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Abstract
Systematic characterization of aerosol over the oceans is needed to understand the aerosol effect on climate and on transport of pollutants between continents. Reported are the results of a comprehensive photosensitive (optical) and physical characterization of ambient aerosol in five key island locations of the Aerosol Robotic Network (AERONET) of sun and sky radiometers, spanning over 2–5 yr. The results are compared with aerosol optical depths and size distributions reported in the literature over the last 30 yr. Aerosol found over the tropical Pacific Ocean (at three sites between 208S and 208N) still resembles mostly clean background conditions dominated by maritime aerosol. The optical thickness is remarkably stable with mean value of ta(500 nm) 5 0.07, mode value at tam 5 0.06, and standard deviation of 0.02–0.05. The average A˚ ngstro¨m exponent range, from 0.3 to 0.7, characterizes the wavelength dependence of the optical thickness. Over the tropical to subtropical Atlantic (two stations at 78S and 328N) the optical thickness is significantly higher: ta(500 nm) 5 0.14 and tam 5 0.10 due to the frequent presence of dust, smoke, and urban–industrial aerosol. For both oceans the atmospheric column aerosol is characterized by a bimodal lognormal size distribution with a fine mode at effective radius Reff 5 0.11 6 0.01 mm and coarse mode at Reff 5 2.1 6 0.3 mm. A review of the published 150 historical ship measurements from the last three decades shows that tam was around 0.07 to 0.12 in general agreement with the present finding. The information should be useful as a test bed for aerosol global models and aerosol representation in global climate models. With global human population expansion and industrialization, these measurements can serve in the twenty-first century as a basis to assess decadal changes in the aerosol concentration, properties, and radiative forcing of climate.