Launched in December 1999, NASA's Multi-angle Imaging SpectroRadiometer (MISR) has given researchers the ability to observe the Earth from nine different views for the last 22 years. Among the many advancements that have since resulted from the launch of MISR is progress in the retrieval of aerosols from passive space-based remote sensing. The MISR operational standard aerosol (SA) retrieval algorithm has been refined several times over the last 20 years, resulting in significant improvements to spatial resolution (now 4.4 km) and aerosol particle properties. However, the MISR SA still suffers from large biases in retrieved aerosol optical depth (AOD) as aerosol loading increases. Here, we present a new MISR research aerosol (RA) retrieval algorithm that utilizes over-land surface reflectance data from the Multi-Angle Implementation of Atmospheric Correction (MAIAC) to address these biases. This new over-land and over-water algorithm produces a self-consistent aerosol and surface retrieval when aerosol loading is low (AOD <0.75); this is combined with a prescribed surface algorithm using a bounded-variable least squares solver when aerosol loading is elevated (AOD >1.5). The two algorithms (prescribed + retrieved surface) are then merged as part of our combined surface retrieval algorithm. Results are compared with AErosol RObotic NETwork (AERONET) validation sun-photometer direct-sun + almucantar inversion retrievals.Over land, with AERONET AOD (550 nm) direct-sun observations as the standard, the root mean squared error (RMSE) of the MISR RA combined retrieval (n=11563) is 0.084, with a correlation coefficient (r) of 0.935 and expected error of $\pm (\mathrm{0.20}\times [\mathrm{MISR}\mathrm{AOD}]+\mathrm{0.02}$). For MISR RA retrieved AOD >0.5 (n=664), we report an Ångström exponent (ANG) RMSE of ∼0.35, with a correlation coefficient of 0.844. Retrievals of ANG, fine-mode fraction (FMF), and single-scattering albedo (SSA) improve as retrieved AOD increases. For AOD >1.5 (n=66), FMF RMSE is <0.09 with correlation >0.95, and SSA RMSE is 0.015 with a correlation coefficient of ∼0.75.Over water, comparing AERONET AOD to the MISR RA combined retrieval (n=4596), MISR RA RMSE is 0.063 and r is 0.935, with an expected error of $\pm (\mathrm{0.15}\times [\mathrm{MISR}\mathrm{AOD}]+\mathrm{0.02})$. ANG sensitivity is excellent when MISR RA reported AOD >0.5 (n=188), with an RMSE of 0.27 and r=0.89. Due to a lack of coincidences with AOD >1 (n=21), our conclusions about MISR RA high-AOD particle property retrievals over water are less robust (FMF RMSE =0.155 and r=0.94, whereas SSA RMSE =0.010 and r=0.50).In general, better aerosol particle property constraints can be made at lower AOD over water compared to our over-land retrievals. It is clear from the results presented that the new MISR RA has quantitative sensitivity to FMF and SSA (and qualitative sensitivity to non-sphericity) when retrieved AOD exceeds 1, with qualitative sensitivity to aerosol type at lower AOD, while also eliminating the AOD bias found in the MISR SA at higher AODs. These results also demonstrate the advantage of using a prescribed surface when aerosol loading is elevated.

中文翻译：

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新的 MISR 研究气溶胶反演算法：陆地和水上气溶胶粒子特性的多角度、多光谱、有界变量最小二乘法反演

NASA 的多角度成像光谱辐射计 (MISR) 于 1999 年 12 月发射，在过去的 22 年中，研究人员能够从九个不同的角度观察地球。自从 MISR 发射以来取得的许多进步中，有一项是在从被动天基遥感中回收气溶胶方面取得的进展。MISR 运行标准气溶胶 (SA) 反演算法在过去 20 年中经过多次改进，显着提高了空间分辨率（现在为 4.4 公里）和气溶胶粒子特性。然而，随着气溶胶负载的增加，MISR SA 在检索的气溶胶光学深度 (AOD) 中仍然存在较大偏差。这里，我们提出了一种新的 MISR 研究气溶胶 (RA) 反演算法，该算法利用来自多角度大气校正实施 (MAIAC) 的陆地表面反射率数据来解决这些偏差。当气溶胶负荷较低时，这种新的陆上和水上算法会产生自洽的气溶胶和地表反演（AOD <0.75 ); 当气溶胶负载升高 (AOD >1.5 )时，这与使用有界变量最小二乘求解器的规定表面算法相结合 。然后将这两种算法（规定的 + 检索的表面）合并为我们的组合表面检索算法的一部分。结果与 AERONET 验证太阳光度计直射太阳 +  almucantar 反演反演进行了比较。在陆地上，以 AERONET AOD（550 nm）直射太阳观测为标准，均方根误差（RMSE）的MISR RA 组合检索 ( n =11563 ) 为 0.084，相关系数 ( r ) 为 0.935，预期误差为$\pm (\mathrm{0.20}\times [\mathrm{MISR}\mathrm{AOD}]+\mathrm{0.02}$). 对于 MISR RA 检索到的 AOD  >0.5 ( n =664)，我们报告了 Ångström 指数 (ANG) RMSE ~0.35，相关系数为 0.844。ANG、精细模式分数 (FMF) 和单散射反照率 (SSA) 的检索随着检索的 AOD 的增加而提高。对于 AOD  >1.5 ( n =66 )，FMF RMSE <0.09，相关性 >0.95，SSA RMSE 为 0.015，相关系数为∼0.75。水上，将 AERONET AOD 与 MISR RA 联合反演进行比较 ( n =4596) , MISR RA RMSE 为 0.063，r为 0.935，预期误差为 $\pm (\mathrm{0.15}\times [\mathrm{MISR}\mathrm{AOD}]+\mathrm{0.02})$. 当 MISR RA 报告 AOD >0.5 ( n =188 )、RMSE 为 0.27 和 r =0.89时，ANG 灵敏度非常好 。由于缺乏与 AOD  >1 ( n =21 ) 的巧合，我们关于 MISR RA 高 AOD 粒子属性反演的水的结论不太稳健（FMF RMSE  =0.155和r =0.94，而 SSA RMSE  =0.010和r = 0.50). 一般来说，与我们的陆上回收相比，可以在较低的 AOD 下对水上的气溶胶粒子特性进行更好的约束。从呈现的结果中可以清楚地看出，当检索到的 AOD 超过 1 时，新的 MISR RA 对 FMF 和 SSA 具有定量敏感性（以及对非球形度的定性敏感性），在较低的 AOD 下对气溶胶类型具有定性敏感性，同时还消除了 AOD 偏差在更高 AOD 的 MISR SA 中发现。这些结果还证明了在气溶胶负荷升高时使用指定表面的优势。