Development of near-sea-level Langley calibration algorithm for aerosol optical depth measurement

• Main Author: Chang, Jackson Hian Wui
• Format: Thesis
• Language: English; English
• Published: 2014
• Subjects: QC851-999 Meteorology. Climatology Including the earth's atmosphere
• Online Access: https://eprints.ums.edu.my/id/eprint/40568/1/24%20PAGES.pdf; https://eprints.ums.edu.my/id/eprint/40568/2/FULLTEXT.pdf; https://eprints.ums.edu.my/id/eprint/40568/

Aerosol optical depth (AOD) represents the total attenuation of solar terrestrial radiation caused by aerosol. In long-term monitoring networks, accurate measurement of AOD is difficult due to the lack of frequent calibration of the spectrometer. This is because conventional Langley calibration is usually performed at high mountains for clear and stable atmosphere and regular access to high altitudes is inefficient in terms of accessibility and economical prospects. Therefore, a near-sea-level Langley calibration algorithm is developed to allow frequent calibration feasible even at low altitude. It uses the combination of Perez-Du Mortier (PDM) model and statistical filter to constrain the Langley extrapolation to get closest possible extraterrestrial constant over a wide range of wavelengths. To further contain the wavelength-dependent error due to varying extraterrestrial constant, the Ratio Langley method is combined with the proposed algorithm to improve the prediction accuracy. In this way, more accurate AOD can be estimated by reanalysis of the calibrated volume spectrum using Beer-Lambert law. A total of 568 useful solar spectral data had been collected using ground-based spectrometer for the validation purposes. It is found that the AODs predicted by the proposed algorithm agree well to the reference values obtained from i-SMARTS model with high linearity and small error <3% for all wavelengths. The consistency of the proposed method is also validated with good resultsover two study areas (n=241) with different location, day, and time. Overall results implied that the application of the proposed algorithm in near-sea-level Langley calibration is proven feasible for AOD measurement.