Aerosol optical thickness (AOT) was retrieved using the Advanced Very High Resolution Radiometer (AVHRR) PATMOS-x Level-2b gridded radiances and the two-channel algorithm of the National Climatic Data Center (NCDC). The primary retrieval product is AOT at 0.63？μm channel. AOT is also retrieved at 0.83？μm or 1.61？μm channel for consistent check. The retrieval was made during day time, under clear sky and snow-free conditions, and over the global oceans. The spatial resolution is degree grid and the temporal resolution is both daily and monthly. The resultant AVHRR AOT climate data record (CDR) spans from August 1981 to December 2009 and provides the longest aerosol CDR currently available from operational satellites. This dataset is useful in studying aerosol climate forcing, monitoring long-term aerosol trends, and evaluating global air pollution and aerosol transport models over the global ocean. 1. Introduction Aerosols affect the climate through direct and indirect effects. For example, they affect the surface and atmospheric radiation budgets directly through scattering and absorbing shortwave radiation [1, 2]. They also affect climate indirectly through the interaction with clouds and precipitation in a number of intricate ways [3, 4]. Uncertainty in aerosol effects on climate is one of the largest uncertainties in climate forcing [5]. A long-term aerosol dataset with climate quality is essential in reducing the uncertainty of aerosol effect on climate. Satellite remote sensing of aerosols is the only way to provide aerosol data coverage over the globe for multidecadal time periods [6, 7]. Advanced Very High Resolution Radiometer (AVHRR) measurement is the longest operational global satellite observation of the Earth’s atmosphere and surface. Observed radiances/reflectances have been widely used to derive long-term products of atmospheric variables, such as cloud and aerosol (see, e.g., [8–12]). AVHRR radiances have been carefully recalibrated recently using more accurate Moderate Resolution Imaging Spectroradiometer (MODIS) radiances to achieve a better quality [13–15]. The newly calibrated radiances are aggregated into degree orbital grid in the AVHRR Pathfinder Atmospheric Extended (PATMOS-x) Level-2b climate data product [14]. Using the AVHRR PATMOS-x Level-2b top of atmosphere (TOA) radiances and the two-channel aerosol retrieval algorithm [12, 16] of the National Climatic Data Center (NCDC), aerosol optical thickness (AOT) has been retrieved over global oceans from August 1981 to December 2009, which forms the longest aerosol CDR currently
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