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Analysis of Visible/SWIR surface reflectance ratios for aerosol retrievals from satellite in Mexico City urban area

机译:墨西哥城市区卫星气溶胶回收的可见/ SWIR表面反射率比分析

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摘要

The surface reflectance ratio between the visible (VIS) and shortwaveinfrared (SWIR) radiation is an important quantity for the retrieval of theaerosol optical depth (τ) from the MODIS sensor data. Based onempirically determined VIS/SWIR ratios, MODIS τ retrieval usesthe surface reflectance in the SWIR band (2.1 µm), where theinteraction between solar radiation and the aerosol layer is small, topredict the visible reflectances in the blue (0.47 µm) and red (0.66 µm) bands. Therefore, accurate knowledge of the VIS/SWIR ratio isessential for achieving accurate retrieval of aerosol optical depth fromMODIS. We analyzed the surface reflectance over some distinct surface coversin and around the Mexico City metropolitan area (MCMA) using MODIS radiancesat 0.66 µm and 2.1 µm. The analysis was performed at 1.5 km×1.5 km spatial resolution. Also, ground-based AERONET sun-photometer dataacquired in Mexico City from 2002 to 2005 were analyzed for aerosol depth and other aerosol optical properties. In addition, a network ofhand-held sun-photometers deployed in Mexico City, as part of the MCMA-2006Study during the MILAGRO Campaign, provided an unprecedented measurement ofτ in 5 different sites well distributed in the city. We foundthat the average RED/SWIR ratio representative of the urbanized sitesanalyzed is 0.73±0.06 for scattering angles 140° and goes up to0.77±0.06 for higher ones. The average ratio for non-urban sites wassignificantly lower (approximately 0.55). In fact, this ratio stronglydepends on differences in urbanization levels (i.e. relative urban tovegetation proportions and types of surface materials). The aerosol opticaldepth retrieved from MODIS radiances at a spatial resolution of 1.5 km×1.5 kmand averaged within 10×10 km boxes were compared with collocated 1-hτ averaged from sun-photometer measurements. The use of the newRED/SWIR ratio of 0.73 in the MODIS retrieval over Mexico City led to asignificant improvement in the agreement between the MODIS andsun-photometer AOD results; with the slope, offset, and the correlationcoefficient of the linear regression changing from (τ=0.91τ+0.33, =0.66) to (τ=0.96 τ−0.006, =0.87).Indeed, an underestimation of this ratio in urban areas lead to asignificant overestimation of the AOD retrieved from satellite. Therefore,we strongly encourage similar analyses in other urban areas to enhance thedevelopment of a parameterization of the surface ratios accounting for urbanheterogeneities.
机译:可见(VIS)辐射和短波红外(SWIR)辐射之间的表面反射率是从MODIS传感器数据中检索气溶胶光学深度(τ)的重要参数。根据经验确定的VIS / SWIR比,MODISτ检索使用SWIR波段(2.1 µm)中的表面反射率,其中太阳辐射与气溶胶层之间的相互作用较小,从而预测蓝色(0.47 µm)和红色(0.66)的可见反射率。 µm)波段。因此,对VIS / SWIR比率的准确了解对于从MODIS准确获取气溶胶光学深度至关重要。我们使用0.66 µm和2.1 µm的MODIS辐射分析了墨西哥城都会区(MCMA)和周围一些明显的表面覆盖层的表面反射率。以1.5 km×1.5 km空间分辨率进行分析。此外,还分析了2002年至2005年在墨西哥城获得的地面AERONET太阳光度计数据,以了解气溶胶深度和其他气溶胶光学特性。此外,作为在MILAGRO运动期间进行的MCMA-2006研究的一部分,在墨西哥城部署的手持式日光光度计网络提供了前所未有的度量,可测量全市5个不同地点的τ。我们发现,对于散射角<140°,分析的代表城市化站点的平均RED / SWIR比为0.73±0.06,对于较高的散射角,上升至0.77±0.06。非城市场所的平均比率明显较低(约0.55)。实际上,该比率在很大程度上取决于城市化水平的差异(即相对的城市植被比例和地表材料类型)。从空间分辨率为1.5 km×1.5 km的MODIS辐射中取回并在10×10 km盒子内平均的气溶胶光学深度与从太阳光度计测量得到的并列的1-hτ进行了比较。在墨西哥城的MODIS检索中使用新的0.73的RED / SWIR比,导致MODIS与太阳光度计AOD结果之间的协议有了显着改善;线性回归的斜率,偏移和相关系数从(τ=0.91τ+ 0.33,= 0.66)变为(τ= 0.96 τ−0.006,= 0.87)。实际上,对该比例的低估导致对从卫星获取的AOD的估计过高。因此,我们强烈建议在其他城市地区进行类似的分析,以促进考虑到城市异质性的表面比率参数化的发展。

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