a large number of related species to improve our understanding of the factors controlling ozone in the lower stratosphere and in the troposphere.

Box 8.1

Findings

Current understanding of atmospheric ozone has been reviewed by WMO (1999). Three priorities for future space-based observations emerge from the WMO report: (1) improved observation of ozone concentrations at altitudes below 20 km, (2) measurement of ozone concurrently with related species, and (3) better understanding of the long-term trend in ozone. How do NASA and NPOESS plans meet the science requirements?

The research plans of NASA target the first two of these priorities but not the third; there is no commitment by the agency to ensure continuity of observations, as is needed for long-term trend studies. Indeed, the ability to detect long-term trends from satellite observations over the past decade has been largely serendipitous, and yet it has proven crucial for assessing human effects on atmospheric ozone.

The NPOESS environmental data record (EDR) objectives for the operational Ozone Mapping and Profiler Suite (OMPS) instrument (see Table 8.1) provide specifications comparable to those of the new generation of research instruments and would make NPOESS a powerful source of information for detecting long-term trends in atmospheric ozone. The EDR thresholds (Table 8.1) are sufficient for detecting long-term trends in ozone columns but inadequate for detecting trends in the vertical distribution of ozone because of the coarse vertical resolution (5 km) and the insufficient precision below 15 km. Resolving the vertical distribution of ozone trends is critical to interpreting trends in the total column and assessing ozone radiative forcing.

The following observations for the OMPS sensor on NPOESS are intended to ensure its usefulness for monitoring long-term trends in ozone:

  • The OMPS should significantly exceed the EDR thresholds and provide or approach the EDR objectives below 25 km.

  • There should be a 1-year overlap between successive OMPS launches to allow sensor intercomparison and guarantee long-term traceability.

  • Calibration and validation of the OMPS must be viewed as a critical activity to be maintained throughout the lifetime of the instrument. It should be led by a group independent of the OMPS team.

  • Any changes made to the retrieval algorithm should be followed by reprocessing of the entire record of OMPS observations to preserve the integrity of the record for long-term trend analyses. This requirement implies in particular that the raw radiances from OMPS should be archived.

The recent WMO (1999) report gives a detailed discussion of the measurement capabilities of the space- and ground-based instruments used to assess long-term trends in ozone concentration. The principal instruments are summarized in Table 8.2. The accuracies, precisions, and instrument drifts in Table 8.2 were verified by intercomparisons with other ground- and space-based instruments (WMO, 1999). Additional space-based data for ozone are available from the TIROS-N operational vertical sounders (TOVS) on NOAA polar-orbiting satellites from 1979 to the present. Comparison with other sounders indicates that TOVS are sensitive only to trends in lower stratospheric ozone. TOVS ozone retrievals are complicated by cloud effects, water vapor absorption, and surface emissivity; intersatellite instrument differences further complicate the interpretation of the data. Because of these problems, TOVS data have been of little use for long-term trend analyses.

Trends in total ozone columns for the period 1978 through 1998 have been analyzed using data from the TOMS and SBUV instruments as well as from the Dobson and Brewer ground-based networks (WMO, 1999). The different records are in good agreement:



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