Dust Aerosol Retrieval Over the Oceans With the MODIS/VIIRS Dark Target Algorithm: 2. Nonspherical Dust Model. Issue 10 (23rd October 2020)

• Record Type: Journal Article
• Authors: Zhou, Yaping; Levy, Robert C.; Remer, Lorraine A.; Mattoo, Shana; Espinosa, W. Reed
• Is Part Of: Earth and space science. Volume 7:Issue 10(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-10-23
• Subjects: Remote sensing -- dust aerosol retrieval -- MODIS -- VIIRS -- dust optical properties; Space sciences -- Periodicals; Geophysics -- Periodicals; 500.5
• DOI: 10.1029/2020EA001222
• Languages: English
• ISSNs: 2333-5084

Abstract

Abstract: The Dark‐target (DT) aerosol algorithm retrieves spectral Aerosol Optical Depth (AOD) and other aerosol properties from Moderate‐resolution Imaging Spectrometer (MODIS) reflectance observations. Over the ocean, the DT algorithm is known to contain scattering‐angle‐dependent biases in its retrievals of AOD, Angstrom Exponent (AE), and Fine Mode Fraction (FMF) for dust aerosols. Following a two‐step strategy to improve the DT retrieval of dust over ocean, for which the first step is to identify dusty pixels (reported in "Part 1"), in this "Part 2, " we report on construction of a new dust model lookup table (LUT) and the strategy for applying it within the existing DT algorithm. In particular, we evaluate different characterizations of dust optical properties from a variety of frameworks and databases, and compare them with the current DT retrieval assumptions. Substituting the standard operational LUT with a spheroid dust model with identified dusty pixels shows significant improvement when compared with collocated AERONET‐identified dusty pixels. Specifically, the application of the new dust model to dusty pixels reduces their AOD bias from 0.06 to 0.02 while improving the fraction of retrievals within expected error from 64% to 82%. At the same time, the overall bias in AE is reduced from 0.13 to 0.06, and the scattering‐angle‐dependent AE bias is largely eliminated. In testing on two full months of data (April and July), the new retrieval will reduce the monthly mean AOD by up to 0.1 and 0.2 in the north Atlantic and Arabian seas, respectively. The average AE and FMF are also reduced in these dust heavy regions. Key Points: MODIS Dark Target Ocean aerosol retrieval algorithm has a long‐standing retrieval bias for dust aerosol This study implements a nonspherical dust aerosol model to better represent dust optical properties The new algorithm reduces biases in AOD, AE, and FMF in dusty pixels and causes an overall reduction in AOD in major dust regions over ocean