Study of Cloud Contamination on the Aerosol Optical Thickness (AOT) Trend Derived from AVHRR AOT Climate Data Record (CDR)

Sub-pixel cloud contamination is one of the major issues plaguing passive satellite aerosol remote sensing. Its impact on the aerosol optical thickness (AOT) retrieval has been analyzed by many studies. However, the question of how it influences the AOT trend remains to be answered. In this paper, four AVHRR AOT datasets from 1981-2009 over global oceans for four different definitions of clear-sky, respectively, are produced and a global analysis of the effect of cloud contamination on the AVHRR AOT retrieval as well as on its trend is then performed by comparing the results from the four datasets. It was found that cloud contamination imposes not only a positive bias on AOT values but also a positive bias on its long-term trend such that negative trends become less negative and positive trends become more positive. A cloud probability value of ≤ 1% has been identified as an optimal criterion for producing AVHRR AOT climate data record (CDR). In order for a satellite aerosol product to be useful and reliable in aerosol trend detection, the cloud contamination effect on aerosol trends needs to be studied and evaluated carefully along with the effects of calibration error, surface disturbance, and aerosol model assumptions.