An Introductory Study on the Calibration of CE318 Sunphotometer

The indoor and field calibration methods of CE318 sunphotometer are mainly explored and the calibration results of the two methods are compared. Based on integrating sphere radiation calibration method, an experiment is carried out for the calibration of the sky scattering radiation channels at eight different wave bands of CE318 sunphotometer. Langley calibration method and standard instrument relative calibration method are used to calibrate sun direct radiation channels at different wave bands of CE318 sunphotometer. The results of this experiment show that the calibration results of sky scattering radiation channels at 670 nm, 870 nm, 1020 nm and the original calibration of the same instrument are fundamentally equal with the discrepancy less than 6%. It has high reliability. Unfortunately the result of the wave band of 440 nm is higher than the original calibration, and the departure range is about 18.9%. Based on the calibration results, it can be concluded that there are good feasibility and reliability in using integrating sphere for calibrating the sky scattering radiation channels. However, there is a little difference between the calibration results and theoriginal calibration. Considering the fact that aging filters might be led to by using conditions and measurement for long periods outdoors, some aerosol particles may fall on the filters which are inside the sensor head, even because the filters run very long time, so the filters are made broken. Generally speaking, the calibration results of the sun direct radiation channels are slightly larger than theoriginal calibration. The results show that the filters of some channels are aging, so it is necessary to replace them to ensure observation precision. In addition, the comparisons between the Langley method and standard instrument relative calibration method are made. It shows from the comparison results that good weather conditions on the day of calibration are required by the Langley method, which include clear sky, cloud-free and stable atmosphere, and very accurate standard instrument is required by the standard instrument relative calibration method. Because the standard instrument relative calibration method is more advantageous than the Langley method from the comparing results, the standard instrument relative calibration method is employed by the present study to calibrate the CE318 sunphotometer in the CARSNET (China Aerosol Robot Sunphotometer NETwork) of China Meteorological Administration to guarantee the precision of the observation data.