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STAR / SMCD / EMB Aerosol Remote Sensing

Single- and multi-channel algorithms

thumbnail: AOD scattergramComparisons of the single- and multi-channel, aerosol products reported in the Clouds and the Earth's Radiant Energy System (CERES) datasets showed systematic differences that were partly attributed to differences in sampling and cloud screening. A study was conducted that tried quantifying the aerosol optical depth (AOD) differences when the above differences were absent and exactly the same clear radiances were inputted to the aerosol algorithms used to generate the two products. This was accomplished by retrieving AOD with the single- channel algorithm at 22 oceanic locations from the reflectance data in the MODIS Atmosphere Parameters Subset Statistics (MAPSS) dataset for the period of 2000-2007, and then by comparing them to the corresponding MODIS AOD data reported in MAPSS. On average, the mean differences were wavelength and platform dependent (Figure 1). The single-channel 644-nm average AOD was larger by 0.004-0.015 (~2-9%) than that from the multi-channel algorithm. The mean AOD at 1632 nm from both algorithms were very similar from Terra, but the single-channel AOD from Aqua at 2119 nm was  lower by 0.02 (~24%). Slight dependence of the mean differences on the scattering angle was observed, which was partially explained by the differences between the retrieved aerosol model in the multi-channel retrieval, and the fixed aerosol model used in the single-channel algorithm [Laszlo et al., 2008].

Figure 1. Scatterplots of the single-channel aerosol optical depth (AODS) versus the multichannel aerosol optical depth (AODM) for the short (a, c) and long (b, d) wavelength channels for Terra (Figures 1a and 1b) and Aqua (Figures 1c and 1d).
Scatterplot of AODs

Note that although the results are plotted in log-log scale, the regression analyses and corresponding statistics were calculated in the linear scale. AODS was retrieved from the 10-km central pixel reflectance in MAPSS, while AODM was obtained directly from MAPSS for the corresponding pixel. The correlation coefficient (r), the 1:1 line, and the line and coefficients of the standard and orthogonal linear fits are also shown. The scale represents the number of AOD values.

References:

Laszlo, I., H. Liu, A. Ignatov (2008), Comparison of single- and multi-channel aerosol optical depths derived from MAPSS data, J. Geophys. Res., 113, D19S90, doi:10.1029/2007JD009664.