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

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 NH 3 , (NH 4 ) 2 SO 4 , NH 4 HSO 4 , NH 4 NO 3 , NH 4 Cl, Na 2 SO 4 , H 2 SO 4 , K 2 SO 4 and KCl, with solute concentrations corresponding to water activities a w = 0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, T het ( a w ), deviate strongly from T het Δ a w het ( a w ), the values calculated from the water-activity-based approach (where T het Δ a w het ( a w ) = T melt ( a w + Δ a w het ) with a constant offset Δ a w het with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH 3 and NH 4 + salts (molalities ≲ 1 mol kg −1 corresponding to a w ≳ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of microcline in pure water ( T het ( a w = 1)) and 5.5 K above T het Δ a w het ( a w ), revealing NH 3 and NH 4 + to significantly enhance the IN of the microcline surface. Conversely, more concentrated NH 3 and NH 4 + solutions show a depression of the onset temperature below T het Δ a w het ( a w ) by as much as 13.5 K 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 T het ( a w ) T het Δ a w het ( a w ) even at very small solute concentrations. In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from Δ a w het = const. indicates specific chemical interactions between particular solutes and the microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K + available on the microcline surface with externally added cations (e.g., NH 4 + ). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH 3 molecules hydrogen bonded on the microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen fraction in dilute NH 3 and NH 4 + solutions. Moreover, we show that aging of microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near-neutral solutions since most of it recovers when aged particles are resuspended in pure water. In contrast, exposure to severe acidity (pH ≲ 1.2) or alkalinity (pH ≳ 11.7) damages the microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for IN in airborne dust containing microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and acidic conditions to a 5 K enhancement during condensation freezing when microcline particles experience high humidity ( a w ≳ 0.96) at warm (252–257 K) and NH 3 /NH 4 + -rich conditions.