The environmental implications of stratospheric ozone depletion were just emerging when the Nimbus-7 satellite was launched in 1978. This marked the beginning of space-based ozone monitoring with observations from the Solar Backscatter Ultraviolet (SBUV), Total Ozone Mapping Spectrometer (TOMS), and Satellite Aerosol and Gas Experiment (SAGE) instruments. Since that time a National Plan for Stratospheric Monitoring (NOAA, 1989) was put into place. Its keystone was the continuation of ozone observations on the NOAA polar orbiting satellite series with the SBUV2 instrument. Parallel to this plan, NASA has been flying improved TOMS and SAGE instruments on U.S. and Russian environmental satellites. The Upper Atmosphere Research Satellite (UARS) launched in 1991 carried instruments to measure several stratospheric parameters, including ozone. The Global Ozone Monitoring Experiment (GOME), a new multispectral nadir instrument launched on the European Earth Resources Satellite (ERS-2) in April 1995, is expected to provide data on tropospheric ozone columns and related constituents.

These space-based observations are supplemented by several other long-term data sets. The Dobson and Brewer networks of ground-based spectrophotometers provide total ozone column measurements at more than 150 sites. Ozonesondes launched from sites around the world, at frequencies that vary from twice a month to three times a week, provide detailed vertical profiles of ozone from the surface to the middle stratosphere (Logan, 1999; Logan et al., 1999). Aircraft campaigns sponsored by NASA and other agencies measure ozone concurrently with