Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements
Accuracy assessments of aerosol optical properties retrieved from Aerosol Robotic Network (AERONET) Sun and sky radiance measurements
复制标题
DOI：
10.1029/2000jd900040
复制
发表时间：
2000-04-27
影响因子：
4.4
通讯作者：
Slutsker, I
中科院分区：
文献类型：
--
作者：
Dubovik, O;Smirnov, A;Slutsker, I
对气溶胶机器人网络（AERONET）地基日-天扫描辐射计测量的辐射量反演气溶胶光学特性的敏感性进行了研究。这些研究的重点是测试一种新的反演概念，用于同时从直接和漫射辐射的光谱测量中获取气溶胶尺寸分布、复折射率和单散射反照率。分析了大气辐射模式中随机误差、仪器偏移和已知不确定性对反演的扰动。太阳或天空通道校准错误，天空辐射测量时方位角指向不准确，以及计算表面反射率不准确被认为是误差源。考虑了这些误差对具有双峰大小分布（弱吸收水溶性气溶胶、吸收生物质燃烧气溶胶和沙漠尘埃）的三种典型和光学上不同的气溶胶特性的影响。在反演中，假设气溶胶粒子是具有相同复折射率的多分散均匀球体。因此，我们还研究了在非球形尘埃气溶胶以及具有不同折射率的外部或内部混合球形颗粒的情况下，具有这种假设的反演如何偏差检索。分析表明，如果反演包括光谱光学深度和全太阳表面天空辐射的数据组合（散射角的角覆盖达到100度或更大），则成功地检索了所有气溶胶特征（尺寸分布、复折射率和单散射反照率）。即使在测量中存在相当强的系统或随机不确定性的情况下，大多数遥感应用的检索精度也是可以接受的。主要的限制与所有气溶胶类型的低光学深度情况的表征有关，在这种情况下，气溶胶光学深度的直接辐射测量可能会出现较高的相对误差。此外，测试结果表明，天空辐射中散射角覆盖范围的减小（散射角为75度或更小）会导致折射率实际信息的丢失。精确的方位角指向对尘埃的表征至关重要。非球形尘埃颗粒的散射需要特殊的分析，因此，通过球体对气溶胶的近似，我们可以通过反演光谱光学深度和全太阳表面的天空辐射度来得出单散射反照率。反演仅在前40度散射角测量的天空辐射，其中非球形影响较小，可以精确地检索非球形颗粒的气溶胶大小分布。
Sensitivity studies are conducted regarding aerosol optical property retrieval from radiances measured by ground-based Sun-sky scanning radiometers of the Aerosol Robotic Network (AERONET). These studies focus on testing a new inversion concept for simultaneously retrieving aerosol size distribution, complex refractive index, and single-scattering albedo from spectral measurements of direct and diffuse radiation. The perturbations of the inversion resulting from random errors, instrumental offsets, and known uncertainties in the atmospheric radiation model are analyzed. Sun or sky channel miscalibration, inaccurate azimuth angle pointing during sky radiance measurements, and inaccuracy in accounting for surface reflectance are considered as error sources. The effects of these errors on the characterization of three typical and optically distinct aerosols with bimodal size distributions (weakly absorbing water-soluble aerosol, absorbing biomass-burning aerosol, and desert dust) are considered. The aerosol particles are assumed in the retrieval to be polydispersed homogeneous spheres with the same complex refractive index. Therefore we also examined how inversions with such an assumption bias the retrievals in the case of nonspherical dust aerosols and in the case of externally or internally mixed spherical particles with different refractive indices. The analysis shows successful retrieval of all aerosol characteristics (size distribution, complex refractive index, and single-scattering albedo), provided the inversion includes the data combination of spectral optical depth together with sky radiances in the full solar almucantar (with angular coverage of scattering angles up to 100 degrees or more). The retrieval accuracy is acceptable for most remote sensing applications even in the presence of rather strong systematic or random uncertainties in the measurements. The major limitations relate to the characterization of low optical depth situations for all aerosol types, where high relative errors may occur in the direct radiation measurements of aerosol optical depth. Also, the results of tests indicate that a decrease of angular coverage of scattering (scattering angles of 75 degrees or less) in the sky radiance results in the loss of practical information about refractive index. Accurate azimuth angle pointing is critical for the characterization of dust. Scattering by nonspherical dust particles requires special analysis, whereby approximation of the aerosol by spheres allows us to derive single-scattering albedo by inverting spectral optical depth together with sky radiances in the full solar almucantar. Inverting sky radiances measured in the first 40 degrees scattering angle only, where nonspherical effects are minor, results in accurate retrievals of aerosol size distributions of nonspherical particles.