Based on an improved algorithm, which uses channel 1 and 2 radiances of the Advanced Very High Resolution Radiometer (AVHRR), we retrieved aerosol optical thickness and Angstrom exponent over the ocean. Data from AVHRR instruments on board NOAA-7, -9, -11, and -14 satellites were processed and a preliminary aerosol climatology for 1983 - 1998 was complied (http://gacp.giss.nasa.gov/retrievals). Uncertainties in the AVHRR radiance calibration (particularly in the deep-space count value) are among the major factors limiting the retrieval accuracy. Manifestations of this include an observed discontinuity in exponent at the transition between NOAA-9 and NOAA-11 and a trend in Angstrom exponent during NOAA-11 lifetime. In order to produce a consistent multi-satellite climatology we are developing a technique to "equalize" data from different satellites. It consists of adjusting calibration coefficients in order to satisfy the following requirements: a) a continuity must be achieved at transitions between successive satellites and b) global mean values of aerosol optical thickness should be equal over comparable periods of time. We discuss the application of this technique to the available data set. Our aerosol record reveals a seasonal cycle with wintertime maxima and summertime minima in the globally averaged aerosol optical thickness. The Northern hemisphere mean optical thickness systematically exceeds that averaged over the Southern hemisphere. Zonal means of the optical thickness exhibit an increase in the tropical regions of the Northern hemisphere associated with annual desert dust outbursts and a spring time increase at middle latitudes of the Northern hemisphere. Increased aerosol loads observed at middle latitudes of the Southern hemisphere are probably associated with higher sea salt particle concentrations.