We present an improved version of GACP (Global Aerosol Climatology Project) algorithm which uses channel 1 and 2 radiances of the Advanced Very High Resolution Radiometer (AVHRR) to retrieve aerosol optical thickness and Angstrom exponent over the ocean. We specifically discuss recent changes in the algorithm as well as the results of a sensitivity study analyzing the effect of several sources of retrieval errors not addressed previously. Uncertainties in the AVHRR radiance calibration (particularly in the deep- space count value) may be among the major factors potentially limiting the retrieval accuracy. On the other hand, the performance of two-channel algorithms weakly depends on a specific choice of the aerosol size distribution function. The updated algorithm is applied to a 10-year period of observations (July 1983 - Aug 1994), which includes data from NOAA-7, NOAA-9 (February 1985 - November 1988),and NOAA-11 satellites. The results are posted on the world wide web at http:gacp.giss.nasa.gov/retrievals. The NOAA-9 record shows no discernable long-term trends in the global and hemisphere averages of the optical thickness and Angstrom exponent. On the other hand, there is a discontinuity in the Angstrom exponent values derived from NOAA-9 and NOAA-11 data and a significant temporal trend in the NOAA-11 record. The latter are unlikely to be related to the Pinatubo eruption and may be indicative of a serious calibration problem.
We discuss the methodology of interpreting channel 1 and 2 AVHRR radiance data to retrieve tropospheric aerosol properties over the ocean and describe a detailed analysis of the sensitivity of monthly average retrievals to the assumed aerosol models. We use real AVHRR data and accurate numerical techniques for computing single and multiple scattering and spectral absorption of light in the vertically inhomogeneous atmosphere-ocean system. Our analysis shows that two-channel algorithms can provide significantly more accurate retrievals of the aerosol optical thickness than one-channel algorithms and that imperfect cloud screening is the largest source of errors in the retrieved optical thickness. Both underestimating and overestimating aerosol absorption as well as strong variability of the aerosol refractive index may lead to regional and/or seasonal biases in optical thickness retrievals. The Angstrom exponent appears to be the most invariant aerosol size characteristic and should be retrieved along with optical thickness as the second aerosol parameter.