Abstract
Abstract. Non-ideal angular response of a spectroradiometer is a well-known error source of spectral UV measurements and for that reason instrument specific cosine error correction is applied. In this paper, the performance of the cosine error correction method of Brewer spectral UV measurements in use at the Finnish Meteorological Institute (FMI) is studied. Ideally, the correction depends on the actual sky radiation distribution, which can change even during one spectral scan due to rapid changes in cloudiness. The FMI method has been developed to take into account the changes in the ratio of direct to diffuse sky radiation and it derives a correction coefficient for each measured wavelength. Measurements of five Brewers were corrected for the cosine error and the results were compared to the reference travelling spectroradiometer (QASUME). Measurements were performed during the RBCC-E (Regional Brewer Calibration Center – Europe) X Campaign held at El Arenosillo, Huelva (37∘ N, 7∘ W), Spain, in 2015. In addition, results of site audits of FMI's Brewers in Sodankylä (67∘ N, 27∘ E) and Jokioinen (61∘ N, 24∘ E) during 2002–2014 were studied. The results show that the spectral cosine error correction varied between 4 and 14 %. After that the correction was applied to Brewer UV spectra the relative differences between the QASUME and the Brewer diminished even by 10 %. The study confirms that the method, originally developed for measurements at high latitudes, can be used at mid-latitudes as well. The method is applicable to other Brewers as far as the required input parameters, i.e. total ozone, aerosol information, albedo, instrument specific angular response and slit function are available.
Highlights
Brewer spectroradiometers (Brewer), currently manufactured by Kipp and Zonen B.V. and formerly by SCI-TEC Instruments Inc., measure total ozone, spectral UV radiation, aerosol optical depth (AOD) and sulfur dioxide (SO2) in more than 40 countries all over the globe (Kerr et al, 1985; Bais et al, 1996)
This paper studies if the cosine error correction method used at the Finnish Meteorological Institute (FMI) (Lakkala et al, 2008) could be used to respond to this need
3 Results The cosine error corrected Brewer irradiances were compared with the irradiance measured simultaneously by the QASUME unit during the comparison campaign in Huelva in 2015 and during the UV comparisons of the site audits in Finland
Summary
Brewer spectroradiometers (Brewer), currently manufactured by Kipp and Zonen B.V. and formerly by SCI-TEC Instruments Inc., measure total ozone, spectral UV radiation, aerosol optical depth (AOD) and sulfur dioxide (SO2) in more than 40 countries all over the globe (Kerr et al, 1985; Bais et al, 1996). This work studies the non-ideal angular response of the Brewer UV measurements: a well known and important source of uncertainty. Irradiance measurements should be proportional to the cosine of the angle θ between the direction of the incident ra-. K. Lakkala et al.: Cosine error correction of the Brewer UV diation and the normal of the radiometer’s diffuser. Lakkala et al.: Cosine error correction of the Brewer UV diation and the normal of the radiometer’s diffuser The deviation from this ideal angular response is called the cosine error
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