Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions - Part 1: The K-feldspar microcline

Abstract

Potassium-containing feldspars (K-feldspars) have been considered as key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar microcline, which is highly IN active. Freezing of emulsified droplets with microcline suspended in aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities aw =0.9-1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, Thet(aw), deviate strongly from T Δahetwhet (aw), the values calculated from the water-activitybased approach (where T Δahet whet (aw) = Tmelt(aw+Δahet w) with a constant offset Δahet w with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH+4 salts (molalities ≲1 mol kg-1 corresponding to aw ≳0.96), we find IN temperatures raised by up to 4.5K above the onset freezing temperature of microcline in pure water (Thet(aw = 1)) and 5.5K above T Δahet whet (aw), revealing NH3 and NH+4 to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH3 and NH+4 solutions show a depression of the onset temperature below Δahet whet (aw) by as much as 13.5K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH+4 as cation exhibit nucleation temperatures Thet (aw)