Abstract
Beijing is one of the largest metropolitan areas in the world with relatively high aerosol loading. The population of Beijing is approximately 21.5 million based on statistics from 2014. In order to improve the air quality of Beijing by monitoring and better understanding of high aerosol loading at fine spatial resolution, an extended version of the Look Up Table (LUT) aerosol retrieval algorithm from PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar) measurements of total intensity and polarization was tested over this region. Instead of using the surface reflectance model introduced in the GRASP (Generalized Retrieval of Aerosol and Surface Properties) algorithm, the assumption of spectral reflectance shape invariance principle is used to separate the total radiance contribution of surface and aerosols. Case studies were conducted in Beijing and evaluated preliminarily using the coincident AERONET measurements. The results indicate a significant agreement with a slope of 1.083 and a correlation coefficient of 0.913. A high Gfrac (fraction of accurate retrievals) of 78% is also observed. Analysis on the retrieval accuracy illustrates that the algorithm capability depends significantly on the data quality index, as the AOD (Aerosol Optical Depth) retrieval accuracy is relatively lower for the data with quality index less than 0.75.
Highlights
Radiative forcing caused by aerosols is thought to be one of the largest uncertainties in the radiative forcing of the earth’s climate [1]
The algorithm fits the complete set of POLDER/PARASOL
The spectral reflectance shape invariance principle was first developed by Flowerdew and Haigh, 1995 [24], which suggested that the shape of the surface Hemispherical Directional Reflectance Factor (HDRF) should be nearly spectrally invariant because the surface scattering elements are much larger than the wavelengths of the scattered light
Summary
Radiative forcing caused by aerosols is thought to be one of the largest uncertainties in the radiative forcing of the earth’s climate [1]. Better estimates of the perturbations to earth’s radiation budget require accurate optical and physical properties of aerosols such as spectral aerosol optical depth (AOD, ±0.02), size distribution (effective radius ±10%, effective variance ±40%) and single scattering albedo (SSA, ±0.03) [7,8] Determination of these parameters on a global scale can be achieved by means of satellite remote sensing. The algorithm fits the complete set of POLDER/PARASOL observations in all spectral channels (except 763, 765, and 910 nm dominated by gaseous absorption) for up to 16 observation geometries including both measurements of total radiances and linear polarization Based on this strategy, the algorithm aims at retrieval of an extended set of aerosol microphysical parameters including the aerosol volume size distribution (in bins), the total volume concentration of aerosol, the complex aerosol refractive index, mean height of aerosol layer, and the fraction of nonspherical scatterers.
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