Correction of direct irradiance measurements of Brewer spectrophotometers due to the effect of internal polarization

Details Correction of direct irradiance measurements of Brewer spectrophotometers due to the effect of internal polarization Correction of direct irradiance measurements of Brewer spectrophotometers due to the effect of internal polarization

Jan 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006georl..3302806c&link_type=abstract

Geophysical Research Letters, Volume 33, Issue 2, CiteID L02806

Physics

5

Atmospheric Composition And Structure: Aerosols And Particles (0345, 4801, 4906), Atmospheric Composition And Structure: Radiation: Transmission And Scattering, Atmospheric Composition And Structure: Instruments And Techniques

Scientific paper

Due to the combined effect of two polarization sensitive elements, the entrance window and grating, the sensitivity of Brewer spectrophotometers for direct-sun measurements changes with solar zenith angle (SZA). We determined this SZA-polarization dependence with four independent methods, which agreed within +/-1.5%. For SZA < 50° this effect is negligible. At SZA = 60°, 70°, and 80° the Brewer's sensitivity is reduced by 1%, 4%, and 10%, relative to SZA = 35°, when the direct solar irradiance is perpendicular to the entrance window. Differential absorption algorithms for retrieving trace gases (e.g., ozone) are unaffected since the polarization effect is almost wavelength independent. However, systematic errors are introduced in Langley extrapolations (2-4% overestimation of the zero air mass factor), retrievals of aerosol optical depth (overestimation of 0.01-0.04), and aerosol single scattering albedo. Therefore, Brewer direct irradiance measurements should be corrected for the SZA-polarization dependence. The effect in sky-radiance measurements can be removed only by hardware modifications.

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