Abstract
Aerosol is one of the important geophysical parameters that determine the earth’s radiation budget, energy balance and hydrological cycle. The “Deep Blue” Moderate Resolution Imaging Spectro-radiometer (MODIS) Aerosol Optical Depth (AOD) retrieval algorithm was designed to complement existing “Dark Target” Ocean and Land algorithms to be able to retrieve AOD over bright land surface. Using level 2 AOD data from five Aerosol Robotic Network (AERONET) stations over the study location of North Africa (0°S - 40°N, 30°W - 60°E), comparative accuracy assessments are made for combined MODIS AOD aboard Terra and Aqua satellites and US Navy Aerosol Analysis and Prediction System (NAAPS) forecast AOD data. The aerosol transport and vertical mixing over the region are investigated at different altitudes up to 3000 m above ground level using Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT). The MODIS validation result shows highest correlation in the Sub-Sahel (0.811) followed by Sahel (0.726) and then Sahara region (0.662). Furthermore, the combined retrieval algorithm of Terra and Aqua MODIS shows statistically significant discrepancies from AERONET AOD values in term of mean, t-test value, index of agreement and fractional error. The comparison of NAAPS predicted soil dust to AERONET AOD fared best in December to February (DJF) season for the Sahel region and June to August (JJA) season for the Sahara when the dust emission and transport are at the peak. However, median ratios of NAAPS to AERONET AOD indicated bias in some island sites in the Atlantic Ocean which may be due to the presence of sea salt over the site. The analysis carried out in this study reveals that both MODIS retrieval algorithm and NAAPS model could be improved by incorporating some local aerosol sources from the study area.
Highlights
One of the long observing space-borne instruments that acquire Aerosol Optical Depth (AOD) is the MODerate resolution Imaging Spectro-radiometer (MODIS), which is a 36-band spectrometer covering land and ocean and widely used by researchers [1]
The analysis carried out in this study reveals that both Moderate Resolution Imaging Spectro-radiometer (MODIS) retrieval algorithm and Navy Aerosol Analysis and Prediction System (NAAPS) model could be improved by incorporating some local aerosol sources from the study area
Statistical tools were employed to test the significant difference between the mean AOD of the three measurement methods
Summary
One of the long observing space-borne instruments that acquire AOD is the MODerate resolution Imaging Spectro-radiometer (MODIS), which is a 36-band spectrometer covering land and ocean and widely used by researchers [1]. The combination of two instruments (radiometer) in flight aboard the two ships giving daily near-global observations, makes the MODIS sensors an enticing choice for such an aerosol dataset for researchers. The MODIS retrieval algorithm for AOD considers assumptions regarding surface reflectance, aerosol properties, meteorological conditions etc. Each one has a significant role it plays in AOD retrieval over different aerosol regions. Reference [3] evaluates the uncertainties of Collection 6 over Land and Water. Uncertainties are shown over coastal regions, where runoff and/or biological activities create issues for the surface boundary conditions
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