Preliminary Assessment and Verification of the Langley Plots Calibration of the Sun Photometer at Mt Foyeding Observatory, Beijing

An assessment and verification of the Langley calibration method of the Sun photometer at Mt Foyeding (MFYD) Observatory in Beijing was performed. We explored whether the Langley plot calibration is practicable for this mountainous site by analyzing the aerosol climatology and carrying out a case study. Then, the aerosol optical depth (AOD) results were verified under the reference of AERONET AOD. The results showed that satisfactory atmospheric conditions are present on winter mornings, characterized by a smaller average AOD (similar to 0.09-0.14) and a lower range ratio (similar to 36.97-63.38%) than in the afternoons and over a whole day. The six days selected as the case study all showed stable atmospheric conditions characterized by daily average triplets of <2% for all wavelengths. The residual sum of squares for V-0 lambda at all wavelengths was <0.0002 and the residual standard deviation was <0.2%. A large improvement was found in the linear regression at morning relative to the statistics obtained over the whole day, when the coefficient of determination and residual standard deviation were promoted by 0.22-2.90% and similar to 2.76-23.32, respectively. The final V-0 lambda value was derived from 31 days of observation and the deviations from the reference V-0 lambda were about -1.69, -1.29, -0.81, -0.42, -0.34, -0.22, -0.63 and -0.36% at 340, 380, 440, 500, 675, 870, 1020 and 1640 nm, respectively. The regression analysis of the AOD validation showed a perfect AOD performance, with 100% of the retrievals lying within the expected error (0.05 +/- 10%) from 380 to 1640 nm and 99.99% for the 340 nm band. Good AOD agreement (correlation coefficients > 0.998) and residual standard deviation values ranging from similar to 0.006 to 0.011 were observed, with the relative mean bias varying from 0.999 to 1.066. The mean biases were concentrated within +/- 0.02 for the ultraviolet bands and within +/- 0.01 for the other bands; therefore, the results of this preliminary assessment and verification indicated that the Langley plots method is suitable for photometer calibration at the MFYD Observatory.

Automated summary

The Langley calibration method for Sun photometers at the MFYD Observatory in Beijing was assessed and verified, showing satisfactory results for atmospheric conditions, particularly in winter mornings. The aerosol optical depth results were validated against AERONET AOD, demonstrating good agreement and accuracy across different wavelengths.