卫星偏振测量是气溶胶遥感的一种重要手段.气溶胶模型的准确性是影响卫星遥感气溶胶参数精度的关键因素之一.在卫星反演气溶胶算法中, 若忽略气溶胶粗模态贡献(星载偏振传感器气溶胶反演的一种常用假设)或选错气溶胶类型, 均会带来反演结果的误差.基于六种典型的气溶胶类型(沙尘型、生物质燃烧型、乡村背景型、污染大陆型、污染海洋型和重污染型)模型, 模拟研究了气溶胶模态和类型选择对卫星近红外偏振通道反演气溶胶光学厚度(AOD)的影响.利用矢量的辐射传输模式, 模拟分析了六种气溶胶类型在865 nm波长的大气偏振反射分布函数(BPDF); 发现大气BPDF与气溶胶粒子尺度密切相关, 粗模态对大气BPDF的贡献远小于细模态; 粗、细模态同时存在时, 大气BPDF反而小于仅细模态时的BPDF.在此基础上, 分析了“忽略粗模态贡献”和“选择错误气溶胶类型”两种情况下AOD的反演误差, 得到如下结论：(1)忽略气溶胶粗模态贡献, 会导致反演的细模态气溶胶光学厚度(AODf)偏小.六种典型气溶胶类型模型情况下, AODf反演结果可偏低12.3%～35.7%, 其中沙尘型气溶胶时AODf反演误差最大, 污染大陆型气溶胶时AODf反演误差最小.(2)若气溶胶类型选择错误, 反演的AOD可能偏大或偏小, 取决于与气溶胶类型对应的大气BPDF的差别.测试的六种气溶胶类型中, 沙尘型与重污染型的大气BPDF差别最大, 二者互换(即“选择错误”)时, AOD反演误差最大, 分别可达220.3%或-60.6%; 乡村背景型与污染大陆型的大气BPDF差别最小, 两者互换时, AOD反演误差最小, 分别为7.1%和-3.0%.研究结果对于发展新一代星载偏振传感器及其气溶胶反演算法研究具有参考价值.

Satellite polarimetric observation is an important approach for remote sensing of atmospheric aerosols. The validity and adaptability of aerosol type model is one of key factors affecting polarimetric remote sensing of aerosol properties. For an aerosol retrieval algorithm, either neglecting coarse mode aerosols (one of common assumption of aerosol retrieval from satellite polarimetric sensors) or employing incorrect aerosol types can bring errors to the retrieval results. In this paper, the influence of aerosol modes and choice of aerosol types (six typical types: desert dust, biomass burning, background/rural, polluted continental, polluted marine, and dirty pollution) on the retrieval of aerosol optical depth (AOD) were investigated based on near-infrared polarized satellite channel at 865 nm. Based on the vector radiative transfer simulation results, the atmospheric bidirectional polarized reflection distribution functions (BPDF) of the above six aerosol types at 865 nm were analyzed. The results demonstrate that atmospheric BPDF depends greatly on aerosol particle’s size, while influence of coarse mode on atmospheric BPDF is much smaller than that of fine mode. Compared with the atmospheric BPDF associated with only one fine mode, the BPDF associated with both fine and coarse modes shows smaller BPDF. With these simulation results, AOD retrieval errors were analyzed under two circumstances, neglecting coarse mode aerosols and employing an incorrect aerosol type. The results show that: (1) Neglecting coarse mode can lead to underestimation of the retrieved fine mode AOD (AODf). The AODf errors are in the range of -12.3% to -35.7% for different aerosol types. The maximum AODf error associates with desert dust aerosol type while the minimum with polluted continental type. (2) The use of an incorrect aerosol type can bring large AOD retrieval error, which depends on bias of atmospheric BPDF related to the incorrect aerosol type. Among all six aerosol types, the maximum BPDF difference occurs between desert dust and dirty pollution aerosol types, which yields the maximum AOD retrieval errors of -60.6% and 220.3%, given mischoosing one versus another. The minimum AOD retrieval errors can be -3.0% and 7.1% occurring between background/rural and polluted continental types, which have the smallest BPDF difference. The results of this study can help to develop the next generation spaceborne polarimetric sensors and corresponding aerosol retrieval algorithms.