known from the atmospheric rise, but the contributions of individual source components are not. Surprisingly, the main sink for methane, oxidation by tropospheric OH, appears to be stable over at least the most recent decade.

  • How does the photochemical breakdown of methane contribute to other chemical and radiative processes in the atmosphere on dec-cen timescales? The photochemical breakdown of CH4 consumes OH and produces water throughout the atmosphere; in the stratosphere even small amounts of water vapor are extremely efficient contributors to the global greenhouse effect. Thus, dec-cen changes in methane can be expected to alter the vertical distribution of water vapor in the higher troposphere and lower stratosphere, thereby changing radiative forcing. The ice particles that form at these altitudes also provide sites for heterogeneous chemistry. The feedbacks among these chemical processes and atmospheric dynamics are largely unknown.

  • Why is N2O increasing on dec-cen timescales? Records of atmospheric N2O show a steady rise since the nineteenth century, but a quantitative budget of N2O sources and sinks that explains this rise cannot be developed from the existing scarce observations and limited understanding of processes. The global nitrogen cycle has been heavily altered by human activities, particularly by the widespread application of high-nitrate fertilizers. Processes related to soils have likely been more disrupted by recent human activity than oceanic denitrification/nitrification rates. Industrial N2O production also is a likely cause of the recent increase.

  • Why has tropospheric ozone increased since the nineteenth century and are further increases likely? What are the controls on the abundance of tropospheric ozone? To what degree are precursors, photochemistry, transport, and dilution important? Although trends over recent decades are unclear, tropospheric ozone has almost certainly increased substantially since the last century. Because ozone is a pollutant with documented health and ecosystem impacts and a greenhouse gas, we need to be able to predict its concentrations. Knowing the interactions among precursors, transport, and photochemistry also is critical.

Stratospheric Ozone
  • How does the coupling between chemistry, dynamics, and radiation in the lower stratosphere and upper troposphere operate on dec-cen timescales? For example, what do we need to know to predict the timing of ozone recovery accurately? How do changes in the stratosphere affect atmospheric circulation and the surface radiation balance? Ozone distribution influences the vertical temperature structure and dynamics of the lower stratosphere because of its absorption of UV radiation. These dynamics in turn determine the distribution of other atmospheric con-

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