processes that determine the size, concentration, and chemical characteristics of atmospheric aerosols.

  • Toxics and nutrients. Document the rates of chemical exchange between the atmosphere and ecosystems of critical economic and environmental import, and elucidate the extent to which interactions between the atmosphere and biosphere are influenced by changing concentrations and depositions of harmful and beneficial compounds.

INTRODUCTION

The chemistry of the Earth's atmosphere has emerged as a central theme in studies of global change. Atmospheric chemistry provides the scientific foundations to understand a number of phenomena that are part of global change. These



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