Hygroscopic properties of particles nebulized from water extracts of aerosols collected at Chichijima Island in the western North Pacific: An outflow region of Asian dust

We present 2 year measurements of hygroscopic properties of water-soluble matter (WSM) extracted from marine aerosols from remote Chichijima Island in the western North Pacific during 2001-2002. Hygroscopic growth factors (g) of WSM were measured by a hygroscopicity tandem differential mobility analyzer with initial dry particle diameter of 100 nm. The observed g at 90% relative humidity (RH), g(90%), ranged from 1.42 to 1.89 with an average of 1.790.11. The g values are significantly lower than that of seawater (2.1) and slightly lower than those previously reported for marine aerosols (>1.8), probably due to the atmospheric processing associated with chlorine depletion; mean Cl-/Na+ molar ratio (1.100.23) was smaller than seawater (1.18), and organometal interaction (e.g., formation of water-insoluble calcium oxalate, g(90%)<1). Inorganic salts accounted for 87-98% of WSM. Na+ and Cl- are two major species, contributing 63% of total inorganic ion mass. The calculated aerosol water content (V-w/V-dry) at 85% RH during hydration experiment ranged from 1 to 3.66 (mean of 2.750.81). V-w/V-dry is negatively correlated with organic mass fraction, indicating that organics may suppress the hygroscopicity of the marine aerosol particles. The declined g(90%) and Cl-/Na+ molar ratio and increased abundance of water-soluble organics in spring demonstrated that the atmospheric mixing of anthropogenic pollutants (e.g., NO3, SOx) and water-soluble organics can decrease the growth factor of marine aerosols. The two case studies of spring aerosols demonstrated that Asian dusts were internally mixed with hygroscopic species in different ways, depending on their transport pathway.