Estimation of aerosol single scattering albedo from solar direct spectral radiance and total broadband irradiances measured in China

[1] Aerosol single scattering albedo (wo) is a primary factor dictating aerosol radiative effect. Ground-based remote sensing of wo has been employed most widely using spectral sky radiance measurements made from a scanning Sun photometer. Reliable results can be achieved for high aerosol loadings and for solar zenith angle >50 . This study presents an alternative method using spectral direct radiance measurements or aerosol optical depths together with total sky irradiance to retrieve wo. The method does not require sky radiance data that can only be acquired by the expensive scanning Sun photometer. The method is evaluated using extensive measurements by a suite of instruments deployed in northern China under the East Asian Study of Tropospheric Aerosols: An International Regional Experiment (EAST-AIRE) project. The sensitivities of the retrieval to various uncertain factors were first examined by means of radiative transfer simulations. It was found the retrieval is most sensitive to cloud screening, total irradiance and the Angstrom Exponent (AE), but only weakly depends on surface albedo and the fine structure of aerosol size distribution. Using 1 year of rigorously screened clear-sky measurements made at the Xianghe site, the retrieved wo values were found to agree with those retrieved from the Cimel Sun photometer by the AERONET method to within 0.03 (RMS), and 0.003 (mean bias). As part of the differences originate from different sky views seen by the Sun photometers and pyranometer under comparison, a further test was conducted by using total sky irradiances simulated with the retrieved aerosol properties from the AERONET. The resulting estimates of wo agree to within 0.01–0.02 (RMS differences) and 0.002–0.003 (mean bias). These values are better measure of the true retrieval uncertainties, as they are free from any data mismatch. The characteristics of wo retrievals were discussed.