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Discussion papers | Copyright
https://doi.org/10.5194/amt-2017-433
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Jan 2018

Research article | 11 Jan 2018

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Measurement Techniques (AMT) and is expected to appear here in due course.

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

Akihiro Uchiyama1, Tsuneo Matsunaga1, and Akihiro Yamazaki2 Akihiro Uchiyama et al.
  • 1Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Ibaraki, 305-8506, Japan
  • 2Meteorological Research Institute, Japan Meteorological Agency, Tsukuba, Ibaraki, 305-0052, Japan

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 degree 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 thickness is less than 0.5 at 550nm 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 degrees 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, 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.

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Akihiro Uchiyama et al.
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Atmospheric aerosols are an important constituent of the atmosphere. Measurement networks using radiometers such as SKYNET have been developed. There are two constants that we must determine. One of them is the solid view angle (SVA) of the radiometer. The problems related to SVA were investigated. It was shown that the conventional method can cause a systematic underestimation, and an improved method was proposed. Utilizing the results of this paper, SKYNET data will become more reliable.
Atmospheric aerosols are an important constituent of the atmosphere. Measurement networks using...
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