Carefully designed observations (with or without specific tracer compounds or suites of tracer compounds) can be used in conjunction with diagnostic and/or observation-based models to independently infer a number of phenomena: long-term trends and regional and seasonal variability in short-lived free radical species not amenable to continuous, spatially extensive monitoring; urban, regional, and global-scale emission inventories of ozone precursors; and the sensitivity of ozone and other photochemical oxidants to ozone precursor compounds. At the same time, the diagnostic and observation-based interpretation of field measurements will require adequate laboratory definition of the fundamental mechanisms involved in atmospheric processes.

The development and deployment of monitoring networks, along with analysis of resulting data, are needed to establish the chemical climatology of ozone, other photochemical oxidants, and their precursors. This climatology will help establish temporal and spatial trends and shorten the time required to unequivocally observe a response in ozone to changes in the concentration of its precursor compounds. These networks must include components capturing the roles that meteorology and dynamics play in the redistribution of airborne chemicals. Moreover, a comprehensive chemical climatology for photochemical oxidants must include data from the free troposphere as well as the surface. It is thus likely that these networks will require the use of balloon sondes, robotic aircraft, and spacebased platforms in conjunction with newly developed instrumentation based on small, lightweight, low-power technology.

Linking Scientific Results with Integrated Assessments

Integrated assessments draw from a wide range of scientific information and disciplines to provide more comprehensive guidance on scientific and technical matters to the decision-making community. The research strategy in atmospheric chemistry should support these assessments by providing analytical and modeling tools that can readily support these integrated assessments.

Atmospheric Aerosols and UV/Visible Radiation

Compelling evidence has emerged that aerosols play a central role in control of both the UV/visible exposure at the Earth's surface and the balance between albedo and retention of infrared radiation in the atmosphere. Aerosols demand specific attention.

Minute amounts of particulate matter in the stratosphere, along with increased levels of anthropogenic chlorine, are responsible for the Antarctic ozone hole and probably for the less dramatic but nevertheless significant global-scale ozone depletion. The atmospheric haze associated with industrial activity near major cities is now believed to partially mask the expected increase in surface temperature associated with greenhouse gas increases. Atmospheric aerosols also



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