Abstract
Abstract. MODIS (MOderate-resolution Imaging Spectroradiometer) retrievals of aerosol optical depth (AOD) are biased over urban areas, primarily because the reflectance characteristics of urban surfaces are different than that assumed by the retrieval algorithm. Specifically, the operational "dark-target" retrieval is tuned towards vegetated (dark) surfaces and assumes a spectral relationship to estimate the surface reflectance in blue and red wavelengths. From airborne measurements of surface reflectance over the city of Zhongshan, China, were collected that could replace the assumptions within the MODIS retrieval algorithm. The subsequent impact was tested upon two versions of the operational algorithm, Collections 5 and 6 (C5 and C6). AOD retrieval results of the operational and modified algorithms were compared for a specific case study over Zhongshan to show minor differences between them all. However, the Zhongshan-based spectral surface relationship was applied to a much larger urban sample, specifically to the MODIS data taken over Beijing between 2010 and 2014. These results were compared directly to ground-based AERONET (AErosol RObotic NETwork) measurements of AOD. A significant reduction of the differences between the AOD retrieved by the modified algorithms and AERONET was found, whereby the mean difference decreased from 0.27±0.14 for the operational C5 and 0.19±0.12 for the operational C6 to 0.10±0.15 and -0.02±0.17 by using the modified C5 and C6 retrievals. Since the modified algorithms assume a higher contribution by the surface to the total measured reflectance from MODIS, consequently the overestimation of AOD by the operational methods is reduced. Furthermore, the sensitivity of the MODIS AOD retrieval with respect to different surface types was investigated. Radiative transfer simulations were performed to model reflectances at top of atmosphere for predefined aerosol properties. The reflectance data were used as input for the retrieval methods. It was shown that the operational MODIS AOD retrieval over land reproduces the AOD reference 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 the modified C5 algorithm shows a good performance with a retrieved value of AOD = 0.86.
Highlights
Aerosol particles influence various aspects of the Earth’s ecosystem
A significant reduction of the differences between the aerosol optical depth (AOD) retrieved by the modified algorithms and AERONET was found, whereby the mean difference decreased from 0.27 ± 0.14 for the operational Collection 5 (C5) and 0.19±0.12 for the operational Collection 6 (C6) to 0.10±0.15 and −0.02 ± 0.17 by using the modified C5 and C6 retrievals
The results have shown a relative mean bias (RMB; ratio of AOD from MODerate-resolution Imaging Spectroradiometer (MODIS) and AOD derived from sun photometer at “Beijing_RADI” AERONET site) of 0.97, whereas the standard MODIS algorithm overestimated the AOD with a RMB of 1.12
Summary
Aerosol particles influence various aspects of the Earth’s ecosystem. By scattering (Charlson et al, 1987, 1992; Tegen et al, 2000) or absorption (Jacobson, 2001; Ramanathan et al, 2001) of solar radiation, aerosol particles influence the Earth’s radiation budget, perturbing cloud and precipitation cycles and the global climate (Charlson et al, 1987; Yu et al, 2007; Menon et al, 2002; Houghton et al, 2001). Wong et al (2011) developed an aerosol retrieval algorithm for Hong Kong and the Pearl River Delta region with a spatial resolution of 500 m They determined the surface reflectance with a minimum reflectance technique which increased the slope of the linear regression coefficient between AODs from MODIS and sun photometer measurements from 0.75 to 0.83. These properties and the corresponding modification of reflection assumptions in the operational C5 and C6 retrieval algorithms are presented in Sect.
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