Crystallization of Sulfate and Nitrate Coatings on Tropospheric Mineral Particles

Many aqueous atmospheric particles contain insoluble inclusions such as soot or mineral dusts. Mineral components include silicates, aluminosilicates, and iron oxides. Large regions of the troposphere are infused with mineral dust particles from northern Africa and Gobi deserts. There are also local regional emissions in air areas such as the southwestern USA. These materials are crystalline and could provide well-ordered atomic arrays that impart local order into nearby aqueous solutions and thus increase the frequency of critical cluster formation.

Our first paper on heterogeneous nucleation required the invention of aerosol technology to create mineral dust particles having coatings of ammonium sulfate. We showed that aqueous ammonium sulfate layers crystallize at 57% relative humidity (RH) on Al₂O₃, at 59% RH on ZrO₂, and at 65% RH on TiO₂ (298 K), which can be compared to 35% when only homogeneous nucleation is active.

In that work, we aerosolized commercial particles of Al₂O₃, ZrO₂, and TiO₂, so we were unable to adjust the size and chemistry of these particles. Hence, we developed aerosol technology for controlling the diameters of Al₂O₃, Fe₂O₃, and SiO₂ particles (50 to 500 nm) and the sulfate or nitrate coating thickness (1 to 100 nm). We then studied the dependence of the crystallization relative humidity on oxide diameter and chemistry, developing a model of lattice matching for heterogeneous nucleation to explain the observed CRH series.

Our results are expressed as quantitative equations useful for incorporation in larger scale atmospheric models:

where AS and AN denote ammonium sulfate and ammonium nitrate coatings and D_c and D_h denote the diameters of Al₂O₃ (corundum) and Fe₂O₃ (hematite) inclusions.