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

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.