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Optimized Broadband Extinction Method for Retrieving 500 nm AOD with Long-Term Direct Solar Radiation: Model Test and Application
Jinyuan Xin2,3, Yongjing Ma1,2, Ruixia Liu4, Lingbin Kong1,2, Lili Wang2, Yining Ma1,2, Wenyu Zhang1
1 College of Atmospheric Sciences, Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province, Lanzhou University, Lanzhou 730000, China
2 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
3 Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, Chengdu University of Information Technology, Chengdu 610225, China
4 Key Laboratory of Radiometric Calibration and Validation for Environmental Satellite, China Meteorological Administration; National Satellite Meteorological Center, Beijing 100081, China
- A new method is improved to retrieve AOD at 500 nm with the direct solar radiation.
- Compared with the observation site, OBEM has high precision to retrieve AOD.
- OBEM is available to reconstruct the historical database of AOD at 500 nm.
- OBEM implements the homogenization for the historical and current AOD at 500 nm.
- OBEM AOD exhibited a slight decline trend in the recent 18 years in Beijing.
A traditional broadband extinction method is improved by introducing the cloud-screening module and aerosol modal adjustment module, and the new approach is known as optimized broadband extinction method (OBEM). Based on OBEM, it could retrieve 500 nm aerosol optical depth (AOD) database with the use of direct solar radiation. Comparison of the monthly average AOD from OBEM and Chinese Sun Hazemeter Network (CSHNET) for 2006–2010 over Beijing site shows that the retrievals and observations display a high degree of consistency. The correlation equation is Y = 1.00 × X – 0.01 with a correlation coefficient R2 of 0.83 and root-mean-square error (RMSE) of 0.08. The largest relative error for the yearly average value is only 3.85%. So it is clear that the OBEM could serve as an effective tool to retrieve highly accurate historical 500 nm AOD data and implement homogenization with the current observational 500 nm AOD monitored by satellites and sun-photometers. The yearly average AOD of Beijing in 1993–2010 is 0.58 ± 0.03, and the seasonal AODs in MAM (March–April–May) and JJA (June–July–August) are obviously higher than SON (September–October–November) and DJF (December–January–February), with values of 0.70 ± 0.14 and 0.64 ± 0.16, respectively. The long-term 500 nm AOD reveal significant periodic inter-annual characteristics with a slight downward trend, and the irradiance also decreased due to the extinction of increasing total-cloud amount.
Aerosol optical depth at 500 nm; Optimized broadband extinction method; Cloud screening module; Aerosol modal adjustment module.