Ahmad Ziauddin, Franz Bryan A, McClain Charles R, Kwiatkowska Ewa J, Werdell Jeremy, Shettle Eric P, Holben Brent N
NASA Goddard Space Flight Center, Greenbelt Road, Greenbelt, Maryland 20771, USA.
Appl Opt. 2010 Oct 10;49(29):5545-60. doi: 10.1364/AO.49.005545.
We describe the development of a new suite of aerosol models for the retrieval of atmospheric and oceanic optical properties from the SeaWiFS and MODIS sensors, including aerosol optical thickness (τ), angstrom coefficient (α), and water-leaving radiance (L(w)). The new aerosol models are derived from Aerosol Robotic Network (AERONET) observations and have bimodal lognormal distributions that are narrower than previous models used by the Ocean Biology Processing Group. We analyzed AERONET data over open ocean and coastal regions and found that the seasonal variability in the modal radii, particularly in the coastal region, was related to the relative humidity. These findings were incorporated into the models by making the modal radii, as well as the refractive indices, explicitly dependent on relative humidity. From these findings, we constructed a new suite of aerosol models. We considered eight relative humidity values (30%, 50%, 70%, 75%, 80%, 85%, 90%, and 95%) and, for each relative humidity value, we constructed ten distributions by varying the fine-mode fraction from zero to 1. In all, 80 distributions (8 Rh×10 fine-mode fractions) were created to process the satellite data. We also assumed that the coarse-mode particles were nonabsorbing (sea salt) and that all observed absorptions were entirely due to fine-mode particles. The composition of the fine mode was varied to ensure that the new models exhibited the same spectral dependence of single scattering albedo as observed in the AERONET data. The reprocessing of the SeaWiFS data show that, over deep ocean, the average τ(865) values retrieved from the new aerosol models was 0.100±0.004, which was closer to the average AERONET value of 0.086±0.066 for τ(870) for the eight open-ocean sites used in this study. The average τ(865) value from the old models was 0.131±0.005. The comparison of monthly mean aerosol optical thickness retrieved from the SeaWiFS sensor with AERONET data over Bermuda and Wallops Island show very good agreement with one another. In fact, 81% of the data points over Bermuda and 78% of the data points over Wallops Island fall within an uncertainty of ±0.02 in optical thickness. As a part of the reprocessing effort of the SeaWiFS data, we also revised the vicarious calibration gain factors, which resulted in significant improvement in angstrom coefficient (α) retrievals. The average value of α from the new models over Bermuda is 0.841±0.171, which is in good agreement with the AERONET value of 0.891±0.211. The average value of α retrieved using old models is 0.394±0.087, which is significantly lower than the AERONET value.
我们描述了一套新的气溶胶模型的开发情况，该模型用于从海色宽视场传感器（SeaWiFS）和中分辨率成像光谱仪（MODIS）传感器中反演大气和海洋的光学特性，包括气溶胶光学厚度（τ）、埃指数（α）和离水辐射率（L(w)）。新的气溶胶模型源自气溶胶机器人网络（AERONET）观测数据，具有双峰对数正态分布，比海洋生物学处理组之前使用的模型分布更窄。我们分析了公海和沿海地区的AERONET数据，发现模态半径的季节变化，特别是在沿海地区，与相对湿度有关。通过使模态半径以及折射率明确依赖于相对湿度，这些发现被纳入到模型中。基于这些发现，我们构建了一套新的气溶胶模型。我们考虑了八个相对湿度值（30%、50%、70%、75%、80%、85%、90%和95%），对于每个相对湿度值，我们通过将细模态分数从零变化到1构建了十种分布。总共创建了80种分布（8个相对湿度×10个细模态分数）来处理卫星数据。我们还假设粗模态粒子是非吸收性的（海盐），并且所有观测到的吸收完全归因于细模态粒子。细模态的组成有所变化，以确保新模型表现出与AERONET数据中观测到的相同的单散射反照率光谱依赖性。对SeaWiFS数据的重新处理表明，在深海区域，从新的气溶胶模型反演得到的平均τ(865)值为0.100±0.004，这更接近本研究中使用的八个公海站点的τ(870)的平均AERONET值0.086±0.066。旧模型的平均τ(865)值为0.131±0.005。将从SeaWiFS传感器反演得到的月平均气溶胶光学厚度与百慕大群岛和瓦勒普斯岛的AERONET数据进行比较，结果显示两者非常吻合。事实上，百慕大群岛上81%的数据点和瓦勒普斯岛上78%的数据点在光学厚度上的不确定度在±0.02范围内。作为对SeaWiFS数据重新处理工作的一部分，我们还修订了替代校准增益因子，这使得埃指数（α）的反演有了显著改进。新模型在百慕大群岛上的α平均值为0.841±0.171，与AERONET值0.891±0.211吻合良好。使用旧模型反演得到的α平均值为0.394±0.087，明显低于AERONET值。
