Abstract
ABSTRACTA 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
Highlights
Atmospheric aerosols play an important role in the terrestrial climate system and air quality monitoring
We propose the optimized broadband extinction method (OBEM) model for retrieval of the aerosol optical depth (AOD) at 500 nm with the introduction of a cloud-screening module and an aerosol modal adjustment module
The cloud-screening results from 2009 and 2010 are 253 and 248, with corresponding observational clearness values of 257 and 248, among which the misjudgment percentages (B*/total*) of the two years are 9.49% and 9.68%, and the erroneous rejection percentages (C*/total*) are 10.89% and 9.68%, respectively. Another critical point that should be considered in this cloud screening process is that the sunshine duration and relative humidity (RH) datasets are obtained at the Beijing Observatory, whereas the hazemeter observations are collected in the yard of the Iron Tower, Institute of Atmospheric Physics (IAP)
Summary
Atmospheric aerosols play an important role in the terrestrial climate system and air quality monitoring. Based on OBEM, it could retrieve 500 nm aerosol optical depth (AOD) database with the use of direct solar radiation.
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