Direct Sun Spectral Measurements with the Brewer Spectrophotometer: Calibration and Optical Depth Retreival
S. Kazadzis, A.F. Bais, N. Kouremeti, V. Amiridis, M. Blumthaler, B. Schallard
[abstract] Direct ultraviolet spectral measurements are performed at Thessaloniki, Greece (40.5N, 22.9E), with a Brewer MKIII spectrophotometer since 1997. Three different methods for the absolute calibration of the direct spectral measurements in the wavelength range of 290-365 nm are presented. Spectral measurements of the direct irradiance are very important in many atmospheric sciences applications. They are useful for the determination of the aerosol optical depth, as well as of the column abundance of atmospheric species absorbing on the ultraviolet. Also, various modeling studies benefit from such measurements, since its parameterization is much easier than that of the global or the diffuse radiation. Direct spectral measurements could be useful for the determination of the actinic flux, in combination with the global irradiance and also for other applications that the percentage of the direct contribution to the global irradiance is needed (ex. cosine correction for non ideal angular response). The laboratory calibration which is applied to the global irradiance diffuser has to be transferred to the direct irradiance port. This spectral function is referred to this work as calibration factor (CL). The calibration factor can be calculated with one of the following methods: Method 1: Simultaneous measurements of the direct sun irradiance with the direct irradiance port and the global port have been obtained. The diffuse (scatter) radiation is blocked on the global irradiance port with a use of specially designed box that is placed on top of the instruments diffuser. Measurements took place for a certain solar zenith angle (30o) and for clear sky and aerosol free atmospheric cases. Method 2: A shadowing disk is used for blocking the direct radiation while the global port is used. Simultaneous measurements of the global, the diffuse (global port) and the direct irradiance with the direct port are obtained. Measurements of these three components for a single wavelength were performed by a small rotation of the instrument on the azimuthal axis for measuring the direct and the diffuse irradiance with the global port. Method 3: The third method is a calibration that was performed in the laboratory based on the known irradiance of a 1000W DXW lamp source. The lamp was positioned to a distance where the dimensions of the lamp and the lamp-instrument distance, could simulated the direct solar measurements. The irradiance of the lamp at 50 cm was known so the global irradiance at the various distances was calculated with the help of the inverse square law.
[keywords] Brewer spectrophotometer
8th Biennial WMO/GAW Brewer Users Group Meeting, 2003
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