The inter-comparison of MODIS, MISR and GOCART aerosol products against AERONET data over China

Abstract

Aerosols in East Asia have significant direct and indirect effects on the Earth's climate change. Several aerosol products have been generated to provide long-term monitoring of aerosol properties, but the discrepancies among them make the reliability of the products in doubt which leads to the uncertainty in the understanding of the long-time series of aerosols in East Asia, and especially in China. In this study, we investigate the applicability of Aerosol Optical Thickness (AOT) products derived from the measurements or model results of MODIS, MISR and GOCART through a statistical comparison, with Aerosol Robotic Network (AERONET) measurements over four sites located in China during a long-term period (2001–2011). Analysis shows that (1) only 43.06% of MODIS retrieved AOT values fall within ±(0.15τ+0.05) of the paired validation data from the AERONET for most sites, lower than the global averaged level (>66%) announced by Levy and Remer, in addition, MODIS/Terra shows a decreasing trend (−0.009/yr.) while MODIS/Aqua shows an increasing trend (+0.0012/yr.) and provides more stable AOT retrievals than MODIS/Terra during 2001–2011; (2) about 66.82% of MISR retrieved AOTs fall within the error envelop and MISR also shows a high correlation around 0.88 with AERONET data; (3) only 26.82% of GOCART model calculated AOTs fall within ±(0.15τ+0.05) of the paired validation data from the AERONET. GOCART also shows a low correlation (R=0.22–0.60) with AERONET data and thus cannot be well applied to total column AOT. Comparison of MODIS, MISR and GOCART products over the four sites in China and four sites located separately in Europe, North Africa and USA is carried out, which shows lower retrieval accuracy and larger discrepancy of products in China region. This is attributed to the fewer match-ups, larger aerosol load, moderate level of aerosol absorption and the complex nature of the aerosol mixtures. The analysis indicates the need for multiple-viewing-angle measurements of both intensity and polarization to provide the relevant aerosol parameters with sufficient accuracy for climate research.