Wang Yi, Wang Jun, Levy Robert C, Xu Xiaoguang, Reid Jeffrey S
Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, IA 52242, USA.
Center of Global and Regional Environmental Research, The University of Iowa, Iowa City, IA 52242, USA.
Remote Sens (Basel). 2017;9(6):595. doi: 10.3390/rs9060595. Epub 2017 Jun 12.
We present a new approach to retrieve Aerosol Optical Depth (AOD) using the Moderate Resolution Imaging Spectroradiometer (MODIS) over the turbid coastal water. This approach supplements the operational Dark Target (DT) aerosol retrieval algorithm that currently does not conduct AOD retrieval in shallow waters that have visible sediments or sea-floor (i.e., Class 2 waters). Over the global coastal water regions in cloud-free conditions, coastal screening leads to ~20% unavailability of AOD retrievals. Here, we refine the MODIS DT algorithm by considering that water-leaving radiance at 2.1 µm to be negligible regardless of water turbidity, and therefore the 2.1 µm reflectance at the top of the atmosphere is sensitive to both change of fine-mode and coarse-mode AODs. By assuming that the aerosol single scattering properties over coastal turbid water are similar to those over the adjacent open-ocean pixels, the new algorithm can derive AOD over these shallow waters. The test algorithm yields ~18% more MODIS-AERONET collocated pairs for six AERONET stations in the coastal water regions. Furthermore, comparison of the new retrieval with these AERONET observations show that the new AOD retrievals have equivalent or better accuracy than those retrieved by the MODIS operational algorithm's over coastal land and non-turbid coastal water product. Combining the new retrievals with the existing MODIS operational retrievals yields an overall improvement of AOD over those coastal water regions. Most importantly, this refinement extends the spatial and temporal coverage of MODIS AOD retrievals over the coastal regions where 60% of human population resides. This expanded coverage is crucial for better understanding of impact of aerosol particles on coastal air quality and climate.
我们提出了一种新方法，用于在浑浊的沿海水域利用中分辨率成像光谱仪（MODIS）反演气溶胶光学厚度（AOD）。这种方法补充了现行的暗目标（DT）气溶胶反演算法，该算法目前在有可见沉积物或海底的浅水区（即2类水域）不进行AOD反演。在全球无云条件下的沿海水域，海岸筛选导致约20%的AOD反演不可用。在此，我们对MODIS DT算法进行了改进，认为无论水体浑浊度如何，2.1μm处的离水辐射均可忽略不计，因此大气顶2.1μm处的反射率对细模态和粗模态AOD的变化均敏感。通过假设沿海水域浑浊水体上的气溶胶单次散射特性与相邻开阔海洋像素上的相似，新算法可以在这些浅水区反演AOD。对于沿海水域的6个AERONET站点，测试算法产生的MODIS - AERONET共定位对多了约18%。此外，将新的反演结果与这些AERONET观测结果进行比较表明，新的AOD反演结果在沿陆地和非浑浊沿海水域的精度与MODIS业务算法反演的相当或更好。将新的反演结果与现有的MODIS业务反演结果相结合，可使沿海水域的AOD总体得到改善。最重要的是，这种改进扩展了MODIS AOD反演在沿海地区的空间和时间覆盖范围，而全球60%的人口居住在这些沿海地区。这种扩大的覆盖范围对于更好地理解气溶胶颗粒对沿海空气质量和气候的影响至关重要。
