Assessment and comparison of three years of Terra and Aqua MODIS Aerosol Optical Depth Retrieval (C005) in Chinese terrestrial regions

The latest MODIS aerosol optical depth (AOD) retrieval algorithm (C005) based on analyses of global AERONET data for both Terra and Aqua satellites has gradually supplanted the older algorithm (C004) since 2006. However, due to few AERONET sites in China, assessing applications of MODIS products over Chinese terrestrial regions for both Terra and Aqua is very necessary, including evaluating and comparing differences between Terra and Aqua. In this study, three years of Terra and Aqua AODs (C005) were analyzed over Chinese terrestrial regions using long-term and large-scale ground-based network observation data, from the Chinese Sun Hazemeter Network (CSHNET). The results show that C005 MODIS aerosol products (C005) have enough accuracy over China, with the same correlation coefficient (R) ∼ 0.86, the slope ∼ 1.006 and 0.990, the offset ∼−0.043 and − 0.038, 53% and 50% of collocated data within the expected errors for Terra and Aqua, respectively, though there existed differences in different ecosystems and geographic locations. Overall, the MODIS products underestimated by 0.03 or 9% for Terra, 0.02 or 6% for Aqua, and more underestimation (11–16%) was noted in summer and autumn, which is based on annual and seasonal means of all sites. For different Angstrom exponent (α) ranges, the MODIS underestimated by 0.03 or 8% at the moderate α, and 0.07 or 27% for the fine (α > 1.5) and coarse (α < 0.5) modes, which dominated for both Terra and Aqua. Remarkably, there are no very significant differences between Terra and Aqua AODs based on long-term statistical Terra and Aqua AODs for different Angstrom exponent (α) ranges and regional monthly averages Over China, which confirms that either the Terra or Aqua estimate of AOD over China is a valid representation of the daily average, though there may be a diurnal cycle near aerosol emission regions. Moreover, the magnitude of the differences of regional monthly mean Terra and Aqua AODs varied from region to region and increased with aerosol loading, but there was no significantly consistent negative or positive bias between Terra and Aqua AODs in each region over China.