Hygroscopic properties of saline mineral dust from different regions in China

Saline mineral dust particles, emitted from saline topsoil in arid and semi-arid regions, contribute significantly to tropospheric aerosol particles. However, hygroscopic properties of saline mineral dust particles, especially for those found in regions other than North America, are poorly understood. In this work we investigated hygroscopic properties of thirteen saline mineral dust samples collected from different locations via measuring sample mass change as different relative humidity (RH, up to 90%), and measured their chemical and mineralogical compositions using ion chromatography and X-ray diffraction. The mass growth factors at 90% RH, defined as the sample mass at 90% RH relative to that at <1% RH, were found to display large geographical variations, spanning from ~1.02 to 6.7, and the corresponding single hygroscopicity parameters (κ) were derived to be in the range of <0.01 to >1.0. The saline components (mainly Na⁺, Cl^- and SO₄^2-) contained by saline mineral dust particles largely determined their hygroscopicity, and the predicted mass growth factors at 90% RH using an aerosol thermodynamic model (ISORROPIA-II), agreed with measured values within 20% for most of samples examined, though larger discrepancies also occurred for three samples. Our results improve our understanding in hygroscopicity of saline mineral dust particles and thus their heterogeneous chemistry and ability to serve as cloud condensation nuclei to form cloud droplets.