Enhanced ice nucleation efficiency of microcline immersed in dilute NH 3 and NH 4 + -containing solutions

Potassium containing feldspars (K-feldspars) have been considered 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 ice nucleation onset temperatures, T het ( a w ) deviate strongly from T het ∆ a w het ( a w ), the values calculated from the water-activity-based approach (where T het ∆ a w het ( a w ) = T melt ( a w + ∆ a w het ) with a constant offset ∆ 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 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 ∆ 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 ∆ 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 ∆ a w het ( a w ) even at very small solute concentrations. In all these cases, the fraction of water volume frozen heterogeneously displays a decrease as solute concentration increases. This deviation from ∆ 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 re-suspended in pure water. In contrast, exposure to severe acidity (pH ~ 11.7) damages the microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for ice nucleation on airborne dust containing microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and NH 3 /NH 4 + -free 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.