Why Do Sulfuric Acid Coatings Influence the Ice Nucleation Properties of Mineral Dust Particles in the Atmosphere?

Laboratory studies with supermicrometer particles have shown that mineral particles coated with sulfuric acid are relatively poor ice nuclei. We investigated this phenomenon, which is of atmospheric relevance, by probing the structure of water at the mineral-aqueous acid interface as a function of the sulfuric acid concentration using sum frequency generation vibrational spectroscopy. We found that ordered water structures at water/mica interfaces drastically diminished at molarities of sulfuric acid equal to 0.5 M and totally disappeared when the molarities reached 5 M. The decrease in ordered water structures at the interface was caused by a combined effect of the decreased mica surface potential at low pH, the adsorption of sulfates on mica, and the lack of free water molecules in high concentrations of acidic solution. The good ice nucleation ability above liquid water saturation is correlated with the presence of structured water, suggesting that structured water at the interface may be needed for efficient heterogeneous ice nucleation.