Measured and Modeled Light Scattering Values for Dry and Hydrated Laboratory Aerosols

Abstract

Closure experiments were completed to compare measured and modeled aerosol optical properties and their dependence on controlled relative humidity (RH) and wavelength of light. NaCl, (NH4)2SO4, and NH4NO3 aerosol particles with approximate geometric mass mean diameters of 0.2 ?m and geometric standard deviations of 1.7 were tested as part of this study. High evaporative losses (up to 40%) were observed for NH4NO3 aerosol at this particle size range due to heating, and the results from these tests have been excluded from the closure analysis. Aerosol optical properties were measured with a RH-scanning nephelometry system (humidograph) and modeled with a Mie?Lorentz light scattering model. Particle size distributions were measured with a scanning differential mobility analyzer. Closure between the measured and modeled values of the total light scattering coefficient (σsp), backscatter ratio (b), and Ångström exponent (å) for dry (low RH) aerosols was achieved within 0.0%?5%, 4%?15%, and 3%?17%, respectively. The values of f(RH), hemispheric b, and å at 80% RH agreed within 2%?27%, 1%?27%, and 1%?28%, respectively. Correcting for nephelometer nonidealities, including a heating artifact, improved the agreement between the measured and predicted σsp values at RH = 80% from 35% to 13% for the TSI nephelometer at the maximum heating condition, and from 18% to 11% for the Radiance Research, Inc. (RR), nephelometer. Accurate quantification of the closure for these optical properties is important when establishing visibility standards, and assessing the progress toward meeting those standards, as well as reducing the uncertainties in estimating radiative forcing due to those aerosols.