A pure marine aerosol model, for use in remote sensing applications

Abstract

Retrievals of aerosol optical depth (AOD) and related parameters from satellite measurements typically involve prescribed models of aerosol size and composition, and are therefore dependent on how well these models are able to represent the radiative behavior of real aerosols. This study uses aerosol volume size distributions retrieved from Sun-photometer measurements at 11 Aerosol Robotic Network (AERONET) island sites, spread throughout the world's oceans, as a basis to define such a model for pure (unpolluted) maritime aerosol. Volume size distributions are observed to be bimodal and approximately lognormal, although the coarse mode is skewed with a long tail on the low-radius end. The relationship of AOD and size distribution parameters to meteorological conditions is also examined. As wind speed increases, so do coarse-mode volume and radius. The AOD and Ångström exponent show linear relationships with wind speed, although with considerable scatter. Links between aerosol properties and near-surface relative humidity, columnar water vapor, and sea surface temperature are also explored. A recommended bimodal maritime model, which is able to reconstruct the AERONET AOD with accuracy of order 0.01–0.02, is presented for use in aerosol remote sensing applications. This accuracy holds at most sites and for wavelengths between 340 nm and 1020 nm. Calculated lidar ratios are also provided, and are in the range of other studies, although differ more strongly from those currently used in Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) processing.