The instrument constant of sky radiometers (POM-02) – Part 1: Calibration constant
Abstract. Ground-based networks have been developed to determine the spatiotemporal distribution of the optical properties of aerosols using radiometers. In this study, the precision of the calibration constant (V0) for the sky radiometer (POM-02) that is used by SKYNET was investigated. The temperature dependence of the sensor output was also investigated, and the dependence in the 340, 380, and 2200 nm channels was found to be larger than for other channels and varied with the instrument. In the summer, the sensor output had to be corrected by a factor of 1.5 % to 2 % in the 340 and 380 nm channels and by 4 % in the 2200 nm channel in the measurements at Tsukuba (36.05∘ N, 140.13∘ E), with a monthly mean temperature range of 2.7 to 25.5 ∘C. In the other channels, the correction factors were less than 0.5 %. The coefficient of variation (CV, standard deviation/mean) of V0 from the normal Langley method, based on the data measured at the NOAA Mauna Loa Observatory, is between 0.2 % and 1.3 %, except in the 940 nm channel. The effect of gas absorption was less than 1 % in the 1225, 1627, and 2200 nm channels. The degradation of V0 for wavelengths shorter than 400 nm (−10 % to −4 % per year) was larger than that for wavelengths longer than 500 nm (−1 to nearly 0 % per year). The CV of V0 transferred from the reference POM-02 was 0.1 % to 0.5 %. Here, the data were simultaneously taken at 1 min intervals on a fine day, and data when the air mass was less than 2.5 were compared. The V0 determined by the improved Langley (IML) method had a seasonal variation of 1 % to 3 %. The root mean square error (RMSE) from the IML method was about 0.6 % to 2.5 %, and in some cases the maximum difference reached 5 %. The trend in V0 after removing the seasonal variation was almost the same as for the normal Langley method. Furthermore, the calibration constants determined by the IML method had much higher noise than those transferred from the reference. The modified Langley method was used to calibrate the 940 nm channel with on-site measurement data. The V0 obtained with the modified Langley method compared to the Langley method was 1 % more accurate on stable and fine days. The general method was also used to calibrate the shortwave-infrared channels (1225, 1627, and 2200 nm) with on-site measurement data; the V0 obtained with the general method differed from that obtained with the Langley method of V0 by 0.8 %, 0.4 %, and 0.1 % in December 2015, respectively.
