AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Effect of Relative Humidity (RH) on Sulfate Aerosol Optical Properties Using Cavity Ring-Down Spectroscopy
XIJING ZHU, Dean Atkinson, Portland State University
Abstract Number: 614 Working Group: Aerosol Chemistry
Abstract Inorganic nitrate and sulfate compose a major fraction of the ambient aerosol and there is a strong correlation between visibility degradation and sulfate mass concentration. Inorganic salts are hygroscopic by nature and can grow rapidly in size with increasing relative humidity (RH); the optical properties depend strongly on particle size. In contrast with some other salts, sulfate undergoes 2 ionization steps generating sulfate, bisulfate and sulfuric acid; all of which display quite different hygroscopic behavior. While the pure salts have been studied extensively, the hygroscopicity of bisulfate/ sulfate mixtures is more uncertain. The chemical composition of the aerosol will determine the refractive indices of dry particles and water uptake, thus it is necessary to understand the chemical effect combined with the RH effect for sulfate aerosol and how sulfate aerosol behaves absent the complications of ambient aerosol. Laboratory-generated mixed composition aerosol systems are studied using a Humidity-Controlled Cavity Ring-Down extinction instrument. The extinction coefficients of varying composition salt mixtures of NH$_4HSO$_4, (NH$_4)$_2SO$_4, NaHSO$_4 and Na$_2SO$_4 are measured simultaneously at three widely spaced wavelengths (355,532 and 1064nanometer) and three RH conditions (~25%, ~45% and ~80%). Using three wavelengths, the effective radius of the particle size distribution can be derived using the approach derived by O’Neill for Aeronet data[Atkinson et al., 2010]. The RH-dependent radii can then be used to determine the particle hygroscopicity parameter. These experimental results are compared to existing hygroscopic aerosol models with an ultimate goal of better understanding of the behavior of sulfate aerosols in the ambient atmosphere.