利用多角度POLDER偏振资料实现陆地上空大气气溶胶光学厚度和地表反照率的同时反演 I.理论与模拟

Simultaneously Retrieving Aerosol Optical Depth and Surface Albedo over Land from POLDER's Multi-angle Polarized Measurements I: Theory and Simulations

陆地上空标量辐射对地表反射率和大气气溶胶散射都具有很强的敏感性,而偏振反射只对大气气溶胶敏感,对地表不敏感。根据这个原理并结合POLDER(POLarization and Directionality of Earth Reflectance)资料的特点,作者提出综合利用标量辐射和偏振反射信息来实现陆地上空大气气溶胶和地表反照率的同时反演。首先,利用多角度偏振辐射观测提取大气气溶胶光学参数,再利用标量辐射测量对偏振反演结果作进一步筛选和订正,同时获得地表反射率。数值模拟试验结果证明,仅利用偏振信息只能获取大气气溶胶信息,而且其结果误差较大,特别是对于散射作用较强的短波长通道如670 nm误差更大,但经过标量辐射订正后的结果得到明显改善,气溶胶光学厚度和地表反射率与真实值之间相关系数都达到0.99以上。为提高查找表的计算效率,提出并建立了反演方案所需要的半参数化数值表,利用内插方法寻求气溶胶光学厚度和地表反射率的数值解的反演方法。

The remote sensing of aerosol optical thickness and surface albedo has long been a challenge in the earth observation since both the atmosphere and surface contribute to the space-borne measurements, and they are difficult to be separated from the measurements, especially over land where radiances from the surface reflectance and atmospheric scattering are comparable. The satellite measured intensities are sensitive to both atmospheric scattering and surface albedo, while polarized radiances are only sensitive to the atmospheric scattering. Based on this principle, the aerosol optical parameters are first given through look-up table method, and then they are corrected with the total intensity measurements. The POLDER （POLarization and Directionality of Earth Reflectance） multi-angle polarized measurements are simulated and then used in retrieval simulations, results show large errors for aerosol optical parameters if only polarized measurements are used in retrieval algorithm, especially for short wavelength channel of 670 nm, in which multiple scattering effect is relatively strong. The results can be improved if intensities are also be used, and meanwhile surface albedo over land can be given. The statistics show that the correlation coefficients between the retrieved and simulated values, both for aerosol optical depth and surface albedo, are above 0. 99. New semi-parameterized look-up tables are also established to speed up the efficiency of retrieval algorithm.