Comparison of single‐channel and multichannel aerosol optical depths derived from MAPSS data

Abstract

Previous comparisons of the single‐channel and multichannel aerosol products reported in the Clouds and the Earth's Radiant Energy System (CERES) Single Scanner Footprint (SSF) data sets showed systematic differences that were partly attributed to differences in sampling and cloud screening. This study concentrates on quantifying the aerosol optical depth (AOD) differences when the above differences are absent and exactly the same clear radiances are inputted to the aerosol algorithms used to generate the two products. This is accomplished by retrieving AOD with the single‐channel algorithm at 22 oceanic locations from the reflectance data in the Moderate Resolution Imaging Spectroradiometer (MODIS) Atmosphere Parameters Subset Statistics (MAPSS) data set for the period of 2000–2007 and then by comparing them to the corresponding MODIS AOD data reported in MAPSS. Comparisons of AODs are performed for two MODIS instruments flown onboard the Terra and Aqua platforms at two wavelengths. On average, the mean differences are wavelength and platform dependent. The single‐channel 644‐nm AODs are larger by 0.004–0.015 (∼2–9%) than those from the multichannel algorithm. The mean AOD at 1632 nm from both algorithms are very similar from Terra, but the single‐channel AOD from Aqua at 2119 nm is lower by 0.02 (∼24%). The mean absolute differences are 0.022–0.025 and do not change much with wavelength or platform. Slight dependence of the mean differences on the scattering angle is observed, which is partially explained by the differences between the retrieved aerosol model in the multichannel retrieval and the fixed aerosol model used in the single‐channel algorithm.