基于多角度多光谱Stokes参数Q和U测量反演气溶胶偏振相函数

偏振相函数是气溶胶重要的光学参数之一，它对气溶胶复折射指数、粒子尺度和形状都十分敏感。多角度多光谱偏振遥感可以有效获取气溶胶偏振相函数信息。新一代CIMEL太阳-天空偏振辐射计CE318-DP作为高精度地基气溶胶偏振遥感仪器已被引入全球气溶胶自动观测网AERONET(AErosol RObotic NETwork)，并作为扩展多波长偏振测量的太阳-天空辐射计观测网SONET(Sun/sky-radiometer Observation NETwork)的主要仪器，已在不同类型气溶胶观测站点积累了多年的偏振数据。但目前偏振反演仅能利用线偏振度或偏振辐亮度。与线偏振度和偏振辐亮度相比，Stokes参数Q和U不仅包含天空光线偏振强度信息还包含偏振方向信息。利用CE318-DP多光谱多角度测量的天空光Stokes参数Q和U反演气溶胶偏振相函数的方法。针对CE318-DP标准主平面偏振观测模式PPP(Polarized Principal Plane)下Stokes参数U对气溶胶特性变化不敏感、信息难以利用的不足，测试了新的平纬圈偏振扫描模式ALMP(ALMucantar Polarization)获取Stokes参数Q和U，并成功应用于偏振相函数的反演。系统分析了340～1 640 nm多光谱通道上典型生物质燃烧型气溶胶和水溶性气溶胶的-P₁₂/P₁₁反演结果并测试了反演方法在晴朗和灰霾不同大气条件下的适用性。无论在主平面还是平纬圈观测几何下，反演结果在可见光和近红外通道上均与真实值具有较好的一致性。进一步讨论了模型中基于气溶胶参数初始值和大气气溶胶参数真实值计算的“大气单次散射/大气散射”的比值近似相等的假设条件在短波通道不能很好地满足是造成紫外波段反演结果偏差较大的原因之一。后续有待进一步提高反演模型在短波通道的适用性，为利用不同光谱通道上-P₁₂/P₁₁的变化特征改进气溶胶微物理参数反演奠定了基础。

Retrieval of the Polarized Phase Function of Aerosol Particles Based on Multi-Angle Multi-Spectral Measurements of the Stokes Parameters Q and U

The polarized phase function is one of the important optical parameters, which is very sensitive to the aerosol complex refractive index, particle size and shape. The multi-angle multi-spectral polarization remote sensing is an effective means for obtaining the aerosol polarized phase function. As a ground-based high-accuracy polarization instrument for aerosol remote sensing, the new generation CIMEL dual-polar sun-sky radiometer CE318-DP has been introduced into the worldwide AErosol RObotic NETwork (AERONET). Meanwhile, as the main instrument of the Sun/sky-radiometer Observation NETwork (SONET) with the extension of multi-wavelength polarization measurements, it has accumulated polarization data for many years over observation stations with different aerosol types. However, the retrieval has been based only on the degree of linear polarization or the polarized radiance up to now. Compared to the degree of linear polarization and the polarized radiance, the Stokes parameters Q and U contain information not only on intensity of linear polarization but also on the orientation of polarization. This study introduces an algorithm to retrieve the aerosol polarized phase function based on the Stokes parameters Q and U of skylight from the multi-angle multi-spectral polarization measurements of the CE318-DP. Considering that the Stokes parameter U is changeless with different aerosol properties for the CE318-DP standard polarization observation scenario PPP (Polarized Principal Plane), which is difficult to be utilized, a new ALMP (ALMucantar Polarization) observation scenario is tested to obtain the Stokes parameters Q and U, then to be applied in retrieval of polarized phase function. As to the typical biomass burning and water-soluble aerosols, the results of -P₁₂/P₁₁ in the channels centered at 340 to 1 640 nm were presented and analyzed systematically. Moreover, the applicability of the inversion algorithm in clear and hazy sky conditions was also tested. For the visible and near-infrared channels, the results were in agreement with the truth values not only for the PPP but also for the ALMP geometries. One of the reasons for the obvious deviation of the results in the ultraviolet (UV) bands was also discussed, which was the assumption of the approximately equivalent “ratios of atmospheric single scattering and atmospheric scattering” based on the initial aerosol parameters and the real aerosol parameters could not be satisfied in the UV bands. The inversion model should be improved to be applied to the short-wave channels in further studies. On this basis, some subsequent researches can be engaged in to utilize the features of multi-spectral -P₁₂/P₁₁ to improve the retrieval of aerosol microphysical properties.