An iterative approach converting extinction enhancement from water uptake to physical growth factor for clay aerosol and the effect of refractive index

Abstract

Mineral dust aerosol is present throughout the atmosphere, though the climate impact of this aerosol type is still not well known. One of many factors that contributes to this uncertainty is how water uptake alters the physical and optical character of the aerosol. A sensitive optical technique, cavity ring down spectroscopy, was used to quantify changes in the extinction cross-section with increasing relative humidity for montmorillonite, Na-rich montmorillonite, and illite aerosol. An iterative approach for converting the measured optical extinction enhancement to physical growth factor over increasing relative humidity was employed to quantify the effect of changing both size and refractive index. The method demonstrates an important application of previously retrieved effective refractive indices allowing for the determination of average particle size for the non-spherical particles from simple Mie theory calculations. The changed refractive index and particle size were used to show how these properties combine and interact to alter the distribution between scattering and absorption for the aerosol. The water uptake shifted the calculated single scattering albedo of the aerosol species more toward scattering when both optical and size changes were accounted for, as opposed to just size; limitations on these calculations are discussed. These findings demonstrate the necessity of understanding the effect of optical properties and size together, especially as many climate models do not account for water uptake properties of clay aerosol. Copyright © 2020 American Association for Aerosol Research.