Optimized Broadband Extinction Method for Retrieving 500 nm AOD with Long-Term Direct Solar Radiation: Model Test and Application

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 = 1.00X – 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