Kaufman, Y. J., L. A. Remer, D. Tanre, R. R. Li, R. Kleidman, S. Mattoo, R. Levy, T. Eck, B. N. Holben, C. Ichoku, J. Martins, and I. Koren, 2005: A critical examination of the residual cloud contamination and diurnal sampling effects on MODIS estimates of aerosol over ocean. IEEE Trans. Geosci. Remote Sens., 43 (12), 2886-2897.
Observations of the aerosol optical thickness (AOT) by the MODIS instruments aboard Terra and Aqua satellites are being used extensively for applications to climate and air quality studies, as indicated from 28 publications in 2004 alone. Data quality is essential for these studies. Here we add to the published MODIS validations by investigating the effects of unresolved clouds on the MODIS measurements of the AOT. The main cloud effect is from residual cirrus that increases the measured AOT by 0.015±0.003 at 0.55 µm. In addition, lower level clouds can add contamination. We examine the effect of lower clouds using the difference between simultaneously measured MODIS and AERONET AOT. The difference is positively correlated with the cloud fraction. However, interpretation of this difference is sensitive to the definition of cloud contamination vs. aerosol growth. If we consider this consistent difference between MODIS and AERONET to be entirely due to cloud contamination we get a total cloud contamination, of 0.025±0.005, though a more likely estimate is closer to 0.020 after accounting for aerosol growth issues. This reduces the difference between MODIS observed global aerosol optical thickness over the oceans and model simulations by half, from 0.04 to 0.02. However it is insignificant for studies of aerosol cloud interaction. We also examined how representative are the MODIS data of the diurnal average aerosol. Comparison to monthly averaged sunphotometer data confirms that either the Terra or Aqua estimate of global AOT is a valid representation of the daily average.
