Characterization of tropospheric aerosols over the oceans with the NOAA advanced very high resolution radiometer optical thickness operational product

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Abstract
The National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) is an instrument on a polar orbiting satellite that provides information on global aerosol distributions. The remote sensing algorithm is based on measurements of backscattered solar radiation which yield a measure of the “radiatively equivalent” aerosol optical thickness τAsat(EAOT) over the oceans. Seasonally composited EAOT data for the period July 1989 to June 1991 reveal many spatially coherent plume‐like patterns that can usually be interpreted in terms of known (or reasonably hypothesized) sources in association with climatological wind fields. The largest and most persistent areas of high EAOT values are associated with wind‐blown dust and biomass burning sources; especially prominent are sources in Africa, the middle East, and the Asian subcontinent. Prominent plumes over the midlatitude North Atlantic are attributed to pollution emissions from North America and Europe. Large plumes attributed to pollution aerosols and dust from sources in Asia are clearly visible over the western and central North Pacific. On a global scale the annually averaged northern hemisphere EAOT values are about 1.7 times greater than those in the southern hemisphere. Considering each hemisphere separately, EAOT values in summer are about twice those in winter. Within the midlatitude band 30°–60° (i.e., where anthropogenic emissions are greatest) the summer/winter ratio is about 3. The temporal variability of monthly mean EAOT in specific ocean regions often shows characteristic seasonal patterns that are usually consistent with aerosol measurements made in the marine boundary layer. Nonetheless, there are features in the EAOT distributions that can not be readily interpreted at this time. The AVHRR EAOT distributions demonstrate that satellite products can serve as a useful tool for the planning and implementation of focused aerosol research programs and that they will be especially important in studies of climate‐related processes.