Analysis of the eight‐year trend in ozone depletion from empirical models of solar backscattered ultraviolet instrument degradation

Abstract

Currently archived ozone data from the Solar Backscatter Ultraviolet (SBUV) spectrometer experiment on the Nimbus 7 satellite has been reported to show large global decreases in the amount of atmospheric ozone, both total content and as a function of altitude, for the period from 1978 to 1987. Analysis of atmospheric albedo data leading to these reported trends was based on empirical models of the SBUV spectrometer and diffuser plate degradation with time. Their combined degradation can be obtained from apparent decreases in measured solar irradiance at wavelengths where little or no long‐term intensity changes are expected. The central problem in analyzing SBUV data is to separately specify the diffuser plate degradation [Husdon et al., 1988]. Ratios of radiance to solar irradiance used to obtain ozone amounts are proportional only to diffuser reflectivity and independent of any spectrometer degradation. Even observations of relatively short‐term solar irradiance changes with time must be based on a model of diffuser plate degradation, since the SBUV data provide no internal means to uniquely specify diffuser plate reflectivity changes at ail wavelengths. A new class of explicitly empirical models have been developed that produce a better fit to all of the SBUV solar flux data. The models have a single free parameter to separately specify the diffuser plate and spectrometer degradation. This parameter must be within a narrow range to bring the calculated ozone trend into approximate agreement with data from the Dobson network, Solar Mesospheric Explorer (SME) and Stratospheric Aerosol Gas Experiment (SAGE) satellites, or with the different trends reported from the Umkehr ground stations. It is shown that outside sources of ozone data must be used to obtain a unique solution from SBUV radiance data within the precision necessary to determine the existence of a global annual ozone decrease. A correction factor is given as a function of time and altitude that brings the SBUV data into approximate agreement with SAGE, SME, and Dobson ozone trends. The total ozone results also agree with a recently developed wavelength pair justification method of internal calibration for the SBUV instrument. In our opinion, the currently archived SBUV ozone data should be used with caution for periods of analysis exceeding 1 year, since it is likely that the yearly decreases contained in the archived data are too large.

Solar UV variations during the decline of Cycle 23

“…A specific example of this situation occurred for the Nimbus-7 SBUV instrument measuring stratospheric ozone, where the instrument design made it difficult to determine the separation of response changes between optical components and the solar diffuser plate. While initial studies using Nimbus-7 SBUV data indicated more rapid atmospheric ozone changes than expected due to increasing concentrations of chlorofluorocarbons (Heath, 1988), further analysis showed that an alternate model of instrument behavior gave SBUV ozone trends that were much more consistent with results from multiple independent data sources (Herman et al, 1990). …”

The Effect of Solar Flux Variations and Trace Gas Emissions on Recent Trends in Stratospheric Ozone and Temperature

“…ROTTMAN, 1988;LEAN, 1987). However, the 11-year solar cycle variation has not yet been measured with sufficient accuracy over an entire cycle, primarily because of difficulties with satellite instrumentation (ROTTMAN, 1988;HERMAN et al, 1990). Currently, the most reliable long term UV data are of the Lyman alpha (La) irradiance at 121.567 nm measured by the Solar Mesosphere Explorer (SME) satellite.…”

NOAA 11 Solar Backscattered Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 1. Observations and long‐term calibration

“…The Nimbus 7 Solar Backscatter Ultraviolet (SBUV) instrument was launched in October 1978 to measure stratospheric ozone profiles and also made daily spectral solar UV measurements until February 1987 in the wavelength region 160-400 nm [Heath et al, 1975]. Deploying the diffuser plate only for solar observations reduced the overall degradation rate, and varying the frequency of solar observations provided data used to model long-term changes in diffuser reflectivity and instrument throughput Herman et al, 1990;Schlesinger and Cebula, 1992]. Daily solar spectral UV data over the range 120-300 nm were taken from January 1982 to April 1989 by the Solar Mesosphere Explorer (SME) satellite Rottman, 1988].…”