The instrument constant of sky radiometers (POM-02) - Part 2: Solid view angle

Abstract

Ground-based networks have been developed to determine the spatiotemporal distribution of aerosols using sky radiometers. In this study, errors related to the solid view angle (SVA) of sky radiometers, which are used by SKYNET, were investigated. The SVA is calculated using solar disk scan data, the measured radiances around the solar direction in 0.1×0.1° increments. These measurements include the scattered light from aerosol and air molecules, as well as the direct solar irradiance, causing errors in the SVA calculation. The influence of these errors was evaluated with simulations. From the results of these simulations if the aerosol optical depth (optical path length) is less than 0.5 (0.58) at 550 nm and the aerosol does not include large particles, such as desert dust particles, then its influence on the SVA calculation was less than 0.5 %. Problems with the software for the SVA calculation were also investigated. First, the data processing does not consider the change of airmass (solar zenith angle) during the solar disk scan measurement. In practice if a measurement is made in the period when the change in airmass is small, then the error is small. Second, before starting data processing, the minimum measured value is subtracted from the measured values, resulting in underestimation of the SVA by 1 % to 4 %. Thirdly, the values between 1.4 and 2.5° are not properly extrapolated, resulting in overestimation of the SVA by 0.6 % to 2.1 %. The second and third error sources partially cancel each other out, and the total error is an underestimation of 0.5 % to 1.9 % of the actual value. Furthermore, the annual trend in the SVA was examined. In both the visible and near-infrared regions (Si photodiode region) and in the shortwave-infrared region (InGaAs photodiode region), this trend cannot be seen in 4 and 8 years of data, respectively. The seasonal variation of the SVA was also examined, but no clear seasonal variation could be detected.