Abstract
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.
Highlights
Atmospheric aerosols are an important constituent of the atmosphere
This indicates that if the measured value of scattered light can be subtracted, the estimation accuracy of the solid view angle (SVA) can be greatly improved. From these results, when we determine the SVA by using the data from the solar disk scan measurement if the aerosol optical depth is less than 0.5 (0.58) and the aerosol does not include large particles such as desert dust particles, the effect of the scattered radiances on the SVA calculation is less than 0.5 %, and is well approximated by
The annual trend of the SVA in the shortwave-infrared channels cannot be seen in 8 years of data, and even if there is a trend, it is smaller than the measurement uncertainty
Summary
Atmospheric aerosols are an important constituent of the atmosphere. Aerosols affect the global climate through the radiation budget both directly and indirectly (e.g., Ramanathan et al, 2001; Lohmann and Feichter, 2005) and human health as one of the main components of air pollution. For ground-based observations, the solar direct irradiance and sky radiances are measured, and the aerosol characteristics are retrieved by analyzing these data. In Part 1 (Uchiyama et al, 2018), the temperature dependence of the sensor output was investigated and the calibration constants determined by the improved Langley method and normal Langley method were compared. An alternative method to determine the calibration constant for the 940 nm channel and the shortwave-infrared channels (1225, 1627, 2200 nm) was shown using on-site measurement data. 3, we describe the problem with the current SVA calculation program This software is attached to the SKYRAD package (Nakajima et al, 1996), which is used to retrieve aerosol parameters from sky radiometer data.
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