Ice nucleation of bare and sulfuric acid-coated mineral dust particles and implication for cloud properties: Ice formation on dust particles

Ice nucleation properties of atmospherically relevant dust minerals coated with soluble materials are not yet well understood. We determined ice nucleation ability of bare and sulfuric acid-coated mineral dust particles as a function of temperature (−25 to −35°C) and relative humidity with respect to water (RHw; 75 to 110%) for five different mineral dust types: (1) Arizona test dust, (2) illite, (3) montmorillonite, (4) K-feldspar, and (5) quartz. The particles were dry dispersed and size selected at 200 nm, and we determined the fraction of dust particles nucleating ice at various temperatures and RHw. Under water-subsaturated conditions, compared to bare dust particles, we found that coated particles showed a reduction in their ice nucleation ability. Under water-supersaturated conditions, however, we did not observe a significant coating effect (i.e., the bare and coated dust particles had nearly similar nucleating properties). X-ray diffraction patterns of the coated particles indicated that acid treatment altered the crystalline nature of the surface and caused structural disorder; thus, we concluded that the lack of such structured order reduced the ice nucleation efficiency of the coated particles in deposition ice nucleation mode. In addition, our single column model results show that coated particles significantly modify cloud properties such as ice crystal number concentration and ice water content compared to bare particles in water-subsaturated conditions. However, in water-supersaturated conditions, cloud properties differ only at warmer temperatures. These modeling results imply that future aged dust particle simulations should implement coating parameterizations to accurately predict cloud properties.