Adaption of the MODIS aerosol retrieval algorithm using airborne spectral surface reflectance measurements over urban areas: a case study

[摘要] MODIS (MOderate-resolution Imaging Spectroradiometer) retrievals of aerosoloptical depth (AOD) are biased over urban areas, primarily because thereflectance characteristics of urban surfaces are different than that assumedby the retrieval algorithm. Specifically, the operational "dark-target"retrieval is tuned towards vegetated (dark) surfaces and assumes a spectralrelationship to estimate the surface reflectance in blue and red wavelengths.From airborne measurements of surface reflectance over the city ofZhongshan, China, were collected that could replace the assumptions withinthe MODIS retrieval algorithm. The subsequent impact was tested upon twoversions of the operational algorithm, Collections 5 and 6 (C5 and C6). AODretrieval results of the operational and modified algorithms were comparedfor a specific case study over Zhongshan to show minor differences betweenthem all. However, the Zhongshan-based spectral surface relationship wasapplied to a much larger urban sample, specifically to the MODIS data takenover Beijing between 2010 and 2014. These results were compared directly toground-based AERONET (AErosol RObotic NETwork) measurements of AOD.A significant reduction of the differences between the AOD retrieved by themodified algorithms and AERONET was found, whereby the mean differencedecreased from 0.27±0.14 for the operational C5 and 0.19±0.12 for theoperational C6 to 0.10±0.15 and -0.02±0.17 by using the modified C5and C6 retrievals. Since the modified algorithms assume a higher contributionby the surface to the total measured reflectance from MODIS, consequently theoverestimation of AOD by the operational methods is reduced. Furthermore, thesensitivity of the MODIS AOD retrieval with respect to different surfacetypes was investigated. Radiative transfer simulations were performed tomodel reflectances at top of atmosphere for predefined aerosol properties.The reflectance data were used as input for the retrieval methods. It wasshown that the operational MODIS AOD retrieval over land reproduces the AODreference input of 0.85 for dark surface types (retrieved AOD = 0.87 (C5)).An overestimation of AOD = 0.99 is found for urban surfaces, whereas themodified C5 algorithm shows a good performance with a retrieved value ofAOD = 0.86.

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