Miecznik Grzegorz, Illing Rainer, Petroy Shelley, Sokolik Irina N
Ball Aerospace & Technologies Corporation, 1600 Commerce Street, Boulder, Colorado 80301, USA.
Appl Opt. 2005 Jul 10;44(20):4186-204. doi: 10.1364/ao.44.004186.
Linearly polarized radiation is sensitive to the microphysical properties of aerosols, namely, to the particle-size distribution and refractive index. The discriminating power of polarized radiation increases strongly with the increasing range of scattering angles and the addition of multiple wavelengths. The polarization and directionality of the Earth's reflectances (POLDER) missions demonstrate that some aerosol properties can be successfully derived from spaceborne polarimetric, multiangular measurements at two visible wavelengths. We extend the concept to analyze the retrieval capabilities of a spaceborne instrument with six polarimetric channels at 412, 445, 555, 865, 1250, and 2250 nm, measuring approximately 100 scattering angles covering a range between 50 and 150 deg. Our focus is development of an analysis methodology that can help quantify the benefits of such multiangular and multispectral polarimetric measurements. To that goal we employ a sensitivity metric approach in a framework of the principal-component analysis. The radiances and noise used to construct the sensitivity metric are calculated with the realistic solar flux for representative orbital viewing geometries, accounting for surface reflection from the ground, and statistical and calibration errors of a notional instrument. Spherical aerosol particles covering a range of representative microphysical properties (effective radius, effective variance, real and imaginary parts of the refractive index, single-scattering albedo) are considered in the calculations. We find that there is a limiting threshold for the effective size (approximately 0.7 microm), below which the weak scattering intensity results in a decreased signal-to-noise ratio and minimal polarization sensitivity, precluding reliable aerosol retrievals. For such small particles, close to the Rayleigh scattering limit, the total intensity provides a much stronger aerosol signature than the linear polarization, inspiring retrieval when the combined signals of intensities and the polarization fraction are used. We also find a strong correlation between aerosol parameters, in particular between the effective size and the variance, which forces one to simultaneously retrieve at least these two parameters.
线偏振辐射对气溶胶的微观物理特性敏感，即对粒径分布和折射率敏感。偏振辐射的分辨能力随着散射角范围的增加和多波长的加入而显著增强。地球反射率（POLDER）任务的偏振和方向性表明，一些气溶胶特性可以从两个可见波长的星载偏振多角度测量中成功推导出来。我们扩展了这一概念，以分析一台在412、445、555、865、1250和2250纳米处有六个偏振通道的星载仪器的反演能力，该仪器测量大约100个散射角，覆盖范围在50度至150度之间。我们的重点是开发一种分析方法，该方法有助于量化这种多角度和多光谱偏振测量的益处。为了实现这一目标，我们在主成分分析框架中采用了灵敏度度量方法。用于构建灵敏度度量的辐射亮度和噪声是根据具有代表性的轨道观测几何形状的实际太阳通量计算的，考虑了地面的表面反射以及一台假想仪器的统计和校准误差。计算中考虑了覆盖一系列代表性微观物理特性（有效半径、有效方差、折射率的实部和虚部、单次散射反照率）的球形气溶胶粒子。我们发现有效尺寸存在一个极限阈值（约0.7微米），低于该阈值，弱散射强度会导致信噪比降低和偏振灵敏度最小，从而无法进行可靠的气溶胶反演。对于如此小的粒子，接近瑞利散射极限时，总强度提供的气溶胶特征比线性偏振要强得多，当使用强度和偏振分数的组合信号时，有助于进行反演。我们还发现气溶胶参数之间存在很强的相关性，特别是有效尺寸和方差之间，这迫使人们至少要同时反演这两个参数。
