Validation of Modis Aerosol Optical Thickness Product Distributed by Nsmc over Seas around China and Its Adjacent Area

MODIS (Terra) aerosol optical thickness product distributed by NSMC were validated over seas around China and its adjacent area using AERONET data from Jan. 2005 to May 2007. Two spatio-temporal matching up schemes were adopted and the results were not quite the same. Using 25km radius and ±0.5h as the sampling window, there were 32 match-ups. The linear fitting results between MODIS and AERONET were y=0.672075x+0.132479(R=0.68), y=0.707179x+0.112427(R=0.65) and y=0.650986x+0.089910(R=0.56) for MODIS 550, 658 and 860nm channels respectively. The RMSE was 0.15, 0.13 and 0.10 respectively. The percentages of retrievals within the expected uncertainties were 62.5%, 46.9% and 46.9% respectively. Using data in spring (Mar. to May, 22 match-ups), the linear fitting results were y=0.804612x+0.050988 (R=0.77), y=0.862697x+0.034432 (R=0.75) and y=0.851294x+0.012003 (R=0.71) respectively. Using 45km radius and ±1.0h as the sampling window, there were 120 match-ups. The validation results were relatively poor with large data dispersion and R of 3 channels were all less than 0.3 while exceeding 0.6 without data from “Taichung” in the linear fitting. The RMSE was 0.23, 0.20 and 0.17 for 550, 658 and 860nm respectively with the best result 0.11, 0.09 and 0.07 at Hong Kong PolyU. The percentages of retrievals within the expected uncertainties were all less than 50% with the best result 61.5% at “NCU Taiwan”. Generally speaking, for the research area, the sampling window of 25km radius and ±0.5h was more suitable than 45km radius and ±1.0h. Although the MODIS AOT products over ocean produced at NSMC can assure certain precision, the validation results are not quite satisfying. Besides the influences of turbid water reflectance near shore, cloud contamination, aerosol model suitability and representative ability of aerosols over ocean by nearby land-based observation, we conclude that calibration of L1B data may be the important error source.