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in air and vacuumon long term measurements ofstratospheric ozone and solar irradiance variations from space.William K. Fowler and Kevin W. KellyBall Aerospace & Technologies Corp.P. 0. Box 1062, Boulder, CO 80306Donald F. HeathResearch Support Instruments, Inc.5500 Central Ave., Boulder, CO 80301ABSTRACT1_n recent years, the consistency ofultraviolet solar spectral irradiance measurements has improved to the extent that broadband averages of solar irradiances agree to within a few percent over ranges of a few hundred nanometers. Over spectralranges from tens to tenths ofnanometers significantly larger disagreements in spectral irradiances are observed. Some wellknow factors which contribute to measurement uncertainties over smaller wavelength intervals are wavelength calibrationerrors, uncertainties in radiometric standards especially in the transition region from one standard to another, and differencesand uncertainties in slit scattering function between instruments.Extensive pre-launch radiometric calibration of SBUV-2 ozone monitoring instruments in air and vacuum have indicatednew sources ofradiometric calibration uncertainties. These are effects of Woods anomalies in grating efficiency andwavelength dependent changes (sometimes with significant structure) in the reflectance ofMgF2 overcoated aluminumsurfaces which seems to be associated with the absorption and desorption ofwater vapor by the MgF2 thin film.These effects combine to produce wavelength dependent radiometric calibration differences between air and vacuumconditions of as much as 10 percent or more. These results indicate that one should not assume that an accurate radiometriccalibration in air is valid in space for the wavelength region of200-400 urn. |
