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

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 "darktarget" 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 Zhongshanbased 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.

A surface reflectance scheme for retrieving aerosol optical depth over urban surfaces in MODIS Dark Target retrieval algorithm

“…MDT does not provide aerosol retrieval over very bright surface (i.e., desert) and over snow and ice regions. In addition, several validation studies have shown that MDT AOD retrievals over urban area are positively biased with respect to AERONET AODs Jethva et al, 2007;Hyer et al, 2011;Gupta et al, 2013;Munchak et al, 2013). These studies have shown that the major source of bias in the MDT over cities is that the city surface does not behave as a vegetated "dark" target.…”

“…Several other research attempts have been made to change the surface scheme in the MDT for particular urban regions and produce better AOD retrievals for those specific cities (de Almeida Castanho et al,2007Li et al, 2005;Wong et al, 2011;Zha et al, 2011;Li et al, 2012;Escribano et al, 2014;Jäkel et al, 2015). Instead of a retrieval focused on only one city, we seek a surface parameterization that is valid for cities across the globe.…”

MOSAiC-ACA and AFLUX - Arctic airborne campaigns characterizing the exit area of MOSAiC

“…Due to an increase of noise at the edges of the measured spectra, the final data are provided for 400 and 1800 nm wavelength, only. The measurement uncertainties are related to the radiometric and spectral calibration and to the correction of the cosine response which sum up to a total wavelength-dependent uncertainty ranging between 3 and 14% 52 .…”

A surface reflectance scheme for retrieving aerosol optical depth over urban surfaces in MODIS dark target retrieval algorithm

“…MDT does not provide aerosol retrieval over very bright surface (i.e., desert) and over snow and ice regions. In addition, several validation studies have shown that MDT AOD retrievals over urban area are positively biased with respect to AERONET AODs (Levy et al, 2010;Jethva et al, 2007;Hyer et al, 2011;Gupta et al, 2013;Munchak et al, 2013). These studies have shown that the major source of bias in the MDT over cities is that the city surface does not behave as a vegetated "dark" target.…”

“…Several other research attempts have been made to change the surface scheme in the MDT for particular urban regions and produce better AOD retrievals for those specific cities (de Almeida Castanho et al,2007Li et al, 2005;Wong et al, 2011;Zha et al, 2011;Li et al, 2012;Escribano et al, 2014;Jäkel et al, 2015). Instead of a retrieval focused on only one city, we seek a surface parameterization that is valid for cities across the globe.…”