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The optical properties of aerosols, i.e., the way in which they scatter light, are coupled to their microphysical characteristics primarily through the surface area. The extinction coefficient is the integral contribution in a unit volume of each particle's cross-sectional area multiplied by its extinction cross section. The extinction cross-section is a function of the particle shape, size, refractive index and wavelength of the incident light. Thus, environmental processes that change these properties of aerosols will change their optical properties. Aerosols are sensitive to relative humidity (RH) changes when a hygroscopic aerosol begins to deliquesce, it's optical properties change as it takes on water. The diameter increases, thus changing the surface area, and the refractive index approaches that of water. This depends, of course, on the solulability of the aerosol. In November and December, 1996 measurements from the NCAR C-130 were made with a number of aerosol instruments over the ocean southwest of Tasmania as part of ACE-1. Total particle concentration was measured with CN counters and size distributions from 0.1 to 20 um were measured with the PMS PCASP and FSSP-300 probes. In addition, refractive index was derived from measurements by the NCAR MASP. These measurements provided the means to study how non-anthropogenic aerosols respond to changes in relative humidity. Further evidence of the coupling between the aerosol microphysical and optical properties s observed in vertical profile comparisons of these properties with measurements from hemispheric radiometers.
The measurements show a clear relationship between aerosol size and relative humidity. The directly measured scattering properties are also strongly correlated. The deliquescence hysteresis is seen as an abrupt increase in size at 60% RH with another group of measurements showing a gradual decrease below 60% RH as particles evaporate. The scattered light measurements show that both forward and back scattered light decreases abruptly at the top of the boundary layer as RH decreases sharply in the free troposphere. Since the CN count shows little change at the boundary layer interface, the light scattering is primarily sensitive to particles larger than about 0.3 um, the lower threshold of the FSSP-300. The short wave radiation reaching the surface, as measured by the upward looking Eppley radiometer, decreases with decreasing altitude, reflecting the increased scattering by aerosols and higher water vapor. Comparison with an atmospheric radiation model indicates that scattering by aerosols dominate extinction of broadband radiation. Finally, there is a moderate correlation between aerosol refractive index and RH, but the variation in refractive index indicates the possibility of an externally mixed aerosol population.